Shape of AOT Reverse Micelles: The Mesoscopic Assembly Is More Than the Sum of the Parts

AOT reverse micelles are a common and convenient model
system for
studying the effects of nanoconfinement on aqueous solutions. The
reverse micelle shape is important to understanding how the constituent
components come together to form the coherent whole and the unique
properties observed there. The shape of reverse micelles impacts the
amount of interface present and the distance of the solute from the
interface and is therefore vital to understanding interfacial properties
and the behavior of solutes in the polar core. In this work, we use
previously introduced measures of shape, the coordinate-pair eccentricity
(CPE) and convexity, and apply them to a series of simulations of
AOT reverse micelles. We simulate the most commonly used force field
for AOT reverse micelles, the CHARMM force field, but we also adapt
the OPLS force field for use with AOT, the first work to do so, in
addition to using both 3- and 4-site water models. Altogether, these
simulations are designed to examine the impact of the force field
on the shape of the reverse micelles in detail. We also study the
time autocorrelation of shape, the water rotational anisotropy decay,
and how the CPE changes between the water pool and AOT tail groups.
We find that although the force field changes the shape noticeably,
AOT reverse micelles are always amorphous particles. The shape of
the micelles changes on the order of 10 ns. The water rotational dynamics
observed match the experiment and demonstrate slower dynamics relative
to bulk water, suggesting a two-population model that fits a core/shell
hypothesis. Taken together, our results indicate that it is likely
not possible to create a perfect force field that can reproduce every
aspect of the AOT reverse micelle accurately. However, the magnitude
of the differences between simulations appears relatively small, suggesting
that any reasonably derived force field should provide an acceptable
model for most work on AOT reverse micelles.