Chapter 1

Introduction

1.1. Introduction to Surfactants

Surfactants are among the most versatile products in the chemical industry. They

appear in such diverse products such as motor oils we used in our automobiles,

detergents for laundry and homes and drilling mud used in prospecting for petroleum.

In spite of wealth of experience in the field, the utilization of surfactants for particular

purpose remains more of an art than a science. The objective here is to provide an

appreciation of the characteristics of the surfactants with the knowledge of chemistry

and physics of the phenomena in which they are used to yield an understanding of the

reasons for their use for certain purposes and in certain products. A surface-active agent

(or, more briefly, surfactant)jsa substance that, when present at low concentration in a

system, has the property of adsorbing onto the surfaces or interfaces of the system and

altering to a marked degree the surface or interfacial free energies of those surfaces (or

interfaces). The term interface indicates a boundary between any two immiscible

phases; the term surface denotes an interface where one phase is a gas, usually air.

The interfacial free energy is the minimum amount of work required to create an

interface. The interfacial free energy per unit area is what we measure when we

determine the interfacial tension between two phases. It is the minimum amount of

work required to create unit area of the interface or to expand it by unit area. When we

measure the surface tension ?.t a liquid, we are me~suring the interfacial free energy per

unit area of the boundary between the liquid and the air above it. Therefore the

minimum work required to create the addition amount of that interface is the product of

the interfacial tension times the increase in area of the interface; W min = 'Y X A.

Surfactant is a substance that at low concentrations adsorbs at some or all of the

interfaces in the system and usually acts to reduce interfacial free energy.

1

Hoar and Schulman in 1943 [1] fIrst reported that oil can be dissolved in bulk water or

water in bulk oil in presence of surfactant and (sometimes co-surfactant) to produce a

clear homogeneous solution. The term micro emulsion was introduced to such systems

and later it was recognized that these systems should be designated as water-in-oil or

oil- in-water micro emulsions according to the nature of the bulk solvent. The

microemulsion is primarily distinguished from the emulsion not by being composed of

smaller droplets but by being subjected to a very restrictive condition so that it is

thermodynamically stable.

Surfactants are surface-active materials containing a polar head group (soluble in

water) and an aliphatic tail (soluble in oil). They optimize their interaction~ by standing

at the oil-water interface and decrease drastically the interfacial energy 'Y [2]. In most

cases this decrease of'Y stops at some point; if we add a larger amount of surfactant this

value stays roughly constant and is definitely non-vanishing. The reason is that, beyond

a certain limiting concentration, the added surfactant prefers to stay in the interface of

the bulk solvent and the other immiscible phases to form micelles or reverse micelles.

This limiting concentration is generally called Critical Micelle Concentration (CMC). If

surfactant is dissolved in water at concentration higher than CMC, it forms micelles

and little amount of oil can be dispersed in the system. By increasing the amount of oil

(i.e. molar ratio of oil, Wo=[oil] I [surfactant]), after some limit (Wo~12) the micelles

are called oil-in-water microemulsion. On the other hand if the surfactant is dissolved

in oil at concentration upper than CMC, it constructs reverse micelles, if little amount

of water is dispersed· in the system while by increasing the water molar ratio (Wo=

[Water]/[surfactant]), (Wo~12) the system is called water-in-oil microemulsion [6].

1.1.2. Classification of Surfactants

For surface activity in a particular system the surfactant molecule must have a chemical

structure that is amphiphilic in that solvent under the conditions of use. Amphiphile

structure is a structure that has both lyophobic group (having very little attraction for

the solvent) and lyophilic group (possessing strong attraction for the solvent). In the

case of water these terms are known as hydrophobic and hydrophilic groups.

2

The hydrophobic group is usually a long chain hydrocarbon residue designated by the

generic symbol R; while hydrophilic group is an ionic or highly polar group.

Depending on the nature of hydrophilic group, surfactants are classified as [3]

1. Anionic surfactants: The surface active portion of the molecule bears a negative

charge, for example, ROC-O-Na+ (soap), RC6~S03-Na+ (alkylbenzene

sulfonate).

2. Cationic surfactants: The surface active portion bears a positive charge, for

example, RNH/cr (salt of a long-chain amine), RN(CH3h+Cr (quaternary

ammonium chloride).

3. Zwitterionic surfactants: Both positive and negative charges may be present in the

surface active portion, for example, R+NH2CH2COO- (long-chain amino acid).

4. Nonionic surfactants: The surface-active portion bears no apparent ionic charge, for

example, RCOOCH2CHOHCH20H (monoglyceride of long-chain fatty acid).

Differences in the nature of the hydrophobic groups are usually less pronounced than

the nature of the hydrophilic group. Generally, they are long-chain hydrocarbon

residues. However, they include such different structures as

1. Straight-chain. Long alkyl chains of eight to eighteen or more carbon atoms derived

from the natural fatty acids.

