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

Sucheol Shin, Adam P. Willard|arXiv (Cornell University)|Jan 31, 2018

Spectroscopy and Quantum Chemical Studies43 references59 citations

TL;DR

This paper introduces 'intrinsic hydropathy'—a new measure of surface hydrophilicity derived from water's interfacial hydrogen bonding structure. Using all-atom molecular dynamics simulations and a mean-field model, the authors show that water's interfacial structure remains homogeneous and hydrophobic-like below a threshold surface-water interaction strength, but becomes heterogeneous above it. The key contribution is a quantitative link between molecular orientation distributions and the energetic component of surface-water interactions that specifically perturb the hydrogen bonding network.

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.

Motivation & Objective

• To understand how surface-water interactions influence the molecular structure and hydrogen bonding network of interfacial water.
• To identify the threshold strength of surface-water interactions that triggers a transition from homogeneous to heterogeneous interfacial water structure.
• To develop a quantitative, physically interpretable measure of hydrophilicity—'intrinsic hydropathy'—based on the energetic contribution of surface interactions to interfacial H-bond reorganization.
• To establish a link between water’s interfacial molecular orientation distribution and the effective strength of surface-water interactions.

Proposed method

• All-atom molecular dynamics simulations of liquid water at a disordered, tunable hydrophilicity surface with variable surface partial charges (α scaling).
• Use of the SPC/E water model with fixed surface water molecules in equilibrium configurations to simulate varying surface polarity.
• Application of a mean-field model of interfacial hydrogen bonding to isolate the energetic component of surface-water interactions that specifically affect H-bond network structure.
• Definition of an order parameter, δλphob, based on the equilibrium distribution of water molecular orientations to quantify interfacial structural heterogeneity.
• Comparison of δλphob with excess chemical potential (∆µex) for cavity formation to classify regions as hydrophobic or hydrophilic.
• Fitting of effective surface density profiles using Gaussian functions to represent the first hydration layer, with parameters (as, σs, ρ0) extracted from density profiles.

Experimental results

Research questions

• RQ1 At what threshold strength of surface-water interactions does water’s interfacial hydrogen bonding structure transition from homogeneous to heterogeneous?
• RQ2 How does the equilibrium distribution of water molecular orientations reflect the energetic contribution of surface-water interactions to interfacial H-bond network reorganization?
• RQ3 Can the structural response of interfacial water be used to define a new, physically meaningful measure of surface hydrophilicity independent of surface details?
• RQ4 Why is hydrophobic behavior so prevalent in aqueous solvation despite the presence of hydrophilic surfaces?

Key findings

• Below a critical threshold of surface-water interaction strength, water’s interfacial structure remains homogeneous and insensitive to surface disorder, resembling that at a hydrophobic surface.
• Above this threshold, the interfacial structure becomes spatially heterogeneous, with regions exhibiting both perturbed and weakly perturbed H-bond networks.
• The equilibrium distribution of water molecular orientations at the interface can be used to quantify the specific energetic contribution of surface-water interactions that reorganize the interfacial hydrogen bonding network.
• This specific energetic component is proposed as a new measure of hydrophilicity, termed 'intrinsic hydropathy', which is independent of surface topography and sensitive only to interaction strength.
• Conditional probability analysis shows that δλphob ≈ 0.1 corresponds to hydrophobic-like interfacial structure, while |δλphob| > 0.1 indicates hydrophilic-like structure.
• The Gaussian-fitted surface density profile parameters (as, σs, ρ0) increase with surface polarity α, indicating enhanced solvent adsorption and a more structured first hydration layer as surface hydrophilicity increases.