CHAPTER SIX

This Chapter:

Raman spectroscopic study was carried out on water in gelatin at 4% (wt./vol.) in gel (2!PC) and sol '(4rPC-6rPC) states at various concentrations (0.5mM, lmM, 5mM, lOmM and 15 mM) of anionic surfactant, Sodium dodecyl sulphate (SDS). The in­phase-collective stretching mode of vibration of hydrogen bonded -OH oscillators, centered around 3250 cm-1 in tetrahedral net­work of water molecules were observed to be significantly affected by temperature and presence of SDS with gelatin as a third com­ponent. In 4% (wt.jvol.) gelatin concentration, which is above the overlap concentration {rv 2% (wt./vol.)}, defects in intermolec­ular networking of water are induced below and above critical micelle concentration (CMC) of SDS. We propose two states: sol-I ( 4rPC) and sol-II (6rPC ). In gelatin, peak amplitude of -OH oscillators which is proportional to nearest neighbor density (co­ordination number), decreased from gel to sol-I state and then it increased by ""' 7% in sol-II state. We observed interesting phe­nomena at, above and below CMC of SDS. The peak amplitudes were seen to decrease with temperature below CMC from gel to sol-I state and then increased ""'7% in sol-II state but above CMC, it exhibited a strong increase from gel to sol-I state and then de­creased by ,..., 6% in sol-II state which is just reverse of the trend observed below CMC. However, at CMC the peak amplitude was observed to be independent of temperature. Continuous shifting of peak centre and full-width at half maxima (FWHM) towards lower values was observed with increasing SDS concentrations in the gel state.

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Chapter 6

Laser Raman Spectroscopic Study of Water in Gelatin-Surfactant Solutions and Gels

6.1 Principle of Raman Scattering

Analysis of absorption or transmission spectra of electromagnetic radiation by organic, inorganic molecule at some particular wavelength has been taken as a most versatile and authentic way to get structure information about the molecule. When light is passed through the organic molecules in a ceratin wavelength range, and the transmitted light is analysed and a plot of absorbance versus frequency is made, then we see that at some particular wavelength there is a. peak. This peak at some particular frequency is due to the absorption of that electromagnetic wave at that frequency due to the electronic transition from ground state to excited state. This absorption of electromagnetic wave is thus equal to the energy level difference between the two levels. The energy transition E 1 -+ E 2 corresponds to the absorption of energy exactly equivalent to the energy of the wavelength absorbed:

llE = ( E2 - Et) = he/ A = hv

wher~, A and v are the wavelength and frequency of the radiation. Radiation of different kind like ultraviolet-visible, infrared, microwave and radio frequency, absorbed by the molecules associated to the different kind of transition like electronic, rotational and vibrational movements of the molecule, electron spin resonance or electron paramag­netic properties, nuclear magnetic resonance, detection of magnetic properties of certain atomic nuclei etc., can be fruitfully diagonosis the chemical structure, its movement very well. At ordinary temperatures organic molecules are in a constant state of vibration,

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each bond having its characteristic stretching and bending frequency. and being capable of absorbing light of that frequency. The vibration of two atoms joined together by a chemical bond can be likened to the vibrations of two balls joined by a spring: using this analogy, we can rationalize different features of infrared or Raman spectra. The vi~ brational frequency is calculated with a reasonable accuracy by using Hook's law which correlates the frequency with bond strength and atomic masses. Since, .

v= Bond strength . h _ 1 ( k ) I/

2

, t us, v-- I Equivalent mass 271' m1m2 (m1 +m2)

(6.1)

where, v = frequency, k = a constant related to the strength of the spring (the force constant of the bond), mt, m 2 = the masses of two balls and m1m 2/(m1 + m2) is often ·expressed as p, the reduced mass. The vibrational frequency of a bond is expected to increase when the bond strength increases, and also when the reduced mass of the system decreases. We can predict that C=C and C=O stretching will have higher frequencies than C-C and C-0 stretching.

