Poly(ethylenimine) as a Subphase Stabilizer of StearicAcid Monolayers at the Air/Water Interface: Surface

Pressure-Area Isotherm and Infrared Spectroscopy Study

Mi-Ja Hwang and Kwan Kim*

Department of Chemistry and Center for Molecular Catalysis, Seoul National University,Seoul 151-742, Korea

Received April 9, 1998. In Final Form: March 5, 1999

The effect of poly(ethylenimine) (PEI) dissolved in water on the surface pressure-area (π-A) isothermof stearic acid (STA) at the air/water interface was investigated. When the concentration of PEI was quitelow, for example, 2.2 × 10-4 g/L (5.1 × 10-6 M in monomeric unit concentration), the isotherm of the STAmonolayer was little affected by PEI. On a concentrated PEI solution, for example, at 2.2 × 10-2 g/L, theisotherm of the STA monolayer exhibited a noticeable variation as a function of subphase pH, particularlyin basic conditions. The isotherm of STA could be obtained very reproducibly even at pH 10.4. At basicpHs, PEI induced the occurrence of a wide plateau region in an isotherm that could be attributed to thecoexistence of liquid-condensed (LC) and liquid-expanded (LE) phases of STA on a water subphase. Inaddition, the collapse pressure of the STA monolayer was raised to 68 mN/m, indicating that the stabilityof the monolayer could be increased dramatically by virtue of an acid-base-type interaction between theamine group of PEI and the carboxyl group of STA. In situ reflection-absorption infrared (RAIR) spectroscopyrevealed that at acidic pHs STA molecules should form two-dimensional crystalline domains even at high“per molecular” area. In contrast, at basic pHs RAIR spectral data dictated that STA molecules werestrongly disordered at high per molecular area, and a near-crystalline structure seemed to form only whenthe per molecular area was lowered to a value corresponding to a pure LC phase in the π-A isotherm ofSTA on the PEI-containing subphase.

1. Introduction

In the past decade, extensive studies have been per-formed to elucidate the structures and intermolecularforces of two-dimensional arrays of molecules at the air/water interface.1 These studies stemmed not only fromintrinsic scientific interest but also from their applicabilityto forming well-ordered ultrathin organic films on solidsubstrates by, for instance, the Langmuir-Blodgett (LB)method.2

Usually, the fabrication of a close-packed assembly ofamphiphilic molecules at an air/water interface by theLangmuir method requires suitable subphase conditionsrelated to the ionic species and its concentration, pH, andtemperature.1,2 In general, such conditions depend on thetypes of amphiphilic molecules. A consensus has arisen,however, that the addition of divalent metallic ions to awater subphase enhances the stability of the amphiphilicmolecules not only at an air/water interface but also afterdeposition onto a solid substrate.3-8

In contrast to metallic ions, the effect of soluble organicand/or polymeric compounds on monolayer stability has

been scarcely studied. Nonetheless, it has been reportedthat poly(ethylenimine) (PEI) dissolved in a water sub-phasegreatly influences the isothermalbehaviorofvariousamphiphilic molecules such as fatty acids, perfluoro fattyacids, and alkyl sulfonate at the air/water interface.9-11

PEI is a highly branched water-soluble polymer with adistribution of primary, secondary, and tertiary aminegroups in the ratio of 1:2:1.12 Owing to the formation ofa polyion complex, the usually condensed fatty acidmonolayers could be altered to expanded compressiblefilms. The stability of monolayers also increased, asindicated by higher collapse pressures.

From a fluorescence microscopy study, Chi et al.11

observed the surface textures of fatty acid monolayers asvarying widely as a function of concentration of PEI,temperature, and chain length of the fatty acid. In thepresent work, the effect of PEI on the surface pressure-area (π-A) isotherm of a typical fatty acid, namely, stearicacid (STA), is investigated in greater depth as a functionof subphase pH and the molecular weight of PEI. An efforthas been made to understand the characteristics of theisotherm data in terms of the pattern of arrangement ofSTA molecules by referring to the reflection-absorptioninfrared (RAIR) spectra taken at the air/water interface.We believe that the present work should also enhance ourunderstanding of the mechanism of monolayer penetrationby bulk-soluble surface-active molecules, which is relevantto a number of fluid-processing and biomedical systems.13

* To whom all correspondence should be addressed. Fax: 82-2-8891568. E-mail: kwankim@plaza.snu.ac.kr.

(1) Roberts, G. Langmuir-Blodgett Films; Plenum Press: New York,1990.

(2) Ulman, A. An Introduction to Ultrathin Organic Films FromLangmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991.

(3) Zasadzinski, J. A.; Viswanathan, R.; Madsen, L.; Garnaes, J.;Schwartz, D. K. Science 1994, 263, 1726.

(4) Simon-Kutscher, J.; Gericke, A.; Huhnerfuss, H. Langmuir 1996,12, 1027.

