Thermodynamic analysis of an asymmetric system: Aqueous sodium dehydrocholate- hexadecyltrimethylammonium bromide mixed micelles

Abstract

Presuming that the thermodynamic behavior of a system is symmetric, as in the Regular Solution Theory (RST or Rubingh's method), may mask relevant characteristics and lead to erroneous conclusions derived from inexact computed properties (micelle composition, intramicellar interaction energy, activity coefficients and excess free energy of mixed micellization). This work addresses a system which seems prima facie strongly non-ideal and asymmetric due to the very different structures of the surfactants: the aqueous solutions of the catanionic system sodium dehydrocholate (NaDHC) and hexadecyltrimethylammonium bromide (HTAB). It has been analyzed with two procedures: the RST and the EOMMM (Equation Oriented Mixed Micellization Modeling). The latter finds the Margules parameters and the micelle compositions that globally minimize the total free energy of micellization. It does not assume symmetry and assures the fulfillment of the Gibbs-Duhem relation.