Evolution from Surface-Influenced to Bulk-Like Dynamics in Nanoscopically Confined Water

We use molecular dynamics simulations to study the influence of confinement on the dynamics of a nanoscopic water film at T = 300 K and rho = 1.0 g cm(-3). We consider two infinite hydrophilic (beta-cristobalite) silica surfaces separated by distances between 0.6 and 5.0 nm. The width of the region characterized by surface-dominated slowing down of water rotational dynamics is similar to 0.5 nm, while the corresponding width for translational dynamics is similar to 1.0 nm. The different extent of perturbation undergone by the in-plane dynamic properties is evidence of rotational-translational decoupling. The local in-plane rotational relaxation time and translational diffusion coefficient collapse onto confinement-independent "master" profiles as long as the separation d >= 1.0 nm. Long-tithe tails in the perpendicular component of the dipole moment autocorrelation function are indicative of anisotropic behavior in the rotational relaxation.