(5) Gericke, A.; Huhnerfuss, H. Thin Solid Films 1994, 245, 74.(6) Shih, M. C.; Bohanon, T. M.; Mikrut, J. M.; Zschack, P.; Dutta,

P. J. Chem. Phys. 1992, 96, 1556.(7) Allara, D. L.; Swalen, J. D. J. Phys. Chem. 1982, 86, 2700.(8) Yazdanian, M.; Yu, H.; Zografi, G.; Kim, M. W. Langmuir 1992,

8, 630.

(9) Kobayashi, K.; Takaoka, K.; Ochiai, S.; Taru, Y.; Takasago, M.Thin Solid Films 1992, 210/211, 559.

(10) Kajiyama, T.; Zhang, L.; Uchida, M.; Oishi, U.; Takahara, A.Langmuir 1993, 9, 760.

(11) Chi, L. F.; Johnston, R. R.; Ringsdorf, H. Langmuir 1991, 7,2323.

(12) Chance, J. J.; Purdy, W. C. Langmuir 1997, 13, 4487.(13) Charron, J. R.; Tilton, R. D. Langmuir 1997, 13, 5524.

3563Langmuir 1999, 15, 3563-3569

10.1021/la9804029 CCC: $18.00 © 1999 American Chemical SocietyPublished on Web 04/24/1999

2. Experimental Section

Stearic acid (C17H35COOH, 99+%), poly(ethylenimine) (MW25000 and 2000; hereafter, these will be denoted, respectively,as PEI25000 and PEI2000), and diethylenetriamine (H2NCH2CH2-NHCH2CH2NH2, 99%), purchased from Aldrich Chemical Co.,were used as received. Other chemicals, unless otherwisespecified, were reagent grade, and triply distilled water, ofresistivity greater than 18.0 MΩ‚cm, was used throughout. Thegold substrates used to obtain the reflection-absorption infrared(RAIR) spectra of LB films of STA on metal surfaces were preparedby the resistive evaporation of titanium and gold at 10-6-10-7

Torr on batches of glass slides.The π-A isotherm of stearic acid (STA) at the air/water

interface was measured with a microprocessor-controlled filmbalance (KSV 3000 Langmuir balance), equipped with a tem-perature controller set at 20 ( 0.3 °C; surface pressure wasmeasured via the Wilhelmy method using a platinum plate.Initially, the Langmuir trough was filled with water to whichaqueous PEI solution was added to a final concentration of (2.2× 10-4)-(2.2 × 10-2) g/L (in monomer units, (5.1 × 10-6)-(5.1× 10-4) M). The pH of the subphase, adjusted by adding HCl orNaOH solution, was measured with a digital pH meter; at specificpHs the isotherms were insensitive to the amount of HCl orNaOH added. Next, 1.55 × 10-3 M STA in chloroform was spreadonto the water subphase; during the drop-by-drop deposition thetip of a 100 µL syringe (Hamilton) was kept within 2 cm of thewater subphase. Once evaporation of the solvent (20 min) tookplace, the surface layer was compressed at a constant rate of0.042 nm2/STA molecule/min.

All infrared spectra were taken with a Bruker IFS 113v FT-IRspectrometer equipped with a globar light source and a liquid-N2-cooled wide-band mercury cadmium telluride detector. Theattenuated total reflection infrared (ATR-IR) spectra of LB filmson ZnSe crystals were obtained with a Specac ATR optics byaveraging 128 interferograms at a 4 cm-1 resolution. The incidentangle of the infrared light (unpolarized) was set at 45° with respectto the plane of the ZnSe crystal such that 12 total internalreflections took place throughout the crystal. The RAIR spectraof LB films on gold substrates were obtained with a Harrickreflection optics by averaging 1024 interferograms at a 4 cm-1

resolution. The incident angle of the p-polarized light (selectedusing a Harrick wire grid polarizer) was set at 80°. The Happ-Genzel apodization function was used in Fourier transformingthe interferograms. The final spectra are represented as -log-(R/R0), where R and R0 are the reflectivities of the sample layerand the bare gold substrate, respectively.

To obtain the RAIR spectra of STA at the air/water interface,the infrared light (unpolarized) was directed toward the air/water interface at an incident angle of 40° by using an ATRoptics customized inhouse for reflection spectroscopy. For thismeasurement, a mini-trough that was 130 × 30 × 10 mm3 wasconstructed of Teflon, and its barrier was moved manually insteps of 5 mm to change the surface area occupied by STAmolecules. Initially, 7.56 × 10-4 M STA in chloroform was spreadonto the water subphase to occupy 0.84 nm2/molecule; 10 minwas allowed for the evaporation of chloroform. Each compressionthus corresponds to a decrease of 0.037 nm2/molecule. Otherexperimental conditions were the same as those employed whenusing the KSV 3000 Langmuir balance. Infrared interferogramswere gathered, starting at 5 min after the barrier movement.Each spectrum was obtained by averaging 1024 interferogramsat a 4 cm-1 resolution. The Blackman-Harris apodizationfunction was used in Fourier transforming the interferograms.The final spectra are represented as -log(R/R0), where R and R0are the reflectivities of the sample monolayer and pure water,respectively.

