the structures of langmuir blodgett films of fatty acids...

Advances in Colloid and Interface ScienceŽ .91 2001 163�219

The structures of Langmuir�Blodgett filmsof fatty acids and their salts

J.B. Peng, G.T. Barnes, I.R. Gentle�

Department of Chemistry, The Uni�ersity of Queensland, Brisbane, Queensland 4072, Australia

Abstract

Recent advances in several experimental techniques have enabled detailed structuralŽ .information to be obtained for floating Langmuir monolayers and Langmuir�Blodgett

films. These techniques are described briefly and their application to the study of films offatty acids and their salts is discussed. Floating monolayers on aqueous subphases have beenshown to possess a complex polymorphism with phases whose structures may be comparedto those of smectic mesophases. However, only those phases that exist at high surface

Ž .pressures are normally used in Langmuir�Blodgett LB deposition. In single LB monolay-ers of fatty acids and fatty acid salts the acyl chains are in the all-trans conformation withtheir long axes normal to the substrate. The in-plane molecular packing is hexagonal withlong-range bond orientational order and short-range positional order: known as the hexatic-Bstructure. This structure is found irrespective of the phase of the parent floating monolayer.The structures of multilayer LB films are similar to the structures of their bulk crystals,consisting of stacked bilayer lamellae. Each lamella is formed from two monolayers of fattyacid molecules or ions arranged head to head and held together by hydrogen bondingbetween pairs of acids or ionic bonding through the divalent cations. With acids the acylchains are tilted with respect to the substrate normal and have a monoclinic structure,whereas the salts with divalent cations may have the chains normal to the substrate or tilted.The in-plane structures are usually centred rectangular with the chains in the trans

Abbre�iations: LB, Langmuir�Blodgett; SPM, scanning probe microscopy; AFM, atomic forcemicroscopy; TEM, transmission electron microscopy; BAM, Brewster angle microscopy; GIXD, grazingincidence X-ray diffraction; SAXS, small angle X-ray scattering; FTIR, Fourier transform infrared;ATR, attenuated total reflection; RAS, reflection�absorption spectroscopy; QCM, quartz crystal mi-crobalance

� Corresponding author. Tel. �61-7-3365-4800; fax: �61-7-3365-4299.Ž .E-mail address: [email protected] I.R. Gentle .

0001-8686�01�$ - see front matter � 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S 0 0 0 1 - 8 6 8 6 9 9 0 0 0 3 1 - 7

( )J.B. Peng et al. � Ad�ances in Colloid and Interface Science 91 2001 163�219164

conformation and packed in a herringbone pattern. Multilayer films of the acids show only asingle-step order�disorder transition at the melting point. This temperature tends to rise asthe number of layers increases. Complex changes occur when multilayer films of the saltsare heated. Disorder of the chains begins at low temperatures but the arrangement of thehead groups does not alter until the melting temperature is reached. Slow heating to atemperature just below the melting temperature gives, with some salts, a radical change inphase. The lamellar structure disappears and a new phase consisting of cylindrical rods lyingparallel to the substrate surface and stacked in a hexagonal pattern is formed. In each rodthe cations are aligned along the central axis surrounded by the disordered acyl chains.� 2001 Elsevier Science B.V. All rights reserved.

Keywords: Fatty acids; Langmuir-Blodgett films; Structure determination; Grazing incidence; X-raydiffraction; Monolayers

Contents

1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1651.1. Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1651.2. Salts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1651.3. Unit cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1651.4. Structure nomenclature of bulk fatty acids . . . . . . . . . . . . . . . . . . . . . . 1661.5. Angle references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167

2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1673. The main characterisation techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168

3.1. The surface film balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1683.2. X-Ray scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1683.3. Neutron scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1703.4. Transmission electron diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1703.5. Vibrational spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

3.5.1. Fourier transform infrared spectroscopy . . . . . . . . . . . . . . . . . . . 1703.5.2. Raman spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171

3.6. Scanning probe microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1714. Monolayers at the air�liquid interface � Langmuir films . . . . . . . . . . . . . . . . . 172

4.1. Isotherms and phase diagrams of Langmuir films . . . . . . . . . . . . . . . . . . 1724.2. Effects of pH and cation on the isotherm and phase diagram . . . . . . . . . . 1734.3. The structures of monolayers in different phases . . . . . . . . . . . . . . . . . . 173

5. Langmuir�Blodgett deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1745.1. Conditions for LB deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1745.2. Deposition of single monolayer LB films . . . . . . . . . . . . . . . . . . . . . . . 1765.3. The speed of LB deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1765.4. Effect of subphase chemistry on the composition of LB films of fatty acid

salts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1766. Structures of single-monolayer LB films of fatty acids and their salts . . . . . . . . . . 177

6.1. Basic structure of single monolayer films � hexatic packing . . . . . . . . . . . 1786.2. Conformation of the hydrocarbon chains . . . . . . . . . . . . . . . . . . . . . . . 180

( )J.B. Peng et al. � Ad�ances in Colloid and Interface Science 91 2001 163�219 165

6.3. Orientation of the long axes of molecules . . . . . . . . . . . . . . . . . . . . . . . 1806.4. Effect of substrates on the structures of monolayer films . . . . . . . . . . . . . 1826.5. Structural correlation between LB monolayers and the parent floating

monolayers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1836.6. Composition of single monolayers of soaps . . . . . . . . . . . . . . . . . . . . . . 185

7. Structures of LB multilayer films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1857.1. Comparison of multilayer and bulk crystal structures . . . . . . . . . . . . . . . 1857.2. Basic structure of LB films of fatty acids and their salts . . . . . . . . . . . . . . 186

7.2.1. In-plane structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1867.2.2. Chain tilt and orientation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1877.2.3. Structure of lamellae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188

7.3. Polymorphism in LB films of fatty acids . . . . . . . . . . . . . . . . . . . . . . . . 1887.4. Evolution of the structure with number of layers . . . . . . . . . . . . . . . . . . 1897.5. Rearrangement of the first layer in multilayer films . . . . . . . . . . . . . . . . 1917.6. Epitaxial growth in multilayers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1917.7. Correlation between the structures of floating monolayers and

deposited multilayers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1937.8. Structures of the outermost layer in multilayer films . . . . . . . . . . . . . . . . 1937.9. Superstructure in LB films of fatty acid salts . . . . . . . . . . . . . . . . . . . . . 195

7.10. Effect of dipping direction on the structure of the LB film . . . . . . . . . . . . 1957.11. Artificial effect on the image in AFM measurements . . . . . . . . . . . . . . . 204

8. Thermally-induced phase changes in LB films of fatty acids and their salts . . . . . . . 2058.1. Thermal behaviour of fatty acid LB films . . . . . . . . . . . . . . . . . . . . . . . 2058.2. Thermal behaviour of LB films of fatty acid salts . . . . . . . . . . . . . . . . . . 206

8.2.1. Monolayer LB films of fatty acid salts . . . . . . . . . . . . . . . . . . . . 2078.2.2. Multilayer LB films of fatty acid salts . . . . . . . . . . . . . . . . . . . . . 2088.2.3. Formation of hexagonally-packed cylindrical structures . . . . . . . . . 211

9. Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214

1. Terminology

1.1. Acids

Abbreviations for acid names are given in Table 1.

1.2. Salts

Abbreviations for salts are formed by replacing the H in the abbreviation for theacid with the chemical symbol for the cation. The charge on the cation is notconsidered, so for example, cadmium stearate is CdSt, not CdSt .2

1.3. Unit cells

In two-dimensional space there are five types of unit cell, but only three arefound in LB films. These are shown in Fig. 1.


Table 1Abbreviations for acid names

Abbreviation Common name Systematic name Chemical Formula

Ž .HMy Myristic acid Tetradecanoic acid CH CH COOH3 2 12Ž .HPa Palmitic acid Hexadecanoic acid CH CH COOH3 2 14Ž .HSt Stearic acid Octadecanoic acid CH CH COOH3 2 16Ž .HAr Arachidic acid Eicosanoic acid CH CH COOH3 2 18Ž .HBe Behenic acid Docosanoic acid CH CH COOH3 2 20

Fig. 2 shows the herringbone packing that arises when the acyl chains are in theall-trans conformation, are unable to rotate about the chain axes, and thus projectnon-circular outlines onto the film plane.

1.4. Structure nomenclature of bulk fatty acids

X-Ray and IR spectroscopic studies of fatty acid crystals have revealed several� �different crystalline forms 1�4 . Those that are relevant to the structures found in

LB films of the acids are described below.A : two dimers in unit cell; the planes of the acyl chains are parallel and there is2

Ž .alternating cis and trans hydrogen bonding of the acid head-groups see Fig. 4 .B: monoclinic; c-axis tilted toward b-axis; larger tilt than A; two dimers in unit

Žcell; planes of the acyl chains perpendicular to one another i.e. a herringbone.pattern ; all trans bonding except for the first C�C bond; cis H-bonding of

carboxylic groups.C: most stable form; monoclinic; c-axis tilted toward a-axis, larger tilt than B;

two dimers in unit cell; herringbone chain pattern; trans H-bonding of carboxylicgroups.

Fig. 1. Two-dimensional arrangements of molecules and corresponding unit cells as observed inLangmuir�Blodgett films.


Fig. 2. Herringbone arrangement of chains.

E: metastable, transforming to B form; similar to B form but has all transbonding in the acyl chains.

1.5. Angle references

Throughout this review the angle of incidence for the radiation used in examin-ing floating and LB films is always, for consistency, measured from the surfaceplane. This is the usual terminology in grazing incidence X-ray and neutronscattering techniques, but differs from the convention in spectroscopy where anglesusually are measured from the surface normal.

The tilt of the acyl chain axis is always measured from the surface normal.

2. Introduction

Over the last two decades the precision and sensitivity of a number of experi-mental techniques have been improved to such an extent that they can now beused to probe the structure and arrangement of molecules in extremely thin films

Ž .such as floating monolayers Langmuir films on an aqueous subphase andŽ .Langmuir�Blodgett LB films on solid substrates. These techniques include X-ray

diffraction and reflection, neutron reflection, infrared and Raman spectroscopy,electron diffraction, and the scanning probe microscopy methods. Larger structurescan be observed by fluorescence and Brewster angle microscopy and by surfaceplasmon excitation, while the quartz crystal microbalance may also provide usefuldata. These developments have led to major advances in our understanding of bothLangmuir and LB films, sometimes confirming ideas that had been based onearlier macroscopic measurements, but often revealing new and hitherto unsus-pected detail.

The early literature on spread monolayers at the air�water interface and on LB� �films was comprehensively reviewed by Gaines 5 in 1966. Later reviews on LB

� � � � � �films have been contributed by Petty 6 and Roberts 7 , while Ulman 8 hasdiscussed both LB and self-assembled films and given a very detailed introduction

Ž . � �to many characterisation techniques. More recently 1997 , Schwartz 9 has pre-sented an excellent review of the structures of LB films. The significance and

� � � �applications of LB films have been surveyed by Roberts 10 , Swalen et al. 11 and� �Peterson 12 .

The present review is focussed on the structures of the LB films of fatty acidsand especially of their salts, and also considers the thermal stability of these films.These materials were selected because of their relatively simple molecular struc-


tures and because they have been investigated more extensively than other sub-stances. Despite their structural simplicity, such materials exhibit rich and complexphase behaviour at the air�water interface and in LB films. They are the archetyp-ical LB film materials and an understanding of their LB film structures is basic toan understanding of all LB films.

By definition, LB films are formed from floating monolayers so the reviewincludes a brief overview of the structures of Langmuir films of these materials.First, however, it is necessary to survey briefly the technical advances that havemade the examination of thin-film structures possible.

3. The main characterisation techniques

3.1. The surface film balance

The surface film balance has been the principal instrument for manipulating and� �examining floating monolayers since its invention by Pockels 13 just over a

century ago. Over that time there have been steady improvements in precision andconvenience culminating in recent years with the introduction of computer controland data collection. For our present purposes, however, the most significant

� �advance was probably the introduction by Gaines 14 of the paper Wilhelmy platefor measuring surface pressure. Such plates, made of high-quality filter or chro-matography paper, are always wet when in contact with the aqueous subphase inthe Langmuir trough so the contact angle of the subphase liquid on the platesurfaces is always zero. This feature enables the plate to be used reliably in boththe compression and expansion of a monolayer and permits accurate automaticcontrol of surface pressure. Such control is particularly important when themonolayer must be held at constant surface pressure while some other measure-ment, such as those described below, is made. It is also essential for properLangmuir�Blodgett deposition.

3.2. X-Ray scattering

Ž .The X-ray scattering elastic scattering techniques for determining the struc-tures of floating monolayers and LB films include diffraction and reflectivity

� �measurements 15 . A diffraction pattern is a Fourier transform of the periodicdistribution of electron density in the film and thus provides information about the

1 3 ˚Ž .lattice structure of the film. Since Langmuir and LB films are very thin 10 �10 Athey are always supported on substrates which are much thicker than the films

Žthemselves, so the incident beam must be applied at a very low angle grazing.incidence in order to generate diffraction signals of sufficient intensity for obser-

vation.Ž .Grazing incidence X-ray diffraction GIXD requires a very bright source of

X-rays and during the last few decades the brilliant beams from synchrotronbending magnets and insertion devices have become available. The incident angle


must be below the critical angle for total external reflection and is usually less than0.2�. The penetration depth of the resulting evanescent wave is of the same orderas the thickness of a few monolayers so it is scattered by the Langmuir or LB film.Such scattering results in diffraction if the film has a repeating pattern of electrondensity.

In-plane diffraction is produced by a two-dimensional lattice structure in planesparallel to the substrate surface. In a Langmuir or LB monolayer there is no

Ž .periodic structure along the normal to the surface plane the z-direction soconstructive interference of the scattered waves cannot occur in this direction.

Ž .Diffraction can, however, occur in the surface plane the xy-plane . Most con-densed monolayers consist of domains with various orientations of the structuralpattern, so they are like a two-dimensional powder. Thus in the xy-plane there arealways some domains which are correctly oriented with respect to the X-ray beamto fulfil the Bragg condition for diffraction:

n�d �hk 2sin�

Ž .where d is the distance between the reflecting ‘planes’ or lines in 2D space , h,khkare the relevant Miller indices for the xy-plane, � is the wavelength, n � 1, 2, 3, ...,

Ž .and � is the angle in the surface plane between the beam and the reflecting‘planes’. The diffracted beam makes an angle 2� to the incident beam so bymeasuring this angle d can be determined.

However, even monolayers are not true 2D systems. Mostly they are formedfrom long-chain molecules and the orientation of the chains contributes to thediffraction. If the chain axes are normal to the surface the diffraction spots lie inthe xy-plane, but if the chains are tilted away from the normal the diffraction spotsare shifted along the z-direction and the magnitude of this shift can be used to

� �calculate the tilt angle of the chains 16 .In-plane correlation lengths can be calculated from the full width at half

maximum of the diffraction peaks, but allowance must be made for the instrumen-tal contribution.

For multilayer LB films there is a repeat structure in the z-direction. This resultsin a set of diffraction spots with the same 2� scattering angle in the xy-plane butshifted in the z-direction with vertical scattering angles � . The spacing of thesefspots gives the repeat distance in the z direction and hence information on thelayer structure of the LB film.

It is usual to report scattering angles in terms of the wave-vector transfer Q,calculated as follows:

1�22Ž .Q � 2�� 1 � cos � � 2cos� cos2�Ž .x y f f

Ž .Q � 2�� sin�z f


The diffraction patterns from Langmuir and LB films can be observed byscanning with a position sensitive detector and Soller collimator aligned normal tothe film plane. With LB films much faster results can be obtained by using imaging

� �plates 17,18 .The reflectivity signal is the Fourier transform of the gradient of electron density

Ž .along the normal z of the film surface and yields the d-spacing along the normal.The Q signal may be scanned by changing either the wavelength or the incidentzangle. It is much stronger than the diffraction signal so laboratory X-ray generatorsmay be used.

3.3. Neutron scattering

� �With neutron scattering 19 the scattering centres are the atomic nuclei ratherthan electrons. The scattering power of a nucleus depends on its scattering length,which varies considerably from one element to another and is different fordifferent isotopes of the same element. In particular, the deuterium nucleus has alarge positive scattering length whereas the hydrogen nucleus has a low negativevalue. Thus, one can use deuteration techniques to highlight part of a molecule orselected molecules or monolayers in a film and then with neutron scatteringinvestigate the film structure � a technique known as contrast variation. Availableneutron beams are not bright enough to generate useful diffraction patterns soonly grazing-incidence reflectivity is available. The neutron scattering techniquethus yields only the lattice constant and the structure along the direction normal tothe film surface.

3.4. Transmission electron diffraction

Ž . � �Transmission electron diffraction TED 20 can be used to determine thein-plane structure of an LB film and the orientation of the molecular long axis. Inprinciple, electron diffraction is much the same as X-ray diffraction, but relative toX-rays, the interaction of electrons with matter is much stronger, and thus thepenetration power of electrons through matter is much smaller. Consequently, adiffraction pattern can only be obtained with a very thin sample: an LB film mustbe deposited on a very thin and homogeneous amorphous substrate. Usually theelectron beam is normal to the film surface. Heat damage to the film at the pointwhere the electron beam impinges is inevitable, so an electron beam of low energyand a short exposure time are required.

3.5. Vibrational spectroscopy

3.5.1. Fourier transform infrared spectroscopyŽ .Fourier transform infrared FTIR spectroscopy is a very important tool for

investigating the structure of LB films, despite the thinness of the sample, as it ispossible to design the experimental set-up to give a satisfactory signal-to-noise

� �ratio 21 .


Ž .For floating monolayers, FTIR reflection�absorption spectroscopy FTIR-RAS� �has been shown by Dluhy and associates 22,23 and by others to be an effective

tool for investigating the conformations of the alkyl chains of monolayers. In thistechnique the infrared beam is reflected from the monolayer in a Langmuir troughusing an angle of incidence between 30� and 60�. Polarised or unpolarised radiationmay be used. In principle, FTIR-RAS could also be used to study the polar groupsof Langmuir monolayers, but the bands from these groups usually occur in thesame spectral region as water vapour absorption bands and adequate subtraction of

� �the latter is difficult. There are, however, some reports 24,25 that polarisationmodulation of the incident beam and phase-sensitive detection can be used toeliminate the water vapour absorption bands.

To examine LB films two types of infrared measurement are often used:Ž . Ž .attenuated total reflection ATR and grazing incidence reflection GIR . In the

ATR technique, the LB film is deposited onto both sides of an IR-transmittingŽ .crystal e.g. germanium or silicon . The radiation enters the crystal at one end so

that it is incident on the sides at an angle less than the critical angle. It thusundergoes multiple reflections inside the crystal before emerging at the far end.On each reflection the evanescent field of the IR beam penetrates the LB film andabsorption may take place.

� �GIR is performed on a highly reflective metal substrate 26 . When the electricŽvector of the IR radiation is perpendicular to the incident plane the plane of the

.incident and reflected waves , known as s-polarised radiation, there is a phase shiftof approximately 180� on the LB film�metal interface causing the electric fields ofthe incident and reflected radiation to cancel each other. In contrast, where theelectric vector of the IR radiation is in the incident plane, referred as p-polarised

Žradiation, the phase shift is approximately 90� for incidence angles of 2�5� relative. � �to the surface plane 27 . Thus IR absorption takes place when the vibrations

possess a transition dipole moment with a component perpendicular to thefilm�metal interface. As the s-polarised component of unpolarised radiationcontributes no structural information but gives noise to the detected signal,p-polarised IR light is normally used in the GIR measurements. This method isparticularly sensitive to the orientation of the molecular groups on the substrate.

3.5.2. Raman spectroscopyTo be Raman active a molecular vibration must cause some change in a

component of the molecular polarisation. Since Raman scattering is an inherently� �weak process, an enhancement technique is usually needed to study LB films 28 .

