THERMODYNAMICS OF MIXED MICELLE FORMATION - SOLUTIONS OF A CATIONIC SURFACTANT, A SULFOBETAINE, AND ELECTROLYTE

Thermodynamic arguments are developed which lead to a micellar-Gibbs-Duhem equation for ionic surfactants. For mixed micelles of ionic and nonionic surfactants this equation can be tested by measuring the ionic surfactant monomer concentration as a function of micelle composition and supporting electrolyte using EMF methods. For the system C12TAB, C-14 sulphobetaine in 0.2 M NaBr, the mixed micelles conform within experimental error to regular solution theory. Studies at a series of electrolyte concentrations show that micellar degrees of dissociation obtained from counterion activities agree with estimates found from the behaviour of the monomer concentration. The dependence of the micellar degree of dissociation alpha on micelle composition is well described by the simple expression (1+n(s))/alpha = 1 + n(s)/alpha(o), where n(s) is the ratio of the ionic to nonionic surfactant and alpha(o) is the degree of dissociation of the pure surfactant.