Dynamics of water molecules at the surface of an aqueous micelle: Atomistic molecular dynamics simulation study of a complex system

The dynamics of water molecules and ions near an aqueous micellar interface is a subject of intense current interest, as such a system serves as a prototype of more complex biological systems, in addition to being important in its own right. We study the dynamics by using an atomistic molecular dynamics simulation of a micelle of cesium pentadecafluorooctanoate (CsPFO) in water. This micellar system is stable over a range of temperature, allowing us to perform a detailed study of the microscopic dynamics of water at the surface of the micelle, at two different temperatures. The dipolar orientational correlation function of the water molecules and the polar solvation dynamics (SD) of cesium ions and tagged water molecules are calculated. Results show that the reorientational motion of water molecules near the micelle is restricted, and as a result exhibits a slow component which is slower than its bulk value by at least two orders of magnitude, in agreement with dielectric relaxation experiments. In addition, the SD of cesium ions is found to be slowed down significantly, again by more than two orders of magnitude compared to that in the bulk water. Through an analysis of partial solvation time correlation functions, we find that the cesium ions are primarily solvated by the polar headgroups, and their SD is thus intimately coupled to micellar dynamics. Both orientational dynamics and SD show strong temperature dependence