Laser Raman spectroscopic study of water in gelatin-surfactant solutions and gels

A Raman spectroscopic study was carried out on water in gelatin at 4% w/v i n gel (25 degrees C) and sol (40-60 degrees C) states at various concentrat ions (0.5, 1, 5, 10 and 15 mM) of anionic surfactant, sodium dodecyl sulfat e (SDS). The in-phase collective stretching mode vibration of hydrogen-bond ed -OH oscillators, centered around 3250 cm(-1) in a tetrahedral network of water molecules, was observed to be significantly affected by temperature and the presence of SDS. According to our observation this may be due to th e thinning of the hydration water around the gelatin molecules due to stron g thermal agitation. The peak center of the collective bands of water decre ased linearly with SDS concentration in the gel state which implied that wi th the increase in concentration of SDS, the -OH oscillators gradually lost their attachment to gelatin chains and were replaced by SDS molecules. Ult imately this resulted in a thinning of the hydration layer around the gelat in and the oscillation frequency of-OH oscillators moved towards 3250 cm(-1 ) at 1 mM SDS concentration resulting in increased coupling of -OH oscillat ors to form the tetrahedral network at the critical micelle concentration ( cmc) of SDS. The variation in the peak amplitudes and the systematic revers al of their trend about the cmc axis was surprising. At 40 degrees C the am plitude of the peak at 3250 cm(-1) increased drastically due to a possible coil expansion by about 7-8% which accommodated more interstitial water int o the pseudonetwork leading to an increase in the number of nearest neighbo rs and for about 6% increase in the C value. However, at the cmc the peak a mplitude was observed to be independent of temperature. Continuous shifting of the peak center and full width at half-maxima towards lower values was observed with increasing SDS concentrations in the gel state.