Hydration of metal surfaces can be dynamically heterogeneous and hydrophobic

Abstract

We have applied molecular dynamics and methods of importance sampling to study structure and dynamics of liquid water in contact with metal surfaces. The specific surfaces considered resemble the 100 and 111 faces of platinum. Several results emerge that should apply generally, not just to platinum. These results are generic consequences of water molecules binding strongly to surfaces that are incommensurate with favorable hydrogen-bonding patterns. We show that adlayers of water under these conditions have frustrated structures that interact unfavorably with adjacent liquid water. We elucidate dynamical processes of water in these cases that extend over a broad range of timescales, from less than picoseconds to more than nanoseconds. Associated spatial correlations extend over nanometers. We show that adlayer reorganization occurs intermittently, and each reorganization event correlates motions of several molecules. We show that soft liquid interfaces form adjacent to the adlayer, as is generally characteristic of liquid water adjacent to a hydrophobic surface. The infrequent adlayer reorganization produces a hydrophobic heterogeneity that we characterize by studying the degrees by which different regions of the adlayers attract small hydrophobic particles. Consequences for electrochemistry are discussed in the context of hydronium ions being attracted from the liquid to the metal-adlayer surface. E xtended metal interfaces play a fundamental role in aqueous electrochemistry, a field of principal importance in the advancement of renewable, clean energy sources (1, 2). In many processes that occur at metal interfaces, such as electrolysis, corrosion, and electrocatalysis, water is ubiquitous, often acting as both solvent and reactant (3). Although many studies exist that detail the behavior of water across small length scales and timescales (4-7) and at low temperatures (8-10), at present there is little understanding of the large length scale correlations and emergent behavior of water on metal surfaces, even though such effects are likely to influence function in important ways (11)(12)(13)(14). Here, we address this gap in knowledge with a theoretical model of the interactions between water and a metal surface. Specifically we illustrate how a metal surface can impose geometrical constraints within the adlayer of water, leading to a composite metal-water interface that is hydrophobic on large length scales. We further show how defects within the hydrogen-bonding patterns of the adlayer create transient regions of hydrophobic behavior that exist on small length scales and over long timescales. These results offer a microscopic explanation and generalization of previous experimental observations that have inferred hydrophobicity of a platinum surface at low temperatures (15-17).To study the aqueous metal interface we use a molecular model (6, 18) that neglects explicit electronic degrees of freedom beyond accounting for electronic polarization of the metal. Despite its relative simplicity, the model is in reaso...

Characterizing heterogeneous dynamics at hydrated electrode surfaces

“…References 1-14 are examples. Our recent work in this area 14 highlighted the role of interfacial properties occurring over nanosecond timescales. In this paper, we examine this dynamics in further detail, considering models of Pt-water interfaces and showing structural relaxations in the water ad-layers proceed exceedingly slowly via mechanisms that are spatially heterogeneous.…”

Evaluation of the constant potential method in simulating electric double-layer capacitors

“…Vatamanu and co-workers [28][29][30][31] investigated ionic liquid electrolytes with carbon or gold electrodes using the smooth particle mesh Ewald (SMPE) 32,33 method to simplify the calculation. The hydration of metal-electrode surfaces was examined by Limmer et al 34,35 In all of these studies, detailed comparisons of the CPM with the fixed charge method have been lacking. In a recent paper, Merlet et al 36 examined the differences between CPM and FCM simulations as measured by the relaxation kinetics in EDLC with nanoporous carbide-derived carbon electrode and the electrolyte structure at interface in EDLC with planar graphite electrode.…”

Simulation of Adsorption Processes at Metallic Interfaces: An Image Charge Augmented QM/MM Approach

“…As discussed in Ref. 16 , the reorganization in the adlayer occurs at timescales larger than nanoseconds. Obtaining dynamical properties at these timescales with ab initio MD is out of scope, but possible to address with the IC-QM/MM approach.…”

“…16 . We obtain very similar results for the profile of the water density and the structure of the adlayer.…”

“…The Gaussian charge method developed by Siepmann and Sprik 15 treats the image charges as dynamical variables and realizes the mutual modification of induced and adsorbate charges. The Siepmann-Sprik scheme has been successfully applied to water/platinum interfaces [15][16][17] as well as ionic liquids at platinum 18 and graphene 19,20 electrodes within a purely classical frame.…”