Thermodynamics of solutions of interacting aggregates by methods similar to surface thermodynamics. Part 3.—Solutions of ionic surfactants in the absence and presence of electrolyte with a common counterion

Abstract

Methods developed previously are applied to solutions of an anionic surfactant plus electrolyte with a common counterion. Expressions which allow for solution non-ideality are obtained for the effects of temperature, pressure and electrolyte concentration on the c.m.c. These have almost the same form as those obtained from the mass-action model but, in the present case, the “micellar degree of dissociation” is clearly defined without making any distinction between bound and free counterions. It is shown that the concentration of nonionic surfactant ions may decrease with increasing surfactant concentration above the c.m.c. in the absence of added salt; that changes in the micelle aggregation number N are related to micelle poly-dispersity through expressions resembling those for non-interacting uncharged micelles, and that N can be expected to increase on addition of salt. It is argued that micellar weights obtained from light scattering are probably fairly reliable for concentrations well above the c.m.c. Inter-relationships between three possible expressions for the free energy of micellisation of ionic surfactants and the corresponding quantity for nonionics are explored. The fact that changes in the free energy of micellisation per CH₂ group for ionic surfactants obtained from the mass action model nearly equal those for nonionics is explained.