An increase in the signal can be achieved by depositing films onto the surfaces of� �noble metals such as Au or Ag 29 .

3.6. Scanning probe microscopy

Ž .Various scanning probe microscopy SPM techniques have become importanttools in the study of LB films in recent years. They all involve scanning a very fineprobe tip over the surface of the sample in a raster pattern, measuring the


interaction, and collating the information into an image of the surface. Underoptimum conditions the techniques approach molecular resolution but only scan a

Ž 2 2 .very small area of surface 100 nm �1 mm . Scanning tunnelling microscopyŽ .STM has very high lateral resolution but is limited to monolayers on conducting

Ž .substrates or to conducting films. Atomic force microscopy AFM does not havethis limitation, and can provide direct information on the surface morphology ofthe sample. All of the SPM techniques yield structural information about only thetop surface of a film.

AFM measures the interactions between probe and surface arising from van derWaals, electrostatic, frictional, capillary, or magnetic forces. Unlike electron mi-croscopy, it does not require a vacuum and can be operated in a wide variety ofenvironments.

4. Monolayers at the air �� liquid interface — Langmuir films

� � � � � �In the early work of Pockels 13 , Rayleigh 30 , and Langmuir 5,7,31 , Langmuirfilms of fatty acids with different chain lengths were shown to occupy the same

˚2Ž .molecular area approx. 20 A , and to be one molecule thick with the moleculesoriented with the polar functional group immersed in the water and the longnon-polar acyl chain directed nearly vertically up from the interface. Such mono-molecular films, or monolayers, at the air�liquid interface are now called Lang-muir films.

4.1. Isotherms and phase diagrams of Langmuir films

A Langmuir film is like a two-dimensional system. The compression isotherm ofŽ .a Langmuir film refers to a plot of surface pressure � as a function of area per

ˆŽ .molecule A . Surface pressure is the difference in surface tension between theŽ . Ž .clean surface of the subphase and the film-covered surface :0

� � � 0

With a surface film balance the surface pressure and the area per molecule maybe monitored during film compression or expansion. Traditionally, the isotherm

Ž . Ž .can be divided into four regions: gaseous G phase; liquid expanded LE or L1phase corresponding to a liquid-like state but not always observed; and the

Ž . Ž . Ž .condensed C phases, liquid condensed LC or L and ‘solid’ S . However, recent2work has shown that the phase behaviour of monolayers of long-chain compoundsis much more complex than this simple assignment implies.

The phase diagram of a monolayer can be obtained by measurements of eitherthe isotherms at different temperatures or the isobars at different surface pres-sures. The phase behaviour of monolayers of long-chain acids, esters and alcohols

� � � �have been investigated by Harkins et al. 32�35 , Stenhagen et al. 36,37 , Lundquist� � � � � �38,39 and recently by Peterson and co-workers 40 and Lawrie and Barnes 41 .


These studies indicate a complex pattern of phases in which the moleculararrangements cannot be deduced from the traditional macroscopic measurementsof surface pressure, molecular area, surface viscosity, and surface potential.

Some phase transitions are not detectable in the compression isotherm, but mayŽ . � �be observed by techniques such as Brewster angle microscopy BAM 42 and

� �GIXD 43 .

4.2. Effects of pH and cation on the isotherm and phase diagram

The pH of the subphase affects the ionisation of the carboxylic acid in a floatingmonolayer and consequently the interaction of the acid with cations in thesubphase. With a sufficiently high pH, divalent cations tend to cause a contractionof the monolayer with the LC � S phase transition occurring at a lower surface

� �pressure than at low pH or with the LC phase not appearing 44 . Cadmium and Pbhave a more covalent character to their bonding with carboxylic acids and produce

� �greater contraction of the monolayers than do other divalent ions 45 .Trivalent cations have not been extensively studied, but there are some recent

reports in which the rare earth cations, europium and terbium, have been shown to� �produce an expansion in the monolayer 46 , whereas yttrium gives a contraction

� �47 .

4.3. The structures of monolayers in different phases

The precise nature of the phases at molecular level was only revealed whengrazing incidence X-ray diffraction with brilliant synchrotron X-ray beams wasintroduced to study floating monolayers. The pioneering work is due to Dutta and

� � � �co-workers 48,49 and to Mohwald, Peterson, and their co-workers 50 . Fig. 3¨shows a phase diagram of n-docosanoic acid in which the phases correspond toeither liquid crystal or solid crystal states. Similar phase diagrams are found forother long-chain acids, shifted systematically along the temperature and surface

� �pressure axes according to the chain length 51 . The structures of the monolayer indifferent phases are listed in Table 2. However, phases other than those listed inTable 2 may exist. For example, the Ov phase is a phase in which the chains are

� �packed hexagonally but tilt towards the next nearest neighbour 42,43,52 . Peterson� � � � � �et al. 53 and Kaganer et al. 54 as well as Riviere et al. 55 have discussed in´

detail the floating monolayer phases and structures.It should be noted that the term phase is normally applied to a thermodynamic

equilibrium state. In the case of Langmuir films the ‘phases’ above the equilibriumŽ .spreading pressure e.g. LS, S occurring at high surface pressures are not strictly in

equilibrium states, but since, in practice, they are stable relative to the experimen-tal timescale, they may still be considered as phases.

In reality they are metastable states, and with time may change to an equilibrium� �state. Recently, for example, Kondrashkina et al. 54 studied with X-ray reflectivity

the time dependence of the structure of a floating monolayer of arachidic acid


Table 2aCondensed monolayer phases and the corresponding structures

Phase Name Structure Smectic In-plane area2˚Ž .category per chain A

bŽ .L Liquid-condensed Centred rectangular NN I 19.82� cŽ .L Liquid-condensed Centred rectangular NNN F or H 19.82

dŽ .LS Super-liquid Hexagonal V BH 19.8Ž .S Solid Centred rectangular V E 19.2Ž .CS Closed-packed solid Centred rectangular V Xtal 18.6

a � �Data from Petty 6 .b NN: chains tilt towards nearest neighbour.c NNN: chains tilt towards next nearest neighbour.d V: chains are normal to the film surface.

Ž . Ž . Ž .HAr on water and on aqueous solutions of CdCl and Pb NO pH � 5.3 . At2 3 230 mN m�1 and 21�C the films were initially in the S phase. When the relaxationtime was 500 s, only a couple of reflectivity minima were observed, correspondingto the d spacing from a monolayer, but if the relaxation time was greater than0011000 s, a set of peaks corresponding to a bilayer d-spacing appeared and theintensity developed further with time. This indicates that bilayer or multilayerdomains form and grow gradually in the film as the system approaches anequilibrium state.

Langmuir films in condensed phases consist of randomly oriented domains of4 6 ˚ � �10 �10 A in size 57 . In each domain the molecules are arranged with the same

bond1 orientation. The positional correlation lengths have been reported to be˚ � �approximately 100 A 48,58 .

5. Langmuir–Blodgett deposition

In 1920, Langmuir introduced the technique for transferring a floating monolayerto a solid surface by slowly raising the hydrophilic solid through the liquid surface

� � � �covered with the monolayer 59 . Later Blodgett 60 succeeded in transferringŽ .successive layers onto the same solid support a glass plate by vertically dipping

the plate in and out of a monolayer-covered liquid surface. The transfer of afloating monolayer onto a solid substrate is thus called Langmuir�Blodgett, or LB,

� �deposition. In addition to the vertical deposition mode Langmuir and Schaefer 61suggested a horizontal deposition mode by which the floating monolayer is trans-ferred to a hydrophobic solid surface by allowing the horizontal solid surface totouch the monolayer. Only one monolayer may be deposited by this method.

1The term ‘bond’ used in this context does not refer to a chemical bond but to the line joining twoneighbouring molecules.


Ž � �.Fig. 3. Phase diagram of a n-docosanoic acid monolayer on water after Peterson 56 .

5.1. Conditions for LB deposition

LB deposition of, for example, fatty acids and their salts, is normally performedat 20�40 mN m�1 at room temperature: that is, the floating monolayer is in the S,LS or L phase. While the floating monolayer is being transferred onto the2substrate during LB deposition, the area of the floating monolayer is normallyreduced continuously in order to keep the surface pressure constant. A simpleparameter to characterise the quality of the deposition, the transfer ratio, isemployed and defined as

decrease in area of Langmuir monolayerTR �

area of the transferred film on solid substrate

There are three types of LB deposition: Y, X and Z type. When a solid plate isinserted in and out of a monolayer-covered liquid surface, it is often found thatonce the first layer has been deposited, an additional layer is deposited each timethe plate enters or is removed from the liquid: i.e. TR � 1 on both the upstrokeand the down stroke. This two-way deposition is called Y-type deposition. Undercertain conditions a layer is deposited only as the plate enters the liquid and isreferred as X-type deposition. Conversely, the deposition is called Z-type if it onlytakes place as the plate is withdrawn from the liquid. Actually there has been littlepure X- or Z-type deposition reported. With the salts of fatty acids Y or XY-typedeposition usually occurs. XY-type deposition refers to TR � 1 on downstrokesand 0 � TR � 1 on upstrokes.

The mechanisms of the asymmetric X, XY and Z depositions are not quite clearbut two models have been presented. One suggests the overturning of molecules in

� �the outermost monolayer under water 62 , the other postulates the peeling of partof the outermost monolayer as the film passes through the air�liquid interface� �63,64 .

� �By X-ray reflectivity, Choi et al. 65,66 observed the LB deposition of CdAr to


change from Y-type, through XY-type, to X-type during 39 passages with anaverage of approximately 27.5 layers being finally deposited. The reflectivity dataindicate that there is a distribution of layer thicknesses ranging from 23 to 31 layerswith 27 and 29 layers dominating. They performed a successful Monte Carlo

� �simulation of this pattern based on the detachment mechanisms of Honig 67 and� �Peng 63 .

5.2. Deposition of single monolayer LB films

When only a single LB monolayer is required the deposition must be performedby raising the substrate upward through the monolayer-covered surface. For

Ž .satisfactory deposition TR � 1 the substrate surface must be hydrophilic andcarefully prepared.

When further layers are to be deposited, the first layer may be formed byupward motion on a hydrophilic substrate or by downward motion on a hy-drophobic substrate. The final layer is always deposited on an upstroke.

5.3. The speed of LB deposition

Although a slow deposition speed has often been regarded as essential for the� �production of high quality LB films, Peterson and associates in 1983 68 showed

that speeds up to 10 mm s�1 could be used in certain cases. The upper limit is setby the rate of drainage of the water between the substrate and the film beingdeposited. If the withdrawal rate is lower than a certain critical value the filmemerges completely dry, but at rates above this critical value there are streaks ofwater trapped beneath the deposited film and drainage of this water damages thefilm. The critical drainage rates are high when an acid is being deposited on aprevious acid layer, but are much slower for deposition directly onto a hydrophilicsolid surface. Large effects on drainage rates arising from the subphase chemistrywere also observed.

5.4. Effect of subphase chemistry on the composition of LB films of fatty acid salts

The composition of LB films of fatty acid salts depends on both the pH valueand the concentration of the cation in the subphase when the LB films are built up.

� � � �Vogel et al. 69 using FTIR and Petrov et al. 70 with neutron activation analysisand photometric measurements studied the dependence on subphase pH of the

Ž .cation content in multilayers of cadmium and barium arachidate CdAr and BaArŽ �4 �5 .with a constant concentration of cation in the subphase normally 10 �10 M .

� � � Ž .�Sastry et al. 71 using XPS X-ray photoelectron spectroscopy , Peltonen et al.� � Ž . � � Ž .72 using XPS and Kobayashi et al. 73,74 using both XPS and FTIR-RASquantitatively investigated this dependence with lead, calcium, cadmium, barium

Ž . � �and manganese arachidate PbAr, CaAr, CdAr and MnAr films. Bolbach et al. 75Ž .with both SIMS secondary ion mass spectrometry and XPS studied the effect of

pH value on the composition of the bilayer of BaAr.


All of these results are in agreement and show that to obtain a 100% combina-tion of the carboxy anions with divalent cations in the film the pH value requiredfor lead is 4, for cadmium 6.5, for manganese 7.0, for calcium 7.5 and

� �for barium 9. Ahn and Franses 76 using Flory�Huggins dilute solution theoryand the Stern�Gouy�Chapman equation calculated the compositions of such LBfilms as a function of pH and obtained good agreement with the results from theabove experiments.

The incorporation of multivalent ions into LB films is complicated by thehydrolysis and hydration of the ions in the subphase solution. Over a range of pHvalues the complex pattern of hydrolysis for Zn2� in solution leads to changes in

� �the composition and stability of spread arachidic acid monolayers 77 . Increasingthe pH causes the prominent LC phase of the un-ionised acid to contract anddisappear by pH 6.6 as the zinc salt forms in the floating monolayer, but withfurther increases in pH the monolayer exhibits a complex pattern of instability.

Ž .When these latter films were deposited immediately after formation 25 layers ,X-ray diffraction patterns with sharp peaks were observed, but the positions of thepeaks were strongly dependent on pH and indicated changes in bilayer spacing and,by inference, packing arrangements and angle of tilt.

� � � �Calcium palmitate 78 and calcium stearate 79 LB films showed no evidence ofhydrolysis but did contain at least one molecule of water of hydration for eachmolecule of calcium dicarboxylate.

Maximum drainage speeds for dry monolayer deposition were found by Veale� �and Peterson 80 to vary markedly with pH, cation concentration and cation type.

There is a small number of publications on the deposition of fatty acids whereŽ .the cation in the subphase is trivalent. Yttrium stearate YSt gives high quality

� �multilayers at high pH, and, using FTIR and QCM, Zotova et al. 47 found thatŽ .YSt was deposited at pH values around 5.6, but Y OH St was formed at pH 6.4.3 2

� �Y-Type deposition has also been reported 46 for arachidic acid monolayers onsubstrates containing the trivalent europium and terbium ions, but there was noevidence for the incorporation of hydroxide into these films.

6. Structures of single-monolayer LB films of fatty acids and their salts

Study of the structures of LB films of fatty acids and their salts started as earlyas 1935 when Blodgett successfully transferred floating monolayers to solid subs-

1 4 ˚� � Ž .trates 60,81,82 . However, as LB films are so thin 10 �10 A , it was only whenbrilliant synchrotron X-ray sources, high performance Fourier transform infraredŽ .FTIR spectroscopy and scanning probe microscopy became available in the lasttwo decades, that their structures could be comprehensively investigated.

As mentioned in Section 5.1, LB films of fatty acids and their salts are alwaysprepared at high surface pressures and nearly always at room temperature:conditions where the floating monolayers are in the LS, S or CS phase. To obtain asingle LB monolayer, the deposition is always on a hydrophilic substrate by upward


movement so the molecules in such films are always oriented with their polargroups towards the substrate and their hydrocarbon chains outward.

Generally at room temperature the molecules in single monolayer LB films arehexagonally packed with short-range translational order and long-range bond

Žorientational order regardless of the deposition conditions see, however, Section.6.5 . The molecular chains are in the all-trans conformation and on average normal

to the surface of the substrate. This is often called hexatic-B packing and corre-sponds to a smectic BH structure.

Sometimes, if there is a strong coupling between the monolayer and the surfaceof the substrate, the orientation of the monolayer structure may be correlated tothat on the surface of the substrate. This is known as epitaxial growth during theLB deposition.

6.1. Basic structure of single monolayer films � hexatic packing

The earliest successful study of the structures of single monolayer LB films was� �carried out in 1937 by Havinga and De Wael 83 . Using electron diffraction they

studied BaSt and BaAr films deposited at a pH of 7.0 on either nitrocellulose orgold films. With these samples a single crystal-like hexagonal diffraction pattern

˚Ž .was obtained, giving a length parameter intermolecular spacing of 4.79�4.80 A˚2and an area per molecule of 19.9�20.0 A . They therefore suggested that the

molecules are packed hexagonally in the films. These results agree very closely with� � � �those published recently 84,85 . In 1938 Germer and Storks 86 , also using

electron diffraction, investigated the structures of both monolayer and multilayerŽ .films of BaSt and HSt no dipping conditions were reported . They found that the

molecular chains in the single monolayer films are roughly normal to the filmŽ .surfaces tilt from the normal within 5�10� , and are closely packed but, contrary to

the results of Havinga and De Wael, they are arranged randomly.Ž . � �Many years later 1985 Bonnerot et al. 87 , with electron diffraction, studied

monolayer and multilayer films of docosanoic and �-tricosenoic acids and foundthat the monolayer films had a hexagonal structure with a domain size less than 10�m.

Ž . � �Fischer and Sackmann 1986 88 studied HAr single monolayers with electronmicroscopy and electron diffraction. The films were deposited on Formvar-coatedcarbon and silica layers. They reported that the structure of the film depends onthe temperature and surface pressure of the floating monolayer during the LB

Ž .deposition. If the film is deposited at temperature T below 24�C and surfaceŽ . Ž �1 .pressure � less than a so-called � approx. 20 mN m , or if T 25�C, thec

structure of the film will have twofold symmetry due to chain tilting. If the film ismade at � � � and T � 24�C it will have hexagonal symmetry.c

� �Electron diffraction was also used by Garoff et al. 84 in 1986 to study singlemonolayer LB films of CdSt deposited at a surface pressure of approximately 10mN m�1 and pH of 6.8 on a silica-coated amorphous carbon film. Based on thedevelopments in both theory and experiment on the melting of two-dimensional

� �solid and liquid crystals since 1970 89�92 , they deduced that the films have a fully


˚hexagonal symmetric structure with a d-spacing of 4.20 A which gives an area per˚2molecule of 21 A , and that the molecules in the films have a long-range bond

orientational order up to a millimetre but a short-range translational order only of˚40 A. They suggested that the structural feature is determined by the head groups

of the molecules, while the chains tilt approximately 25� from the normal of thefilm surface forming a so-called micellar cluster composed of approximately 100molecules.

� �At the same time, Vogel and Woll 93,94 also studied single monolayer LB films¨� Ž .�with low energy electron diffraction LEED . They transferred monolayers ofboth methyl stearate and HAr onto Au, Cu or Ag substrates at approximately 20mN m�1. They reported that the films have a hexagonal structure with an

˚ ˚2intermolecular distance of 4.9 A and an area per chain of 20.8 A . The correlation˚length is up to 140 A. They found that there is a strong correlation between the

orientation of the hexagonal structure and that of the single crystalline substrate,indicating epitaxial growth, and that the orientation is maintained across the wholefilm. However, there is no such correlation if the substrate is polycrystalline.

� �In 1989 Riegler 85 , with electron diffraction, investigated the structure andthermal stability of single monolayer LB films of CdSt, CdAr and CdBe. He foundthat the films of the three soaps have the same structure as Garoff had reported,but the diffraction intensity decays with increasing temperature before melting ofthe films. Therefore, he suggested that the molecular chains as well as the headgroups are responsible for the long-range bond orientational order.

The structures of single monolayer films have also been studied with GIXDusing synchrotron X-ray sources, FTIR and Raman spectroscopy, and other meth-

� �ods. The GIXD measurements of the films of CdAr 17,58 , heneicosanoic acid� � � �95 , and CdSt and PbSt 96,97 on silicon wafers or polished glass slides show thatonly a single diffuse in-plane diffraction peak can be observed, indicating that thefilms have a symmetric hexagonal structure with the chains perpendicular to the

˚2film surface. The area per molecule is approximately 19.4�19.7 A . The in-plane˚correlation lengths were reported to be approximately 30�50 A.

IR measurements of single monolayer LB films of long-chain fatty acids and� �their salts also indicate that the films have hexagonal structure. Kimura et al. 2

used an FTIR-ATR technique to study single monolayer and multilayer films ofHSt deposited on a germanium ATR plate at room temperature and 20 mN m�1

Ž .L phase . The CH scissoring band from the single monolayer film shows only a2 2singlet at 1468 cm�1, corresponding to hexagonal packing with the hydrocarbonchains freely rotating about their long axes, while the multilayers exhibit a doubletat 1473 and 1465 cm�1 due to a rectangular in-plane structure. These results were

� �confirmed by Umemura et al. 1 with FTIR-RAS measurements, but the filmswere deposited at 30 mN m�1 on silver-coated glass slides. Blaudez et al. with

� �FTIR-RAS 98�100 found that the hexagonal symmetry of the single monolayerfilms is independent of whether the hydrophilic or hydrophobic ends of themolecules contact the substrate surface.

