Water Organization and Dynamics on Mineral Surfaces Interrogated by Graph Theoretical Analyses of Intermolecular Chemical Networks

Abdullah Ozkanlar,Morgan Kelley,Aurora Clark

doi:10.3390/min4010118

Abstract

Intermolecular chemical networks defined by the hydrogen bonds formed at the α-quartz|water interface have been data-mined using graph theoretical methods so as to identify and quantify structural patterns and dynamic behavior. Using molecular-dynamics simulations data, the hydrogen bond (H-bond) distributions for the water-water and water-silanol H-bond networks have been determined followed by the calculation of the persistence of the H-bond, the dipole-angle oscillations that water makes with the surface silanol groups over time, and the contiguous H-bonded chains formed at the interface. Changes in these properties have been monitored as a function of surface coverage. Using the H-bond distribution between water and the surface silanol groups, the actual number of waters adsorbed to the surface is found to be 0.6 H2O/10 Å2, irrespective of the total concentration of waters within the system. The unbroken H-bond network of interfacial waters extends farther than in the bulk liquid; however, it is more fluxional at low surface coverages (i.e., the H-bond persistence in a monolayer of water is shorter than in the bulk) Concentrations of H2O at previously determined water adsorption sites have also been quantified. This work demonstrates the complementary information that can be obtained through graph theoretical analysis of the intermolecular H-bond networks relative to standard analyses of molecular simulation data.

Highlights

Adsorbed water plays an essential role in the reactivity of many minerals
It is the ability of water to form hydrogen bonds, and the complex network of these hydrogen bonds, that underlies its interaction with hydrophilic mineral surfaces and strongly influences surface diffusion and reactivity through passivation and shielding of other adsorbates to reactive sites
In recent work [4], we have developed a graph theoretical formalism for interrogating the structure and dynamics of intermolecular interactions from molecular simulation data, including that for hydrogen bonds

Summary

Introduction

Adsorbed water plays an essential role in the reactivity of many minerals. Interfacial behavior is strongly influenced by the chemical composition at the mineral surface (oxidation state, the presence of defects, surface hydrophilic or hydrophobic groups), which can be probed using a variety of spectroscopic (e.g., X-ray photoelectron spectroscopy) as well as microscopic (e.g., scanning tunneling microscopy) and mass spectrometric methods (e.g., resonant ionization mass spectrometry). Classical statistical molecular simulations (molecular dynamics–MD, or Monte Carlo–MC) have historically played an important role in dissecting the molecular underpinnings of interfacial water organization that influence experimentally observed reactivities [3]. Large data sets are often analyzed and the average values are utilized to gain chemical insight. There is much utility in this approach, and it can be used to gain insight into localized interactions, surface clustering and adsorption sites of a water on the surface. It is the ability of water to form hydrogen bonds, and the complex network of these hydrogen bonds, that underlies its interaction with hydrophilic mineral surfaces and strongly influences surface diffusion and reactivity through passivation and shielding of other adsorbates to reactive sites

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