Influence of Surface Polarity and Confinement Length Scale On the Dynamics of a Nanoscopic Water Film

We use Molecular Dynamics simulations to study the influence of surface polarity and confinement on the dynamics of a nanoscopic water film at T = 300 K and ρ = 1.0 g cm^-3. We consider two infinite, atomically detailed silica surfaces reproducing the chemistry of β-cristobalite. By means of a dimensionless parameter, k, we scale the surface Coulombic charges and model systems ranging from hydrophobic apolar (k = 0) to hydrophilic (k = 1.0). We find that both rotational and translational dynamics exhibit a non-monotonic dependence on k characterized by a maximum in the in-plane diffusion coefficient, D_||, at values between 0.6 and 0.8, and a minimum in the rotational relaxation time, τ_R, at k = 0.6. The slow dynamics observed in proximity of the hydrophobic apolar surface are a consequence of β-cristobalite templating an ice-like water layer. The fully hydrophilic surfaces (k = 1.0), on the other hand, result in slow interfacial dynamics due to the presence of dense but disordered water that forms strong hydrogen bonds with surface silanol groups. In order to study the effect of confinement length scale, we consider a water film confined by hydrophilic (k = 1.0) silica surfaces separated by distances between 0.6 and 5.0 nm. We find that 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 ²master² profiles as long as d ³ 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.