(331b) Multicomponent Diffusion in Nonionic Micellar Solutions with Very Hydrophobic Solutes

When a hydrophobic solute, such as a drug or nutrient, diffuses in the presence of micelles or surfactant aggregates, the partitioning of solute into the oily interior of the aggregates strongly affects the rate of diffusion of both the solute and the surfactant. In this work, the effect of decane on the ternary diffusion coefficient matrix and the micelle diffusion coefficient in aqueous solutions of nonionic surfactant (C₁₂E₁₀) has been investigated with Taylor dispersion and dynamic light scattering techniques. Here, surfactant and decane diffuse together exclusively as decane-containing micelles and so one might expect both surfactant and solute to diffuse down their respective gradients with the micelle gradient diffusion coefficient. The Taylor results show that surfactant does indeed diffuse down the surfactant gradient with the micelle gradient diffusion coefficient. However, our results show that decane diffuses down a decane gradient with a diffusivity that is four times less than the micelle gradient diffusion coefficient. Furthermore, strong diffusion coupling, comprising decane diffusion down a surfactant gradient and surfactant diffusion up a decane gradient, was also observed, with cross diffusivities that were on the order of or larger than the main diffusivities. In order to understand the origins of these remarkably strong multicomponent effects, diffusion in these micellar solutions was modeled as gradient diffusion in a polydisperse system of interacting spheres that consist of various numbers of solute and surfactant molecules. This theory allows us to predict the ternary diffusivity matrix from measured values for the surfactant aggregation number, the hydration index and the decane-free infinite dilution micelle diffusivity, acquired using both static and dynamic light scattering techniques.