Water Dynamics at Rough Interfaces

TL;DR

This study combines molecular dynamics simulations and NMR experiments to explore water dynamics at rough, low-mobility interfaces, revealing slowed relaxation, a transition in temperature dependence, and a universal beta process affecting biological and nanoporous systems.

Contribution

It provides new insights into water relaxation mechanisms at rough interfaces, highlighting the transition from bulk-like to interface-dominated dynamics and the role of the beta process.

Findings

Water relaxation slows near rough interfaces even without attractive interactions.

A crossover from Vogel to Arrhenius behavior occurs at interfaces.

A universal beta process with E_a=0.5 eV dominates at low temperatures.

Abstract

We use molecular dynamics computer simulations and nuclear magnetic resonance experiments to investigate the dynamics of water at interfaces of molecular roughness and low mobility. We find that, when approaching such interfaces, the structural relaxation of water, i.e., the $\alpha$ process, slows down even when specific attractive interactions are absent. This prominent effect is accompanied by a smooth transition from Vogel to Arrhenius temperature dependence and by a growing importance of jump events. Consistently, at protein surfaces, deviations from Arrhenius behavior are weak when free water does not exist. Furthermore, in nanoporous silica, a dynamic crossover of liquid water occurs when a fraction of solid water forms near 225 K and, hence, the liquid dynamics changes from bulk-like to interface-dominated. At sufficiently low temperatures, water exhibits a quasi-universal $\beta$ process, which is characterized by an activation energy of $E_a\!=\!0.5$ eV and involves anisotropic reorientation about large angles. As a consequence of its large amplitude, the faster $\beta$ process destroys essentially all orientational correlation, rendering observation of a possible slower $\alpha$ process difficult in standard experiments. Nevertheless, we find indications for the existence of structural relaxation down to a glass transition of interfacial water near 185 K. Hydrated proteins show a highly restricted backbone motion with an amplitude, which decreases upon cooling and vanishes at comparable temperatures, providing evidence for a high relevance of water rearrangements in the hydration shell for secondary protein relaxations.