3. Results and Discussion

3.1. Isotherm of STA on Pure Water. A “classical”π-A isotherm of STA on a pure water subphase at 20 °Cis shown in Figure 1 (dotted line). The isotherm clearlyexhibits the gas/solid and solid/solid-phase transitionsupon increasing the surface pressure. The solid-condensedphase collapses at ∼50 mN/m. When the linear portion of

the solid-condensed phase is extrapolated to zero surfacepressure, the intercept is given by 0.22 nm2/molecule, ingoodagreementwith the literature.14,15 The π-A isothermstaken at pH 3.6 (HCl solution) and 9.0 (NaOH solution)are almost the same as that taken on pure water (see thedashed and solid lines in Figure 1). The collapse pressureis seen, however, to increase up to ∼59 mN/m at pH 9.0,implying that the stability of the STA monolayer isenhanced in a slightly basic condition. Nonetheless, atpH higher than 9, the π-A isotherm becomes veryirregular because of the dissolution of STA into thesubphase; that is, the mean molecular area correspondingto the onset of the surface pressure increase decreasesgradually such that the isotherm is no longer obtainedreproducibly.

Hereafter, we will present the observation made on theisotherm of STA affected by the presence of PEI in thewater subphase. In this study, two different PEI sampleswere employed; one with a higher molecular weight(25000) and the other with a comparatively much lowermolecular weight (2000). Furthermore, two differentconcentrations were considered for each PEI that had beendissolved in the water subphase; one at 2.2 × 10-2 g/L (5.1× 10-4 M in monomeric unit concentration) and the otherat 2.2 × 10-4 g/L. Since the observation made on thePEI25000-containing subphase is barely different from thaton the PEI2000 solution, we will focus on the behavior ofSTA found mainly on the PEI25000 subphase.

3.2. Isotherm of STA on an Aqueous PEI25000Solution. The π-A isotherms of STA on the 2.2 × 10-2

g/L PEI25000 containing water subphase in acidic conditionsat 20 °C are shown in Figure 2. In acidic conditions, PEIdid not greatly affect the isotherm of STA. Nevertheless,at pHs 4.4 and 6.7 the collapse pressure of STA was raisedfrom ∼50 to 52 and 56 mN/m, respectively, while at pH< 3.4 the collapse pressure was lowered to 45 mN/m. Inaddition, in the presence of PEI at higher pHs, the liquid-condensed phase of STA became less clearly defined; thetransition from a liquid-condensed to a solidlike structureoccurred at lower pressures, as is likely in the presenceof divalent metal ions.4,5

In basic conditions, the effect of PEI on the isotherm ofSTA was seen more distinctly. Figure 3 shows the π-Aisotherms of STA on the 2.2 × 10-2 g/L PEI25000 containingan aqueous subphase at pHs 7.8-10.4 at 20 °C. The

(14) Gericke, A.; Huhnerfuss, H. J. Phys. Chem. 1993, 97, 12899.(15) Datta, A.; Sanyal, M. K.; Dhanabalan, A.; Major, S. S. J. Phys.

Chem. B 1997, 101, 9280.

Figure 1. Surface pressure/area isotherms of stearic acid ona subphase of water at (a) pH ∼6 (on pure water, dotted line),(b) pH 3.6 (dashed line), and (c) pH 9.0 (solid line) at 20 °C.

3564 Langmuir, Vol. 15, No. 10, 1999 Hwang and Kim

isotherms in basic conditions are completely different fromthose in acidic conditions (Figure 2), specifically in theregion of wide surface area. In addition, the isotherm atpH 8.9 (Figure 3c) does not match up with its counterparttaken at pH 9.0 without PEI (Figure 1c). In the presenceof PEI, the onset of the first pressure increase occurs atca. 0.7 nm2/molecule at pH∼9 while in its absence it occursat ∼0.27 nm2/molecule. PEI induces the isotherm of STAto change from an incompressible monolayer to anexpanded one; for instance, at pH 8.9, a wide plateau regionis observed between 16 and 22 mN/m. The latter regionshould correspond to a domain where liquid-condensed(LC) and liquid-expanded (LE) phases could coexist. Thisimplies that, in the presence of PEI, a solid-condensedform arises from a gas/solid coexistence region via anadditional fluid-solid-phase transition. In addition, in thepresence of PEI the collapse pressure of the STA monolayeris raised to 68 mN/m, indicating that the stability of themonolayer increases dramatically on a PEI-containingwater subphase in agreement with Chi et al.11

It is remarkable that the isotherm of STA is obtainedvery reproducibly even at pH 10.4 in the presence of 2.2× 10-2 g/L PEI25000. As can be noticed from Figure 3e, theonset of the first pressure increase occurs at a higher permolecular area at pH 10.4 than at pH 8.9 (i.e., at 0.7 nm2/molecule at pH 8.9 but at 1.2 nm2/molecule at pH 10.4).The plateau region at pH 10.4 is, however, decreasedcompared to the case at pH 8.9; the domain of coexistenceof LC and LE phases is observed between 24 and 27 mN/

m. Nonetheless, the π-A isotherm feature in the solid-condensed region is almost the same at the two pHs. Thecollapse pressure of the STA monolayer is similarlyobserved around 68 mN/m, implying that the monolayerof STA is very stable even at pH 10.4.

