Aqueous solvation of amphiphilic solutes exhibits a rich and complex behaviour that is only partially understood. Recently, various femtosecond resolved spectroscopy and molecular simulation studies probed the dynamics of water molecules solvating small amphiphilic species. These studies provided strong evidence of a slowdown of the hydrating water molecules but the mechanism underlying this change in the local dynamics is still under debate. We addressed this issue with a high-level computational study of aqueous tetramethylurea solutions using state-of-the-art force field and ab initio molecular dynamics methods, considering different concentrations and focusing on the relation between dynamical and structural properties. Our simulations revealed a concentration dependence on the slowdown effect that was maximal for both the translational and reorientational diffusion at a solute/water number-ratio of 0.1. Furthermore, we show that the water molecules hydrogen bonded to the hydrophilic part of the solute have a pronounced subdiffusive behavior, surprisingly without showing an enhancement of these hydrogen bonds. We expect our findings to help elucidate two major points: clarify the role that molecular crowding plays in the dynamics of water, which is important for understanding the hydration layer of macromolecules and alcohols in aqueous solution; and to provide a pictorial description of the dynamics of inhomogeneous aqueous systems on the atomistic level. 

Multiscale study of the dynamics of water molecules around small amphiphilics

246th National Meeting (2013)

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