Structure, Dynamics and Phase Behaviors of Cationic Micellar Solutions:

Abstract

In this work, the combination of Raman spectroscopy and Small Angle Neutron scattering (SANS) have been used for the first time, to understand the aggregational behavior of micelles in micellar solutions under different external conditions like temperature, pressure, surfactant concentration and the electrolyte concentration. This comprises the aggregation to solid phase and the growth of micelles due to electrolytes.The dynamics of the micellar assemblies were analyzed with the help of Raman spectroscopy. The characteristic vibrational bands, their frequency positions, widths, peak height ratios of selected bands are found to be very sensitive to external parameters like temperature, pressure and electrolyte concentration. At ambient conditions, the Raman spectra resembles the spectra of hydrocarbons with broad vibrational bands and almost equal amount of trans and gauche conformations indicating the disorder in the micelles. An increase in the trans to gauche ratio and hence the ordering of the surfactants was achieved by decrease in the temperature of the micellar solutions. An increase of pressure has the similar kind of influence on the spectral behavior. The phase transitions observed with the influence of temperature and pressure are indicated in the discontinuities of the peak frequency, peak width and peak height ratios. Neutron scattering provides the structural information of the micelles. The intermicellar distance as well as their shape were calculated directly from the SANS intensity profile. SANS experimental data were fitted with well known Hayter-Penfold model to extract the structural parameters such as aggregation number, charge and size of the micelles.Aqueous micellar solution of tetradecyltrimethylammonium bromide (TTAB) at ambient conditions shows an increase of aggregation number with increase in surfactant concentration. With the application of temperature and pressure the phase transition from micellar solution to solid phase was evidenced in the decrease in the SANS intensity profile. This phase transition was directly proven by the use of Raman spectroscopy, where, the spectra after the phase transition resemble that of the dry powder TTAB surfactant. At ambient conditions aqueous micellar solutions of various alkyltrimethylammonium bromide surfactants (with alkyl chain length from C12 to C18) show an increase of micellar length and hence, the aggregation number with increase in alkyl chain length. Thermal behavior of surfactant with alkyl chain length C16 and C18 shows an intermediate metastable gel phase on cooling and was clearly demonstrated by Raman spectra and neutron scattering.