As implied in the above discussion, the isotherm of STAis very dependent on the pH of the PEI-containing watersubphase. The isotherm at pH 7.8 (Figure 3a) is quitesimilar to that at pH 6.7 (Figure 2d) albeit that the collapsepressure is distinctly ca. 12 mN/m higher at pH 7.8 thanat pH 6.7. As the pH of the subphase is increased from 7.8to 8.2, the onset of the first pressure increase is affectednoticeably. The onset area increases continuously fromca. 0.4 nm2/molecule at pH 8.2 to 1.2 nm2/molecule at pH10.4. Around pH 10 the isotherm is no longer subjectedto a noticeable change. The most dramatic change inisotherm occurred at the interval between pHs 8 and 9.Along with the increase in the onset area, the wide plateauregion becomes more distinct at higher pHs. The surfacepressure at which a transition from the LE phase to theLE/LC coexistence region occurs is also raised on theincrease of the subphase pH.

So far, we have mentioned the isotherm of STA in thepresence of 2.2 × 10-2 g/L PEI25000. With PEI at 2.2 × 10-4

g/L, the isotherm of STA was also affected by the subphasepH. The LC phase became less clearly defined and thecollapse pressure increased upon the increase of thesubphase pH. However, no distinct LE phase was observedin this case even at higher pHs. The π-A isotherm becamevery irregular at pH 10.4, owing to the dissolution of STAinto the subphase. Hence, the concentration of PEIappeared to be a crucial factor in inducing the appearanceof the LE/LC coexistence region and in stabilizing theSTA monolayer on an aqueous solution.

3.3. Isotherm of STA on an Aqueous PEI2000 Solu-tion. Figure 4 shows a series of π-A isotherms of STA onthe 2.2 × 10-2 g/L PEI2000 containing an aqueous subphaseat 20 °C. The isotherm patterns are much the same asthose obtained on the 2.2 × 10-2 g/L PEI25000 solutions.The isotherm at acidic pHs is quite similar to that on purewater. Once again, the isotherm of STA was obtained veryreproducibly even at pH 10.2; the LE/LC coexistence regionas well as the LE phase were clearly identified. Upon theincrease of subphase pH, the onset of surface pressureincrease occurred at a higher per molecular area; in thepresence of PEI2000, the plateau region due to the coexist-ence of the LE and LC phases was relatively wider andoccurred at somewhat lower pressure than in the presenceof PEI25000. The collapse pressure was barely susceptible

Figure 2. Surface pressure/area isotherms of stearic acid ona subphase containing 2.2 × 10-2 g/L PEI25000 at (a) pH 2.4, (b)pH 3.4, (c) pH 4.4, and (d) pH 6.7 at 20 °C.

Figure 3. Surface pressure/area isotherms of stearic acid ona subphase containing 2.2 × 10-2 g/L PEI25000 at (a) pH 7.8, (b)pH 8.2, (c) pH 8.9, (d) 9.8, and (e) pH 10.4 at 20 °C.

Figure 4. Surface pressure/area isotherms of stearic acid ona subphase containing 2.2 × 10-2 g/L PEI2000 at (a) pH 3.9, (b)pH 8.9, (c) pH 9.8, and (d) pH 10.2 at 20 °C.

Stabilizer of Stearic Acid Monolayers Langmuir, Vol. 15, No. 10, 1999 3565

to the molecular weight of PEI occurring at ∼68 mN/m.The isotherm was subjected to a small change around pH10. The isotherm of STA taken on the 2.2×10-4 g/L PEI2000-containing aqueous subphase was barely different fromthat on the similarly diluted PEI25000-containing subphase;the LE phase was no longer seen even at higher pH.

As mentioned briefly in the Introduction section, Chi etal.11 made much the same observation as we did at 20 °C.The amount of PEI they dissolved in water (2.13 × 10-3

M in monomeric units) was about 4 times larger than thatemployed in this work (5.1 × 10-4 M ) 2.2 × 10-2 g/L).The molecular weight of PEI they used was ca. 1800, butthey did not mention the pH of the subphase. Nevertheless,their isotherm feature was very similar to ours obtainedon a 2.2 × 10-2 g/L PEI2000 solution at pH 10.2; the onsetof the first pressure increase as well as the plateau regionoccurred at a similar per molecular area.

