Temperature dependence of water dynamics at an aqueous micellar surface: Atomistic molecular dynamics simulation studies of a complex system

In order to study the temperature dependence of water dynamics at the surface of a self-organized assembly, we perform long atomistic molecular dynamics simulations of a micelle of cesium pentadecafluorooctanoate in water at two different temperatures, 300 and 350 K. Since this micellar system is stable over a range of temperature, a detailed study of the microscopic dynamics of water at the surface of the micelle at both temperatures could be performed. The diffusion and dipolar orientational correlation function of the water molecules and the polar solvation dynamics of cesium ions at the micellar surface are calculated as a function of their location from the micellar surface. Our study reveals a strong temperature dependence. The relaxation of both the time correlation functions are highly nonexponential, and become very slow at 300 K. It is found that while the slowness in the orientational time correlation function originates partly from the formation of bridge hydrogen bonds between the polar head groups (PHG) of the micelle and the water molecules, the solvation dynamics slows down primarily due to the interaction of the positive cesium ions with the negatively charged PHGs. (C) 2002 American Institute of Physics.