When a sample is illuminated with an intense beam of light at frequency v, an oscillating dipole is included in the molecules, even if v is a frequency that cannot cause electronic or vibrational excitation. For the sample to be infrared active, it is necessary to induce dipole by the radiation. Taking Born-Oppenheimer approximation to separate electronic and nuclear motion, a pure vibrational transition can be written as

lllo(r, R)d>v(R)---+ lllo(r, R)<Pv'(R) (6.2)

where, <Pv and <Pv'( R) refer to two different vibrational states; rand Rare the coordinates of electronics and nuclei, respectively; and 1110 is the ground-state electronic wave func­tion. If the nuclei are stationary, the eletric dipole operator J.l depends only on electronic coordinates, and the transition dipole is

(6.3)

This equation is zero because the second integral over R is zero for its stationary state. Thus for symmetric stretching when the electronic cloud is being stretched equally on both sides of the nuclei, the induced dipole moment is zero because one sided electron stretching is compensated by the other sided stretching. But in the case of asymmetric stretching when the electron cloud is stretched towards one direction causes the change in the molecular position i.e, nuclear motion, thus it is infrared active. So far, the infrared spectroscopy is described. But in the Raman spectroscopy the picture is completely different. The infrared active molecule may be Raman inactive and vice versa. Actually, Raman spectroscopy deals with the polarization of the vibrating molecules not induced dipole moment as in the case of infrared. If we describe the time variation of the electric field of the light as E(t) = Eocos27rvt, i.e., the dipole induced in the molecule will be

p(t) = a-(v).Eo cos 21rvt (6.4)

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In principle, the polarizability a is a tensor but for what follows it is sufficient to ignore this and treat it just as a number, o(v).

If the molecule is vibrating with time, its polarizability is not constant. As the nu~ clei move, the electrons respond, and at each nuclear position a different polarizability results. We can describe this simply by writing

a(v) = a0 (v) + a'(v) cos 21rv't (6.5)

where, o 0 gives the polarizability of the molecule in its equilibrium nuclear configuration, a'(v) describes the change in polarizability with nuclear motion, and, v' is a vibrational frequency. Combining Eq. (6.4) and(6.5), we have

J.L( t) Eo [o0 ( v) + a' ( v) cos 271' v't] cos 271' vt

Eoo0 (v) cos 21rvt + Eoo'(v) cos 21rv't cos 21rvt

J.L(t) + J.L 1(t) (6.6)

The first term is just the normal induced dipole oscillating frequency v. This oscillation gives rise to light scattering and the refractive index of the medium. The second term produces a Ramn spectrum. This can be seen if one uses the identity cos A cos B (1/2)[cos(A +B)+ cos( A- B)] to rewrite it as

J.L 1(t) = 2{Eoo(v)[cos 2:r(v + v')t +cos 21r(v- v')t]} (6.7)

A dipole oscillating at a particular frequency results in the emission of radiation at that frequency. The intensity of radiation of proportional to (J.L') 2

• Thus when a vibrating sample is illuminated at frequency v, emission will be observed at frequencies v + v' and v - v'. The location of the emission peaks (relative to the exciting frequency) permits measurement of the vibrational frequency v'. The spectral band at lower energy than the exciting light is called the Stokes band, and the band at v + v' is called the anti-stokes band. As the population at the higher level is lower than the population at the lower level according to Boltzmann statistics, the numbers of electrons taking part in transition for the Stokes line is higher than the anti-Stokes line. So in Raman spectroscopy what is mostly measured is Stokes line.

6.1.1 Modes of vibration

Molecules with many atoms possess large numbers of different vibrational modes. For a nonlinear molecule with n atoms, the number of vibrational modes is (3n - 6), so that methane molecule theoretically possesses 9 and ethane has 18. But does the methane molecule have 9 absorption bands? Figure 1 shows that for a single methylene group several vibrational modes are available, and many atom joined to two other atoms will

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STRETOUNG MODES FOR CH 2

~etrlr: Anttsymmetric

In plane deformation Out of plane deformation

+ + +

Rode Twlet Weg

Figure 6.1: Vibration modes in methylene groups. Similar AX2 groups (-NH2 , -N02 ,

etc.).

undergo comparable vibrations. Each of the different vibration modes may give rise to different absorption band, so that C H2 group give rise to two C-H stretching bands at v611m. and Vanti.· Other vibrations may not give rise to absorption, since some may have frequencies outside the normal' infrared region and some of them may be come at the same frequency region (degeneracy) and their absorption bands will overlap.