� �AFM observations on CdAr monolayers 101,102 clearly show the hexaticstructure of the films. Areas of hexagonally packed molecules with fully extended


acyl chains normal to the surface are interspersed by less ordered regions whichare thinner due to the likely presence of some gauche bonds. There is no evidence

� �for the micellar cluster pattern postulated by Garoff et al. 84 , although thesurface pressures were higher than they used.

Core level loss spectroscopy, electron energy loss spectroscopy, and near-edgeŽ . � �X-ray absorption fine structure NEXAFS were used by Rajagopal et al. 103 on

LB monolayers of CdAr, PbAr, CaAr, ZnAr and BaAr. The cation had littleinfluence on the spectra but there was a correlation between the electronegativityof the cations and the NEXAFS results.

� �In contrast with the above results on single monolayer structure, Lesieur 104has reported that, with transmission X-ray diffraction measurements, a singlemonolayer film of HBe on collodion membrane exhibits a rectangular in-planestructure similar to that of multilayer films.

6.2. Conformation of the hydrocarbon chains

As mentioned in Section 3.5 the information on chain conformation is oftenprovided from vibrational spectroscopy measurements. Most of the spectroscopystudies indicate that the chains in both monolayer and multilayer films of fattyacids and their salts at room temperature are in the all-trans conformation.

Ž .However, it has also been reported see below that a considerable number ofgauche defects exist in the chains of the first few monolayers deposited.

� �Kimura et al. 2 with FTIR-ATR found that the band progressions appearingbetween 1400 and 1180 cm�1 from both monolayers and multilayers of HSt showidentical frequencies. The band progression arises from the CH wagging vibra-2tions of chains in the all-trans conformation.

� �Using Raman spectroscopy, Dierker et al. 105 studied 1 to 27 layer films of� ŽCdSt. Based on the bands of the CH stretching mode the symmetric stretch at2

�1 .2843 cm , which relates to the crystalline symmetry of the unit cell; and theŽ �1 . �antisymmetric stretch at 2882 cm , which is sensitive to the chain conformation ,

they concluded that the first few layers have a greater tendency for hexagonalpacking and a greater number of static gauche defects compared to succeeding

� � � � Ž .layers. However, Naselli et al. 106 Rabolt et al. 107 with GIR-FTIR and Rabe� � Ž .et al. 108,109 with Raman spectroscopy studied the thermal behaviour of LB

films of mono- and multilayers and reported that gauche defects occur only at highŽ .temperature � 90�C .

6.3. Orientation of the long axes of molecules

The structure and orientation of the acyl chains has been investigated by avariety of techniques, and the results do not always appear to be in agreement.However, in most cases the conflict arises from the fact that different techniquesmay respond to different aspects or portions of the film. For example, diffractiononly occurs from the ordered domains of a film, whereas most measures of filmthickness yield an average value. A monolayer of CdAr appears to consist of


domains of hexagonally-packed acyl chains, fully extended and normal to thesurface, separated by thinner disordered regions where the chains are either tilted

� �or possess gauche bonds 102 . GIXD would only detect the hexagonally-packeddomains, whereas measurements of film thickness such as X-ray reflectivity andellipsometry would give a value less than the length of the fully extended molecule.

� �GIXD measurements of the single monolayer films of CdAr 58 , CdSt, PbSt� � � �96,97 , and heneicosanoic acid 95 show that the molecular chains are perpendicu-lar to the surface of the substrate because the in-plane diffraction shows only a

Ž .single peak at Q � 0 see Section 6.1 . However, FTIR, X-ray reflectivity andzother measurements of single monolayer films have given inconsistent results onthe chain orientation. Some lead to the same conclusion as the GIXD measure-ments, but others indicate that the chains tilt significantly from the normal of thefilm surface.

The observation of the CH stretching vibrations with polarised infrared ATR2� �spectroscopy by Kimura et al. 2 indicates that the dichroic ratios of the bands

corresponding to antisymmetric and symmetric vibrations at 2920 and 2850 cm�1,respectively, from a single monolayer film of HSt are the same, implying that thechains are uniaxially oriented and approximately normal to the substrate. Ume-

� �mura et al. 110 also reported from FTIR-RAS measurements that the chains insingle monolayer films of both CdSt and CaSt are normal to the surface of the

Ž .substrate silver-coated glass . The same conclusion was drawn by Nakanaga et al.� �111 in their study by photoacoustic FTIR with single and multilayers of CdAr on

� � Žglass plates, and by Blaudez et al. 98,99 with an FTIR-RAS study of deuterated.and protonated monolayers and multilayers on CaF crystals. With near-edge2

Ž . � �X-ray absorption fine-structure NEXAFS measurements, Hahner et al. 112,113¨studied the structures of single monolayers of CdAr and CaAr on silicon wafers,and reported that the molecular chains in both films orient on average perpendicu-lar to the substrate surface.

However, there are also reports of the chains in single monolayer films beingoriented in other than the vertical configuration. X-Ray reflectivity measurements� �97 show that the thickness of a single monolayer of PbSt on a silicon wafer is

˚approximately 20 A, less than the length of a fully extended molecule, implying thatthe chains are inclined from the normal of the film surface. Ellipsometry measure-

� � � �ments 108 show similar results. Arndt et al. 114 studied the structures of bothsingle monolayer films and multilayers of protonated and perdeuterated CdAr ongold with GIR-FTIR. By comparing the variation of the CH stretching band2intensities from 1 to 13 monolayers they suggested that either the single monolayerhas a higher degree of disorder compared with multilayers, or that the chains tiltfrom the normal of the film surface. The same conclusion was obtained by Duschl

� �et al. 115 with plasmon surface polariton field-enhanced Raman spectroscopy of� �CdAr. Jones et al. 116 , in their study of multilayers of �-tricosenoic acid on

Ž .silicon using reflection high energy electron diffraction RHEED observed that,due to the substrate effect, the chains tilt randomly in the first few layers, while inthe successive layers a single direction of tilt develops.

The reported tilt angles of the chains in single monolayer films are quite


� �different from author to author. Allara and Swalen 117 used GIR-FTIR spectros-copy to study the orientation of LB films of CdAr of one to 10 monolayers onsilver-coated glass substrates. They reported that the tilt angle of the chain axes isless than 8� from the normal to the film surface. As mentioned in Section 6.1,

� � Ž .Garoff et al. 84 with TED reported that the chains in a single monolayer film ofCdSt on a SiO�amorphous carbon substrate tilt from the surface normal by 25�.

� �The NEXAFS measurements by Rabe, Outka et al. 118�120 indicate that thechains in a monolayer of CdAr are normal to the surface of the film, but the

� �inclination in a monolayer of CaAr is 33�. However, Ahn and Franses 121 usingpolarised FTIR-ATR spectroscopy measured the tilt angles of the acyl chains in

Žsingle monolayer films of PbSt, CaSt and CdSt made at pH � 6.0 and � � 19 mN�1 .m and reported that the average tilt angles for the latter two soaps are

Ž .approximately 36� on both silicon Si�SiO and Ge plates, while for the lead soap2it is approximately 23�. Their results also imply that the tilt angle is not related tothe substrate of the film but to the specific cation. With NEXAFS Kinzler et al.� �122 reported that the tilt angle of the chains in a single monolayer on silicon

� �wafer is 33� for CdAr and 29� for CaAr. Recently, Peng, 123 with GIXD-IPmeasurements has found that the chains in CdBe and PbAr LB single monolayerson silicon wafers are, on average, tilted from the normals to the films by 23� and20�, respectively.

6.4. Effect of substrates on the structures of monolayer films

As mentioned above, single monolayer LB films of fatty acids and their saltsusually have a hexagonal in-plane structure with short-range positional order andlong-range bond orientational order irrespective of the substrate: whether crys-

Ž � � � � � � � �.talline e.g. single crystals of certain metals 93,84 , CaF , 98,99 , ZnSe 99 , Ge 22Ž � � � �.or amorphous e.g. glass slides 95 , Si�SiO 58,97,98 . However, orientational2

epitaxy and even strain epitaxy were observed in some single monolayer films,depending on both the substrate and the nature of the cation in the film, or ineffect, depending on the strength of the coupling between the monolayer and thesurface of the substrate.

� �As mentioned in Section 6.1, Vogel and Woll 93 found orientational epitaxial¨growth of the monolayers of both pure HAr and its mixture with methyl arachidatedeposited on single crystals of Cu, Au, and Ag, but there was no such correlationwhen the films were deposited on the same metals with polycrystalline structures.

� �Schwartz and Viswanathan et al. 124,125 found with AFM that the couplingbetween the monolayer and the substrate lattice and the nature of the counterionhave significant effects on the molecular packing in the monolayer film. Forexample, they observed that single monolayers of CdAr and BaAr on mica do notexhibit periodic structure, whereas films of PbSt show a rectangular structure witha long-range positional order and with one set of lattice rows in the sameorientation as the substrate. Films of MnAr had a rectangular structure but only ashort-range positional order. On mica, both PbSt and MnAr monolayers have aconsiderably greater lattice spacing and molecular area than do multilayers of


these materials, indicating a strong coupling of the first layer to the mica lattice˚Ž .mica has a hexagonal lattice with a 5.2-A nearest-neighbour spacing . On the

Ž .other hand, with a silicon wafer SiO surface as the substrate, neither film2showed any net structure. However, hexatic structures have been observed for

� � � �CdAr on mica by Peng and Barnes 102 and on Si�SiO by Foran et al. 17 .2� � ŽBlaudez et al. 98,99 in their FTIR-RAS measurements with deuterated and

.hydrogenated CdAr found that the first monolayer may have a rectangulararrangement when it is deposited on an orthorhombic commensurable substrate, orif it is deposited at a low temperature where the intralayer interactions arestrengthened.

� �Kinzler et al. 126 with NEXAFS reported that the tilt angle of the chains inCdAr monolayers on Ag is 15�, but is approximately 29� on Si wafers with eitherhydrophobic or hydrophilic surfaces. Thus, the substrate seems to affect theorientation of the molecules in monolayer films. On the other hand, GIXD data

� �for CdAr on Si wafers 17 indicate a chain tilt of no more than 0.5�.

6.5. Structural correlation between LB monolayers and the parent floating monolayers

The transfer of a monolayer from a liquid surface to a solid surface significantlychanges the interaction between the head groups of the monolayer molecules andthe substrate. As a result, the structure of the film before and after the transfercould be different. Even in the case of a floating monolayer a change in theinteraction between the headgroups and between the headgroups and subphasegives rise to a significant change in the phase behaviour of the monolayer. Forexample, with floating monolayers of fatty acids, the phase behaviour and thestructures mainly depend on the hydrocarbon chains because the coupling betweenthe head groups is very weak. As this coupling becomes stronger, the phasebehaviour of the floating monolayer may no longer be dominated by the chains.Thus the introduction of a divalent cation into the subphase leads to the loss of the

� �liquid condensed phases at room temperature 127 .As discussed above, the arrangement of the hydrocarbon chains in the single

monolayer LB films of fatty acids and their salts on various substrates has, ingeneral, hexagonal symmetry unless there is a strong coupling between the subs-trate and the monolayer. These films are always deposited at high surface pres-sures.

An important investigation of the correlation between the structure of a singlelayer LB film and that of the floating monolayer was reported by Shih et al. in 1993� �95 . Using GIXD, they studied the structures of single monolayers of hene-icosanoic acid on polished glass slides, and found that irrespective of the phase of

Ž .the floating monolayer L , S or LS the single monolayer LB films always2Žexhibited a hexagonal structure with short-range positional order correlation

˚.length � 30 A and with the chains normal to the substrate. The spacing between˚ ˚2Ž .the adjacent chains was 4.74 A 19.5 A per chain for the films deposited from the

˚ ˚2Ž .S and LS phases but 4.82 A 20.1 A per chain from the L phase. These two2� �spacings correspond to a rotator phase of a long-chain paraffin 128 . Although this


small difference in the spacing may arise from the structure of the parentŽmonolayer the structure in the L phase is a distorted hexagonal symmetry with2

the chains tilted from the substrate normal by 25� and an area per chain of 21.6˚2 .A , there is little correspondence in structural symmetry or even in area permolecule before and after deposition, as long as no water is transferred to the

� �substrate together with the monolayer 127 .The maximum withdrawal rate for fully-drained deposition has been shown by

� � Ž .Peterson 68,80 to depend on the nature of the hydrophilic substrate. These rates�4 �1 � 4 �6 �1varied from 8 � 10 m s for InP 111 to 3 � 10 m s for SiO .2

However, most studies were performed on LB films that had been removed fromcontact with the floating monolayer and its subphase and dried. When the

Ž .substrate glass or silicon wafer was raised through the subphase surface, deposit-ing a monolayer, but was still kept in contact with the liquid subphase the GIXD

� �results were quite different 129 . The freshly formed LB monolayer then had thesame structure as the floating monolayer from which it had been formed: the L ,2

Ž .L , S, or LS Rotator II phases were all transferred unchanged. After the2substrate had been detached and dried the hexagonal LS phase structure wasalways observed. This result suggests the presence of a thin film of water betweenthe substrate and the LB monolayer before contact with the bulk subphase wassevered and the film dried. This result appears to conflict with the drainage speed

� �measurements of Peterson and associates 68,80 , but as they used visual observa-tion it is probable that a thin layer of water could not have been detected.

The AFM measurements on single monolayer films of CdAr, BaAr, PbSt and� �MnAr by Schwartz, Viswanathan, and coworkers 124,125 , described in Section 6.4,

also provide some evidence that the structures of single monolayer LB films do notdepend on the structures of the floating monolayer but do depend on theinteraction between the head groups and the surfaces of the substrates and on the

� �intralayer interaction. Sikes and Schwartz 130 , using AFM and transmission IRspectroscopy, studied single monolayers of pentadecanoic and hexadecanoic acidson mica. The films were transferred from the L phase. They reported that the2deposited films are more condensed than before deposition and have a structuresimilar to that in the LS phase of the floating monolayer.

� �However, Peterson and coworkers 131�133 have reported some results on thestructural correlation which are quite different from those mentioned above. WithTED they studied single monolayer films of HBe deposited on amorphous subs-

� Ž .�trates of Formvar and PMMA poly methyl-methacrylate . They observed that the� �films deposited on Formvar 131 from L , L and CS phases had a disordered,2 2

untilted centred rectangular symmetry, but the unit cell parameters were scatteredover a range larger than the experimental error. The actual packing was different

� �from any structure observed in the floating monolayer. On PMMA 132 theyobserved a variety of structures, from untilted hexagonal to tilted rectangularstructures, depending on the deposition surface pressure. In a later paper theyreported that the tilt angle of the molecular chains in the LB monolayers was

� �highly correlated with that in the floating monolayer 133 . With high energyelectron reflection they studied the tilt angle of the chains in HBe monolayers,


deposited from the L phase at surface pressures from 1.5 to 20 mN m�1 on silicon2oxide substrates. The corresponding tilt angles are 33��1.5�, a few degrees less thanthose in the floating monolayer. They suggested that the lower tilt angle on thesolid substrate is because the interaction between the film and the solid surface

�1 � �changes the effective surface pressures by a few mN m 133,134 .

6.6. Composition of single monolayers of soaps

There are several possibilities for the chemical composition in a single monolayerfilm of a soap such as CdSt. The general expectation is that each divalent cationcombines with two carboxy groups and hence two hydrocarbon chains. However,Garoff et al. reported that Auger analysis of single monolayer LB films of CaSt ongold showed that only one stearate anion combined with a cation and that chloridewas the second counterion associated with each calcium ion. The association of asingle alkanoate ion with a divalent cation also has been observed for CaBe single

� � � �monolayer LB films on alumina 135 . Leveiller et al. 136 with GIXD and X-rayreflectivity studied an uncompressed monolayer of arachidic acid on a solution ofdilute cadmium chloride at a pH value of 8.8 and low surface pressure. Theirresults show that a CdOH� layer formed beneath the arachidate monolayer andwas bound to the arachidate layer in a stoichiometry close to 1:1. Therefore, forfatty acid salts it is most likely that each of the long-chain anions associates withone divalent metal cation both in floating monolayers and in single monolayer LB

Ž .films with the head groups of the molecules in contact with the substrate surfaces .

7. Structures of LB multilayer films

The basic structure of LB multilayer films will be considered in three sections:the structure of the central portion of the film excluding layers close to thesubstrate and close to the outer surface, the structure of the first layer, and thestructure of the outer layers. Discussion of other aspects will follow.

7.1. Comparison of multilayer and bulk crystal structures

The structures of the central portions of multilayer LB films of fatty acids andtheir salts are similar to the structures of their bulk phases. The bulk phases have alamellar structure, where the arrangement of the molecules is illustrated in Fig. 4� �137,138 . In the case of fatty acids a lamella consists of two layers of acidcombining with each other through hydrogen bonds between the carboxyl groups,forming dimers, with the chains tilting at an angle to the normal of the plane of thehead groups. In the case of salts, a lamella consists of two layers of long-chaincarboxyl ions combining with one layer of cations. The chains may either be tiltedor parallel to the normal of the plane of the head groups. In principle, the chainsare in the all-trans conformation, but as it is possible to form mixed crystals withacids of different chain lengths in any proportion with the interlayer spacing being


Ž .Fig. 4. Schematic diagram of the molecules arranged in a lamella in a LB film: a a lamella in a fattyŽ .acid salt film; b a lamella in a fatty acid film showing two kinds of dimers; in cis and trans

conformation.

the weighted average for the two acids, it is clearly possible for the longer chains to� �fold over and fill the spaces above the shorter chains 139 .

In LB films all the monolayers are parallel to each other and to the substrate.However, the intralayer and interlayer positional correlation lengths in the LB

2 3 ˚films are only 10 �10 A: much smaller than those of single crystals of their bulkphases. Therefore, an LB film is more like a stack of highly-oriented quasi-2Dpowder layers rather than a large crystal. At room temperature the hydrocarbon

� �chains, as mentioned in Section 6.2, are in the all-trans conformation 1,2,106�109 .Mixed films with acids of different chain lengths do not appear to have beenstudied.

An important question is whether the deposition type, X, XY, Y, or Z, affectsthe structure of the LB film. Evidence will be presented in Section 7.2 that showsthat the basic structure is always head-to-head, tail-to-tail irrespective of thedeposition type.

7.2. Basic structure of LB films of fatty acids and their salts

In multilayer films the structural features to be considered are: in-plane struc-ture; chain rotation or orientation around the chain axis; chain tilt and direction oftilt; the alignment of chains within a lamella; and the stacking and alignment oflamellae.

7.2.1. In-plane structure� �As early as 1935 Clark et al. 81 reported from X-ray measurements that the


basal spacings of CaSt multilayers have virtually the same values whether the� �samples are prepared in bulk or by LB deposition. Stephens et al. 140 , using

electron diffraction, studied LB films of PbSt with 76 layers. They found that thelattice constants and the symmetry of the film structure are the same as those frompowder samples determined with X-ray diffraction: a centred rectangular structure

˚ ˚ ˚2 �1Ž .4.96 A � 7.38 A giving 18.30 A molecule .� �With electron microscopy Trurnit and Schidlovsky 141 confirmed the lamellar

configuration shown in Fig. 4. It has also been found that the structures of LB filmsŽ .with the same composition are independent of the deposition type Y or X , as

Ž . � �reported by Holley and Bernstein with X-ray diffraction 142 and by LangmuirŽ . � �with contact angle measurements 143 .

