A SELF-CONSISTENT MULTICOMPONENT ACTIVITY-COEFFICIENT MODEL FOR IONICMICELLAR SURFACTANT SOLUTIONS

The proposed model incorporates a distribution of micellar sizes self- consistently. The model is based on a mass-action equilibrium approach that includes micelle-micelle interactions as a function of size for a multicomponent surfactant solution consisting of micellar aggregates , monomer, counterions, and added electrolyte. The primary solution no nidealities are accounted for in the multicomponent model with exclude d-volume and electrostatic interactions as a function of aggregate siz e. In addition, the model accounts for the Donnan equilibrium existing between an ionic surfactant solution and the electrolyte solution fro m which it is separated by a semipermeable membrane. Surfactant soluti ons of sodium dodecyl sulfate (SDS) and cetylpyridinium chloride (CPC) in 0.01 M NaCl are studied over the concentration range from the crit ical micelle concentration up to volume fractions of 0.19 (0.87 M) for SDS and 0.16 (0.56 M) for CPC. In comparison to the predictions of an ideal solution multiple-chemical equilibrium constant model, the acti vity-coefficient model predicts increased growth and polydispersity of the aggregates for both CPC and SDS at higher surfactant concentratio ns. Micellar interactions enhance the growth of the micelles due to ex cluded-volume effects that favor growth and electrostatic repulsions t hat oppose it. Micellar aggregates are found to be slightly globular f or both SDS and CPC; however, a clear spherical to globular transition is predicted for the CPC micelles. The multicomponent model reflects experimental osmotic pressure data successfully.