6.2 Introduction to hydrogen bonding and water structure

The unique physical and chemical behaviour of water is due to its hydrogen bonds. These bonds account for the extremely high boiling point of water in comparison to other liquids. The hydrogen sulfide molecule, for example, is more massive than the water molecule, has nearly the same geometry, and sulfide is in the same group as oxygen in the table of elements. However hydrogen sulfide is a gas at room temperature and is not capable of making hydrogen bonds. If water were a gas at room temperature, life as we know it would be impossible. The fact that water has its density maximum not in the solid but in the liquid phase (at 4°C) results from the fact that the network is not static; rather it is a fluctuating network of flickering hydrogen bonds. If this were not so-if ice sank rather than floated,-many of the earth's oceans would be permanently frozen solid.

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The hydrogen bond is not a normal bond; it is a shared electron bond. ~t can be regarded as a weak covalent bond, meaning that the energy needed to break it is low-about 15 KJjMol. compared with 492 KJ/Mol. to break one H-0 bond in H-0-H. This is the

H&

-·- ....... . /-~-. ·. \

\ .,

' , , I

I

·, ,, I

····,-~·· --.... -_.,.. \ / '

~ I \... j ., .,./ ....... - ..

H···-----

Figure 6.2: Hydrogen bond between two molecules. Two small circles are two unpaired electrons.

reason why thermal agitation is sufficient to cause hydrogen bonds to ""flicker" on and off rapidly in liquid water. The hydrogen bond gives water its unique properties. To understand the hydrogen bond, we first take a close look at the water molecule itself. The water molecule is composed of two hydrogen (H) atoms and one oxygen (0) atom (Fig. 6.2). Figure 6.2 may lead to the misconception that the water molecule is flat or planar, which it is not. Rather, it has a tetrahedral structure as shown in Figure 6.3. In gaseous state the H-0-H structure has a V-shape and the angle between two H molecules is 104.5°. The two H-atoms in their neutral state each have only one electron. The neutral oxygen atom has six electrons in its outer shell and needs two further electrons to fill all eight places available in this shell to satisfy the so called octet rule. The 0-H bond is a normal covalent bond; each atom contributes one electron to that of its neighbor. The hydrogen atoms each contribute their one and only electron, while the oxygen contributes its two unpaired electrons. Two of these bonds in the structure H-0-H fill up the two vacancies in the oxygen electron structure to complete the octet. Each shared electron pair is concentrated between the two atoms along the bond axis between H and 0. The H-0-H structure has a V-shape (Fig. 6.2). The two "bubbles" represent the two unshared electron pairs. The tetrahedral structure of water. The center ball represents the oxygen atom, the two outer black balls the hydrogen atoms, and the two clouds the unpaired electrons. Now we come to the hydrogen bond. It

126

6 2-

Figure 6.3: (i) Bond structure of water molecule; (ii) Tetrahedral configuration of water molecules and (iii) Diamondoid networks of water molecules

is what we call a shared electron bond i.e., a special case of the coordinative covalent bond. The oxygen has two unshared electron pairs that do not contribute to the H-0 bonds of the water molecule. These electron pairs are oriented outwards, away from the H-atoms, as shown in Figure 6.2. An unshared electron pair can interact with a nearly naked hydrogen nucleus (proton) to form what is called a coordinative covalent bond, or hydrogen bond. The hydrogen nucleus appears nearly naked from outside the water molecule, because the hydrogen electron is (mostly) on the inside of the molecule, located along the H-0 bond. In a single water molecule the -OH bond is around 1.00 A and the hydrogen bond made by oxygen molecule of one molecule and hydrogen molecule of other molecule is 1. 76 A. It is only because of the fact of former is covalent bond and later is hydrogen bond where electron pair is given by oxygen molecule. In liquid state all the water molecules are interconnected by hydrogen bond and make a diamondoid or tetrahedral network structure. The distance between two nearest non-hydrogen bonded oxygen molecules is about 4.76 A. That is why in solid state i.e., in ice maximum space was lying vacant.