Ž .Dutta and coworkers introduced the grazing incidence X-ray diffraction GIXDtechnique for studying the in-plane and out-of-plane structures of LB films� �144,145 . They reported that the in-plane structures of the films of lead myristate,

˚ ˚Ž .stearate and behenate, are the same, a rectangular structure 4.96 A � 7.38 A , in� �agreement with that reported by Stephens et al. 140 , and that the d spacing is a001

Ž .double bilayer four-layer thickness. They also suggested that the lead atoms shift� �by half of one b spacing from one lead layer to the next and more recently 97

proposed a chain arrangement which shows a lateral shift of the chains from layerto layer by a�4 along a or b�4 along b and that the chains are packed in a

� �herringbone arrangement. Later Pietsch et al. 146 , also with GIXD, determinedthe structure of LB films of both CdAr and PbSt. They reported that intensity

Ž .analysis showed a quarter-spacing a�4 lateral shift of the chains only towardsNNN, i.e. along the a direction.

� �Sasanuma et al. 147 studied the structure of multilayers of CdSt using X-rayreflectivity. Their intensity analysis also indicates that the chains are arranged inthe unit cell with a herringbone configuration.

The observation of polarised infrared spectra of multilayers of HSt on germa-� �nium by Kimura et al. 2 indicates that the dichroic ratio of the CH antisymmet-2

ric stretching vibration band is different from that of the CH symmetric stretching2band, implying that a biaxial orientation of the hydrocarbon chains is present in thefilms. The CH scissoring vibration bands at 1473 cm�1 give the strongest intensity2

Ž .with polarisation parallel to the y-axis parallel to the dipping direction , and at1465 cm�1 give the strongest intensity with polarisation parallel to the x-axis. Thisreflects a tendency for the two crystallographic axes to align with respect to the

Ž .dipping direction see Section 7.10 . The above dichroic nature is independent ofthe number of layers in the multilayer films, indicating that the structure isconstant as the number of layers is increased beyond a bilayer.

7.2.2. Chain tilt and orientation� �Rabolt et al. 107 using transmission- and GIR-FTIR studied polarised IR

Ž .spectra of multilayers of HAr and CdAr protonated and deuterated species . Theyreported that the acid layers form a monoclinic structure with the chains inclinedto the surface normal by 25�. On the other hand, the salt film has an orthorhombicsymmetry with the chains perpendicular to the film surface and with each unit cell


containing two chains. Since there was no anisotropy in transmission measure-ments with polarised IR, they concluded that the two chains orientate perpendicu-

Ž .larly to each other, i.e. the chains are packed in a herringbone pattern Fig. 2 .Ž . � �Similar suggestions were presented by Malik et al. with X-ray diffraction 97 , and

Ž . � � � �Schwartz et al. with AFM 148 . With FTIR, Shimomura et al. 149 studied LBfilms of protonated CdAr and a mixture of protonated and deuterated CdAr. Theyalso reported that in both cases the multilayers on ZnSe disks have an orthorhom-bic subcell structure. In their FTIR measurements of LB mono- and multi-layers of

� � � �cadmium soap, Umemura et al. 150 and Blaudez et al. 99 obtained similar lattice� �symmetries to those from X-ray diffraction measurements 58 . Other spectroscopy

measurements indicated that the multilayer films of fatty acids and their salts�exhibit structural properties similar to their bulk forms 1,2,98�100,105,106,108�

�111,117,149�160 .

7.2.3. Structure of lamellaeThere are a few reports from Blasie and coworkers which provide a very

different configuration of the multilayer structures. With X-ray diffraction and arefinement technique they concluded that asymmetry is present in the CdArbilayers: the monolayers deposited on downstrokes have a greater electron density

˚ ˚Ž . Ž . � �profile length 30.8 A than those deposited on upstrokes 24.6 A 161�163 . Theyalso reported different thermal behaviour of the two types of deposited monolay-ers, an aspect which will be considered in Section 8.

7.3. Polymorphism in LB films of fatty acids

Polymorphism is commonly present in bulk long-chain fatty acids. There areŽ .several crystalline forms observed: A A-super, A A and A , B, C, and E,1, 2 3

depending on the tilting and conformation of the chains, and among them, form C� �has been reported to be most thermodynamically stable 4,153,164,165 . Similar

polymorphism has been observed in LB films of fatty acids.The earliest study of the LB film structures of fatty acids was carried out by

� �Germer and Storks 166 in 1937. With electron diffraction they demonstrated thatLB multilayers of HSt have a structure identical to the crystalline bulk form

˚ ˚Ž .corresponding to form C a � 9.4 A, b � 5.0 A and � 57� . Peterson and Russell� �167 also using electron diffraction observed two crystalline forms coexisting in

� ��-tricosenoic acid multilayers. Recently, Leuthe et al. 168,169 studied the struc-tures of multilayers of HSt, HAr and HBe with X-ray reflectivity and polarised

Žreflection microscopy. They found that all the LB films of the three acids on.silicon wafers , like their bulk forms, contain three crystalline forms, � , and of

different proportions even though the films were deposited from different phasesŽ .of the floating monolayer CS, L L . The molecular chains tilt by 18�21� from2 , 2

the surface normal in the � phase, 25�28� in the phase, and 35�36� in the phase. The in-plane domain size is approximately 1 �m diameter. After annealing

� �at 60�65�C only the phase was observed. Kamata et al. 170 , using both X-ray


diffraction and FTIR, found that two crystalline forms, A and C, coexist inmultilayer films of HSt and CdSt.

� � � �With FTIR spectroscopy Kimura et al. 2 and Umemura et al. 1 observed onlyone phase in LB films of stearic acid. The chains are tilted from the film normal by

� � Ž .an angle of 30� on Ge plates 2 form C or a lower angle on silver-coated glass� � Ž .slides 1 form A . Their results concerning chain tilting are in accord with those2

� � Ž .reported by Clark et al. 81,171 with X-ray diffraction, glass slides as substrates ,� � � Ž .� � �Nesterenko et al. 172 ellipsometry, films on Cd Hg Te 111 and Chollet 160x 1�x

Ž . � �infrared, films on CaF plates . Chollet and Messier 152 , with an FTIR dichoism2study, reported a form C-like structure for multilayers of HBe with chain tilt angleof 23� and of �-tricosenoic acid with the tilt angle of 18�. With ZnAr the X-ray

Ž .diffraction powder camera pattern depends on the pH at which the layers weredeposited: at pH � 6.6 a chain tilt of � 31� is indicated, but at 7.2 � pH � 7.5 anadditional form with tilt of � 19� is observed, at pH � 7.52 one phase with tilt 0�is present, and at pH 7.8 the phase with tilt of � 31� is again found.

Although there are some inconsistencies in the assignment of the crystallineforms reported from author to author, the structural similarity of multilayer LBfilms of fatty acids with their bulk phases is established.

ŽThere are two possible configurations for the carboxy dimer, cis and trans as.shown in Fig. 4 . As mentioned above, the hydrocarbon chains have the all-trans

Žconformation in LB films of the fatty acids at room temperature except form B in.which the C �C bond is in the gauche conformation . It is reported that bulk fatty2 3

acid crystals of form B or E only have the cis isomer, while the crystals of C form� �have both cis and trans dimers in dynamic equilibrium 153�156 . In LB films of

Ž . �1one and three monolayers HSt on silver, the coupling mode around 1300 cm ofthe trans-stearic isomer is prominent, but as the number of monolayers in the filmincreases, the band progression due to CH wagging modes, characteristic of the2

� �cis isomer, becomes dominant 1 .

7.4. E�olution of the structure with number of layers

Although the molecules in the first monolayer are usually hexagonally packedwith vertical chains and with short-range positional order and long-range bondorientational order, the molecular chains in the subsequent bilayer or bilayers havea close packing with a long-range positional order and form the bulk latticestructure. This is the main conclusion drawn from X-ray and electron diffraction,FTIR and AFM measurements. However, there is other evidence which indicates astructural transition over a few layers before the multilayers attain the bulk latticestructure.

� �Tippmann-Krayer et al. 58 , with GIXD, studied the layer dependence of thestructure of LB films of CdAr. As mentioned above, with a single monolayer film

˚�1 ˚Ž .on a silicon wafer only a single diffuse peak at Q � 1.52 A d � 4.13 A wasx yobserved. Addition of a bilayer on the single monolayer gave rise to two sharper

˚�1 ˚Ž .in-plane peaks at Q values of 1.544 and 1.675 A d � 4.07 and 3.75 A indexedx yŽ . Ž .as 11 and 02 , respectively, and indicating that the film has a rectangular


˚ ˚2Ž .structure with close packing a � 4.85 and b � 7.50 A, area per chain � 18.1 A .The same in-plane structure was observed in a 21-layer film. With the 21-layer film

Ž .they did not observe peaks with even values of l in the 11l diffraction, so theysuggested that the metal cations are randomly arranged between the head groups.

� �Malik et al. 96,97 observed the structural evolution with number of layers of bothPbSt and CdSt. They reached the same conclusion on the layer dependence asTippmann-Krayer et al. However, for the lead soap they reported that the aliphaticchain layers and the lead layers have similar in-plane lattice structure. The in-planecorrelation length grows with the number of monolayers in the lead soap film

˚ ˚Ž .approx. 250 A in the three-layer film and 400 A in the five-layer one but stays the˚Ž . � �same approx. 130 A in the cadmium soap films 96 .

� �Pietsch et al. 173�175 , with X-ray diffuse scattering, studied the lateral correla-Žtion and the roughness of the interfaces between the bilayers in PbSt films nine

.and 30 layers and in CdSt multilayers. They reported that the lateral correlation˚ ˚length is approximately 120�130 A and the roughness is approximately 2�3 A.

� � Ž .Kinzler et al. 122 with NEXAFS, SAXS small angle X-ray scattering and highenergy electron diffraction studied one, three, nine and 25 layers of CdAr LB films.They reported that the monolayer film shows a substantial degree of disorder,while in the films of three or more layers the molecules are vertically oriented tothe substrate surface in an all-trans conformation.

A different pattern for the structural evolution was reported by Bonnerot et al.� � Ž . Ž87 . With IR spectroscopy LB films on aluminium substrates , TED LB films on

. Žcarbon films and reflection high-energy electron diffraction LB films on carbon.films they studied the structure of 1�21-layer LB films of HBe and �-tricosenoic

acid. They reported that as the number of layers is increased, the symmetry of LBfilms becomes rectangular, and the axes of the chains, perpendicular to thesubstrate in the first monolayer, tilt progressively to reach a limit of 23� for HBe,and 19� for �-tricosenoic acid. This structural transition covers seven layers.

� �Highfield et al. 176 using neutron reflectivity measurements of multilayers ofdeuterated CdAr on glass slides found that a reduction in the thickness of only thefirst few layers gives a good fit to the data from the samples with even numbers of

� �layers. Takamura et al. 177 also reported that there is a dependence of chain tilton the number of monolayers of MnAr on quartz glass. They used X-ray reflectivitymeasurements and found that the chain tilt angle was 11.8� in single, two- andthree-layer films, 21.1� in five- and nine-layer films, and 26.9� in a 19-layer film.

� �Schwartz et al. 124,125,178 , in their AFM measurements found that the firstmonolayer of PbSt deposited on mica has a larger in-plane d-spacing than that inits bulk multilayers. The structures of the layers subsequently deposited graduallyevolve to the native bulk structure and this transition becomes complete after theaddition of four bilayers on the first monolayer. Similar evolution of the lamellarstructure was also found in LB films of MnAr. The authors refer to this pheno-menon as strained-layer van der Waals epitaxy of the growth of the films. Theyattribute it to the strong coupling between the substrate and the first monolayer

Ž � �.and the nature of the counterion e.g. high electronegativity 124 .


7.5. Rearrangement of the first layer in multilayer films

There are some inconsistent reports on whether the structure of the firstmonolayer is retained after the deposition of the subsequent bilayer. The results

Ž .from FTIR measurements HSt, HBe and �-tricosenoic acid show that the firstmonolayer in a multilayer film maintains its own hexagonal structure and vertical

� � � �chain orientation on the substrate 2,88 . Malik et al. 97 , using GIXD, found thatthe correlation lengths along the direction of the film normal in one-, three- and

˚five-layer films of PbSt were 24, 50 and 100 A, suggesting that the first monolayerremains distinct from the successive deposited layers in the structure. Blaudez et

� �al. 98,99 in their FTIR-RAS measurements also confirmed this argument.� �However, there is a report by Kinzler et al. 122 showing that the deposition of a

bilayer on the first monolayer results in a rearrangement of the molecules in thefirst monolayer. With X-ray reflectivity they found that after addition of a bilayeronto the first monolayer the thickness of the first monolayer was increased

˚ ˚Žsignificantly from less than 20 A to 26.6 A, almost the full molecular length of 27.6˚.A , indicating that the subsequent LB deposition induces a higher degree of orderin the first monolayer.

� �Recently, Peng 123 with GIXD-IP studied the structures of one- to five-layerLB films of CdBe. It was reported that after a bilayer film is deposited on the firstmonolayer, the diffraction spot due to hexagonal symmetry in the first layerdisappears and only a diffraction pattern ascribed to a rectangular structure isobserved. This supports the suggestion that the molecules in the first monolayerrearrange as the following bilayer is deposited.

7.6. Epitaxial growth in multilayers

Two types of epitaxy have been observed in lamellar materials, strained layer andvan der Waals epitaxy. In strained layer epitaxy, the first monolayer of an adsorbedfilm, which has a bulk lattice constant no more than a few percent different fromthat of the substrate, replicates the in-plane structure of the substrate exactly. Thesubsequent layers gradually relax to the bulk structure of the adsorbate bydeveloping dislocations. In van der Waals epitaxy, the adsorbed layer is orientedwith respect to the substrate and has both long-range positional and orientationalorder, but maintains its bulk structure which significantly differs from that of thesubstrate. The latter is common when the adsorbate forms a bulk structure withstrong intralayer interactions but weak and non-specific interlayer interactions. Formost LB multilayer films, the growth after the first monolayer appears to be

� �primarily van der Waals type epitaxy 179�181 , in particular, orientational epitaxy.In the case of strong coupling between the first monolayer and substrate surface astrained layer�van der Waals epitaxy is also observed.

� �Prakash et al. 182 studied the structure of PbSt of more than 200 layers onmica with X-ray diffraction. They found that although the structure of the filmŽ . Ž .centred rectangular symmetry differs from that of mica hexagonal symmetry theorientation of the in-plane lattice structure of the film is unambiguously related to


one row of the hexagonal in-plane structure of the substrate. Later on Schwartz et� �al. 124,125 with AFM studied the evolution of the structure of the same system

through one, three, five, seven and nine layers. They reported that the firstŽ .monolayer shows a well-defined orientation a set of strained lattice rows closely

matching that of the substrate although the bulk lattice has a large mismatch withmica. The other lattice constants of the monolayer are not as highly strained and

Ž .are far from commensurate with the substrate see Section 6 . Subsequent bilayersshow an orientational epitaxial growth but the strained lattice progressively turnsto the bulk structure as the number of layers is increased. The orientational epitaxyin the monolayer is similar to van der Waals epitaxy. These two examples indicatethat the two types of epitaxy may exist during LB deposition. The authors suggestthat PbSt monolayers have very strong intralayer interactions which hold themonolayer together and lead to long-range positional order. However, their AFMimages show that no lattice structure could be observed for single monolayers on

Ž .silicon wafers with amorphous surface irrespective of the type of soap, and thisconflicts with the above argument about the strong intralayer interaction inmonolayers of the lead soap.

� �Epitaxial growth was found by Veale et al. 183 with saturated fatty acids fromC to C and 22-tricosenoic acid when up to 200 layers were deposited on the18 23first LB layer on Si wafers that had been made hydrophobic. This property was

Ž .used to develop and display for microscopic observation the crystallites character-istic of the initial monolayer.

� �With X-ray reflectivity Leuthe et al. 168 observed the epitaxial growth ofŽ .multilayers 25 layers of HBe. As mentioned previously they found three polymor-

Ž .phic structures denoted � , and coexisting in the film with grain size of theorder of micrometers, and epitaxial film growth leading to an interlayer correlationup to 10 bilayers. This means that the epitaxy is not only orientational but alsopositional. After annealing at 65�C for several minutes only a single structure ofform was observed with grains up to tens of micrometers in size. When 20 freshmonolayers were deposited on the surface of the annealed film, the fresh filmretained the domain size and the tilt direction of the molecules of the underlyingannealed film, but still contained the three phases like those observed in theunannealed film. Therefore, the epitaxial growth only reproduces the morphologi-

Žcal features, and to some extent the orientation, of the underlying film confirming� � .earlier work of Peterson et al. 134 , see Section 6 .

The orientationally epitaxial growth dictated by the early monolayers is also� �supported by the TED results from Bohm et al. 184 . With a 21-layer film of CdAr¨

at room temperature they observed the distinct diffraction spots of a rectangularlattice structure. The spots are sharp but arc-like, covering a few degrees. Thisclearly indicates that the layers are correlated with orientational order, as arandom bond orientation from layer to layer would cause sharp diffraction rings.Nevertheless, the positional correlation in multilayers of CdAr only covers a couple

� �of bilayers normal to the surface 58 . The TED results from a multilayer of� ��-tricosenoic acid reported by Peterson et al. 167 also show orientational epitaxy.

� �It has been confirmed by GIXD measurements 48,185 , that the domains in


floating monolayers are randomly oriented. The epitaxial growth of multilayersthus clearly indicates that during LB deposition the monolayer must rearrange tomatch the structure of the underlying layers of the substrate.

7.7. Correlation between the structures of floating monolayers and deposited multilayers

Although there is little evidence showing structural correlation between themonolayer before and after deposition, some papers suggest that aspects of themorphology of the floating monolayer may be preserved in its LB multilayer.

� �Peterson and co-workers 134 used the fact that annealing could create largerorientational domains in floating monolayers to prepare an LB monolayer of�-tricosenoic acid with half from the annealed monolayer and the other half froma fresh monolayer. After 170 additional layers had been deposited on the firstmonolayer the distinct difference in the domain size between the two half areaswas clearly observed with polarised reflection microscopy; the domains in the halffrom the annealed monolayer were clearly larger than those in the other half.

� �Similar effects were observed 186 when part of the substrate had previously beencoated with a bilayer of stearic acid. This portion showed large crystallites whereasthe portion without the stearic acid bilayer showed very small crystallites.

� �In addition, there is a single set of results reported by Leuthe et al. 169 whichshows that the polymorphic structure of HBe multilayers is related to the phases ofthe floating monolayer before deposition. The multilayers deposited from the L 2phase mainly exhibited structures of the and forms, while the films depositedfrom the L and CS phases were dominated by the structure of the form.2

� �Peterson and Russell 186 in 1985 presented an epitaxial evolution theory, whichis based on the assumption that epitaxy improves crystallinity so that an orientationwhich is only slightly preferred in the first monolayer comes to dominate as themultilayer film is deposited.

7.8. Structures of the outermost layer in multilayer films

It is reported that some holes with depths of one- to two-bilayer thicknesses and2 3 ˚10 �10 A diameter have been found on the surfaces of multilayers using AFM

measurements. However, reports on the structural regularity in the top layer arefar from identical: some results indicate that the top layer has a lattice structure,but others show that the top layer is quite disordered.

The AFM measurements of BaAr and other soap multilayer films by Schwartz et� � � � � �al. 187�189 , Schaper et al. 190 and Bourdieu et al. 191 show that there are

˚some irregular holes of a few hundred Angstroms in diameter and of approximatelyone- or two-bilayer thicknesses in depth on the surface of the films, but that the

Ž 2 3top monolayers have lattice structures with long-range positional order 10 �10˚.A .

