Molecular Dynamics Simulations of the Interior of Aqueous Reverse Micelles

Abstract

Aqueous reverse micelles, which are surfactant aggregates in nonpolar solvents that enclose packets of aqueous solution, have been widely studied experimentally and theoretically, but much remains unknown about the properties of water in the interior. The few previous molecular dynamics simulations of reverse micelles have not examined how the micelle size affects these properties. We have modeled the interior of an aqueous reverse micelle as a rigid spherical cavity, treating only the surfactant headgroups and water at a molecular level. Interactions between the interior molecules and the cavity are represented by a simple continuum potential. The basic parameters of the model micelle size, surface ion density, and water content are based on experimental measurements of Aerosol OT reverse micelles but could be chosen to match other surfactant systems as well. The surfactant head is modeled as a pair of atomic ions: a large headgroup ion fixed at the cavity surface and a mobile counterion. The SPC/E model is used for water. The simulations indicate that water near the cavity interface is immobilized by the high ion concentration. Three structural regions of water can be identified: water trapped in the ionic layer, water bound to the ionic layer, and water in the bulklike core. The basic properties of bulk water reemerge within a few molecular layers. Both the structure and dynamics of water near the interface vary with micelle size because of the changing surface ion density. The mobility of water in the interfacial layers is greatly restricted for both translational and rotational motions, in agreement with a wide range of experiments.