127

The hydrogen bond is approximately 30 times weaker than a normal covalent bond, because only one of the contributing atoms is supplying electrons to it; the two electrons stay mainly concentrated near the oxygen. Because the hydrogen bond is so weak, it is easisly broken. At room temperature, thermal energy is enough to break hydrogen bonds. In liquid water the whole network of hydrogen bonds "flickers," each bond making and breaking again in a millionth of a microsecond. It. is this network of flickering hydrogen bonds that gives liquid water its unique properties. Consequently, all the coordinative covalent bond is not necessarily weak. The nitrogen atom in the ammonia molecule has one lone electron pair. When a hydrogen ion is added to make the ion, the fqur hydrogen atoms share equally in the resulting bond structure. As a results, the bond energy between Nand each of the four H-atoms is far stronger than the hydrogen bond between water molecules and is not broken by thermal agitation at room temperature.

In the next section, we shall try to discuss what happens when polymer (gelatin in our case) is added with water. In this case, the polymer which has both +ve and -vely charged site in its backbone. Will polymer stretch -OH bonds and induce some defects in tetrahedral structure? What will happen when anionic surfactant is added to this system? What role will be played by micelle in this complex scenario?

6.3 Introduction to water/gelatin system

Water is the most significant constituent of living cells and its unique physical and chemical properties are being studied most extensively over the last several decades. The unique feature of solvation of water has been recognized and it is one of the most studied liquids. The hydrogen bonds (H-bonds) which play the most crucial role in many interactions and conformational changes are the focus of attention of many scientists. Water, which is interconnected by weak H-bonds forms tetrahedral network. In liquid state, two models of water structures have been commonly accepted (i) where water molecules are connected to neighboring molecules and form a long non-terminating lin­ear chain and other (ii) where water molecules through intermolecular hydrogen bonding form tetrahedral networks [1]. Both these models attributed the water structure to the bending and stretching of intermolecular H-bonds. The profile of -OH stretching mapped by Raman spectroscopy is known to be useful in providing indepth understanding of the microscopic thermodynamical environment of macromolecular sols, gels and networks. The properties of water soluble polymers are largely dependent on the water-polymer interactions. Similarly, the visco-elastic characteristics of polymeric networks and gels depend on the physico-chemical properties of the solvent medium. The polymer-solvent interaction decides the structure of gels and networks in a given thermodynamic en­vironment. This interaction provides a distinction between the bulk. interstitial and hydration water. The water manifests itself in deciding the conformations and macro-

128

scopic properties of polymers in solutions. The property of polymer in aqueous solution shows different trend due to the interaction with water.

Recently, much attention has been paid to probe the effect of water on polymers like Poly­acrylamide, Polyethylene glycol, Polyacrylic acid, various native proteins and H20-D:20, H20 2-H20 systems by IR [2-3], NMR [4] and other techniques. But as molecular vibra­tions have smaller relaxation times than the rotational arrangements of \Vater molecules, hence Raman spectroscopy is the most versatile method to study the structure of water in liquid phase.

This -OH stretching of water has been studied by Giguere et al. and Hare et al. [5-7] tnost recently but the stretching of -OH oscillators in H20-D20, H20rH20 systems were studied long time before by Green et al. [8-10]. Although many reports are avail­able on structure breaking of water by introducing electrolytes like KCl, NaCl etc., yet polymer-water system with surfactant as a third component has been sparsely studied notwithstanding the fact that surfactants have all the capability to deform the tetrahe­dral network of water structure. Reports on chemically cross-linked polymer gels with water as solvent and on gelatin [11] are readily available but to our knowledge, physical gels with surfactants that form fragile cross-links still remC~in largely unexplored. Here we have studied the effect of SDS on the gelation dynamics of gelatin. \Vhich is a biopoly­mer using water as a solvent at different temperatures (gel, sol-I and sol-II) tuning the surfactant concentration as an additional parameter.

