Micelles as a Confined Liquid: Standard and Two-Dimensional Kinetics of Solute Rotation in Surfactant Micelles

Abstract

The dynamics inside a surfactant micelle show clear confinement effects: solute rotation slows and develops a nonexponential relaxation. Multiple rate constants─rate dispersion─must be involved. These effects have been widely attributed to solutes confined to an outer layer of strongly ordered hydrocarbon chains, a layer similar to a Langmuir-Blodgett film. In this picture, confinement effects result from a surface phenomenon enhanced by the high surface area of a nanometer-sized particle. This paper presents results that are inconsistent with strong chain ordering. Varying the solute shows that most of the confinement-induced slowing is at the center of the micelle, not near its surface. Two-dimensional (two-relaxation-period) kinetics show that rate dispersion is mostly due to rate heterogeneity among different solute molecules. A homogeneous component of the stretched dynamics is also identified and is attributed to the coupling of the solute rotation with conformational relaxation of the hydrocarbon chains. In a picture consistent with these results, the micelle is unstructured, and the solutes are more widely distributed than in the conventional model. Confinement effects are not due to surface effects; they are due to the clash in the lengths of the hydrocarbon chain and the particle diameter. This study is enabled by a nonparametric, “data-first” approach that enhances the interpretation of kinetics, especially two-dimensional kinetics.