An isotropic model for micellar systems: Application to sodium dodecyl sulfate solutions

A theoretical description of ionic micellar solutions, which are considered as a mixture of two fully dissociated electrolytes with a common ion, is p roposed. The solution is modeled within the framework of an improved primit ive model, in which the solution permittivity as well as the charge and siz e of the solutes are allowed to vary with surfactant concentration. The MSA theory is used to estimate the electrostatic interactions, and the Carnaha n-Starling approximation is employed to evaluate the hard-sphere contributi ons to the osmotic and activity coefficients. The experimental osmotic coef ficients of sodium dodecyl sulfate (SDS) solutions are reproduced by using a value of 1.9 + 4C(T) nm for the micellar radius, CT being the total surfa ctant molar concentration. This is the only adjustable parameter in the the ory, and the agreement with published estimations of the size of SDS micell es is excellent.