Gelatin is a polypeptide obtained from denatured collagen comprising of all 20 amino acid residues such as glysine, proline. glutamic acid, aspartic acid, lysine and arginine etc., in different proportions [11]. The lysine and arginine groups comprising:::::: 7.5% of residues and are positively charged and these form complexes with the polar head group of anionic surfactants such as SDS. 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 -58% of the chain to be neutral. In gel state the individual chains are inter-locked \vith each other by hydrogen bonds forming triple helices at junction points and in sol state, these helices break up to assume random coil conformations. The gelation concentration ( C*) for the gelatin used. is typically around 2% (wt.jvol.) [12,13]. The polar sit('S of gelatin chains attract the oppositely polarized site of water molecules and these sites on the chain are screened off from the interaction with other -OH oscillators. These water molecules which are attached to the polymer chains are hydration water or associated water and induce a defect in tetrahedral network of water molecules. So the strength of the collective motion gets destroyed due to these defects and decrease as ( 1 - Pc )2

• \\"here PG is the mole fraction of positive and negatively charged gelatin residues.

Gelatin in gel state forms a triple helix netv.·ork that can accommodate huge amount of water in it. The water molecules in these water pools inside the gel feel reduced effects

129

from surrounding gelatin chains because most of the polar sites are screened off but their motions are restricted due to the presence of polymeric network which ultimately pro­duces a deformation in the continuous network structure of water molecules. This water is termed as interstitial water or bound water and other than those which a.re totally out of the influences of gelatin chain are called as bulk water. Any sort of attachment changes the environment of the -01-I oscillators which correspondingly changes the vi­brational frequency, peak center, peak amplitude and the collective feature value of the -OH oscillators making it possible for Raman spectroscopy to observe it as a. shifted Raman band.

6.4 Experimental Procedure:

6.4.1 Materials:

Gelatin was purchased from M/S Loba.-Chemie (Indo-Astra.nal, India.) with sharp molec­ular weight Mw at distribution~ 106 g.mol-1 . Aqueous solution was prepared by soaking the sample nearly for 45 minutes at ......, 55°C to remove the history effects. The solvent was double distilled Milli-Q (Millipore, USA) grade water. To prevent bacterial con­tall!ination 0.1mM Sodium azide was added. Details of sample preparation have been discussed in Maity et al. [14]. SDS used was from SISCO research laboratories Pvt. Ltd. (Bombay, India.) of batch no. TP/419331. The samples were excited with Argonion (J-Watt) laser light source of wavelength 488 nm. The incident light was linearly polar­ized (using a. pola.riser) a.nd focussed to a. spot size of ......, 10 J.lm on the sample. The laser power a.t sample site is ......, 25 m 'W so that sample in the experimental time window does not get bleached. The excited light from the sample was collected by a.n elliptic mirror a.nd then passed through an analyzer whose optic axis was set parallel or perpendicular to the optic axis of the polariser depending on the requirement of the scattering geom­etry.1 The light was then passed through a. SPEX-1877E triplemate spectrophotometer. A CCD camera located at the exit slit of the spectrophotometer measured the intensity of the emitted light at different wavenumbers. The CCD camera was connected to a PC for display a.nd data was stored through a previously loaded software DM3000R. The parallel and perpendicular intensity components of the luminescence for each samples were scanned separately with time duration of 1 minutes. The gelation temperature of 4% (wt.jvol.) gelatin solution has been reported to be Tgel......, 30°C [12].

130

2500 3000

2500 3000

-1 ro (em )

-1 ro (em )

3500 4000

3500 4000

Figure 6.4: (TOP) Iu(w) and h(w), Raman intensity profile of -OH stretching in gelatin gel state (2SOC ). (Bottom)Iu(w) and h(w)in gelatin sol state (4rPC ). Notice that the h(w) component at all temperatures is highly polarised.

6.4.2 Methods:

We studied the parallel (Ill) and perpendicular (J.L) components of intensity of the scattered light by rotating the analyzer infront of CCD camera. The collective intensity Ic was extracted from

(6.8) .

where p is degree of depolarization. Each measurement of l11 and I.l was taken at least three times and all these spectra were seen to be reproducible. The stored spectra were decomposed into individua.l peaks by using a fitting algorithm which applies a.