3.4. Interaction Scheme of STA with PEI. In theisotherms of phospholipids such as DPPC, the LE- to LC-phase transition via an LE/LC coexistence region isfrequently observed.16,17 The transition from an LE phaseto an LE/LC coexistence region is also observed for fattyacids having shorter alkyl chains or at higher temper-atures.14,18-20 With short chains, the intermolecular vander Waals interaction is too weak to induce cohesion offatty acid molecules. Higher temperature enhances thethermal motion of the fatty acid, resulting in a lesscondensed monolayer. In conjunction with these argu-ments, the presence of the LE phase and the LE/LCcoexistence region as well as the increase in surfacepressure for their interconversion observed in this workcould be related to the increase in the amount of disorderof STA monolayers at the air/water interface. Namely,STA monolayers are supposed to be more disordered uponinteracting with PEI, especially at a higher per moleculararea in basic conditions; this view is partly supported bythe in situ RAIR spectral measurement (see section 3.6).

To obtain information on the interaction of STA withPEI, the ATR-IR spectra were recorded for the LBmultilayers deposited on ZnSe crystals. A representativeATR-IR spectrum is shown in Figure 5 which correspondsto a six-layered LB film of STA, prepared on ZnSe in aY-type at 40 mN/m in the presence of 2.2×10-2 g/L PEI25000at pH 8.9; when the spectrum was taken, a single-layeredLB film was used as a reference material to avoid theadsorption effect of PEI on ZnSe. The broad band at∼3300cm-1 and the distinct band at 1553 cm-1 can be assigned,respectively, to the stretching and the bending modes ofamine groups of PEI. The band at 1401 cm-1 can beassigned to the COO- symmetric stretching mode of STA.When the LB films are prepared at higher pHs, the bandsdue to PEI become more distinct, implying that the amountof PEI incorporated in STA layers increases as the pH ofthe subphase increases. These infrared spectral dataindicate not only that the interaction of STA with PEI isquite strong but also that STA is present as a deprotonatedspecies while PEI is present as a protonated species.

It is well-known that a water-soluble amphiphile canform a stable monolayer with the help of a suitablestabilizing material in the water subphase;9,21 that is, thewater-soluble perfluoroundecanoic acid forms a stable

monolayer at high pH in the presence of PEI.9 Recallingthe ATR-IR spectral data mentioned above, similarreasoning may be applied to the present system. Namely,the prevention of STA from dissolving into a watersubphase by PEI at high pH should be due to theirelectrostatic interaction (i.e., an acid-base-type interac-tion). This point may be assessed more clearly by referringto the pKa values of STA and PEI. The pKa value of STAon a water subphase was previously thought to be 5.6,22

but it has been reported recently that the value is verydependent on the nature of the cation present.23 Althoughthe pKa value of the arachidic acid monolayer has beenreported to be about 11,24 the carboxylic group is supposed,in general, to be almost deprotonated at pH > 8. On theother hand, the pKa values of PEI are not definitive,25 butthe extent of protonation of the amine groups of PEI wouldbe approximated as a function of pH by referring to thepKa values of alkylamines as 10∼11. Although notquantitative, most of the amine groups of PEI should beprotonated at pH < 8. Accordingly, in acidic conditionsneither an H-bonding nor an ionic interaction will befavorable between PEI and STA since both are present asprotonated species. The ionic interaction will be dominantin the pH region between 8 and 9 since most of the aminegroups of PEI are still protonated in that pH region. Owingto a buffer action, the isotherm of STA will be subjectedto a small change around pH 10. The degree of incorpora-tion of carboxylic acid-derivatized Ag colloids into the fattyamine LB films was reported also to be a maximum at ca.pH 9; the extent decreased as the pH departed from 9,implying the importance of pH for the electrostaticinteraction at the air/water interface.26,27 Considering thatthe solidlike structure of STA monolayers is sustained upto near 68 mN/m, such an acid-base-type interaction mustbe very strong. The ATR-IR spectral data in Figure 5 arealso consonant with this view. On the other hand, sincethe protonated nitrogen atoms are linked through theethylenic units, one may not need to worry about their

(16) Gericke, A.; Simon-Kutscher, J.; Huhnerfuss, H. Langmuir 1993,9, 3115.

(17) Mitchell, M. L.; Dluhy, R. A. J. Am. Chem. Soc. 1988, 110, 712.(18) Pallas, N. R.; Pethica, B. A. Langmuir 1985, 1, 509.(19) Ruckenstein, E.; Li, B. J. Phys. Chem. 1996, 100, 3108.(20) Guyot-Sionnest, P.; Hunt, J. H.; Shen, Y. R. Phys. Rev. Lett.

1987, 59, 1597.(21) Liu, M.; Kira, A.; Nakahara, H. Langmuir 1997, 13, 4807.

(22) Betts, J. J.; Pethica, B. A. Trans. Faraday Soc. 1956, 52, 1581.(23) Aveyard, R.; Binks, B. P.; Carr, N.; Cross, A. W. Thin Solid

Films 1990, 188, 361.(24) Oishi, Y.; Takashima, Y.; Suehiro, K.; Kajiyama, T. Langmuir

1997, 13, 2527.(25) Suh, J.; Lee, S. H.; Kim, S. M.; Hah, S. S. Bioorg. Chem. 1997,

25, 221.(26) Sastry, M.; Mayya, K. S.; Patil, V.; Paranjape, D. V.; Hegde, S.