� �Arndt and Bubeck 114 in their GIR-FTIR study of monolayer and multilayerŽ .films of CdAr protonated�perdeuterated found that the addition of a bilayer to

the first monolayer significantly increases the ordering of the film, while the first


monolayer and the outermost monolayer in the stack have disordered or chain tilt� �structures. Allain et al. 192 , with X-ray diffraction, studied multilayer films of

HBe and concluded that the top monolayer of the film is poorly filled and quite� �disordered. A similar conclusion was drawn by Blasie et al. 193,194 in their X-ray

scattering measurements of CdAr multilayers. With high-resolution meridionalX-ray diffraction they studied the structures of one-, two-, three- and five-bilayerLB films of CdAr on glass slides coated with octadecyltrichlorosilane. Using aPatterson function deconvolution technique they reported that on average thebilayer appears to become more disordered as the number of bilayers in the film isincreased. They also announced that with a refinement technique the profilestructure of individual monolayers in the multilayer could be distinguished. Theyconcluded that only the monolayer on the surface of each multilayer is disordered,and that ordering of the surface monolayer can be induced by the deposition of an

� �additional monolayer 194 . This conclusion conflicts with the results reported by� � Ž .Malik et al. 96,97 as mentioned in Section 7.4 that in CdSt and PbSt of three-

˚and five-layer films the in-plane correlation lengths are over 130 A and they evenincrease or remain constant with increase of the number of the monolayers in thefilms.

� � � �Wiesler et al. 195 and Englisch et al. 196 with neutron reflectivity studied thestructures of films which were deposited with alternative layers of deuterated andprotonated PbSt or BaSt. The intensity of the reflectivity from the multilayersshows that an intermixing between the two kinds of monolayer takes place at roomtemperature. They suggested that there must be some holes present in theoutermost layer of the film and as the subsequent monolayer is deposited on it, themolecules from the newly deposited monolayer would fill in the holes, giving rise to

� � � � � �the mixing. Buhaenko et al. 197 , Grundy et al. 198 and Stroeve et al. 199 alsoobserved such intermixing behaviour. However, in their FTIR studies of deuterated

� �and protonated cadmium arachidate multilayers Shimomura et al. 149 reportedthat only at high temperature could the interdiffusion of the two species beobserved.

� �Light scattering due to excited surface plasmon was used by Aoki et al. 200 tomeasure the surface roughness of LB films of HAr on silver. The results aresomewhat inconclusive, but suggest that there is a dependence of roughness on thenumber of layers deposited. The authors conclude that the surface roughnessincreases with the number of monolayers.

The transition from Y-type deposition to XY-type may be a cause of surface� �roughness. With scanning electron microscopy Peng et al. 64 demonstrated that

the surface of a XY-type multilayer of CdSt is much more uneven than that of a� �Y-type film. Choi et al. 65 measured the X-ray reflectivity profile of a CdAr film

after 39 passes through the floating monolayer and found that the finished filmcomprised regions of various thicknesses in the range 23�31 layers.

Instability and reorganisation in LB films leading to the spontaneous formation,from a uniformly thick film, of holes and multilayer steps have been reported by

� �Takamoto et al. 201 . The process was followed by AFM. The kinetics depend onthe chain length of the fatty acid and on the substrate.


Alternating hydrogenated and deuterated bilayers of BaSt examined with neu-� �tron and X-ray reflectometry 202 revealed that the interlayer roughness was

highly conformal.

7.9. Superstructure in LB films of fatty acid salts

LB films of fatty acid salts observed with AFM usually exhibit various superlat-tice structures which depend on the cations in the salts. There appear to be noreports of superstructures in the bulk phases. The superlattice structures observedin the LB films emerge in the form of height modulation of the molecules in thefilms.

� �Zasadzinski and coworkers 178,187�189 reported that there are three struc-tures coexisting in multilayers of BaAr. Two of them are in-plane superstructuresŽ .3 � 1 and 2 � 2 with the chains tilting by 26� and 19�, respectively. The thirdstructure is unstable with the chains perpendicular to the film surface. MnAr filmsalso show a 3 � 1 structure with chains tilting by 19� to the next nearest neighbour� �124 . A 4 � 2 structure was found in a CaAr film with the chains tilting by 26� in

� �the direction between NN and NNN 187 . In addition, a chiral structure wasreported in this film. In MgAr films there were two superstructures observedŽ .4 � 2 and 2 � 2 with the chains normal to the film surface. In CdAr films abuckling type superstructure was observed with a long spacing of approximately

˚ � � � �19.2 A 203 . With GIXD-IP Peng et al. 204 found that there is a 5 � 1superstructure present in CdAr multilayers at room temperature. Some of thesuperlattice structures are not stable on heating. For BaAr films only the 3 � 1 is

� �stable under heating, 191 while after annealing the superstructures in both CaAr� �and MnAr films transformed into structures of rectangular symmetry 204 .

� �The effects of pH and cation were investigated by Sigiyama et al. 205 . Thesuperstructure changes with pH and is attributed to the mix of acid and salt, thepure acid projecting to a slightly higher level than the salt. The salt is assumed toform between the divalent cation and two next nearest neighbour carboxylateanions in the same layer: a very different arrangement from that usually accepted

Ž .where the cation is shared by anions in adjacent layers see Section 7.2 .There are many more studies on the crystalline structures of LB films of fatty

acid salts than on that of the fatty acids themselves. These are listed in Tables 3and 4.

7.10. Effect of dipping direction on the structure of the LB film

There are some reports about the orientation of the domains in multilayersrelative to the direction of substrate motion during deposition of the films. Koyama

� � � � � � Ž .et al. 158 , Kimura et al. 2 and Zhang et al. 233 FTIR reported that thecrystallographic a-axis in HSt multilayers has a tendency to orient parallel to thedirection of withdrawal of the substrate during deposition. Similar results have also

� � � �been reported by Bonnerot et al. 87 and Chollet et al. 152 with HBe and

()

J.B.P

engetal.�

Ad �ancesin

Colloid

andInterface

Science91

2001163

�219196

Table 3Structures of LB films of fatty acid salts at room temperature with techniques related to diffraction or reflectivity of X-rays, electrons or neutrons

� �Salt No. of Deposition Method Parameters of First author�year reflayers conditions structure

˚ � �CaSt 45�187 On glass X-Ray diffraction d � 47.55 A Clark�1935 81001

˚ � �CaSt Up to 35 pH � 6 on glass X-Ray diffraction d � 49.4 A Clark�1936 171001

˚ � �PbSt As above pH � 4 on glass As above d � 49.6 A Clark�1936 171001

˚ � �BaSt 1 Nitrocellulose Electron d � 4.79 A Havinga�1937 83˚gold diffraction d � 4.80 A˚BaAr nitrocellulose d � 4.79 A

pH � 7

˚ � �BaSt On glass As above d � 50.31 A Holley�1937 142001

˚ � �Ba-CuSt 301�3000 On glass As above d � 50.25 A Holley�1938 206001

˚Ž � �Ba-CuSt 1100 On glass assumed, X-Ray diffraction d � 50.47 A Bernstein�1938 207001

˚.CaSt 300 but not stated d � 50.12 A001

� �BaSt 2, 4, 6 Organic foil Electron Hexagonal Germer�1938 86˚diffraction a � b � 4.85 A

˚Ba-CuSt 1100�100 On glass X-Ray diffraction d � 50.45 A Bernstein�1940001

˚Ž . � �328 assumed d � 50.25 A 207,208001

˚CuSt 1100�100 d � 47.2 A001

˚404 d � 46.38 A001

˚319 d � 46.91 A001

˚316 d � 47.15 A001

()

J.B.P

engetal.�

Ad �ancesin

Colloid

andInterface

Science91

2001163

�219197

Ž .Table 3 Continued


˚BaSt 300 d � 49.92 A001

˚d � 50.3 A001

˚PbAr d � 56.25 A001

˚PbSt 76 on 11 pH � 5.8 Electron a � 4.96 A, M or O Stephens�1969˚ � �of BaSt Glass slides diffraction b � 7.38 A 140

˚c � 47 A

˚ � �BaSt 150 pH � 7.7 X-Ray reflectivity 2 d � 98.6 A Charles�1971 209001

˚PbSt 100 pH � 6.0 2 d � 100.4 A001�1 ˚PbC 15�40 mN m PbC : 70.0 A,12,14 12

˚PbC Float-glass PbC : 80.5 A24,30 14

˚ � �BaSt 185 pH � 7.7�7.9 Ge X-Ray diffraction d � 49.06 A Takenaka�1972 210001

˚ � �BaSt 2�60 Ag-coated glass X-Ray diffraction d � 47.0 A, Lesslauer�1972 211001HW of 1 bilayer � 1�2HW of 30 bilayer

˚ � �BaSt 40, 43, 60 X-Ray diffraction d � 46.3 A Lesslauer�1974 212001

˚BaPa bilayers d � 36.2 A001

˚MgSt d � 48.5 A001Ž .0%, rh

˚d � 51.6 A001Ž .100%, rh

()

J.B.P

engetal.�

Ad �ancesin

Colloid

andInterface

Science91

2001163

�219198



� �MnSt 1�11 pH � 6.5 X-Ray reflectivity Roughness: Pomerantz�1980 2132 2˚² :Glass, quartz, Z � 1 A ,

Ž .silicon wafer 11 layers on Si2 2˚² :Z � 4 A ,

Ž .1 layer on Si˚ � �MnSt 1, 3, 11, 37, Silicon Neutron d � 50 A Nicklow�1981 214001

43 scattering

˚ � �CdAr 2, 4, 6, 8, pH � 6.0 Neutron Even: d � 49.2 A Highfield�1983 176001�1 ˚10, 20, 25, 29.5 mN m reflectivity Odd: d � 49.5 A001

59 On glass

� 4 � �CdSt 11 InP 100 RHEED Orthorhombic Russell�1984 215˚a � 4.96 A˚b � 7.40 A˚c � 2.54 A

˚ � �PbSt 200 On glass slides GIXD a � 4.96 A Prakash�1984 144˚ � �PbMy b � 7.38 A 1985 145˚ Ž .PbBe d � 94 A PbSt001

� �CdSt 1 pH � 6.8 Electron Hexagonal Garoff�1986 84�1 ˚10 mN m diffraction d � 4.20 A10

SiO-coated carbon Chain tilt:� 25�

˚ � �FeSt 40 Si X-Ray diffraction d � 50 A Prakash�1987 216001

�1 ˚Ž . � �HSt 21 20 mN m X-Ray d � 39.9 A w Kamata�1988 170001

˚Ž .& d � 46.4 A s001

()

J.B.P

engetal.�

Ad �ancesin

Colloid

andInterface

Science91

2001163

�219199



˚CdSt pH � 5.8 FTIR d � 5.03 A001�130 mN m

Glass slides

˚ � �CdMy 10 pH � 5.5 X-Ray diffraction d � 4.005 A Fromherz�1988 217001�1 ˚CdPa 30 mN m 4.485 A

˚CdSt Glass slides 5.015 A˚CdAr 5.475 A˚CdBe 5.980 A˚CdTe 6.415 A

˚ � �CdBe 40 pH � 3.0 X-Ray d � 53 A Buhaenko�1988 197001

˚pH � 5.3 & d � 58 & 60 A001

˚pH � 6.3 neutron d � 60 A001�125 mN m reflectivity

Si, glass

�1 ˚ � �BaPa 21 20, 30, 35 mN m , SAXS d � 44.6A Kajiyama�1989 218001

˚BaSt Glass slides d � 50.6 A001

˚BaBe d � 61.6 A001

˚ � �CaSt 51 pH � 6.5 X-Ray reflectivity; d � 50.2 A Bloch�1989 219001�130 mN m Near total external

On glass slides fluorescence

˚ � �CdAr 9, 13 pH � 6.8 & 7.3 Soft X-ray d � 54.7 A Jark�1989 220001�130 mN m reflectivity

On quartz crystal

()

J.B.P

engetal.�

Ad �ancesin

Colloid

andInterface

Science91

2001163

�219200Ž .Table 3 Continued


˚ � �CdPa 34 pH � 6.6�6.9 SAXS d � 45.1 A Merle�1991 221001�1 Ž .25 mN m tilt: 12�

˚CdBe d � 60.1 A001

CdAr 1 pH � 7.0 GIXD Hexagonal Tippmann-Krayer�1992�1 ˚ � �30 mN m d � 4.13 A 58

3, 21 On Si�SiO Orthorhombic2

˚a � 4.85 A˚b � 7.50 A

˚c � 55 A

�1 ˚ � �PbSt 11 25 mN m , on Si SAXS d � 50.3 A Hohne�1994 222¨001NiSt pH � 6.9

˚MgSt 16 pH � 6.5 d � 48.9 A001

˚pH � 8.5 d � 50.0 A001

� �PbSt 5, 11, 19 pH � 6.5 GIXD Orthorhombic Barberka�1994 223�1 ˚25 mN m a � 4.96 A

˚On Si b � 7.32 A˚c � 50.5 A

� �CoSt 25 pH � 5.8�6.2 X-Ray reflectivity Hexagonal Luo�1994 224�1 ˚25 mN m d � 50.0 A001

On CaF , Ge or2quartz

˚ � �FeSt 100 pH � 5.8, Si & PET X-Ray diffraction d � 50.0 A Ando�1995 225001�130 mN m

()

J.B.P

engetal.�

Ad �ancesin

Colloid

andInterface

Science91

2001163

�219201



˚ � �MgSt 28 pH � 8.5 X-Ray diffraction d � 52.2 A Rapp�1995 226001�1 ˚25 mN m d � 49.0 A,001

after heating

� �CdSt 21 pH � 6.8 X-Ray reflectivity d � 50.4, 50.2 Sasanuma�1990 227001�1 ˚ � �30 mN m and 45.5 A 1995 147

˚On glass slides a � 4.96 A˚b � 7.40 A˚c � 50.4 A

˚ � �BaSt 40 pH � 7.3 Neutron and d � 50.4 A Wiesler�1995 195001�128 mN m X-ray reflectivity

On Si & glass

� �PbSt 1 pH � 5�6.5 GIXD Hexagonal, Malik�1995 97�1 ˚35 mN m a � 4.75 A

3, 5 Si�SiO Orthorhombic2

˚a � 7.38 A˚b � 4.96 A

d betweenPb layers:

˚ Ž .50.3 A 3 layer˚ Ž .50.6 A 5 layer

˚ � �PbSt 21 pH � 6.5 Neutron & d � 51 A, X-ray Englisch�1995 196001�1 ˚25 mN m X-ray reflectivity d � 50 A, neutron001

()

J.B.P

engetal.�

Ad �ancesin

Colloid

andInterface

Science91

2001163

�219202



˚ � �CdBe 20 pH � 6.2 Neutron & d � 60 A Grundy�1990 198001�135 mN m X-ray reflectivity Higher dipping rate,

greater intensity

˚ � �CdSt 1 pH � 5�6.5 GIXD a � 7.57 A Malik�1996 96�1 ˚3, 5 35 mN m b � 4.84 A

Si�SiO2

�1 ˚ � �MnAr 1, 2, 3, 40 mN m X-Ray reflectivity d � 53.8 A Takamura�1996 177001

˚5, 9, Quartz glass 51.3 A˚19 49.0 A

˚ � �PbSt 19, 30 GIXD a � 4.96 A Pietsch�1996 146˚BaSt b � 7.32 A

˚CdSt c � 101 AChain tilt: 4�

˚ � �HBe 1 No � values X-Ray diffraction a � 4.94 A Lesieur�1996 104˚Ž .2, 6, 10 were reported transmission b � 7.34 A˚80 c � 53.2 A

�1 ˚PbSt 80 26 mN m X-Ray diffraction a � 4.96 A Yang and Qiao�1998˚ � �b � 7.35 A 228˚c � 51.0 A

˚ � �PbSt 19, 30 GIXD a � 4.96 A Pietsch�1996 146˚BaSt b � 7.32 A

˚CdSt c � 101 AChain tilt: 4�


Table 4Structures of LB films of soaps determined with AFM

Salt No. of Substrate and force used Parameters of structure First authorlayers

˚Ž . � �CdAr 4 On Si hydrophobic surface 5.2 � 4.7 A Meyer�1991 229�8 2˚10 N 24.2 A �chain

Orthorhombic

Ž . � �BaAr 2 Si hydrophobic Distorted hexagonal Bourdieu�1991 230�8 2˚10 N 20.3 A �chain

˚Thickness: 54 A

CdAr 1 Mica, Si None Schwartz�1992, 1993�8 ˚ � �2�5 10 N 4.82 � 7.48 A 148,231

˚d � 55 A001Orthorhombic

� �CdAr 2,5 Si Buckling Garnaes�1992 203�8 ˚10 N d � 19.0 A

˚CdAr 3, 5, 7 Si 7.4 � 4.8 A Florsheimer�1993¨� �8�20 nN Orthorhombic 232

˚ � �PbSt 1 Mica 4.47 � 9.22 A Schwartz�1993 124˚3 5.14 � 7.52 A˚5 4.97 � 7.39 A˚7 4.93 � 7.28 A˚3 Si 4.92 � 7.28 A

1 NoneCdAr 1 Mica None

˚3 4.82 � 7.48 A˚3 Si 4.82 � 7.48 A

˚MnAr 1 Mica 4.6 � 8.7 A˚3 4.95 � 7.91 A˚5 4.81 � 8.12 A˚3 Si 4.77 � 8.34 A

1 None

˚ � �CdSt 5 Mica 7.4 � 4.5 A Schaper�1993 190˚CdAr 10�100 nN 7.3 � 4.6 A˚CdBe 7.3 � 4.7 A˚CdC 7.9 � 4.6 A24



Salt No. of Substrate and force used Parameters of structure First authorlayers

˚BaAr 3 Mica 3 � 1: 15.2 � 4.4 A, Schwartz�1993� � � 66�, chain tilt: 26�; 188,189

˚2 � 2: 9.4 � 9.4 A, � 66�, chain tilt: 19�;

2˚Untilted: area�mol.� 20 A

� �BaAr 2 Si Annealed Bourdieu�1993 191˚4.2 � 5.0 A, � 64.6�

˚ Ž . � �CdAr 1 Mica 4.8 � 8.1 A hexagonal Peng�1994 102

˚ Ž . � �HSt and 1 Mica 4.2 � 4.3 A hexagonal Kajiyama�1994 101ligno-ceric

�acid

˚CaAr 3 10 nN 4 � 2: 18.6 � 8.9 A, � 71� Viswanathan�1994˚Ž . � �local: 4.6 � 4.5 A, � 79� 187

� ��-tricosenoic acid multilayers, and Blaudez et al. 99 with CdAr films as well as� �Leuthe et al. 169 with HSt, HAr and HBe films.

� �Peterson et al. 80,167,234 , in their electron diffraction study of �-tricosenoicacid multilayers, also found a correlation between the orientation of the structureof the film and the dipping direction. Furthermore, they reported that the tilt angle

Žof the chains in the �-tricosenoic acid multilayers becomes greater from 15 to 25�. Žfrom the surface normal as the deposition surface pressure is increased from 25

�1 . � � � �to 42 mN m 235 . However, Rabolt et al. 107 , using polarised FTIR, studiedmultilayers of both HAr and CdAr and reported that with polariser rotation therewere no relative intensity changes or frequency shifts detected, indicating thatthere was no preferential tilt direction in the films. Similarly, changing theorientation of a multilayer film of CdAr relative to the X-ray beam did not alter

� �the GIXD pattern 236 .

7.11. Artificial effect on the image in AFM measurements

Before ending the review of the structure of multilayer LB films it is importantto note the occurrence of artefacts in the determination of structure with AFM.There have been many studies on the structures of LB films of fatty acid salts with

ŽAFM and STM since 1990 when these techniques became widely available see.Table 4 . With AFM�STM local in-plane lattice structures of the films may be

˚ ˚Ž .obtained at the molecular level e.g. in an area 200 A � 200 A . Depth measure-ments of holes in the films can provide information on the bilayer thickness. The

Ž .first 1991 publications which demonstrated the topography at molecular resolu-


� � � �tion were contributed by Meyer et al. 229 and Bourdieu et al. 230 withmultilayers of CdAr and BaAr, respectively. The former reported that the filmŽ .four-layer CdAr film had an orthorhombic or monoclinic structure but the

˚ ˚ Ž .in-plane lattice constants were 5.2 A and 4.7 A one chain per unit cell , quitedifferent from that determined with electron diffraction from CdSt multilayers

˚ ˚Ž . � �4.96 A and 7.40 A, two chains per unit cell 214 . The authors suggested that thedifference originates from the distortion of the molecules or the lattice by thescanning process.