. linear combination of Gaussian and Lorentzian functions to the composite spectra. The detailed procedure is described elsewhere [9]. We normalized the collective band and

131

retrieve the C value which reflects the collective nature of the -OH oscillators as follows

C = 1 lc(w)dw/ 1 Iu(w)dw (6.9)

where w is the wave number.

6.5 Results and Discussions:

The assignment of Raman bands at different frequency regions is well defined. The high frequency band around 3450 cm-1 , w1 , is due to the symmetric stretching of -OH oscilla­tors and the low frequency band is the first overtone of the bending mode (2w2 ) while the we~k shoulder around 3585 cm- 1 is asymmetric stretching of -OH oscillators. In a wa­ter molecule oxygen atom has two pairs of electrons which make the oxygen atom more electronegative while the two hydrogen atoms are electropositive. So these free electron pairs can make a weak coupling with electropositive site. It may be intramolecular i.e., water-water or intermolecular i.e., water with electropositive site of a foreign atom. The degree:of coupling depends strongly on the distance between the two atoms and also the electr~positivity of the other atom. As the temperature increases, the translational a.nd rotational motion of the atom increases its amplitude which ultimately decreases the strength of the H-bonds and the ordering gets decreased. Introducing gelatin which has +ve a.nd -ve sites in its backbone, it can strongly couple with -OH oscillators of water and make it more ordered or structured by reducing the degree of freedom of -OH oscil­lators. Thus gelatin can inhibit the ordering of -OH oscillators in the solution. Again by introducing an anionic surfactant like SDS which has -ve polar head one can screen off those polar sites of gelatin and observe its impact on ordering of -OH oscillators below,

· at and above CMC of SDS. In next three sections, these are discussed for three diffent physical situations relevant to our studies; ,.y pure gelatin (ii) gelatin with SDS below CMC (iii) gelatin with SDS above CMC.

6.5.1 Pure gelatin:

A set of typical spectra taken below and above Tgel are shown in Fig. 4. Both these spectra were normalized to same baseline. It is seen from the figure that forT > Tgel

(i.e., 40°C) and T < T gel (i.e., 25°C) the spectra has two distinct peaks centered around 3250 cm-1 and 3450 cm-1 with a shoulder centred around 3650-1 • These values agreed well with previously reported features [15]. The shoulder at 3585.5cm-1 in gel state was observed to be shifted towards lower value of 3563cm-1 in sol-I state. The lower frequency peak (i.e., around 32.50 cm- 1 ) which is due to the in-phase-collective motion

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2500

-- 25°C 40°C

v 60°C

3000 ·1 c.o (em )

3500 4000

Figure 6.5: J11(w) Raman spectra of gelatin in gel (2/fJC) and sol states (4fPC and 6(/JC ). Here the peak amplitude at 6(/JC is almost equal to the same at 2/fJC.

of '-OH oscillators was observed to be highly polarized compared to the peak centered around 3450 cm- 1

. The peak amplitude of I1.(w) spectra at 25°C was observed as 13% stronger than that at 40°C, implying that on increasing the temperature (gel melting) the attachment of -OH oscillators with gelatin chains got destroyed due to temperature mediated weakening of H-bonds which showed decrease in the peak amplitude.

In Fig. 5 we have shown all l11(w) spectra taken at three different temperatures in pure gelatin. The peak amplitude of symmetric stretching bands at 34.50cm-1 first decreases from gel to sol-I state and then increases in sol-11 state. It is interesting to observe that the band at 3250 cm- 1 is decreasing from gel to sol-I state at 40°C but increasing back to a higher value at sol-II state i.e .. 60°C. Figure 5, where we plotted Ic(w) versus w also proved the same what we observed from Fig. 5. If we compare the C values given in Table-!, it is observed that from gel state at 25°C to sol state at 40°C it decreases by 14% and from 40°C to 60°C it increases by 13% which again substantiates the features shown by Figs. 4 and .5. It could be interpreted that when the gel melts to sol state, triple helix structures of gelatin get broken exposing more number of charged sites for -OH oscillators which induce deformations in the tetrahedra.! network structure of water through attractions and the interstitial water inside the gelatin network in gel state is decreased by volume due to the fact that gel network turns out to be a pseudonetwork comprised of transient entanglement of chains which will eventually decrease the C value.

