(19e) Solute–Surfactant Interactions in the Transport and Delivery of Solubilized Hydrophobic Compounds

Self-assembled surfactant aggregates, such as micelles and microemulsions, act as carrier particles that modulate delivery of hydrophobic solutes to a targeted phase. The target might be a vapor phase to which food aromas are released in the retronasal cavity, an interface to which amphiphiles adsorb, or cells taking up the solute. Interestingly, the presence of solute within micellar aggregates profoundly affects the rate of micelle–mediated transport, as well as the mechanism for its localized delivery to a second phase.

In the former case of micelle–mediated transport, quantitative prediction of solute and surfactant fluxes due to micellar transport is determined by four components of a 2×2 diffusivity matrix which is highly non-diagonal and concentration dependent. A Taylor dispersion method was used to experimentally investigate this transport for micelle concentrations ranging from dilute to highly concentrated, over a wide range of solute–to–surfactant molar ratios. The micelles comprised decaethylene glycol monododecyl ether surfactants, and the solutes were either decane or limonene. Given independent scattering measurements on effects of the solute on micelle diameter and aggregation number, our data for the diffusivity matrix could be quantitatively predicted, without adjustable parameters, by using Batchelor’s theory for gradient diffusion of polydisperse hard-sphere suspensions, even with highly concentrated mixtures.

This solute transport within micelle carriers is very efficient under many practical conditions, thereby creating a rate–limiting step for delivery near the target phase. Even for hydrophobic aromas, nutrients, phospholipids, or drugs that are very poorly water–soluble, the controlling delivery step can be shown to require solute release from the micelle and diffusion directly through water. In this mechanism, the solubilization thermodynamics plays a key role, by controlling the local distribution of solute inside and outside the micelle. We have developed headspace solid-phase microextraction approaches to capture this solubilization behavior over the entire range of solute concentrations within micelle solutions. The measured behavior is well described theoretically by regular solution theory for ideal–dilute mixtures for the entire range of compositions, from highly dilute to the solubility limit. This resulting ability to quantitatively measure and predict localized solute concentrations in the boundary region near a target phase is a critical element for optimizing delivery of compounds such as flavors, drugs, or environmental toxins.