(312b) Multicomponent Diffusion of Interacting, Nonionic Micelles with Hydrophobic Solutes

Aqueous solutions of micelles are important in many applications because they provide a means to solubilize hydrophobic solutes in water. Here, we examine diffusion in ternary, aqueous solutions of the nonionic surfactant decaethylene glycol monododecyl ether (C12E10) and a hydrophobic solute, either decane or limonene. In solution, the surfactant molecules self-assemble to form micelles swollen by hydrophobic solutes, with essentially no free hydrophobic solute or surfactant in the surrounding solvent. The diffusive behavior of this system is very interesting in that surfactant-solute interactions are strong, and result in a highly non-diagonal diffusivity matrix [D], which depends in part on how strongly micelles grow with an increasing amount of solubilizate along the diffusion pathway.

Ternary diffusion coefficient matrices [D] and morphological parameters, such as the micelle aggregation number, hydrodynamic radius, and hydration index, were measured using the Taylor dispersion method and static and dynamic light scattering techniques, respectively. The matrix [D], for both decane and limonene solutes, was found to be highly non-diagonal, and concentration dependent, over a broad domain of solute to surfactant molar ratios, and micelle volume fractions ranging from dilute to close-packed. Measurements for the average micelle radius and aggregation number indicate a weak dependence on the micelle volume fraction but a strong linear increase with solute-to-surfactant molar ratio, at a rate dependent on the hydrophobic solute type. A recently developed theoretical model, based on Batchelor’s theory for gradient diffusion in dilute, polydisperse mixtures of interacting spheres, was simplified by neglecting local polydispersity, and effectively used to predict [D] with no adjustable parameters. Even though the model originates from dilute theory, the theoretical results were in surprisingly good agreement with experimental data for concentrated mixtures, with volume fractions up to φ ≈ 0.47, indicating the effects of multiparticle hydrodynamic and thermodynamic interactions cancel, resulting in experimental and theoretical predictions that are nearly linear over the entire range of concentration. In addition, the theory predicts eigenvalues D- and D+ that correspond to long-time self and gradient diffusion coefficients, respectively, for monodisperse spheres, in reasonable agreement with experimental data.