2. Branched-chain. Propene, isobutene and some of the isomers of pentene and hexane

can be readily polymerized to a low degree yielding branched-chain mono-olefms

of eight to twenty or more carbon atoms.

3. Long-Chain (Cs-ClS) alkyl benzene residues.

4. Alkyl naphthalene residues (C3 and greater-length alkyl groups).

5. Rosin derivatives. The rosin acids are most often used as, surface active agents, but

they have also been used to make up the hydrophobic groups of derived detergents

and wetting agents. For this purpose they can be used in the same manner as the

fatty acids.

6. High-molecular-weight propylene oxide polymers (polyoxypropyline glycol

derivatives).

7. Long-chain perfluoroalkyl groups.

8. Polysiloxane groups.

3

With such variety of available surfactants one should choose the one which is suitable

for a particular purpose. To do so, it is required to have knowledge of (1) the

characteristic features of currently available surfactants (general physical and chemical

properties and uses), (2) the interfacial phenomena involved in the job to be done and

the role of the surfactant in these phenomena, (3) the surface chemical properties of

various structural types of surfactants and the relationship of the structure of a

surfactant to its behavior in various interfacial phenomena.

1.1.3. The Critical Micelle Concentration (CMC)

We now turn our attention to a fundamental and important property of surfactants

called micelle formation or micellization or critical micelle concentration. This is a

concentration of surface-active solutes in which it starts to form colloidal sized clusters

in solution. Some physical properties of aqueous solution of surfactants that show

abrupt changes in the neighborhood of the CMC are interfacial tension, conductivity"

osmotic pressure, surface tension and detergency [3]. The determination of the value of

the CMC can be made-by use of any of these physical properties but most cOITh'l1only

the breaks in the electrical conductivity, surface tension, light scattering, or refractive

index-concentration curves have been used for this purpose. Some typical CMC values

are listed in Table 1 [3].

Among the factors known to affect the CMC markedly in aqueous solution are (1) the

structure of surfactant, (2) -the presence of added electrolyte in the solution, (3) the

presence of various organic additives in the solution and (4) the temperature of the

solution.

In general, the CMC in aqueous media decreases substantially as the hydrophobic

character of the surfactant increases (i.e. number of carbon atoms in the hydrophobic

group increases to about 16). However when the number of carbon atoms in a straight

chain hydrophobic group exceeds 16, the CMC no longer decreases so rapidly with the

increase in the length of the chain and if the chain exceeds 18 carbons it may remain

substantially unchanged with the further increase in the chain length. This may be due

to the coiling ofthese long chains in the water.

4

Table 1.1. Critical micelle concentration of some common surfactants in aqueous media.

Common Solvent

Temperature CMC Compound Name (OC) (M)

Anionics CSH17S04Na+ SOS H2O 40 I.4xlO-1

C12H2SS04Na+ SDS H2O 40 8.6xlO-3

CsH 1700CCH2CH(S03- AOT H2O 25 6.8x104

Na+)COOCsH17 Cationics

C12H25N(CH3h +Br-- DTAB H2O 25 I.6xlO-2

C14H29N(CH3)3+Br- TTAB H2O 30 3.5xlO-3

C16H33N(CH3)3+Br-- CTAB H2O 25 9.2x104

Zwitterionics CgH17N+(CH3hCH2COO- .... H2O 27 2.5xlO-1 CgH17CH(COO)W(CH3h .... H2O 27 9.7xlO-2

Nonionics CI6H330(C2~OhH .... H2O 25

1.7xlO.{) CI6H330(C2~O)9H .... H2O 25

2.1xlO.{)

1.1.4. Micellar Structure and Shape

The exact structure of the micelles formed and some details of the process of

micellization are still disputed matters. At concentrations not too far above CMC in

aqueous medium (and in the absence of additives that are solubilized by the micelle),

the structure of micelles can be considered to be roughly spherical. The interior region

contains hydrophobic groups of the surfactant molecules, of radius approximately equal

to the length of a full extended hydrophobic group, surrounded by an outer region

containing the hydrated hydrophilic groups and bound water. Changes in temperature,

concentration of surfactant, additives in the liquid phase, and structural groups in the

surfactant all may cause change in the size, shape and the aggregation number of the

micelle with the structure varying from spherical, through rod or disk like to lamellar in

shape. The structure and shape of different type of aggregation of surfactants is shown

in Fig. I. I.

In hydrocarbon medium, the structure of the micelle is similar but reversed. The

hydrophilic heads settle in the interior region and outer region comprises the

5

hydrophobic groups and hydrocarbon. Dipole-dipole interactions hold the hydrophilic

heads together in the core. In concentrated solutions, ten times the CMC or more,

micelles are generally not spherical. At least in some cases, the surfactant molecules are

believed to form extended parallel thick sheets of two molecules (lamellar micelles)

with the individual molecules oriented perpendicular to the plane of the sheet. In

aqueous solution, the hydrophilic heads of the surfactant molecule form the two parallel

surfaces of the sheets and the hydrophobic tails comprise the interior region. In non­

polar media, the hydrophobic groups of the surfactant molecules comprise the surfaces

of the sheets and the hydrophilic groups comprise the interior. In both cases, solvent

molecules occupy the region between parallel sheets of surfactants. In concentrated

solution, surfactant molecules may also take the form of long cylinders packed together

for example, in hexagonal array [3] and surrounded by solvent. These ordered array of

extended micellar structure is called liquid crystalline phase.