Until now images from the surface of LB films at molecular resolution haveŽ .been obtained by the contact scanning mode repulsive force regime . In this mode

the scanning movement of the probe over the surface may push the molecules onthe film surface aside, i.e. a shear deformation may be produced. Schaper et al.� �190 reported that the bilayer thickness obtained with AFM by measuring thedepth of holes in the films differs significantly from that determined with X-raydiffraction.

� �Florsheimer et al. 232 found large deviations in measurements with AFM of the¨in-plane lattice constants and suggested that to obtain the correct in-plane parame-

� �ters of a unit cell, specific scanning directions must be chosen. Schwartz et al. 231adopted a statistical approach to obtain lattice constants for various soap films.

� �However, even with the statistics 188,189,191 care must be taken and artificial� �factors 237 must be avoided when one works with AFM to determine the

parameters of unit cells of organic crystals. For example, with AFM an annealedBaAr multilayer has been determined to have a centred rectangular structure with

˚ ˚ Ža � 4.2 A, b � 5.0 A and � 64.6� but note that these parameters are applicableto the oblique unit cell description of Fig. 1 rather than the centred rectangular

. � �case 191 . However, with GIXD annealed BaAr films showed a structure of˚ ˚oblique symmetry with lattice constants of a � 4.38 A, b � 4.65 A and � 82.3�

� �204 . The discrepancy could be attributed to a disturbance of the moleculararrangement by the AFM probe. Such a perturbation cannot occur with GIXD.

8. Thermally-induced phase changes in LB films of fatty acids and their salts

Study of the thermal behaviour of monolayers and multilayers of long-chainorganic amphiphiles has revealed a complex pattern of phase changes. Singlemonolayer LB films differ from multilayer films of the same kind, due to thecoupling of the monolayer to the substrate. For films of fatty acid salts, heatingfirst excites disorder in the hydrocarbon chain portion, then as the temperaturegoes above 100�C, disorder of head groups begins. For pure acid films only onesingle-stage order�disorder transition is observed near the melting point of thebulk solid.

8.1. Thermal beha�iour of fatty acid LB films

There are few publications on the thermal behaviour of pure fatty acid LB films.


Only a single step order�disorder transition is observed at the melting point. Thereis also a single report about thermally induced crystallisation in LB films of HBe.

� �One of the early studies was performed by Menter and Tabor 238 , who usedrelaxation electron diffraction to investigate the effect of heating on LB films oflong-chain fatty acids. They concluded that the short-range order in LB films isretained below the melting point of the bulk materials, but randomness in theposition and then in the orientation of the alkyl chains increases as the tempera-ture rises up to and above the melting temperature.

� �Umemura et al. 1 studied the order�disorder transition in LB films of HSt onsilver-coated glass slides with FTIR-RAS. For a 21-layer film, as the temperatureincreases the intensity of the asymmetric and symmetric CH stretching bands is2enhanced slightly up to 68�C and then increases dramatically. They also reportedthat there is a strong effect of the number of layers on the melting point observed.The melting transition occurs at approximately 60�C for a single monolayer film;for a three-layer film it is 2�C lower; then rapidly increases as the number of layersincreases up to approximately 68�C with a 21-layer film, just below the melting

Ž .temperature of the bulk solid 72�C . The authors reported that a similar depen-� �dence had also been observed in CdSt and CaSt LB films 110 .

� �Zhang et al. 233 investigated the FTIR transmission spectroscopy of LB filmsof HSt of 21 layers in the range from room temperature to 80�C. Their mainconclusion was that the order�disorder transition involves a decrease in the alkylchain density with orientational disorder and a trans�cis transition of the dimers inthe film. In their study of the thermal behaviour of HAr multilayers with FTIR,

� �Kobayashi et al. 239 also reported that as the temperature is increased, the chainsare more tilted.

With X-ray reflectivity measurements of the thermal behaviour of a 40-layer film� �of HBe, Buhaenko et al. 197 observed a thermally-induced crystallisation. At

room temperature a set of reflection peaks was detected which gives a d spacing001˚ Žof 53 A corresponding to a tilt of the chain axis from the substrate normal by 28�,

.possibly due to a B-form structure . As the temperature was raised to 55�C, a˚ Žsecond set of peaks occurred which gives a d spacing of 48 A corresponding to a001

.tilt angle of 37�, possibly indicating a C-form structure . However, there is noobvious drop in the intensity of the first set of peaks. At 65�C the first set of peakscompletely disappeared, but this disappearance did not cause an increase of theintensity of the second set. Therefore, it is a thermally induced crystallisationrather than a crystal�crystal phase transition in the film. The second set of peaksvanished at 80�C, the melting point of HBe.

8.2. Thermal beha�iour of LB films of fatty acid salts

It has already been shown that the structures of LB films formed from floatingmonolayers of fatty acids on subphases containing divalent cations depend on thenumber of layers deposited. Because of their different structures it is to beexpected that the thermal behaviour of monolayer LB films would differ from that


of multilayer films. Furthermore, multilayer films that have been heated to highŽ .temperatures � 100�C undergo a major structural change with the formation of a

cylindrical phase. The discussion is therefore divided into three sections: monolayerfilms, multilayer films at lower temperatures, and formation of the cylindricalphase at higher temperatures.

8.2.1. Monolayer LB films of fatty acid salts� �Riegler 85 used electron diffraction to study the thermal behaviour of single

monolayer LB films of CdSt, CdAr and CdBe deposited on Formvar film coatedŽ . �1with amorphous carbon approx. 20 nm thick at 30 mN m . At room temperature

all the films had a hexagonal structure with translational order of approximately˚100 A and orientational order of more than several hundred microns. On heating

there was a sharp decrease in the diffraction intensity starting at 30�35�C for CdSt,50�55�C for CdAr, and 65�70�C for CdBe, which was attributed to the onset ofpretransitional disorder. The decrease was roughly linear with increasing tempera-ture and was not reversible. Extrapolation to zero intensity gave temperatures of55�60�C, 70�80�C, and 90�100�C, respectively, which correspond to the melting

˚Ž .temperatures of the single monolayers. The in-plane correlation lengths � 100 Aremained almost unaltered until the intensity had decreased to less than half thevalue at room temperature, where a small reduction in correlation length began.The author suggested that increased disorder or tilt of the chains is responsible forthe pretransition. Similar results for the single monolayer film of CdAr were

� �obtained by Bohm et al. 184 , but they found that the intensity of the diffraction¨could be at least partially restored below 77�C.

The effect of the substrate on the order�disorder transition was studied by� �Cohen et al. 240 using GIR-FTIR. They prepared single CdAr monolayers on

�aluminium and silver, and a CdAr bilayer on OTS�Al where OTS is octadecyl-Ž .�trichlorosilane C H SiCl . The latter system represents very weak film-to-sub-18 37 3

strate bonding as there is only van der Waals interaction between the hydrocarbontails of the OTS and the CdAr. On heating the CdAr bilayer�OTS system the bandprogression, C�H stretching and COO stretching, changes slightly up to approxi-mately 100�C. At 100�130�C a more abrupt change in the relative band intensitiesoccurs, indicative of randomisation of both the hydrocarbon chains and the headgroups. For the CdAr monolayer film on Al, the C�H stretching band showssignificant changes in relative intensity at approximately 100�C, indicating themelting of the chains, whereas there are no appreciable changes in the COOstretching band up to 125�C due to the interaction between the head groups andthe Al surface. These results suggest that the randomisation of the chains is notaccompanied by changes in the orientation of the head groups. For comparison,they also observed the thermal behaviour of self-assembled monolayer films ofOTS on Al which have covalent binding both in the intralayer and to the substrate.Spectral changes upon heating were far less dramatic for these films, with nodiscontinuities attributable to significant disorientation being detected in the rangeof 25�140�C.


8.2.2. Multilayer LB films of fatty acid salts� � � �In 1985 Naselli et al. 106,241 and Schlotter et al. 242 using both GIR and

transmission FTIR, first studied the two-step melting process of multilayers ofCdAr on silver-coated glass slides. They reported that above 65�C the bands in the

�1 � Ž .� �1 � Ž .�CH stretching region at 2918 cm � CH and 2850 cm � CH begin to2 a 2 s 2Ž .show relative intensity variations, whereas there is little change for the � COa 2

Ž . �1and � CO head group vibrations at 1545 and 1432 cm , respectively. By 90�C, as 2Ž . Ž .substantial increase in the intensities of both � CH and � CH was observed,a 2 s 2

Ž . �1the �CH wagging and twisting vibration � � in the 1175�1375 cm region2had broadened considerably and weakened in intensity, but again there was noappreciable change in the head group vibrations. However, at 125�C more dramaticvariations were detected both in the CH and head group stretching regions. These2results suggest that a progressive disordering in the alkyl chains starts with apremelting stage at 45�60�C and slowly extends up to the melting point of the film.This is supported by their DSC measurements of the film which show that at 70�Cthe trace started to deviate from the baseline and at 110�C the film melted. Theyattributed the disordering of the chains at the premelting stage to the release ofgauche conformations.

� �Umemura et al. 110 studied the order�disorder transition of multilayers ofboth CdSt and CaSt on silver-coated glass slides with FTIR-RAS. They reportedthat for CdSt film at 99�C the doublet at 731 and 720 cm�1 due to the CH rocking2band, which is characteristic of the orthorhombic packing of the hydrocarbonchains, changes into a singlet at 725 cm�1 and suggested that this is due to thedisordered lattice or hexagonal structure. Since at this temperature the band

Ž �1 .progression 1400�1200 cm , which is assigned to the all-trans conformation ofthe alkyl chains, is still present, the all-trans conformation must persist after thechange in the packing pattern. The band progression disappears at 110�C when theintensities of both the symmetric and asymmetric CH stretching bands dramati-2cally increase, indicating chain disorder due to an increase in the number of

Ž . Ž .gauche defects. The intensity ratio of the CH vibrations, I� CH �I� COO , as a2 a 2 sŽfunction of temperature rises rapidly at approximately 108�C in fact, it seems to.occur at approximately 98�C if an extrapolation is employed , irrespective of the

film thickness. They interpreted this change as a transition between solid andliquid�crystalline phases. With CaSt the order�disorder transition occurs at ap-proximately 103�129�C in films of 3�21 layers. In an earlier paper with the same

� �technique 243 they found that the order�disorder transition takes place at 110�Cfor CdSt, 125�C for CaSt, and above 150�C for BaSt and AlSt. They also studied the

�spectral changes when the films were subjected to cyclic thermal treatments i.e.Ž .the samples were heated to the transition temperature for CdSt even to 120�C

� Ž . Ž .then cooled to 30�C . The intensity ratio, I� CH �I� COO , returns to a certaina 2 sextent towards its original value for CdSt, but for BaSt the recovery is lesscomplete than that for CdSt while that for CaSt is in between.

� � � �Barbaczy et al. 244 and Rabe et al. 108,109 using waveguide Raman spectros-copy studied CdAr LB multilayers in the temperature range of �125 to �100�C.The CH stretching band at 2880 cm�1 is very sensitive to the conformation and2


molecular motion of the chains. In particular, a broadening with a high frequencyŽ .shoulder has been attributed to a hindered rotation libration about the chain axis

� �245,246 . They reported that from �90 to �16�C the CH stretching band2remains essentially unaltered, indicating less molecular motion in the low tempera-ture range. It changes dramatically on further heating, broadening slightly andasymmetrically with a high frequency shoulder and losing intensity until disappear-ing at around 100�C. In the C�C stretching region the band at 1080 cm�1 isassigned to gauche conformations, while that at 1060 cm�1 is attributed to the

� �trans counterpart 247 . Their results show that even up to 93�C only the C�C transband is observed. They thus suggested that the order�disorder transition approxi-mately 100�C in CdAr multilayers is mainly characterised by a strong increase inlibration and only to a lesser extent by the formation of gauche conformations.

� �Using Raman spectroscopy, Dierker et al. 105 studied the thermal response ofmultilayers of CdSt, focusing on the CH symmetric and asymmetric stretch bands2in films on hydroxylated silicon. Dramatic modifications of the spectra occur upon

Žapproaching the chain melting temperature of 110�C the correct value seems to be� �.approx. 97�C 248 . They characterised these changes by the ratio

Ž . Ž .I� CH �I� CH and reported that a strong decrease in this ratio had beena 2 s 2Ž .observed, but failed to give the temperature. By 94�C the � CH band has greatlys 2

broadened and decreased in intensity, which is attributed to the transition to theliquid state. This change is accompanied by a strong broadening and an increase in

Ž �1 . Ž .frequency by 5 cm of the � CH band which is attributed to the increaseda 2dynamic population of gauche bonds.

� �Rothberg et al. 249 used a pulsed laser optoacoustic technique associated withIR spectroscopy to study the thermal disordering of single and multilayers of CdSt

Ž .on a transparent substrate sapphire . On heating of a five-layer film up to 65�Cthe symmetric and asymmetric CH stretching bands exhibit very slight broaden-2ings and blue-shifts but a large decrease in intensity, indicating orientationaldisordering of the chains with respect to the film surface normal and therefore thepolarised incident field. Cooling at any point between 20 and 65�C does not restorethe intensity that has been lost to that point. On further heating to 100�C, theintensities of both stretching bands drop to one third of the values at roomtemperature with substantial broadening and shifts to higher frequency: effectswhich were attributed to the trans�gauche conformation transition in the chains� �250 . The loss of intensity between 65 and 100�C is reversed on cooling. For thesingle monolayer film the same thermal behaviour was observed but the introduc-tion of gauche bonds occurs at a much lower temperature, even as low as 45�C.

The thermal behaviour of LB monolayers and multilayers has also been investi-gated with electron diffraction and X-ray and neutron scattering by following thevariations of intensity, peak width and peak position with temperature. Buhaenko

� �et al. 197 were the first to use X-ray and neutron scattering to study the thermalbehaviour of LB films. Three species of 40-layer films were employed in themeasurements: HBe, CdBe, and a mixture of the acid and the salt. For CdBe at

˚room temperature d � 60 A. On heating to 90�C a new set of peaks of d � 58001 001A appeared, implying that a tilted phase had developed in the film. When the


temperature was raised to the order�disorder transition range, 110�C, the peaks˚ ˚for the spacing of 60 A vanished and the 58 A peaks became stronger but not as

˚ Ž .strong as the 60 A peaks had been. At 125�C below the bulk melting point thediffraction pattern from the film vanished. The mixed film exhibited the samediffraction features as the salt film on heating but with a weaker intensity. Theresults from neutron reflectivity measurements are quite similar to those fromX-ray scattering.

� �Merle et al. 221 studied the order�disorder transition of LB films of CdBe andCdPa of 34 layers using SAXS. With CdBe films they obtained the same results as

� �those reported by Buhaenko et al. 197 . With CdPa films they reported that below˚ Ž95�C only one series of reflections of d � 45.1 A is present corresponding to a tilt

.of the chains from the normal of the substrate surface by 12� .CdAr LB films of one and 21 layers have been studied with electron diffraction

� � � � Ž .by Bohm et al. 184 and with GIXD by Tippmann-Krayer et al. 58 see Table 3 .¨The results from the multilayer films show that the layers were deposited epitaxi-ally and had an orthorhombic structure at room temperature. As the temperaturewas raised the symmetry gradually changed to hexagonal as indicated in electrondiffraction by the shifting of the outer diffraction spots towards the inner ones with

� Ž . Ž .decreasing intensity the outer spots are 200 reflections, and inner ones 110˚ ˚� Ž .reflections and in GIXD by changes in d from 4.08 A at up to 95�C to 4.13 A110

˚ Ž .at 100�C, while d increased from 3.75 A at 60�C to merge with the 110 peak at200100�C. This process is reversible as long as the sample is not heated above 105�C.The electron diffraction pattern disappeared at 105�110�C.

� �Fukui et al. 251 studied the temperature dependence of the thickness of LBŽ .films 11 layers of CdAr, CdSt, and CdPa using X-ray diffraction over the

temperature range of �193 to 80�C. The d spacing remains constant below001Ž �5 �1.�40�C the thermal expansion coefficient is approx. 10 K , but above �20�C

˚ ˚it decreases monotonically from approximately 55.4 A to 55.0 A at 80�C for CdAr,˚ ˚ ˚ ˚50.3 A to 49.9 A at 80�C for CdSt, and 45.2 A to 44.9 A at 60�C for CdPa.

Some different conclusions on the thermal behaviour of LB films of fatty acid� �salts were drawn by Richardson and Blasie 162 using high-resolution meridional

X-ray diffraction. The samples were prepared as DDAADD, DAADDD,( ) ( )DD AA DD, and DD AA DD on OTS-coated glass slides, where D refers to a3 10

monolayer of cadmium pentacosa-10,12-diyanoate, A to a monolayer of CdAr.They concluded that the density of a CdAr monolayer depends on whether it isdeposited on a downstroke or an upstroke and attributed the higher density indownstroke-deposited layers to almost perpendicular chains and the lower densityin upstroke-deposited layers to tilted chains. Furthermore, the downstroke-de-

Žposited layers exhibited one-step melting over a narrow temperature range at.approx. 80�C , while for upstroke-deposited layers a two stage melting process was

suggested. In the two-stage process the thickness of the monolayer increases withŽ .temperature to approx. 50�60�C by reduction of the tilt angle of the chains, while

Ž .in the second stage the thickness is reduced due to melting at 75�80�C . Thestructural changes with temperature depend on the total number of bilayers in the


film: one AA bilayer film undergoes much greater structural change upon thermalexcitation than three- and 10 AA-bilayer films.

� �Shimomura et al. 149 with FTIR studied the interlayer diffusion behaviour inLB films as a function of temperature. The LB multilayers were prepared on ZnSeplates as assemblies of alternative bilayers of deuterated and protonated CdAr.

Ž .After annealing at 85�C for 1 h the spectra measured at room temperature showthat 30% of the protonated molecules have diffused into the deuterated bilayers,and vice versa. The extent of diffusion was 25% for an annealing temperature of70�C. The diffusion can be hindered by depositing a polymer bilayer between theCdAr bilayers. For example, with a bilayer of fluorocarbon side chain polymer as abarrier, the interdiffusion is reduced to 5% after annealing at 85�C for 1 h.

8.2.3. Formation of hexagonally-packed cylindrical structuresThe order�disorder transition in LB films of CdBe and CdPa of 34 layers was

� �investigated by Merle et al. 221 using SAXS. Between 96 and 98�C a new phase˚ Žoccurs with a d-spacing of 26.4 A. At 99�C only the new phase exists it could be

� �due to a cylindrical structure 248 although the authors did not give an explanation.for the new phase . Above this temperature the diffraction pattern disappears.

Cooling leads to a partial recovery of the intensity of the diffraction at roomtemperature. In a later study, using electron diffraction, they investigated the

� �structure of the new phase but with samples of CdSt of 10 layers 252 annealed at103�C for 1.5 h. The diffraction was measured after cooling to room temperature.

˚They found three spacings with values of 26.3, 15.2 and 13.1 A which wereattributed to a hexagonal packing of cylinders. They also looked at the structure ofthe films at elevated temperatures, and at 101�C a pair of diffraction spots wasobserved which corresponds to approximately 20 molecular spacings, indicating anintermediate state of the film. Since the transition from the lamellar structure tothe cylindrical structure is found to be a slow process, Merle et al. followed the

� �kinetics of the transition with X-ray reflectivity 253 . They reported that the newphase starts growing only after a certain degree of disorder is reached, and thespeed of the phase transition seems to increase as the temperature approaches themelting point.

