(3bj) Multicomponent Diffusion of Interacting, Nonionic Micelles with Hydrophobic Solutes

Research Interests: Colloidal and coupled transport phenomena, including but not limited to multicomponent diffusion, thermodiffusion, diffusiophoresis, thermophoresis, electrophoresis, and active matter transport.

Ternary diffusion in solutions of nonionic surfactants and hydrophobic solutes is particularly interesting because strong solute and surfactant interactions persist across all micelle concentrations, from dilute to crowded conditions. In this work, Taylor dispersion and dynamic light scattering techniques were used to measure the ternary diffusion coefficient matrix [D], and eigenvalues D₊ and D_-, in crowded aqueous solutions of decaethylene glycol monododecyl ether (C₁₂E₁₀) with either decane or limonene. The results at low solute to surfactant molar ratios indicate that C₁₂E₁₀ and hydrophobic solute diffused down their respective gradients with diffusivities similar to the fast D₊ and slow D_- eigenvalues, respectively. Furthermore, strong diffusion coupling, comprising solute diffusion down a surfactant gradient and surfactant diffusion up a solute gradient, was also observed, with cross diffusivities that were on the order of or larger than the main diffusivities. Measurements of the average micelle aggregation number, hydration index, and hydrodynamic radius, obtained using both static and dynamic light scattering methods, indicate that solute-containing micelles interacted as hard spheres with an average radius and aggregation number that increased linearly with the molar ratio of solute to surfactant for both hydrophobic solutes. A theoretical model, developed using Batchelor’s theory for gradient diffusion in a polydisperse system of interacting hard spheres, was effectively used to predict [D], D₊, and D_- with no adjustable parameters. Comparison with theory indicates the eigenvalues D_- and D₊ correspond to long–time self and gradient diffusion for monodisperse hard spheres.