Fig.1.1. Some possible aggregation behavior of surfactants (A) monolayer, (8) spherical

micelle, (C) rod-like micelle, (0) water-in-oil microemulsion, (E) oil-in-water microemulsion and

(F) unilamellar.

B

---f\~O----~ ---- J/. --~=,,---,~:: --....,-- _on·~--:.:97~.:= --~.99~o-

E

The major forces that govern the self-assembly of surfactants into well defined

structures such as micelles, bilayers, etc., derive from the hydrophobic interaction

between the hydrocarbon tails,which induces the molecules to associate, and the

hydrophilic nature of the head groups, which imposes the opposite requirement that

they remain in contact with water. The attraction forces between hydrophobic tails of

6

the surfactant tends to decrease the interfacial area per molecule a (the head group

area) while the repulsive force between the hydrophilic heads of surfactant try to

mcrease a.

It has been shown previously [4] that, for surfactants of optimal area a, amphiphile

volume v and critical chain length Ie' the value of dimensionless 'packing parameter'

v / ale will determine whether they will form spherical micelles (v / ale <113), cylindrical

micelles (l/3<v/ale<II2), flexible bilayers (1I2<v/ale<1) or reverse micelles

( v / ale> 1). Fig.1.2 shows some of the possible structures and packing shapes that

surfactants can form.

a

v-f1 ..... : .. : ..

~ G (a) (b) (c)

Fig. 1.2. Structures formed and critical packing shape for (a) spherical micelles, (b) cylindrical micelles and (c) spherical reverse micelles.

When a surfactant is dissolved in water, micelle formation occurs only if the

temperature T is above a critical value To (critical micelle temperature) and the

surfactant concentration c is above Co (critical micelle concentration). A qualitative

phase diagram is shown in Fig. 1.3. Region I corresponds to an aqueous solution of

monomeric amphiphile. In this region the amphiphile concentration is too low to make

thermodynamically favorable micelle formation. Micelle formation is not a true phase

separation process, so that there is not a critical concentration, but a narrow

concentration range below which no micelles exist and above which virtually all added

amphiphile enters the micellar state. Region II corresponds to an aqueous solution of

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micelles in dynamic equilibrium with monomers. The monomer concentration in this

region == co. Region III corresponds to an aqueous solution of monomers coexisting

with precipitated hydrated surfactant. It is believed that at the phase boundary between

regions II and III a conformational change of the hydrocarbon chain of the amphiphile

occurs. For T > T 0, the chain is flexible, whereas, for T < To, the chain is rigid. Micelles

will not form below To, because rigid molecules will not easily pack into a micellar

structure. The melting of hydrocarbon chains above To will favor micelle formation.

The point K of intersection between the Co and the To curve is usually called the Kraft

point of the amphiphile solution. Region IV is a two-phase region in which the micellar

solution separates into two isotropic micellar solutions at different concentrations.

80

S 60

I r-.. 6 t)

~:40 b

r: 20

N

____ --- CMC -~-----;

III Liquid crystal

1 Concentration (%)

2

Fig. 1.3. Schematic phase diagram of a dilute amphiphile solution.

1.2. AOT

Arosal OT (AOT) is a registered trade mark of American Cyanamid Company, USA

and is commercial name of sodium salt of bis (2-ethylhexyl) sulfosuccinate

{Na(DEHSS)}. The solubility of water in oil can be remarkably enhanced by the

addition of AOT, forming a simple ternary system of microemulsion composed of

water, oil and AOT. These microemulsions can exhibit different types of structures

such as discrete spherical water droplets, interconnected continuous water channels,

interacting roads, etc. The simplest microstructure of water-AOT -oil is that of spherical

8

water droplets of colloidal dimensions and possesses a small degree of polydispersity.

Having control on the water content, Wo, their size can accurately be controlled.

Microemulsions, which have been mostly investigated, are composed of an ionic

surfactant, an alcohol as a co surfactant and a certain amount of salt for their formation

and stability. Since these microemulsions are composed of four or five components,

their experimental and theoretical progress is not considerable. AOT is a surfactant that

has been recognized [5] as forming reverse micelles in hydrocarbon oil, which take up

substantial amount of water without using any cosurfactant.

