| Abstract
| - Aqueous reverse micelles, which are surfactant aggregates in nonpolar solvents that enclose packets of aqueoussolution, have been widely studied experimentally and theoretically, but much remains unknown about theproperties of water in the interior. The few previous molecular dynamics simulations of reverse micelleshave not examined how the micelle size affects these properties. We have modeled the interior of an aqueousreverse micelle as a rigid spherical cavity, treating only the surfactant headgroups and water at a molecularlevel. Interactions between the interior molecules and the cavity are represented by a simple continuum potential.The basic parameters of the modelmicelle size, surface ion density, and water contentare based onexperimental measurements of Aerosol OT reverse micelles but could be chosen to match other surfactantsystems as well. The surfactant head is modeled as a pair of atomic ions: a large headgroup ion fixed at thecavity surface and a mobile counterion. The SPC/E model is used for water. The simulations indicate thatwater near the cavity interface is immobilized by the high ion concentration. Three structural regions ofwater can be identified: water trapped in the ionic layer, water bound to the ionic layer, and water in thebulklike core. The basic properties of bulk water reemerge within a few molecular layers. Both the structureand 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.
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