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2500 3000 ·I

ro (em )

3500 4000

Figure 6.6: lc(w) of gelatin w. gel (2SJC) and sol states (4fPC and 6rfJC) is plotted against frequency shift.

At 60°C, the scenario changes and due to the thermal vibration of the gelatin chains the probability of constituting the pseudo-network decreases. Ultimately it releases interstitial water, trapped in it which adds to the bulk water and consequently some -OH oscillators get released from pseudo-network, thus contributing to the increase in C value by f"V 13%.

In Fig. 7 temperature dependence of peak center of collective bands have been plotted as a function of SDS concentration. Two important observations can be made from here. In the gel state (T < T 9 e1; 2.5°C), the peak centre for pure gelatin observed at f"V 3286 cm-1

gradually reduces as SDS concentration was raised systematically to a concentration of 15mM. Similarly transformation of gel to sol- I and sol- II states was observed to produce decrease in the peak centre values. This shows that with increasing the temperature the stretching frequency of -OH oscillators lies outside the span of vibron density of states and decoupling of -OH oscillators takes place which shifts peak centre towards lower value. In Fig. 8 and 9 we have plotted peak amplitude and full width at half maxima (FWHM) uersus temperature as a function of surfactant concentrations. The peak amplitude and FWHM both were decreased from gel to sol state. The peak amplitude which is proportional to the nearest neighbors density is expected to decrease from gel to sol state as the gelatin chains induce more decoupling of -OH oscillators from the tetrahedral network but at 60°C due to the thermal agitations it releases some

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3300

3285 -,... I

E 0

3270 .........

~ -c: Q)

~<: 0 3255 ~ ca Q) a.

3240

3225 20 30 40 50 60 70

Temperature (°C)

Figure 6. 7: Peak centre versus tempemture is plotted for gelatin at different molarity of SDS, (o) OmM, (D) 0.5mM, (*) lmM, (•) 5mM and(¢) 15mM. For the sake of clarity of the plot the error bar 1:s shown for only one data point.

of the -OH oscillators from the charged and interstitial sites which again reconstruct the networks to some extent. Note that in Fig. 9 the bandwidth of collective band is decreasing with temperature which is a measure of the disorder in the network.

6.5.2 Gelatin with below SDS and at CMC:

The CMC of SDS is known to be "' 1mM [16]. Let us look at the system below CMC (0.5mM) and at CMC. In these systems, SDS displaces the -OH oscillators from the nearby regions to the interior regions of gelatin chains. So by increasing the SDS con­centration the peak amplitude of the collective band should increase which is clearly seen in Fig. 8 in gel states. When we heated the system from gel to sol-I state the amount of interstitial water and peak amplitude decreased. From sol-I to sol-II state due to the thermal agitation it again increased, confirming network structure making phenomenon as was observed in the case of pure gelatin. If we compare the system with the case of pure gelatin, it is observed that due to the SDS molecules more numbers of -OH oscillators are freed from hydration layer which increases the peak amplitude. But. we notice an anomalous behavior at CMC where spherical micelle of SDS molecules start forming. At this concentration the amplitude remains constant within our experimental

13.5

20 30 40 50 60 70

Temperature tc)

Figure 6.8: Peak amplitude of in-phase-collective band at 3250 cm- 1 of gelatin with (o) OmM; (D) 0.5mM; (*) lmM; (•} lOmM and (o) 15mM SDS is plotted with temperature. Notice the temperature independencF of peak amplitude at SDS concentration equal to its CMC value.