12.1. Physical and Chemical Properties of AOT

Arosal OT (AOT) is the commercial name of sodium salt of bis (2-ethylhexyl)

sulfosuccinate [Na(DEHSS)]. It is a white waxy solid producing a clear, colorless

solution in alcohol and water [6]. Its molecular weight is 444.5 and the linear length of

the molecule is 11 A and the maximum cross-sectional area of the polar head part is

about 55 A2 [7]. The structure of AOT molecule [8] is shown in Fig. 1.4.

Fig. 1.4. Structure of AOT molecule.

9

1.2.2. Aggregational and phase behavior of AOT

A typical phase diagram for water, AOT, isooctane system at 288 oK is shown in Fig.

1.4. The domain of existence of isotropic transparent solutions of AOT/water/oil covers

a large region on the oil-rich side of these systems [5]. By reducing the amount of

isooctane and increasing the AOT concentration and water, the system is converted to

liquid crystals.

Liquid cristalsY,

Two-phase Region

Aqueous phase

Water

AOT

Reverse • Micelle

solution

Iso-octane

Fig. 1.5. Phase diagram of water-AOT-isooctane system at 288 OK.

There are some evidences [10,11] on ternary phase diagram for AOT/water/oil systems

and salt effects upon AOT reverse micelles. The addition of an electrolyte (NaCl or

CaCh) to AOT/water/n-decane reverse micelle lowers the maximum solubility of water

and decreases the relative viscosity.

,/

The solubilization of water in AOT reverse micelles depends on many factors. It

depends on the oil used, because the oil can penetrate into the surfactant interface,

increasing volume. If there is no specific hydrocarbon-head group interaction, the best

hydrocarbon penetration will be those which are molecularly most similar to the

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surfactant tails. Thus the solubility of water in AOT hydrocarbon solutions decreases as

the carbon number of an alkane increases [5].

Electrolytes decrease the solubilization of water in AOT reverse micellar solution. At a

certain AOT concentration in cyclohexane, solubilization of salt solution shows that

WO,max decreases as the NaCl concentration increases.

1.2.3. Application of AOT reverse micellar systems

Reverse micellar solution of AOT has been well exploited in different applications (e.g.

chemical reaction study, enzyme activity study, liquid-liquid extraction of proteins, sol­

gel prepar<:tion, nanoparticles preparation etc.).The system is very attractive because it

is optically transparent and therefore the change in the system could be followed by

different spectrophotometric methods. Moreover both hydrophilic and hydrophobic

components could be solubilized in it and therefore making it possible to study a

system involving hydrophilic and hydrophobic substances. There are parameters like

water pool size, surfactant concentration, pH of the water pool which, could be easily

tuned to effect a change in the system under study.

Depending on concentration of AOT and water added in reverse micelles, the strength

of AOT-water interaction, the size of water pool, the microscopic viscosity and the

polarity of aqueous interior, control the reactivity in reverse micellar solution. A wide

variety of AOT applications has been discussed in a review article by T.K. De and A.

Maitra [5]. Several enzymes and proteins have been solubilized in reverse micellar

solution of AOT-hydrocarbon systems (usually isooctane or n-octane). The simplest

and most frequently used method for solubilization of enzymes in the reverse micellar . solution is the injection method. In this method one prepares hydrocarbon solution of

AOT (50-400 mM) and to it adds directly buffered enzyme solution (-10-100 J.1l). The

enzyme solution must be added gently and in tiny droplets and the reverse micellar

solution becomes clear with gentle hand shaking. The injection method sometimes

leads to the formation of supersaturated solution but the problem can be avoided if

stability diagram of the micellar solution is known a priori.

11

The most accepted model of enzyme hosted reverse micelle is the so-called 'water-shell

model' [11], which favors one-enzyme-per-micelle. According to this model the

enzyme is confined to the middle of the water pool and is protected by a layer of water

from the charged inner wall of the micelle. Several enzymes have been solubilized in

reverse micellar solutions and found to be active. In general, the enzymes solubilized in

AOT reverse micelles are able to maintain activity comparable to that found in aqueous

solution.

One of the recent findings in the field is the gelation of reverse micellar solution [13-

16]. Gelatin is as matrix and hosted together with the drug in the water pool of reverse

micelles. Gelatin is then cross-linked and the drug should remain entrapped in the net.

After removal of the surfactant layer and apolar solvents one practically has a globular

protein which contains the drug in its interior. This 'nano-pallet' is biocompatible and

biodegradable without toxic effects. They are water-soluble after complete removal of

the micellar components and can be prepared in different sizes. Enzymes can be

co solubilized with gelatin 'and remain active in gel form. These systems are potentially

very interesting for pharmaceutical and cosmetic applications.

Synthesis of nanoparticles is a new emerging field in the solid state chemistry. Due to

their small size, these crystallites exhibit unique catalytic behavior [17] and show size

quantization effects [18,19], nonlinear properties [20,21] and unusual luminescence

[22,23]. There are many examples of semiconductors, magnetic or metallic particles

synthesized inside reverse micelles of AOT [24-27].

