Fluctuations of water near extended hydrophobic and hydrophilic surfaces

TL;DR

This study uses molecular dynamics simulations to analyze water density fluctuations near hydrophobic and hydrophilic surfaces, revealing that fluctuations near hydrophobic surfaces resemble those at the water-vapor interface, with increased density depletion.

Contribution

The paper introduces a biased sampling technique for water density fluctuations in molecular dynamics simulations, enabling detailed analysis near different surfaces.

Findings

Density fluctuations near hydrophobic surfaces resemble water-vapor interface behavior.

Probability of density depletion is higher near hydrophobic surfaces than in bulk.

Fluctuations near hydrophilic surfaces are similar to bulk water.

Abstract

We use molecular dynamics simulations of the SPC-E model of liquid water to derive probability distributions for water density fluctuations in probe volumes of different shapes and sizes, both in the bulk as well as near hydrophobic and hydrophilic surfaces. To obtain our results, we introduce a biased sampling of coarse-grained densities, which in turn biases the actual solvent density. The technique is easily combined with molecular dynamics integration algorithms. Our principal result is that the probability for density fluctuations of water near a hydrophobic surface, with or without surface-water attractions, is akin to density fluctuations at the water-vapor interface. Specifically, the probability of density depletion near the surface is significantly larger than that in bulk. In contrast, we find that the statistics of water density fluctuations near a model hydrophilic surface are similar to that in the bulk.