G. J. Phys. Chem. B 1997, 101, 4954.(27) Sastry, M.; Patil, V.; Mayya, K. S. Langmuir 1997, 13, 4490.

Figure 5. ATR-IR spectrum of Y-type LB STA films on ZnSe;six layers of STA were deposited on ZnSe at 40 mN/m in 2.2× 10-2 g/L PEI25000 at pH 8.9 at 20 °C. A single-layered film onZnSe was used as a reference material to avoid the adsorptioneffect of PEI on ZnSe.

3566 Langmuir, Vol. 15, No. 10, 1999 Hwang and Kim

repulsive interaction. In this sense, one can understandthe formation of more close-packed STA monolayers atthe air/water interface in the presence of PEI than in thepresence of metal ions, at least in basic conditions.

3.5. Isotherm of STA on an Aqueous Diethylene-triamine Solution. We have discussed above that theelectrostatic interaction between the carboxylate groupof STA and the protonated amine group of PEI shouldplay the key role in inducing the appearance of the LE/LCcoexistence region at a higher per molecular area of STAat basic pHs. It may then be necessary to know whetherthe polymeric characteristics themselves are importantfor PEI to act as a stabilizer of the STA monolayer. In thisrespect, we have attempted to record the isotherm of STAon a water subphase containing a small-sized aminederivative, i.e., diethylenetriamine (DETA). Even on a2.37 × 10-2 g/L DETA containing subphase, no LE phasewas observed, however, for the STA monolayer (figuresnot shown here). At pH 9, the limiting area obtained byextrapolating the linear portion of the solid-condensedphase to zero surface pressure was ∼0.18 nm2/molecule.The latter value is obviously smaller than the usualmolecular cross-section of STA, i.e., ∼0.19 nm2/molecule(on pure water the intercept is given as 0.22 nm2/molecule;see Figure 1). Although the isotherm at pH 9 appears tobe reproducible, the smaller area seems to imply that STAis dissolved into a water subphase at basic pHs as longas the amine groups are not in a polymeric state. It issupposed that protonated small amine molecules are toorepulsive to be concentrated at the air/water interface.Since the LE/LC coexistence region of STA was notidentified even on a PEI-containing subphase when itsconcentration was quite low (2.2 × 10-4 g/L), there shouldbe a critical threshold even for the polymeric amines toinduce the appearance of such an LE/LC coexistence regionat the air/water interface.

3.6. RAIR Spectra of STA on an Aqueous PEI25000Solution. To obtain further information on the role ofPEI, we have attempted to record the reflection-absorp-tion infrared spectra of STA at the air/water interface asa function of subphase pH. A small-sized mini-trough wasemployed in taking the spectra. The barrier of the troughwas moved manually in a series of steps such that thesurface area changed by 0.037 nm2/molecule per step. Wetested initially how much a π-A isotherm taken in a mini-trough would be different from that in the large continu-ously operated trough used in obtaining Figures 1-4.Figure 6 compares typical isotherms of STA obtained usingthe two troughs in the presence of 2.2 × 10-2 g/L PEI25000at pH 7.5 and 8.9; the solid lines and lines with symbolscorrespond to isotherms obtained with large and mini-troughs, respectively. In using the mini-trough, the surfacepressure became stabilized within 5 min after moving thebarrier by 5 mm; the data with symbols in Figure 6represent quantities measured after 5 min. As can be seenin Figure 6a, the isotherms obtained at pH 7.5 hardlydiffer between the two cases. Examining Figure 6b, forthe isotherms taken at pH 8.9 the surface pressure ismeasured to be slightly greater in the large trough thanin the mini-trough. It is well-documented that the surfacepressures measured via discontinuous compression arelower than the values obtained via continuous compres-sion.28 Nonetheless, the shape of the isotherm in a mini-trough is almost the same as that in a large trough; themean molecular area for which the onset of the firstpressure increase occurs and for which the domain ofcoexistence of liquid-condensed and liquid-expanded

phases starts hardly differs in the two cases. Besides, itis supposed that the surface pressure relaxation effectshould be minimal at low pressures.29 On these grounds,the infrared spectral feature that might be observed usinga mini-trough should reflect the structure of STA in alarge Langmuir trough.

Figure 7a shows a typical RAIR spectrum of STA on a2.2 × 10-2 g/L PEI25000-containing subphase at pH 7.5. Onthe latter subphase without STA, no peak was observedin that region. The peaks in Figure 7a should correspondto the C-H stretching vibrations; infrared spectral peaksin the 1000-1700 cm-1 region could not be identifiedclearly because of the presence of strong fringes. Namely,the peaks at 2918 and 2850 cm-1 in Figure 7a can beassigned to the antisymmetric (νas) and symmetric (νs)stretching modes of the CH2 groups, respectively; the peakpositions are determined by using the center of gravitymethod.30 At this point, it is informative to note that theintensities of the νs and νas bands of the CH2 groups werescarcely different in the ATR-IR spectra of LB monolayersof STA on ZnSe that had been prepared in 1 mM CdCl2and in 2.2 × 10-2 g/L PEI25000 at pH 8.9. The peaks inFigure 7a are thus supposed to arise mostly from the STAmonolayer at the air/water interface.

