Effect of salt and alkyl chain length on the structure of bile salt–cationic mixed surfactant system

The effects of NaCl (0–1.0 M) and alkyl chain length on the micellar size and structures of alkyltrimethylammonium bromide (C n TABr, where n = 12, 14, 16 and 18) in aqueous mixtures with and without trihydroxy bile salt, sodium cholate (NaC) have been investigated by surface tension, viscosity, time-resolved fluorescence quenching and dynamic light scattering measurements. The interaction parameter, β, obtained using Rubingh's theory showed strong interaction between C n TABr and NaC. In the presence of NaCl, the viscosity of C 12 TABr and C 14 TABr remains unaltered with increasing concentration of NaC, while for C 16 TABr and C 18 TABr, a micellar growth occurs at lower NaC concentration (5 mM) resulting a peak in the viscosity. Further addition of NaC gives smaller micelles. The position of the peak in viscosity measurements is independent of temperature (30–50 °C) and NaCl concentration, but is more pronounced in high NaCl concentration and at low temperature. These changes in viscosities are due to changes in the microstructure of the mixed aggregates. The results are well supported by dynamic light scattering and time-resolved fluorescence quenching measurements.