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 ρ = 1.0 g cm−3. We consider two infinite hydrophilic (β-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 ∼0.5 nm, while the corresponding width for translational dynamics is ∼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-time tails in the perpendicular component of the dipole moment autocorrelation function are indicative of anisotropic behavior in the rotational relaxation.