Diffusion in associating nonelectrolyte mixtures: stepwise aggregation and micelle formation

Abstract

Equations have been developed to predict diffusion coefficients and Onsager coefficients for associating nonelectrolyte solutes in binary or multicomponent solutions with any number of association equilibria. The equations are used to interpret previously reported data for binary diffusion with stepwise association of ethanol and N-methylacetamide in carbon tetrachloride solutions. Diffusion of Triton X-100, a non-ionic micelle-forming surfactant, is also analyzed. In contrast to the gradual decrease in the diffusivity caused by stepwise association, formation of micellar aggregates produces a sharp drop in the diffusivity at the critical micelle concentration. The relation between interdiffusion coefficients and intradiffusion coefficients for associating nonelectrolyte solutes is discussed.