Water's interfacial hydrogen bonding structure reveals the effective strength of surface-water interactions

TL;DR

This study investigates how surface-water interactions influence the molecular structure of water at interfaces, revealing a threshold effect where water structure shifts from homogeneous to heterogeneous, and introduces a new measure of hydrophilicity called intrinsic hydropathy.

Contribution

The paper combines molecular dynamics simulations with a mean field model to quantify the energetic component of surface-water interactions and introduces intrinsic hydropathy as a novel measure of hydrophilicity.

Findings

Water structure is resistant to change below a certain interaction threshold.

Above the threshold, water interfacial structure becomes heterogeneous.

The equilibrium distribution of molecular orientations quantifies surface-water interaction energy.

Abstract

The interactions of a hydrophilic surface with water can significantly influence the characteristics of the liquid water interface. In this manuscript, we explore this influence by studying the molecular structure of liquid water at a disordered surface with tunable surface-water interactions. We combine all-atom molecular dynamics simulations with a mean field model of interfacial hydrogen bonding to analyze the effect of surface-water interactions on the structural and energetic properties of the liquid water interface. We find that the molecular structure of water at a weakly interacting (i.e., hydrophobic) surface is resistant to change unless the strength of surface-water interactions are above a certain threshold. We find that below this threshold water's interfacial structure is homogeneous and insensitive to the details of the disordered surface, however, above this threshold water's interfacial structure is heterogeneous. Despite this heterogeneity, we demonstrate that the equilibrium distribution of molecular orientations can be used to quantify the energetic component of the surface-water interactions that contribute specifically to modifying the interfacial hydrogen bonding network. We identify this specific energetic component as a new measure of hydrophilicity, which we refer to as the intrinsic hydropathy.