1.3. Preface to nanocrystals

Electronic states and probabilities of optical transitions in molecules and crystals are

determined by the properties of atoms and their spatial arrangement. Electrons in an

atom possess a discrete set of states, resulting in a corresponding set of narrow

absorption and emission lines. Elementary excitations in an electron subsystem of a

crystal, that is electrons and holes, possess properties of a gas of free particles. In

semiconductors, broad bands of the allowed electron and hole states separated by a

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forbidden gap give rise to characteristic absorption and emission features completely

dissimilar to atomic spectra. What happens on the way from atoms to crystal is the

question that can be answered by a study of the properties of matter in two steps: from

atom to cluster and from cluster to crystal.

The distinctive feature of clusters is the precise number of atoms, which can be

organized in them. These numbers determine unambiguously the spatial configuration,

electronic spectra, and optical properties of clusters. As the particle size grows, the

properties can be described in terms of the particle size and shape instead of dealing

with a particular number of atoms and spatial configuration. These type of

microstructurc3 can be referred to as meso scopic structures as their size is always larger

than the crystal lattice constant but comparable to the de Broglie wavelength of the

elementary excitations. They are often called 'quantum crystallites', 'quantum dots', or

'quasi-zero-dimensional structures'. As the size of these crystallites ranges from one. to

tens of nanometers, the word 'nanocrystals' is widely used as well. This term refers to

the crystallite size, whereas the other terms hint at the interpretation of their electron

properties in terms of quantum confmement effects.

Semiconductor nanocrystals can be fabricated using a number of techniques, differing

in the environment in which nanocrystals being constructed, growth conditions, size

range and size distribution, as well as physical and chemical stability and reliability.

Nanocrystals can be developed in inorganic glasses and crystals, in liquid solutions and

polymers, in crystalline surfaces and reverse micellar vessels.

1.3.1. Growth of nanocrystals in inorganic glass matrices

Fabrication of nanocrystals embedded in a glass matrix by means of diffusion

controlled growth is based on commercial technologies developed for fabrication of

color cut-off filters and photochromic glasses. Cut-off fIlters are just glasses containing

nanometer-size crystallites of mixed II-VI compounds (CdSxSel-x). Growth of

crystallites results from the phase transition in a supersaturated viscous solution. The

process is controlled by diffusion of ions dissolved in the matrix and can be performed

13

in the temperature range T glass< T < T melt. where T glass is the temperature of the glass

transition and T melt is the melting temperature of the matrix. Typically, growth

temperatures range between 550°C and 700 °c depending on the desirable size of the

crystallites and matrix composition.

Diffusion-controlled growth from a supersaturated solution can be described in terms of

three distinct precipitation stages, namely nucleation, normal growth and competitive

growth [28]. At the first stage, small nuclei are formed. At the second stage, crystallites

exhibit a monotonic growth due to jumps of the atoms across the nucleus-matrix

interface. At this stage the supersaturation degree decreases with time and the total

volume of semiconductor phase monotonically increases. Finally, when crystallites are

large enough and the degree of supersaturation is negligible (i.e., almost all ions are

already incorporated in crystallites) the surface tension plays the main role and the

growth dynamics are characterized by a diffusive mass transfer from smaller particles

to larger ones.

1.3.2. Nanocrystals in Porous glasses·

Sol-gel technology offers an alternative way of inorganic glass fabrication that does not

include high temperature treatment. Porous glasses developed in this way contain

nanometer-size voids that can be impregnated with dye molecules or nanocrystals.

Successful attempts have been reported of fabrication of nanocrystals embedded in a

sol-gel matrix [28]. As compared with conventional glasses, porous glasses can be

saturated with semiconductor materials up to very high concentrations thus providing

an opportunity to study interactions between crystallites. Additionally, crystallites in

these matrices are expected to have a smaller number of defects because of the lower

precipitation temperature.

However the sol-gel technique in its present state encounters serious problems in

providing a continuous size control and narrow size distribution. Because of wide size

distribution, optical spectra of these systems are usually rather broad. An exception is

the multistep inorganic-organic processing which provides a relatively narrow size

14

distribution of CdS nanocrystals ranging from 2.1 to 9.2 nm and corresponding to

distinct bands in the optical absorption spectrum that is covering both weak and strong

confinement cases [29].

1.3.3. Nanocrystals in organic solutions and in polymers

Nanometer size crystallites made from II-VI compound can be developed in an organic

environment using a variety of techniques based on organometallic and polymer

chemistry [30]. Basic features of the structures fabricated in this way can be

summarized as follows. A relatively low precipitation temperature (usually not

exceeding 200°C) is favorable to minimize the number of lattice defects. The ability to

cap the crystallite surface by organic groups provides a way to control surface states. It

is possible to obtain isolated clusters or to disperse them in a very thin film with a

subsequent structural analysis by means of X-ray or TEM facilities. Under certain

conditions, an extremely narrow size distribution of clusters can be obtained.

