When a hydrophobic solute, such as a drug, aroma compound or pollutant, diffuses in the presence of micellar aggregates, the partitioning of solute into the aggregate interior can strongly affect the rate of transport of both the solute and the surfactant, creating complex multicomponent diffusion effects. In many such mixtures, surfactant and solute diffuse together exclusively within the solute-containing micelles, and thus one might expect both compounds to diffuse with the micelle diffusion coefficient. In our research, however, measurements using a Taylor diffusiometer or holographic interferometry indicate strong multicomponent effects in solutions of nonionic surfactant with heptane, decane or limonene. Solute diffusion down its own gradient is much slower than that displayed by the surfactant, and strong solute–surfactant coupling occurs, where diffusion of one component is driven by gradients in the other. As a result, the ternary diffusion 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. We found that this behavior could be quantitatively explained by treating the micelles as a hard-sphere dispersion with properties determined by the local conditions within the gradient. Theory by G.K. Batchelor was used to capture hydrodynamic and thermodynamic interactions between the interacting spheres, to predict the four diffusivity components in [D]. Dynamic and static light scattering provided, independently, key structural information on the micelles needed in the theory, and headspace solid phase microextraction allowed us to determine solute chemical potentials and partition coefficients. The resulting theoretical predictions were in surprisingly good agreement with experimental data under all conditions—even in concentrated mixtures—without adjustable parameters. In addition, the theory predicts eigenvalues D– and D+ that correspond to long-time self and gradient diffusion coefficients, respectively, for monodisperse spheres, which relate to the mixture behavior in dynamic light scattering.

Complex diffusion behavior in micelles carrying hydrophobic solutes

ACS Spring 2022

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