OSMOTIC-PRESSURE AND INTERPARTICLE INTERACTIONS IN IONIC MICELLAR SURFACTANT SOLUTIONS

Results are presented for the osmotic pressure of concentrated aqueous micellar surfactant solutions. Using a pressure-nulled membrane osmom eter, we measure the osmotic pressure of solutions of the cationic sur factant, cetylpyridinium chloride (CPC), and the anionic surfactant, s odium dodecyl sulfate (SDS), in 0.01 M sodium chloride. Nanofiltration membranes serve as the semipermeable barrier and permit measurement o f large osmotic pressures over volume fractions that span the micellar region ranging from just above the critical micelle concentration (cm c) up to volume fractions of 0.18. Large osmotic pressures, up to 323 and 250 kPa for SDS and CPC, respectively, are interpreted as evidence of strong intermicellar interactions. To quantify these large osmotic pressures, we develop a self-consistent activity-coefficient model th at includes explicitly the surfactant monomer, micellar aggregates, an d electrolyte molecules. Excluded-volume effects are taken into accoun t using the Boublik-Mansoori equation of state, and intermicellar elec trostatic interactions are modeled using the mean spherical approximat ion (MSA). We combine the activity-coefficient model with an ideal mas s-action model developed previously for the micelle equilibrium consta nts at infinitely dilute aggregate concentrations in the vicinity of t he cmc.(1) The resulting nonideal thermodynamic model is used to descr ibe the equilibrium between the micellar aggregates and the surfactant monomer at elevated concentrations. The Donnan membrane effect, which leads to the redistribution of background electrolyte, is accounted f or within the thermodynamic framework of the model. Successful compari son is made between the proposed self-consistent model and the new exp erimental osmotic pressure data.