It is well-documented that the νas(CH2) and νs(CH2)vibrations are somewhat sensitive to the conformation ofacyl chains;31-33 their peak positions are frequentlyreferred to in order to address the structures as well asthe phase changes of fatty acid and phospholipid mono-layers.5,14,17 As a more close-packed trans-zigzag structure

(28) Hifeda, Y. F.; Rayfield, G. W. Langmuir 1992, 8, 197.

(29) Sakai, H.; Umemura, J. Chem. Lett. 1996, 465.(30) Cameron, D. G.; Kauppinen, J. K.; Moffatt, D. J.; Mantsch, H.

H. Appl. Spectrosc. 1982, 36, 245.(31) Gericke, A.; Mendelsohn, R. Langmuir 1996, 12, 758.(32) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982,

86, 5145.(33) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Ellliger, C. A. J.

Phys. Chem. 1984, 88, 334.

Figure 6. Comparison of surface pressure/area isotherms ofstearic acid measured with Langmuir troughs operated eithercontinuously (large trough) or manually (mini-trough) onsubphases containing 2.2 × 10-2 g/L PEI25000 at (a) pH 7.5 and(b) pH 8.9 at 20 °C.

Stabilizer of Stearic Acid Monolayers Langmuir, Vol. 15, No. 10, 1999 3567

is assumed, the frequencies of the above modes are knownto red-shift; the number of gauche conformers increasesalong with the increase in wavenumber and vice versa.On these grounds, we have plotted in Figure 7b the positionof the νas(CH2) mode observed for STA in the presence of2.2 × 10-2 g/L PEI25000 at pH 7.5 and 8.9 as a function ofsurface area occupied by STA at the air/water interface;reliable data was not obtained at the surface area above0.80 nm2/molecule.

Regardless of the surface area, the νas(CH2) band ofSTA appeared at ∼2918 cm-1 at pH 7.5. In contrast, atpH 8.9 the band appeared at >2924 cm-1 at a high surfacearea and was seen at ∼2919 cm-1 when the surface areawas down to ∼0.20 nm2/molecule; a rather abrupt transi-tion from 2922 to 2919 cm-1 occurred at ∼0.30 nm2/molecule. It is noteworthy that the latter transition pointis very close to that in the π-A isotherm, where the LE/LC coexistence region crosses over the pure LC-phaseregion. This may imply that the infrared spectral featureis in fact consistent with the isotherm data. It would alsobe worthwhile to note that the frequency of the νas(CH2)band observed at pH 7.5 in Figure 7b, 2918 cm-1, is quiteclose to that observable in a crystalline state of STA, 2916cm-1. This suggests that STA molecules spread on waterat pH 7.5 should consist of fully extended trans-zigzagcarbon chains. This in turn implies that two-dimensionalcrystalline domains are formed right after spreading STAon the PEI solution at pH 7.5. On the other hand, the factthat the frequency of the νas(CH2) band is observed at>2922 cm-1 at pH 8.9 until the surface area is reducedto 0.30 nm2/molecule indicates that STA molecules arestrongly disordered in order to assume an LE-like phaseon the PEI solution. In the LE/LC coexistence region, theC-H stretching frequency is subjected to a small change,

and this indicates that, in the plateau region, the domainsize of the STA monolayer increases upon the decrease inits surface coverage without being accompanied by anyconformational change, as revealed by the fluorescencemicroscopy study.11 As the area per STA molecule isdecreased to <0.30 nm2/molecule, the alkyl chains seemto assemble into a fully extended trans-zigzag shape.(Actually, we did not obtain the in situ RAIR spectra inthe regions of the highly condensed phase. Nonetheless,it should be informative that the RAIR spectra of the LBSTA monolayer deposited on gold at 30 mN/m (at ∼0.20nm2/molecule) at various pHs (i.e., pH 7.5, 8.6, and 10.4)were hardly different from one another; the νas(CH2) bandswere all observed at ∼2919 cm-1, and the relative peakintensities of the C-H stretching bands of the CH2 andCH3 groups implied a near-perpendicular orientation ofthe alkyl chains with respect to the gold surface (videinfra). This may indicate that the alkyl chains of STA arein fact to assemble into a fully extended trans-zigzag shapeas the area per STA molecules is decreased to <0.30 nm2/molecule on the subphases containing PEI at both pHs,7.5 and 8.9.)