� �Peng et al. 204,254 have studied with GIXD the thermal behaviour of a varietyŽ .of LB films. Below 95�110�C depending on the chain length Cd soap films have

lamellar structures, with decreasing diffraction intensity as the temperature in-creases. At 97�112�C the lamellar structure disappears and a structure of hexago-nally packed cylinders develops due to the melting of the chains and the rearrange-ment of the headgroups. Each cylinder appears to comprise a core of diameter

˚ 2�approximately 6.7 A composed of Cd ions and carboxy groups, surrounded byrandomly oriented chains. The centres of the cylinders, which are arranged parallel

˚to the plane of the substrate, are spaced 27 A apart, which is much less than abilayer thickness. This was attributed to either the declination of the chains to thecylinder axis or the occurrence of gauche conformations. Films containing othercations were found to behave quite differently.

� �Hohne and Mohwald 222 with SAXS and Normarsky microscopy studied the¨ ¨


thermal behaviour of PbSt, NiSt, and MgSt multilayers on silicon wafers. Their˚Ž . Ž .results show that for PbSt film 12 layers up to 100�C the d spacing 50.3 A001

remains unchanged. At 111�C a coexistence of the room temperature phase and a˚new phase with d � 49.8 A is observed which the authors attribute to a001

cylindrical phase. The diffraction pattern vanishes at 114�C and the film ruptures.On cooling to room temperature the two sets of peaks are regained but with much

˚lower intensity. One corresponds to a d-spacing of 50.3 A, the other to a new˚spacing of 38.2 A. With NiSt film the d spacing and the film thickness001

continuously decrease as the temperature increases. The thickness is reversibleonly at temperatures below 70�C. The layered structure of the film is maintainedup to 170�C. MgSt films exhibit a similar behaviour to NiSt on heating. The

˚ ˚d-spacing starts at 52.8 A at room temperature, then it decreases to 49.5 A at 53�C� �due to the release of absorbed water in the film 226 . The melting transition is

slightly above 100�C.

9. Summary and conclusions

The development over the last twenty years of highly sensitive techniques forstructural studies has, for the first time, enabled detailed examination of singlemonolayers on water surfaces and Langmuir�Blodgett monolayers and multilayersdeposited on solid surfaces. Understandably, much of the early work has beenconcerned with relatively simple compounds, such as the fatty acids and their saltswhich are the subject of this review.

Careful examination of floating monolayers of the fatty acids has revealed acomplex polymorphism with phases whose structures may be conveniently de-scribed by using the classification of smectic phases. In condensed phases the films

4 6 ˚consist of randomly oriented domains approximately 10 �10 A in size, with themolecules in each domain arranged with the same bond orientation, but with

˚positional correlation lengths of approximately 100 A.ŽLangmuir�Blodgett films are usually deposited at high surface pressures 20

�1 .mN m . Y-Type deposition, where a monolayer is deposited on both the downstroke and the up stroke, is usually encountered, but as the number of layersincreases the efficiency on the up stroke sometimes diminishes giving XY deposi-tion. Incorporation of the stoichiometric quantity of a divalent cation into a fattyacid film only occurs when the pH exceeds a certain value that varies with thecation.

There are divergent data on the orientation of LB domains relative to thedipping direction, with some results suggesting that there is a correlation whileothers indicate no preferred orientation.

Generally in single monolayer LB films the acyl chains are in the trans confor-mation and oriented with their long axes normal to the substrate. The moleculesare hexagonally packed and this in-plane packing shows long-range bond orienta-tional order and short-range positional order: often called hexatic-B packing andcorresponding to the smectic BH structure. This structure is obtained irrespective


of the phase of the parent floating monolayer provided the film has been removedfrom contact with the floating monolayer and dried. However, while the LBmonolayer is kept in contact with the parent monolayer it retains the structure ofthat monolayer, probably because of a thin water film between the monolayer andthe substrate. Occasionally, when there is strong interaction between the monolayer

Ž .head-group and the substrate, orientational and rarely strain epitaxy may beobserved. There is also some evidence that larger features such as domain size maybe preserved during LB deposition.

In multilayer LB films the basic structure is similar to that observed in the bulkcrystals: a lamellar structure where each lamella is a bilayer consisting of twomonolayers in a head-to-head arrangement irrespective of the deposition type.With fatty acids, the two monolayers are bound together by hydrogen bonding.With the salts of divalent cations, a lamella consists of two layers of long-chaincarboxyl anions combining with one layer of cations. The correlation length in the

˚film plane is approximately 100�200 A, but only approximately two bilayers in thedirection of the normal. The most stable in-plane structure is usually centredrectangular with the chains in the trans conformation and packed in a herringbonepattern. Films of acids show some chain tilt indicating monoclinic structures and

Ž .occasionally polymorphism different tilt angles, etc. , but studies of the salts withdivalent cations report that the chains are either normal to the substrate or tilted,depending on the cation.

The transition from the hexagonal in-plane structure in monolayer films to thecentred rectangular structure in multilayer films as the number of layers isincreased has been variously reported to occur in the first three layers, or to takeup to nine layers. It is possible that in the latter cases there is strong interactionbetween the substrate and the first monolayer leading to strained-layer epitaxy.There is apparently conflicting evidence as to whether the structure of the firstmonolayer changes as subsequent layers are deposited over it. The top layer of amultilayer film may be relatively disordered and usually contains holes, but thestructure of this layer improves when further layers are deposited on it.

Some multilayer films show evidence of superstructures. This feature seems todepend strongly on the nature of the cation and on the pH of the subphase.

When heated, multilayer films of the acids show only a single step order�dis-order transition at the melting point. The melting temperature tends generally torise as the number of layers increases.

With monolayer LB films of the salts, chain tilt or disorder begins at tempera-tures well below the temperature of the main order�disorder transition. There areno accompanying changes in the head group orientation where there is stronginteraction between head group and substrate.

With multilayer films of the salts the effects of heating are more complex. Againa progressive disordering of the acyl chains begins at a low temperature, but noaccompanying change in the head groups is apparent until the melting temperatureis reached. However, there is evidence from several sources indicating that thedisorder observed at lower temperatures is not caused by the introduction ofgauche conformations: the formation of gauche conformers only occurs as the


temperature nears that of the main order�disorder transition. Annealing at atemperature well below the order�disorder transition temperature can lead tosome diffusion and mixing between the layers of a multilayer LB film.

With multilayer LB films of some, but not all, fatty acid salts, slow heating to atemperature just below the melting temperature leads to the disappearance of thelamellar structure and the formation of a new radically different phase. This newphase consists of cylindrical rods lying parallel with the substrate surface andstacked in a hexagonal pattern. Each rod has the cations aligned along the centralaxis surrounded by the disordered acyl chains.

It is thus apparent that a great deal is now known about the structures of LBfilms of fatty acids and their salts with divalent cations. Some questions have yet tobe answered, but there is nevertheless a sound basis of understanding for theinvestigation of more complex films using the highly sensitive techniques that arenow available.

References

� � Ž .1 J. Umemura, S. Takeda, T. Hasegawa, T. Takenaka, J. Mol. Struct. 297 1993 57.� � Ž .2 F. Kimura, J. Umemura, T. Takenaka, Langmuir 2 1986 96.� � Ž .3 L. Hernqvist, in: N. Garti, K. Sato Eds. , Crystallization and Polymorphism in Fats and Fatty

Acids, Dekker, New York, Basel, 1988.� � Ž .4 R.F. Holland, J.R. Nielsen, J. Mol. Spectrosc. 9 1962 436.� �5 G.L. Gaines, Insoluble Monolayers at Liquid�Gas Interfaces, Interscience, New York, 1966.� �6 M.C. Petty, Langmuir�Blodgett Films. An Introduction, Cambridge University Press, Cambridge,

1996.� �7 G.G. Roberts, Langmuir�Blodgett Films, Plenum Press, New York, 1990.� �8 A. Ulman, An Introduction to Ultrathin Organic Films from Langmuir�Blodgett to Self-Assem-

bly, Academic Press, San Diego, 1991.� � Ž .9 D.K. Schwartz, Surf. Sci. Rep. 27 1997 241.

� � Ž .10 G.G. Roberts, Adv. Phys. 34 1985 475.� � Ž .11 J.D. Swalen, D.L. Allara, J.D. Andrade et al., Langmuir 3 1987 932.� � Ž .12 I.R. Peterson, in: G. Mahler, V. May, M. Schreiber Eds. , The Molecular Electronic Handbook,

Marcel Dekker, New York, 1994.� � Ž .13 A. Pockels, Nature 43 1891 437.� � Ž .14 G.L. Gaines, J. Colloid Interface Sci. 62 1977 191.� �15 J. Als-Nielsen, D. Jacquemain, K. Kjaer, F. Leveiller, M. Lahav, L. Leiserowitz, Phys. Rep. 246

Ž .1994 251.� �16 I. Weissbuch, R. Popovitz-Biro, M. Lahav, L. Leiserowitz, K. Kjaer, J. Als-Nielsen, Adv. Chem.

Ž .Phys. 102 1997 39.� � Ž .17 G.J. Foran, J.B. Peng, R. Steitz, G.T. Barnes, I.R. Gentle, Langmuir 12 1996 774.� �18 G.J. Foran, I.R. Gentle, R.F. Garrett, D.C. Creagh, J.B. Peng, G.T. Barnes, J. Synchrotron

Ž .Radiat. 5 1998 107.� � Ž .19 J. Penfold, R.K. Thomas, J. Phys.: Condensed Matter 2 1990 1369.� �20 B.E.P. Beeston, R.W. Horne, R. Markham, Electron Diffraction and Optical Diffraction Tech-

niques, North-Holland, London, 1972.� � Ž .21 Y.P. Song, M.C. Petty, J. Yarwood, Vibrational Spectrosc. 1 1991 305.� � Ž .22 R.A. Dluhy, J. Phys. Chem. 90 1986 1373.� � Ž .23 M.L. Mitchell, R.A. Dluhy, J. Am. Chem. Soc. 110 1988 712.� � Ž .24 D. Blaudez, T. Buffeteau, J.C. Cornut et al., Appl. Spectrosc. 47 1993 869.� � Ž .25 D. Blaudez, T. Buffeteau, J.C. Cornut et al., Thin Solid Films 242 1994 146.


� � Ž .26 R.G. Greenler, J. Chem. Phys. 44 1966 310.� � Ž .27 J. Yarwood, Anal. Proc. 30 1993 13.� � Ž .28 M.C. Petty, in: G.G. Roberts Ed. , Langmuir�Blodgett Films, Plenum Press, New York, 1990.� � Ž .29 Y.J. Chen, G.M. Carter, S.K. Tripathy, Solid State Commun. 54 1985 19.� � Ž .30 Lord Rayleigh, Phil. Mag. 48 1899 337.� � Ž .31 I. Langmuir, J. Am. Chem. Soc. 39 1917 1848.� � Ž .32 W.D. Harkins, T.F. Young, E. Boyd, J. Chem. Phys. 8 1940 954.� � Ž .33 G.C. Nutting, W.D. Harkins, J. Am. Chem. Soc. 61 1939 2040.� � Ž .34 L.E. Copeland, W.D. Harkins, E. Boyd, J. Chem. Phys. 10 1942 357.� � Ž .35 W.D. Harkins, L.E. Copeland, J. Chem. Phys. 10 1942 272.� � Ž .36 S. Stallberg-Stenhagen, E. Stenhagen, Nature 156 1945 239.¨� � Ž .37 E. Stenhagen, in: E.A. Braude, C.F. Nachod Eds. , Determination of Organic Structures by

Physical Methods, Academic Press, New York, 1955.� � Ž .38 M. Lundquist, Chem. Scr. 1 1971 5.� � Ž .39 M. Lundquist, Chem. Scr. 1 1971 197.� � Ž .40 A.M. Bibo, C.M. Knobler, I.R. Peterson, J. Phys. Chem. 95 1991 5591.� � Ž .41 G.A. Lawrie, G.T. Barnes, J. Colloid Interface Sci. 162 1994 36.� � Ž .42 G.A. Overbeck, D. Mobius, J. Phys. Chem. 97 1993 7999.¨� � Ž .43 I.R. Peterson, G. Brezesinski, B. Struth, E. Scalas, J. Phys. Chem. B 102 1998 9437.� � Ž .44 M.L. Kurnaz, D. Schwartz, J. Phys. Chem. 100 1996 11113.� � Ž .45 M. Yazdanian, H. Yu, G. Zografi, Langmuir 6 1990 1093.� �46 R.F. Silva, M.E.D. Zaniquelli, O.A. Serra, I.L. Torriani, S.G.C. de Castro, Thin Solid Films 324

Ž .1998 245.� � Ž .47 T.V. Zotova, V.V. Arslanov, I.A. Gagina, Thin Solid Films 326 1998 223.� � Ž .48 B. Lin, M.C. Shih, T.M. Bohanon, G.E. Ice, P. Dutta, Phys. Rev. Lett. 65 1990 191.� � Ž .49 M.C. Shih, T.M. Bohanon, J.M. Mikrut, P. Zschack, P. Dutta, Phys. Rev. A 45 1992 5734.� � Ž .50 R.M. Kenn, C. Bohm, A.M. Bibo et al., J. Phys. Chem. 95 1991 2092.� � Ž .51 A.M. Bibo, I.R. Peterson, Adv. Mater. 2 1990 309.� � Ž .52 M.K. Durbin, A. Malik, R. Ghaskadvi, M.C. Shih, P. Zschack, P. Dutta, J. Phys. Chem. 98 1994

1753.� � Ž .53 I.R. Peterson, R.M. Kenn, Langmuir 10 1994 4645.� � Ž .54 E.A. Kondrashkina, K. Hagedorn, D. Vollhardt, M. Schmidbauer, R. Kohler, Langmuir 12 1996¨

5148.� �55 S. Riviere, S. Henon, J. Meunier, D.K. Schwartz, M.-W. Tsao, C.M. Knobler, J. Chem. Phys. 101´ ´

Ž .1994 10045.� � Ž .56 I.R. Peterson, in: G.J. Ashwell Ed. , Molecular Electronics, Research Studies Press, Taunton,

1992.� � Ž .57 C.M. Knobler, R.C. Desai, Annu. Rev. Phys. Chem. 43 1992 207.� � Ž .58 P. Tippmann-Krayer, R.M. Kenn, H. Mohwald, Thin Solid Films 210�211 1992 577.¨� � Ž .59 I. Langmuir, Trans. Faraday Soc. 15 1920 62.� � Ž .60 K.B. Blodgett, J. Am. Chem. Soc. 57 1935 1007.� � Ž .61 I. Langmuir, V.J. Schaefer, J. Am. Chem. Soc. 60 1938 1351.� � Ž .62 E.P. Honig, J.H.T. Hengst, D. den Engelsten, J. Colloid Interface Sci. 45 1973 92.¨� � Ž .63 J.B. Peng, Langmuir 6 1990 1725.� � Ž .64 J.B. Peng, J.B. Ketterson, P. Dutta, Langmuir 4 1988 1198.� � Ž .65 J.W. Choi, K.S. Cho, W.H. Lee, H.S. Lee, Thin Solid Films 327�329 1998 273.� � Ž .66 J.W. Choi, K.S. Cho, H.W. Rhee, W.H. Lee, H.S. Lee, Bull. Korean Chem. Soc. 19 1998 549.� � Ž .67 E.P. Honig, J. Colloid Interface Sci. 43 1973 66.� � Ž .68 I.R. Peterson, G.J. Russell, G.G. Roberts, Thin Solid Films 109 1983 371.� � Ž .69 C. Vogel, J. Corset, M. Dupeyrat, J. Chim. Phys. 76 1979 909.� � Ž .70 J.G. Petrov, I. Kuleff, D. Platikanov, J. Colloid Interface Sci. 88 1982 29.� � Ž .71 M. Sastry, A. Mandale, S. Badrinarayanan, P. Ganguly, Langmuir 8 1992 2354.� � Ž .72 J. Peltonen, M. Linden, H. Fagerholm, E. Gyorvary, F. Eriksson, Thin Solid Films 242 1994 88.¨� � Ž .73 K. Kobayashi, K. Takaoka, S. Ochiai, Thin Solid Films 159 1988 267.


� � Ž .74 K. Kobayashi, K. Takaoka, Bull. Chem. Soc. Jpn. 59 1986 933.� �75 G. Bolbach, M. Plissonnier, R. Galera, J.C. Blais, D. Dufour, H. Roulet, Thin Solid Films´

Ž .210�211 1992 524.� � Ž .76 D.J. Ahn, E.I. Franses, J. Chem. Phys. 95 1991 8486.� � Ž .77 A. Dhanabalan, N.P. Kumar, S. Major, S.S. Talwar, Thin Solid Films 327�329 1998 787.� � Ž .78 B.I. Seo, H. Lee, J.J. Chung et al., Thin Solid Films 327�329 1998 722.� � Ž .79 R.R. McCaffery, S. Bruckenstein, P.N. Prasad, Langmuir 2 1986 228.� � Ž .80 G. Veale, I.R. Peterson, J. Colloid Interface Sci. 103 1985 178.� � Ž .81 G.L. Clark, R.R. Sterrett, P.W. Leppla, J. Am. Chem. Soc. 57 1935 330.� � Ž .82 C. Holley, S. Bernstein, Phys. Rev. 49 1936 403.� � Ž .83 E. Havinga, J. De Wael, Rec. Trav. Chim. Pays-Bas 56 1937 375.� � Ž . Ž .84 S. Garoff, H.W. Deckmann, J.H. Dunsmuir, M.S. Alvarez, J.M. Bloch, J. Phys. Paris 47 1986

701.� � Ž .85 J.E. Riegler, J. Phys. Chem. 93 1989 6475.� � Ž .86 L.H. Germer, K.H. Storks, J. Chem. Phys. 6 1938 280.� � Ž .87 A. Bonnerot, P.A. Chollet, H. Frisby, M. Hoclet, Chem. Phys. 97 1985 365.� � Ž .88 A. Fischer, E. Sackmann, J. Colloid Interface Sci. 112 1986 1.� � Ž .89 J.M. Kosterlitz, D.J. Thouless, J. Phys. C 6 1973 1181.� � Ž .90 B.I. Halperin, D.R. Nelson, Phys. Rev. Lett. 41 1978 121.� � Ž .91 D.R. Nelson, B.I. Halperin, Phys. Rev. B 19 1979 2457.� �92 G.W. Gray, J.W.G. Goodby, Smectic Liquid Crystals, Leonard Hill, Glasgow, 1984.� � Ž .93 V. Vogel, C. Woll, J. Chem. Phys. 84 1986 5200.¨� � Ž .94 V. Vogel, C. Woll, Thin Solid Films 159 1988 429.¨� � Ž .95 M.C. Shih, J.B. Peng, K.G. Huang, P. Dutta, Langmuir 9 1993 776.� � Ž .96 A. Malik, M.K. Durbin, A.G. Richter, K.G. Huang, P. Dutta, Thin Solid Films 284�285 1996

144.� � Ž .97 A. Malik, M.K. Durbin, A.G. Richter, K.G. Huang, P. Dutta, Phys. Rev. B 52 1995 R11654.� � Ž .98 D. Blaudez, T. Buffeteau, N. Castaings, B. Desbat, J.M. Turlet, J. Chem. Phys. 104 1996 9983.� � Ž .99 D. Blaudez, T. Buffeteau, B. Desbat, N. Escafre, J.M. Turlet, Thin Solid Films 243 1994 559.

