Regular solution theory regarding sodium cholate and hexadecyltrimethylammonium bromide or dodecyltrimethylammonium bromide binary mixed micelles

Abstract

The formation of binary mixed micelles of sodium cholate (SC) – hexadecyltrimethylammonium bromide (HDTAB) and sodium cholate (SC) – dodecyltrimethylammonium bromide (DTAB) is investigated in an aqueous NaCl solution (0.300 mol kg−1). According to the 2D ROESY spectrum, SC steroid skeleton is in contact via its convex surface with the hydrophobic core of the micelle. Existing cross peaks indicate the spatial proximity of the SC carboxylate group and HDTAB or DTAB cationic heads, i.e., there are attractive electrostatic interactions. In the HDTAB-SC 2D ROESY spectrum there are cross peaks that may belong to HDTAB surfactants with hydrocarbon chains that have globular conformations. In both mixed micelles, there are synergistic interactions between structurally different surfactants. Dependence of the molar excess Gibbs free energy (gE) on the mole fraction of the surfactant i (xi) from the binary micellar pseudophase in both HDTAB-SC and DTAB-SC systems is described by the symmetric first-order Margules function. From theoretical considerations, it follows that the first-order Margules function that describes the dependence gE=f(xi) can be obtained if the number of microstates in binary mixed micellar pseudophases approaches the number corresponding to the number of microstates of random mixing of structurally different surfactants (HDTAB-SC), or there exists a compensation of molar excess configurational enthropy (a departure from random mixing of different particles at the atomic level) with molar excess conformational enthropy (DTAB-SC).