3.7.OrientationofSTAinaLBFilm.At this moment,it would also be informative to address the orientation ofthe acyl chains of STA in LB films. To obtain suchinformation, RAIRS at a metal surface is very effectiveowing to its simple surface selection rule; only thevibrational modes whose dipole moment derivatives havecomponents normal to the metal surface are exclusivelyinfrared-active.34,35 Figure 8 shows the RAIR spectrum ofthe LB STA monolayer deposited on a gold substrate at30 mN/m in 2.2 ×10-2 g/L PEI25000 at pH 8.6. As mentionedpreviously, the peaks in the 2800-3000 cm-1 should beattributed mostly to the C-H stretching modes of STA;the contribution of PEI should be minimal. Namely, thebands at 2920 and 2850 cm-1 are due to the νas(CH2) andνs(CH2) modes of STA, respectively;34,35 both of them weredownshifted upon the increase of the transfer pressure.These bands have transition dipole moments perpen-dicular to the C-C-C axis of the acyl chain. Both bandsat 2939 and 2879 cm-1 can be assigned to the symmetricC-H stretching modes of the CH3 group in Fermiresonance interactions with the overtone of the antisym-metric CH3 deformation mode. These have transition

(34) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J.Am. Chem. Soc. 1987, 109, 3559 and references therein.

(35) Ahn, S. J.; Son, D. H.; Kim, K. J. Mol. Struct. 1994, 324, 223 andreferences therein.

Figure 7. (a) Infrared spectrum of STA on a subphasecontaining 2.2 × 10-2 g/L PEI at pH 7.5 at 0.25 nm2/molecule.(b) Wavenumber of νas(CH2) band of STA on a subphase of 2.2× 10-2 g/L PEI25000 observed as a function of surface areaoccupied by STA at pH 7.5 (open triangles) and 8.9 (filledsquares). When taking the infrared spectra, the temperaturewas 20 ( 1 °C. Error bars represent the values of standarddeviation.

Figure 8. RAIR spectrum of a LB STA single layer on Au; STAwas deposited on a gold substrate at 30 mN/m in 2.2 × 10-2 g/LPEI25000 at pH 8.6 at 20 °C.

3568 Langmuir, Vol. 15, No. 10, 1999 Hwang and Kim

dipole moments parallel to the C-CH3 bond. The band at2966 cm-1 is assigned to the antisymmetric C-H stretch-ing mode of the CH3 group whose transition dipole momentis perpendicular to the C-CH3 bond. In the transmissioninfrared spectrum of STA dispersed in a KBr matrix, theCH3 group vibrational bands are negligibly weaker thanthe CH2 group vibrational bands. The fact that in Figure8 the νs(CH3) and νas(CH3) bands are much stronger thanthe νs(CH2) and νas(CH2) bands suggests that the acylchains of STA molecules assume nearly perpendicularorientation with respect to the gold substrate. It isnoteworthy that the orientation of the STA monolayercontrasts very much with that of the LB perfluoro-undecanoic acid monolayer prepared similarly in thepresence of PEI. As mentioned previously, in the presenceof PEI, the water-soluble perfluoroundecanoic acid (PFU-DA) can form a stable monolayer at high pH. Kobayashiet al.9 reported that the fluorocarbon chains in LB filmswere, however, considerably tilted with respect to thesurface normal; the molecular area of PFUDA seemed tobe determined by the area of the repeating unit of PEI.The perpendicular orientation of the STA molecules maythus be attributed to a rather stronger intermolecularhydrophobic interaction of the alkyl groups coupled withthe electrostatic interaction between the headgroup of STAand PEI.

4. Conclusion

We have confirmed that PEI dissolved in a Langmuirtrough could be very effective in assembling very rigidmonolayers of fatty acid at the air/water interface.Solidlike STA monolayers seemed to be sustained up toa surface pressure near 68 mN/m, specifically in basicconditions, because of a stronger acid-base-type interac-tion occurring between the amine group of PEI and thecarboxyl group of STA. We could correlate the π-Aisotherm feature of STA with the in situ RAIR spectraldata particularly in the region of a wide per moleculararea. On the basis of these observations combined withdata from the literature collected for similar systems usingfluorescent microscopy,11 atomic force microscopy (AFM),36

electron diffraction, and transmission electron micros-copy,10 the aggregation structure of STA and its interactionscheme with PEI at the air/water interface can be

illustrated by the cartoon shown in Figure 9; in fact,regarding the domain structure of the STA monolayer,we consulted the AFM image of a LB film reported by Chiet al.36 Finally, it should be mentioned that in the presenceof PEI very stable and close-packed Y-type LB multilayersof STA can be assembled on solid substrates with the acylchains of STA being perpendicular to the solid substrate.

Acknowledgment. This work was supported by theKorea Science and Engineering Foundation through theCenter for Molecular Catalysis at Seoul National Uni-versity (SNU) and by the Korea Research Foundationthrough the Research Institute of Basic Sciences at SNU.

LA9804029(36) Chi, L. F.; Anders, M.; Fuchs, H.; Johnston, R. R.; Ringsdorf, H.

Science 1993, 259, 213.

Figure 9. Schematic representation of the aggregation struc-ture of STA and its interaction scheme with PEI at the air/water interface at (a) pH 7.5 and (b) pH 8.9.

Stabilizer of Stearic Acid Monolayers Langmuir, Vol. 15, No. 10, 1999 3569