It is possible to use a polymer fiim-buth~as stabilizer--and matrix [29]. Polyvinyl alcohol

and polymethyl methacrylate can be used as matrix materials. Structures thus obtained

exhibit sharp absorption spectra with clearly pronounced bands and an intense

luminescence edge. An evident advantage of structures like semiconductor-in-an­

organic film is the ability to apply a strong electric field when studying electric field

effects because the thickness of the structure, unlike glass, can be made as small as 10

J.1m or even less. One of the important advantage of this technique as compared, for

example, to the diffusion limited growth in inorganic glass, is low defect concentration

because of low temperature of the synthesis (200-300 °C) and the well-defined and

controllable surface structure.

1.3.4. Nanocrystals in reverse micelles

Microemulsions have aroused much interest as a novel medium for chemical reactions.

Fast aqueous reactions can take place inside reverse micelles in general (preferably

AOT-isooctane reverse micelles). In this method aqueous reactants, A and B, are

15

dispersed in the core of reverse micelles separately forming completely clear solutions

(i.e. reverse micelles of individual reactants). The exchange rate of the reactants inside

the water pools of the reverse micelles has been found to be in the order of 10 ms

which occurs following energetic collisions between the droplets. In fast reactions the

time scale of the reaction is typically 10-8 _1O-6s. Therefore, for fast reactions, the

distribution of reactants among the water pools is frozen on the time scale of reaction

(i.e. no exchange of reactants between droplets take place during the time in which the

reaction is taking place). Mixing the two reverse micellar systems containing reactants

induces reaction. As soon as they exchange their core ingredients, the reaction takes

place. If droplets containing species A and B are composed of n molecules, the

stoichiometric equation for the reaction may be represented as

L@n_~nL@ .. ~Y~E ® where the chemical prqcess occurring within the droplets is A + B -7 C. This equation

shows the sum of n number of exchange processes, in which reaction in some occurs, in

others A, B and C are simply redistributed. Using this technique, controllable

nanometer size ll-VI crystallites can be prepared.

1.4. Doped nanocrystals

Most of studies performed on semiconductor nanocrystals are aimed at developing

perfect crystallites and establishing their intrinsic features. However, purposeful

utilization of resonant impurity transitions in nanocrystals is of great practical interest.

Crystals and glasses doped with transition metals and rare-earth elements (Mn, Ti, Nd,

Er) are widely used as phosphors and lasing media. Therefore, it is required to examine

the radiative properties of these impurities when the latter are embedded in ~

nanocrystals.

The fIrst experiments on manganese-doped ZnS nanocrystals revealed a drastic change

in spontaneous decay rate of Mn2+ ions embedded in nanocrystals as compared with

16

ZnS monocrystals [31]. In a bulk crystal after interband excitation, an electron is

captured by an ion in a period of microseconds with subsequent radiative decay, the

lifetime being about 2 milliseconds. In a nanocrystal, electron capture occurs on a

subnanosecond scale, and the radiative lifetime of Mn2+, was found to be no longer than

a few nanoseconds.

The enhancement of radiative decay rate of excited Mn2+ ions in nanocrystals is a direct

consequence of fast energy transfer of the excited electron-hole pairs of s-p band states

of the host in to the d-state of Mn-ion impurity. This sp-d mixing promotes both

enhanced energy- transfers to the impurity and fast radiative decay. Hybridization of

electron states is a direct consequence of spatial confinement. It occurs when

nanocrystal size is less than the exciton Bohr radius, which is aB = 2.5 nm for zinc

sulfide. It is important that a rather fast capture process prevents both radiative and

nonradiative energy relaxation via unwanted competitive channels such as surface

states and other impurities. In ZnS:Mn nanocrystals the luminescence efficiency· is

rather high and the quantum yield was found to be about 20 percent, showing a

monotonic increase with decreasing size; Thus, doped nanocrystals are believed to offer

a set of commercial phosphors and lasing media in the -near future.

1.5. Proteins

The name protein is derived from a Greek word proteios meaning of prime importance.

Most of the solid matter from which living organism are made has a remarkably

uniform composition. In living organisms, only a few elements are present and they

})ccur in proportion 54% of carbon, 7% of hydrogen, 16% of nitrogen, 22% of oxygen

and 1 % of sulfur. They are of great importance in the construction and chemical

activity of living organisms and are called as proteins. A protein is a linear chain whose

backbone is made up of succession of amino-acid monomers, called residues whose

basic composition is shown in Fig. 1.6.

17

H I

R-C-COOH I

NH2 Fig. 1.6. Single amino acid molecule.

Where COOH is the carboxyl group and NH2 is the amino group. R is the side group

(Radical) of the amino acid and can contain apart from C and H (Hydro carbon), other

atoms like 0, S, N, etc. There are twenty one types of amino acids, which differ in

their side groups. The smallest and simplest one is glycine, where R is H. Under normal

physiological conditions the amino acids exist in doubly ionized form. In this case the

acidic carboxyl group can lose a proton and the basic amino group gain a proton to

form a dipolar ion or zwitterion (Fig. 1.7).

