Molecular modeling and simulations of AOT-Water reverse micelles in isooctane: Structural and dynamic properties

This paper reports results from four nanosecond constant pressure and temperature simulations of sodium di-2-ethylhexylsulfoccinate (AOT) and water reverse micelles (RMs) in an apolar solvent, isooctane. The concentration of our simulated micelles was chosen to fall in a range which in nature corresponds to the L-2 phase of the ternary system. To our knowledge, this is the first study to develop a full molecular model for AOT micelles in an apolar solvent. We address here the problems of the shape of the RM and of its hydrophilic inner core. For the AOT-water system, we obtain nonspherical aggregates of elliptical shape with ratios between major axis, a, and minor axis, c, between 1.24 and 1.41. The hydrophilic inner core is also ellipsoidal with larger a/c ratios. Although experiments indicate that the L2 of the AOT-water-oil system is likely to be polydisperse, we can only simulate monodisperse RMs. Nonetheless, our simulations are capable of reproducing well the dimensions of the water pool and their dependence on W-0, as determined in some small-angle neutron and X-ray scattering experiments. Stimulated by recent experiments showing anomalous behavior of the confined water for AOT-water RMs, we have also investigated the static and dynamic properties of the RM's water inner core. From smaller micelles to larger, we find that the properties of confined water tend to near those of bulk water. In particular, we find that the solvation of the counterions is more effective in larger micelles and that diffusion of water is retarded with respect to bulk more in smaller RMs than in larger.