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 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 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 coordinate-pair eccentricity 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 experiment and demonstrate slower dynamics
relative to bulk water, and suggest 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.