R 0 R 0 I II I II

H-C-C-O-H H-C-C-O I

NH2 r1+ / 3

Fig. 1.7. Transition of amino acid from nonionic to zwitterionic form.

Proteins are formed by polycondensation of amino acids. In each step of condensations

a peptide bond is formed by the loss of one water molecule. When two amino acids

combine this way, they form a single peptide bond and the resulting product is a di­

peptide as shown in Fig. 1.8. In this way, when 3 amino acids combine together, they

form two peptide bonds and the resulting product is a tri-peptide. A tetra-peptide, a

penta-peptide can be formed out of 4 amino acids, 5 amino acids respectively. When

the number of peptide bond is less than 100 it is generally called as a polypeptide and

when it is more than 100, it is called as a protein, although the demarcation is not very

18

sharp. The characteristic molecular masses of individual polypeptide chains in proteins

are of order of 20,000, which corresponds to 150-180 amino acid residues (the average

RI R2 I I

NH;-C-COO- + NH;-C-COO-

I H

RI I

NH;-C-I

H

a II C-N

I H

(di-)peptide bond

I H

R2 I

-C-COO-

I H

molecular weight of an amino acid residue is 120) whereas for proteins it can be much

larger.

Fig. 1.8. Schematic of formation of (di-)peptide bond through polycondensation.

Therefore proteins are made up of many amino acids in which they are bound together

by a peptide linkage forming between the amino group of one amino acid to the

carboxyl group of another.

1.6. Collagen and Gelatin

The polypeptide gelatin, one of the most versatile ingredients of food substances, is a

protein obtained by the partial hydrolysis of collagen, the major structural and

connective protein tissue of the animal kingdom. It is the only natural protein, which is

capable of producing clear, thermo-reversible gels in water at near body temperature. It

is a class of proteinaceous substance that has no existence in nature, but derived from a

parent protein collagen, by one of the many ways involving the destruction of

secondary structure of the collagen [32]. Mostly this is achieved by both chemical and

thermal treatments. This is the reason why it is sometimes called as denatured or

disorganized collagen. Gelatin is used in food systems as a gelling agent, thickener,

19

mm former, protective colloid, adhesive agent, stabilizer, emulsifier and beverage­

fming agent. Because of its use in food, pharmaceutical and cosmetic industries etc., it

has become a broad scientific research area and has more than hundred years of

research background.

Collagen, which is the source of gelatin, has a complex structure. Collagen is the

composite of 30 percent of the total organic matter in mammals and 60 percent of their

total protein contents. Much of the collagen is localized in major tissues such as skin,

bone, tendon but collagen fibers pervade almost every organ and tissue. It is the only

mammalian protein containing large amount of hydroxyprolin and it is extraordinarily

rich in glycine and proline. The sulfur content of collagen is very low. Every third

element of collagen is a glycine residue (27%) and an important amount of amino acid

proline and hydroxyproline residue (25% of the total) is present in collagen protein.

Although the chemical composition of collagen protein is not unique, a typical

sequence of collagen protem is given by

- (Gly -Pro-X)- OR - (Gly - X - Hypro) -

Where X being different amino acids. The side groups of the amino acids also play an

important role in the stability of proteins. Some of them have polar groups (such as OR

, COO-, NH3 +, etc.) which are likely to interact with water molecules and establish

hydrogen bonds. The portion of the charged and uncharged groups varies with pH; an

amino group for example, may be either - NH3 + or - NH2 and a carboxyl group may

either be - COOH or - COO-. The charged groups are hydrophilic and presence of

hydrophobic groups (such as those of proline) give unique characteristics to gelatin.

This protein adopts a three dimensional conformation when dissolved in water which is

a direct consequence of the balance between hydrophilic and hydrophobic interaction,

which in tum results from molecular composition [33].

20

The total length of a collagen strand is 1000 residue long (primary structure). The

individual chain is twisted in left-handed helix, which have 10 residues per 3 turns. The

pitch of the helix is approximately 0.9 nm. The torsion of the chain is such that (C = 0)

and (N- H) groups attached on the main chain are oriented perpendicular to its axis

and are not in position to establish intra-chain hydrogen bond to stabilize the helix. So

the question arises, how else can the helix be stabilized? The answer for collagen is the

triple-helix. The three strands are wrapped into a super right-handed helix, with a pitch

roughly 10 times longer (- 8.6 nm). The presence of "Gly" is required to allow the

three chains came close to each other, while "Pro" and "Hypro" residues enhance the

rigidity. The gradual gentle right-handed twists of the individual strands allow the side

groups of various sizes to come into the structures. Collagen is probably the only

polypeptide that forms triple helix structure in hydrogen bond friendly environment.

