Mutual diffusion coefficients and resistance coefficients for aqueous solutions of sodium alkanoate surfactants

Taylor dispersion is used to measure mutual diffusion coefficients (D) for binary aqueous solutions of sodium hexanoate, heptanoate, octanoate, decanoate, and dodecanoate salts at 25 degrees C. The diffusion coefficients of sodium hexanoate and heptanoate drop smoothly as the salt concentration is raised. For each of the longer-chain sodium alkanoates, D drops sharply in the region of the critical micelle concentration. Activity coefficient data are used to calculate resistance coefficients from the diffusion results. For aqueous sodium decanoate, micellar aggregation produces a 4-fold decrease in the friction acting on the diffusing salt. The mutual diffusion coefficient of aqueous sodium octanoate is compared with previously reported intradiffusion coefficients for sodium and octanoate ions.