� � Ž .100 D. Blaudez, T. Buffeteau, M. Orrit, J.M. Turlet, Thin Solid Films 210�211 1992 648.� � Ž .101 T. Kajiyama, Y. Oishi, F. Hirose, K. Shuto, T. Kuri, Langmuir 10 1994 1297.� � Ž .102 J.B. Peng, G.T. Barnes, Thin Solid Films 252 1994 44.� � Ž .103 A. Rajagopal, A. Dhanabalan, S.S. Major, S.K. Kulkarni, Appl. Surf. Sci. 125 1998 178.� � Ž .104 P. Lesieur, J. Mol. Struct. 383 1996 125.� � Ž .105 S.B. Dierker, C.A. Murray, J.D. Legrange, N.E. Schlotter, Chem. Phys. Lett. 137 1987 453.� � Ž .106 C. Naselli, J.P. Rabe, J.F. Rabolt, J.D. Swalen, Thin Solid Films 134 1985 173.� � Ž .107 J.F. Rabolt, F.C. Burns, N.E. Schlotter, J.D. Swalen, J. Chem. Phys. 78 1983 946.� � Ž .108 J.P. Rabe, V. Novotny, J.D. Swalen, J.F. Rabolt, Thin Solid Films 159 1988 359.� � Ž .109 J.P. Rabe, J.D. Swalen, J.F. Rabolt, J. Chem. Phys. 86 1987 1601.� � Ž .110 J. Umemura, S. Takeda, T. Hasegawa, T. Kamata, T. Takenaka, Spectrochim. Acta 50 1994

1563.� �111 T. Nakanaga, M. Matsumoto, Y. Kawabata, H. Takeo, C. Matsumura, Chem. Phys. Lett. 160

Ž .1989 129.� �112 G. Hahner, M. Kinzler, C. Woll, M. Granze, M.K. Scheller, L.S. Cederbaum, Phys. Rev. Lett. 69¨ ¨

Ž .1992 694.� �113 G. Hahner, M. Kinzler, C. Woll, M. Grunze, M.K. Scheller, L.S. Cederbaum, Phys. Rev. Lett. 67¨ ¨

Ž .1991 851.� � Ž .114 T. Arndt, C. Bubeck, Thin Solid Films 159 1988 443.� � Ž .115 C. Duschl, W. Knoll, J. Chem. Phys. 88 1988 4062.� � Ž .116 C.A. Jones, G.J. Russell, M.C. Petty, G.G. Roberts, Phil. Mag. B 54 1986 L89.� � Ž .117 D.L. Allara, J.D. Swalen, J. Phys. Chem. 86 1982 2700.� � Ž .118 J.P. Rabe, J.D. Swalen, D.A. Outka, J. Stohr, Thin Solid Films 159 1988 275.¨� � Ž .119 D.A. Outka, J.P. Rabe, J.D. Swalen, H.H. Rotermund, Phys. Rev. Lett. 59 1987 1321.� � Ž .120 D.A. Outka, J. Stohr, J.P. Rabe, J.D. Swalen, J. Chem. Phys. 88 1988 4076.¨


� � Ž .121 D.J. Ahn, E.I. Franses, J. Phys. Chem. 96 1992 9952.� � Ž .122 M. Kinzler, A. Schertel, G. Hahner et al., J. Chem. Phys. 100 1994 7722.¨� �123 J.B. Peng, Ph.D. Thesis, The University of Queensland, 1998.� � Ž .124 D.K. Schwartz, R. Viswanathan, J. Garnaes, J.A.N. Zasadzinski, J. Am. Chem. Soc. 115 1993

7374.� � Ž .125 R. Viswanathan, J.A. Zasadzinski, D.K. Schwartz, Science 261 1993 449.� � Ž .126 M. Kinzler, W. Schrepp, C. Woll, M. Grunze, Langmuir 11 1995 696.¨� � Ž .127 M.C. Shih, T.M. Bohanon, J.M. Mikrut, P. Zschack, P. Dutta, J. Chem. Phys. 96 1992 1556.� � Ž .128 E.B. Sirota, H.E.J. King, D.M. Singer, H.H. Shao, J. Chem. Phys. 98 1993 5809.� � Ž .129 M.K. Durbin, A. Malik, A.G. Richter, K.G. Huang, P. Dutta, Langmuir 13 1997 6547.� � Ž .130 H.D. Sikes, D.K. Schwartz, Langmuir 13 1997 4704.� � Ž .131 R. Steitz, E.E. Mitchell, I.R. Peterson, Thin Solid Films 205 1991 124.� � Ž .132 M. Engel, H.J. Merle, I.R. Peterson, H. Riegler, R. Steitz, Ber. Bunsenges. Phys. Chem. 95 1991

1514.� � Ž .133 G. Brezesinski, I.R. Peterson, J. Phys. Chem. 99 1995 12545.� � Ž .134 I.R. Peterson, J.D. Earls, I.R. Girling, G.J. Russell, Mol. Cryst. Liq. Cryst. 147 1987 141.� �135 A. Barraud, A. Rosilio, C. Le Gressus, H. Okuzumi, A. Mogami, in: 8th International Congress

on Electron Microscopy, 1974, p. 682.� �136 F. Leveiller, C. Bohm, D. Jacquemain, H. Mohwald, L. Leiserowitz, K. Kjaer, J. Als-Nielsen,¨ ¨

Ž .Langmuir 10 1994 819.� � Ž .137 R.D. Void, G.S. Hattiangdi, Ind. Eng. Chem. 41 1949 2111.� � Ž .138 T.R. Lomer, K. Perera, Acta Crystallogr. B30 1974 2912.� � Ž .139 V. Luzzati, in: D. Chapman Ed. , Biological Membranes, Academic Press, London, New York,

1968.� � Ž .140 J.F. Stephens, C. Tuck-Lee, J. Appl. Cryst. 2 1969 1.� � Ž .141 H.J. Trurnit, G. Schidlovsky, in: A.L. Houwink, B.J. Spit Eds. , Proceedings of the European

Regional Conference on Electron Microscopy, Delft, 1961.� � Ž .142 C. Holley, S. Bernstein, Phys. Rev. 52 1937 525.� � Ž .143 I. Langmuir, Science 87 1938 493.� � Ž .144 M. Prakash, P. Dutta, J.B. Ketterson, B.M. Abraham, Chem. Phys. Lett. 111 1984 395.� � Ž .145 M. Prakash, J.B. Ketterson, P. Dutta, Thin Solid Films 134 1985 1.� � Ž .146 U. Pietsch, T.A. Barberka, U. Englisch, R. Stommer, Thin Solid Films 284�285 1996 387.¨� � Ž .147 Y. Sasanuma, H. Nakahara, Thin Solid Films 261 1995 280.� � Ž .148 D.K. Schwartz, J. Garnaes, R. Viswanathan, J.A.N. Zasadzinski, Science 257 1992 508.� � Ž .149 M. Shimomura, K. Song, J.F. Rabolt, Langmuir 8 1992 887.� � Ž .150 J. Umemura, T. Kamata, T. Kawai, T. Takenaka, J. Phys. Chem. 94 1990 62.� � Ž .151 T. Buffeteau, B. Desbat, J.M. Turlet, Appl. Spectrosc. 45 1991 380.� � Ž .152 P.A. Chollet, J. Messier, Thin Solid Films 99 1983 197.� �153 M. Kobayashi, Crystallization and Polymorphism of Fats and Fatty Acids, Marcel Dekker, New

York, 1988.� � Ž .154 H. Hayashi, J. Umemura, J. Chem. Phys. 63 1975 1732.� � Ž .155 J. Umemura, J. Chem. Phys. 68 1978 42.� � Ž .156 J. Umemura, J. Mol. Struct. 36 1977 35.� � Ž .157 H. Hayashi, J. Umemura, R. Nakamura, J. Mol. Struct. 69 1980 123.� � Ž .158 Y. Koyama, M. Yanagishita, S. Toda, T. Matsuo, J. Colloid Interface Sci. 61 1977 438.� � Ž .159 T. Ohnishi, A. Ishitani, H. Ishida, N. Yamamoto, H. Tsubomura, J. Phys. Chem. 82 1978 1989.� � Ž .160 P.A. Chollet, Thin Solid Films 52 1978 343.� � Ž .161 R.F. Fischetti, V. Skita, A.F. Garito, J.K. Blasie, Phys. Rev. B 37 1988 4788.� � Ž .162 W. Richardson, J.K. Blasie, Phys. Rev. B 39 1989 12165.� � Ž . Ž .163 S. Xu, M.A. Murphy, S.M. Amador, J.K. Blasie, J. Phys. I France 1 1991 1131.� � Ž .164 F. Kaneko, M. Kobayashi, Y. Kitagawa, Y. Matsuura, Acta Crystallogr. C46 1990 1490.� � Ž .165 F. Kaneko, T. Simofuku, A. Miyamoto, M. Kobayashi, M. Suzuki, J. Phys. Chem. 96 1992 10554.� � Ž .166 L.H. Germer, K.H. Storks, Proc. Natl. Acad. Sci. 23 1937 390.� � Ž .167 I.R. Peterson, G.J. Russell, Phil. Mag. A 49 1984 463.


� � Ž .168 A. Leuthe, L.F. Chi, H. Riegler, Thin Solid Films 243 1994 351.� � Ž .169 A. Leuthe, H. Riegler, J. Phys. D: Appl. Phys. 25 1992 1786.� � Ž .170 T. Kamata, J. Umemura, T. Takenaka, Chem. Lett. 1988 1231.� � Ž .171 G.L. Clark, P.W. Leppla, J. Am. Chem. Soc. 58 1936 2199.� �172 B.A. Nesterenko, V.V. Milenin, O.Y.U. Gorkun, A.A. Stadnik, Z.I. Kazantseva, A.V. Nabok,

Ž .Thin Solid Films 201 1991 351.� � Ž .173 U. Pietsch, T.A. Barberka, W. Mahler, T.H. Metzger, Thin Solid Films 247 1994 230.� � Ž .174 R. Stommer, U. Englisch, U. Pietsch, V. Holy, Physica B 221 1996 284.¨� � Ž .175 R. Stommer, J. Grenzer, J. Fischer, U. Pietsch, J. Phys. D: Appl. Phys. 28 1995 A216.¨� � Ž .176 R.R. Highfield, R.K. Thomas, P.G. Cummins et al., Thin Solid Films 99 1983 165.� � Ž .177 T. Takamura, K. Matsushita, Y. Shimoyama, Jpn. J. Appl. Phys. 35 1996 5831.� � Ž .178 J.A. Zasadzinski, R. Viswanathan, L. Madsen, J. Garnaes, D.K. Schwartz, Science 263 1994

1726.� � Ž .179 K. Ueno, K. Saiki, T. Shimada, A. Koma, J. Vacuum Sci. Technol. A 8 1990 68.� � Ž .180 A. Koma, K. Saiki, K. Sato, Appl. Surf. Sci. 41�42 1989 451.� � Ž .181 F.S. Ohuchi, B.A. Parkinson, K. Ueno, A. Koma, J. Appl. Phys. 68 1990 2168.� � Ž .182 M. Prakash, J.B. Peng, J.B. Ketterson, P. Dutta, Chem. Phys. Lett. 128 1986 354.� � Ž .183 G. Veale, I.R. Girling, I.R. Peterson, Thin Solid Films 127 1985 293.� � Ž .184 C. Bohm, R. Steitz, H. Riegler, Thin Solid Films 178 1989 511.¨� � Ž .185 P. Dutta, J.B. Peng, B. Lin et al., Phys. Rev. Lett. 58 1987 2228.� � Ž .186 I.R. Peterson, G.J. Russell, Br. Polym. J. 17 1985 364.� � Ž .187 R. Viswanathan, J.A. Zasadzinski, D.K. Schwartz, Nature 368 1994 440.� � Ž .188 D.K. Schwartz, R. Viswanathan, J.A.N. Zasadzinski, Phys. Rev. Lett. 70 1993 1267.� � Ž .189 D.K. Schwartz, R. Viswanathan, J.A.N. Zasadzinski, Langmuir 9 1993 1384.� � Ž .190 A. Schaper, L. Wolthaus, D. Mobius, T.M. Jovin, Langmuir 9 1993 2178.¨� � Ž .191 L. Bourdieu, O. Ronsin, D. Chatenay, Science 259 1993 798.� � Ž .192 M. Allain, J.J. Benattar, F. Rieutord, P. Robin, Europhys. Lett. 3 1987 309.� � Ž .193 V. Skita, M. Filipkowski, A.F. Garito, J.K. Blasie, Phys. Rev. B 34 1986 5826.� � Ž . Ž .194 V. Skita, W. Richardson, M. Filipkowski, A. Garito, J.K. Blasie, J. Phys. Paris 47 1986 1849.� �195 D.G. Wiesler, L.A. Feigin, C.F. Majkrzak, J.F. Ankner, T.S. Berzina, V.I. Troitsky, Thin Solid

Ž .Films 266 1995 69.� � Ž .196 U. Englisch, T.A. Barberka, U. Pietsch, U. Hohne, Thin Solid Films 266 1995 234.¨� � Ž .197 M.R. Buhaenko, M.J. Grundy, R.M. Richardson, S.J. Roser, Thin Solid Films 159 1988 253.� � Ž .198 M.J. Grundy, R.J. Musgrove, R.M. Richardson, S.J. Roser, J. Penfold, Langmuir 6 1990 519.� �199 P. Stroeve, J.F. Rabolt, R.O. Hilleke, G.P. Felcher, S. Chen, Mat. Res. Soc. Symp. Proc. 166

Ž .1990 103.� � Ž .200 Y. Aoki, K. Kato, K. Shinbo, F. Kaneko, T. Wakamatsu, Thin Solid Films 329 1998 360.� �201 D.Y. Takamoto, E. TerOvanesyan, D.K. Schwartz, R. Viswanathan, S. Chiruvolu, J.A. Zasadzin-

Ž .ski, Acta Phys. Polonica 93 1998 373.� � Ž .202 H. Kepa, L.J. Kleinwaks, N.F. Berk et al., Physica B 241 1997 1048.� � Ž .203 J. Garnaes, D.K. Schwartz, R. Viswanathan, J.A.N. Zasadzinski, Nature 357 1992 54.� �204 J.B. Peng, G.J. Foran, G.T. Barnes, I.R. Gentle, unpublished data.� �205 N. Sigiyama, A. Shimizu, M. Nakamura, Y. Nakagawa, Y. Nagasawa, H. Ishida, Thin Solid Films

Ž .331 1998 170.� � Ž .206 C. Holley, Phys. Rev. 53 1938 534.� � Ž .207 S. Bernstein, J. Am. Chem. Soc. 60 1938 1511.� � Ž .208 S. Bernstein, J. Am. Chem. Soc. 62 1940 374.� � Ž .209 M.W. Charles, J. Appl. Phys. 42 1971 3329.� � Ž .210 T. Takenaka, K. Nogami, H. Gotoh, J. Colloid Interface Sci. 40 1972 409.� � Ž .211 W. Lesslauer, J.K. Blasie, Biophys. J. 12 1972 175.� � Ž .212 W. Lesslauer, Acta Crystallogr. B30 1974 1927.� � Ž .213 M. Pomerantz, A. Segmuller, Thin Solid Films 68 1980 33.¨� � Ž .214 R.M. Nicklow, M. Pomerantz, A. Segmuller, Phys. Rev. B 23 1981 1081.¨


� �215 G.J. Russell, M.C. Petty, I.R. Peterson, G.G. Roberts, J.P. Lloyd, K.K. Kan, J. Mater. Sci. 3Ž .1984 25.

� � Ž .216 M. Prakash, J.B. Peng, J.B. Ketterson, P. Dutta, Thin Solid Films 146 1987 L15.� � Ž .217 P. Fromherz, U. Oelschlagel, W. Wilke, Thin Solid Films 159 1988 421.¨� � Ž .218 T. Kajiyama, I. Hanada, K. Shuto, Y. Oishi, Chem. Lett. 1989 193.� � Ž .219 J.M. Bloch, W. Yun, K.M. Mohanty, Phys. Rev. B 40 1989 6529.� � Ž .220 W. Jark, G. Comelli, T.P. Russell, J. Stohr, Thin Solid Films 170 1989 309.¨� � Ž .221 H.J. Merle, Y.M. Lvov, I.R. Peterson, Makromol. Chem. Macromol. Symp. 46 1991 271.� � Ž .222 U. Hohne, H. Mohwald, Thin Solid Films 243 1994 425.¨ ¨� � Ž .223 T.A. Barberka, U. Hohne, U. Pietsch, T.H. Metzger, Thin Solid Films 244 1994 1061.¨� � Ž .224 X. Luo, Z. Zhang, Y. Liang, Langmuir 10 1994 3213.� � Ž .225 Y. Ando, T. Hiroike, T. Miyashita, T. Miyazaki, Thin Solid Films 266 1995 292.� � Ž .226 G. Rapp, M.H.J. Koch, U. Hohne, Y. Lvov, H. Mohwald, Langmuir 11 1995 2348.¨ ¨� � Ž .227 Y. Sasanuma, Y. Kitano, A. Ishitani, H. Nakahara, K. Fukuda, Thin Solid Films 190 1990 325.� � Ž .228 B. Yang, Y. Qiao, Thin Solid Films 330 1998 157.� � Ž .229 E. Meyer, L. Howald, R.M. Overney et al., Nature 349 1991 398.� � Ž .230 L. Bourdieu, P. Silberzan, D. Chatenay, Phys. Rev. Lett. 67 1991 2029.� �231 D.K. Schwartz, J. Garnaes, R. Viswanathan, S. Chiruvolu, J.A.N. Zasadzinski, Phys. Rev. E 47

Ž .1993 452.� � Ž .232 M. Florsheimer, A.J. Steinfort, P. Gunter, Surf. Sci. Lett. 297 1993 L39.¨ ¨� � Ž .233 Z. Zhang, Y. Liang, Y. Tian, Y. Jiang, Spectrosc. Lett. 29 1996 321.� � Ž .234 I.R. Peterson, Thin Solid Films 357 1984 178.� � Ž .235 I.R. Peterson, G.J. Russell, J.D. Earls, I.R. Girling, Thin Solid Films 161 1988 325.� �236 J.B. Peng, G.J. Foran, G.T. Barnes, I.R. Gentle, to be published.� � Ž .237 L.F. Chi, L.M. Eng, K. Graf, H. Fuchs, Langmuir 8 1992 2255.� � Ž .238 J.W. Menter, D. Tabor, Proc. R. Soc. London A204 1950 514.� � Ž .239 K. Kobayashi, K. Takaoka, S. Ochiai, Thin Solid Films 178 1989 453.� � Ž .240 S.R. Cohen, R. Naaman, J. Sagiv, J. Phys. Chem. 90 1986 3054.� � Ž .241 C. Naselli, J.F. Rabolt, J.D. Swalen, J. Chem. Phys. 82 1985 2136.� � Ž .242 N.E. Schlotter, J.F. Rabolt, Appl. Spectrosc. 39 1985 994.� � Ž . Ž .243 T. Hasegawa, T. Kamata, J. Umemura, T. Takenaka, Chem. Lett. Japan 1990 1543.� � Ž .244 E. Barbaczy, F. Dodge, J.F. Rabolt, Appl. Spectrosc. 41 1987 176.� � Ž .245 Y. Cho, M. Kobayashi, H. Tadokoro, J. Chem. Phys. 84 1986 4636.� � Ž .246 Y. Cho, M. Kobayashi, H. Tadokoro, J. Chem. Phys. 84 1986 4643.� � Ž .247 B.P. Gaber, W. Peticoals, Biochim. Biophys. Acta 465 1977 260.� � Ž .248 P.A. Spegt, A.E. Skoulios, Acta Crystallogr. 16 1963 301.� � Ž .249 L. Rothberg, G.S. Higashi, D.L. Allara, S. Garoff, Chem. Phys. Lett. 133 1987 67.� � Ž .250 R.A. MacPhail, H.L. Strauss, R.G. Snyder, C.A. Elliger, J. Phys. Chem. 88 1984 334.� � Ž .251 T. Fukui, M. Sugi, S. Iizima, Phys. Rev. B 22 1980 4898.� � Ž .252 H.J. Merle, R. Steitz, U. Pietsch, I.R. Peterson, Thin Solid Films 237 1994 236.� � Ž .253 H.J. Merle, H. Metzger, U. Pietsch, Phys. Scr. T45 1992 253.� � Ž .254 J.B. Peng, G.J. Foran, G.T. Barnes, I.R. Gentle, Langmuir 13 1997 1602.

the structures of langmuir blodgett films of fatty acids...

Documents