error at all temperatures. This behavior has not been reported earlier. Hence, providing a comparison at this stage is impossible yet a plausible explanation can be attributed to this. At CMC, in gel state, the charged sites on gelatin backbone are completely occupied by SDS molecules and the extra SDS molecules that are in solution try to form spherical micelles around the gelatin chains or in the intermediate space. When we allow the system to transform from gel to sol states though the extra charged sites are exposed to the -OH oscillators by denaturation of gelatin yet they are nullified by the SDS molecules which were engaged in micelle formation earlier in gel state. So the peak amplitude should not increase much at CMC. The peak center (Fig. 7) changes only by 7-8 cm- 1 which is within experimental error. When we look at Fig. 9~ the bandwidth of collective band decreases when SDS concentration is increased and it reaches the lowest ever value at CMCS at 60°C. The decrease of bandwidth means the more ordering and coupling of -OH oscillators in network structure at a particular temperature. So if we take a closer view it may be inferred that polymer coupled with surfactant can interact strongly on the network structure of water while increasing the temperature. II we look at the C value (Table-I) then it is observed to increase to at least :3--l% over the same of pure gelatin. Tsukicla r/ al. [1 1 J described the same type of behavior with only gelatin and got the same kind of result that the network construction were made by increasing

136

600

..:::- 400 I

E 0 -~ ::I: ~ u.. 200

0 20 30 40 50 60 70

Temperature (°C)

Figure 6.9: Full width at half maxima (FWHM) for in-phase-collective band at 3250 cm-1

of -OH oscillators in gelatin m·e plotted with temperature for different concentrations of SDS. (o) OmM, (D) 0.5 mM, (*) 1 mM and (•) 5 mM.

'

the temperature of the system. But their interpretation of formation of the clathrate cage around the gelatin chain could not be established though the first interpretation could be made applicable to sol state.

Table-I

C values of the system at different SDS concentrations (Maximum error was ± 2%)

Cone. of SDS Gel state Sol state (mM) 25°C 40°C 60°C 0 0.323 0.276 0.310 0 .. 5 0.296 0.281 0.313 1 0.312 0.310 0.313 :3 0.303 0.351 0.301 10 0.292 0.359 0.295 1.5 0.281 0.372 0.292

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(i) (ii)

Figure 6.10: Two possible "necklace-bead" structures, (i) gelatin wraps around surfactant molecules and (ii) surfaclants aggregates around gelatin chain and form micelles {see refs. 16,17}.

6.5.3 Above CMC of SDS:

From Fig. 8, it is observed that for SDS concentrations above 1 mM, the peak amplitude increases from gel to sol state while it decreases for concentrations below CMC. In the gel state at CMC, the gelatin chains are saturated with SDS molecules and almost all the positive polar sites of gelatin chains are neutralized by the opposite polar sites of the SDS molecules and the neutral sites which comprises"' 58% of the total chain length acts as hydrophobic core of micelle. No extra polar sites are exposed upon heating because the system is full of SDS molecules. The extra positive sites coming due to heating of gelatin chains are also neutralized by SDS molecules present in the system which are not sufficient in the case of systems at and below CMC. This ultimately increases the volume of interstitial water in the network pool. At the neutral position micelle starts growing according to the necklace-bead model [16,17] (see Fig. 10) and the polar site of these micelles are exposed to the interior of the network pool and repulse each other which increases the network size ("' 75 A) by rv 5-7% and not only that it displacel'l the boundary of interstitial water into more deeper side of the pool and the thickness and number of hydrating water molecules that are intact with gelatin chains decreases thus increasing the coordination number of coupled -OH oscillators. Due to this reason the peak amplitude increases. When the temperature increases further due to the molecular agitation some decoupling occurs and the peak amplitude decreases by "" 16% of the value of the same at 40°C. In gel state just beyond CMC a very complex phenomenon was observed. The peak amplitudes are abruptly decreased by -20% at ......., 10 mM and then it suddenly increased. In gel state !!;elatin chains are inter-locked with each other and make triple helices. Beyond CMC the two mechanisms may occur: firstly due to the micelle interaction network size may increase as in our case it was by "' 5-7% and secondly the a.c;;sociation of polar head groups of the micelles with the -OH oscillators

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may occur. Here according to our proposition the second cause d01ninates the scenario because in gel state micelle-micelle interaction is not sufficient to increase coordination number of water molecules by getting more interstitial water into its increased area. As we keep on increasing the concentration of the SDS, th.e population of SDS molecules increase and the polar head group of these get attached with more and more of -OH oscillators which ultimately decouple some of the oscillators from network.

139

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