The collagen triple-helix is stabilized by inter-chain hydrogen bonds, which are

perpendicUlar to the chain axis. The hydrogen bonds can be of several types, either

directly between (C=O) and (N-H) between two adjacent back bone or via water

molecules situated in interstitial positions -inside the t..riple-helix. The overall length of

the triple-helix varies between 120-200 nm. These are arranged in parallel rows, to

build the fibers, which are attached by additional covalent bonds located at both ends of

the roads. These bonds make the collagen fibers insoluble. A clear aqueous solution of

gelatin undergoes gel transition (triple helix formation) at characteristic temperature T g

= 30°C.

There are many ways a collagen can be converted to gelatin. So it will not be

surprising if one gets a gelatin of different nature and character (different T g), when .

extracted from different collagen by different methods. Gelatin is also somewhat

unique among all the proteins due to the absence of appreciable internal order. Gelatin

is composed of glycine, proline, hydroxyproline, glutamic acid, alanine, aspartic acid

and arginine, its proportion is given in Fig. 1.9. and their side functional groups are

given in Fig. 1.10.

21

Glycine 33%

Fig. 1.9. Chemical proportion (amino acid composition) of gelatin.

Arginine NH~

C -NH-CH2 -CH2 -CH2 -

NH{ o II

o II

Aspartic Acid HO - C - CH 2 -

Glutamic Acid HO - C - CH 2- CH 2 -

HOCH- CH2 Proline

Hydroxyproline I I

Fig. 1.10. Chemical structure of some of the functional groups in gelatin.

In spite of the apparent conflicts on the reports available in conversion of collagen to

gelatin, it is possible to draw a single coherent picture that can explain this conversion.

The ordered hydrogen bonded configuration of the collagen molecule can be melted out

readily by heating collagen solutions in acid to about 40°C. The transition is sharp

(alike a gel transition) and complete within a few minutes over a small temperature

interval. The disordered molecule falls apart in one of three ways, illustrated in Fig.

1.11.

22

NATIVE COLLAGEN MONOMER

1

30: chain

' •• ,, 1 ', •

.. ""-: .'. . ' . . .... :' .-, ..... ': .... ~ ,,' ",-,,'

2

~ = 80,000 to 125,000

lor2 1 0: chain 1 pComponent

111, = 160,000 to 250,000

" I

I ••• I ~I .. . ' . ", .

.. '

yComponent

Mf = 240,000 to 375,000

Fig. 1.11. A Schematic diagram of the modes of conversion of monomeric tropocollagen to various types of gelatins, assuming no rupture of-peptide bunds.

If there are no additional restraining bonds between chains (path 1), three randomly

coiled single-strand peptide chains result. The three chains which are not of identical

composition and probably not of equal molecular weight are called ex chains. In those

cases (path 2) where two chains are joined by one or more covalent cross-linkages,

denaturation leads to the appearance of two particles, one an ex chain, the other a two­

stranded molecule, known as ~ component, with approximately twice the molecular

weight of the ex chains. In any particular case, the ~ component may be composed of

two similar or two unlike ex chains. The weight distribution will be 67% ~ and 33% ex.

In the final case (path 3)' it can be imagined that at least two covalent cross-linkages

hold the three chains together. The disordering process melts out all traces of secondary

structure, but the three chains cannot separate and remain as a unit in solution. This

three-chain structure is called 'Y component. Only small amounts of the 'Y component

have been isolated from acid-extractable tropocollagen preparations.

23

The physical parameters associated with the gelatin used in our experiments are given

in Table 1.2 and the charge state of the gelatin molecule is given in pie chart in Fig.

1.12.

Table 1.2. Physical parameters of gelatin molecules in water in sol state at 60 °e. Rg is the radius of gyration, ~ is the hydrodynamic radius, Mw is the Molecular weight, A2 is the second virial coefficient of osmotic pressure, [TJl is the viscosity, Do is the diffusion coefficient at infinite dilution, and (dnlac) is the change in refractive index with concentration.

Parameters Value

Rg 342 A

Rh 190A

Mw 2.12 xlO) g.mor!

A2 86.2 x 104 mol.cc.g-2

[11] 0.30 dL.g-1

Do 2.63 X 10-1 cml.s-I

(anlac) 0.142 ± 0.04 cc.g-1

The lysine and arginine groups comprising ::= 7.5% of residues and are positively

charge. Glutamic and aspartic acid constituting ::= 12.5% of residues give the negative

nature to the chain. Other 6% of residues are strongly hydrophobic by nature leaving

out 68% of the chain to be neutral.

Neutral ~ 68%

Fig 1.12. The pie-chart presentation of charge state of the gelatin molecule.

24

Physical, chemical, and rheological properties of solution and gels of gelatin have been

studied extensively in the recent past by several workers through scattering techniques

(light, x-ray, neutron), differential scanning calorimetry (DSC), relaxation methods

(dynamic light scattering, dielectric relaxation and rheology) and yet, the system still

remains far from being completely understood.

25

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