Molecular dynamics simulation of aerosol-OT reverse micelles.

Molecular dynamics simulations are performed for the reverse micelles (RMs) formed by the surfactant Aerosol-OT (AOT, sodium bis(2-ethylhexyl)sulfosuccinate) in isooctane. The appropriate simulation methodology is identified and applied to the study of the effect of RM size, as quantified by w0 = [H2O]/[AOT], on the structure of the reverse micelle. The radial and intrinsic density profiles, pair densities and pair orientations in the first solvation shell, and water-water hydrogen bonding profiles were constructed. On the basis of these various structural characteristics, we find that the organization of sodium ions, sulfonate headgroup, and water oxygen atoms at the surfactant interface is consistent with a pseudolattice structure for w0 = 2. An increase in the RM size leads to the disruption of this lattice, with more sodium ions dissociating from the sulfonate headgroup and an increase in the aqueous solvation of these two species. The water molecules exist primarily in the interior of the RM and exhibit bulklike properties only for w0 approximately 7.5. Some water molecules and sodium ions exist in the intersulfonate headgroup region and interact with the AOT carbonyl group.