Aqueous reverse micelles, which are surfactant aggregates in nonpolar solve
nts that enclose packets of aqueous solution, have been widely studied expe
rimentally 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 pr
operties. We have modeled the interior of an aqueous reverse micelle as a r
igid spherical cavity, treating only the surfactant headgroups and water at
a molecular level. Interactions between the interior molecules and the cav
ity are represented by a simple continuum potential. The basic parameters o
f the model-micelle size, surface ion density, and water content-are based
on experimental measurements of Aerosol OT reverse micelles but could be ch
osen to match other surfactant systems as well. The surfactant head is mode
led as a pair of atomic ions a large headgroup ion fixed at the cavity surf
ace and a mobile counterion. The SPC/E model is used for water. The simulat
ions indicate that water near the cavity interface is immobilized by the hi
gh 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 in
terface 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 o
f experiments.