The molecular structure of the interface between water and a hydrophobic substrate is liquid-vapor like

With molecular simulation for water and a tunable hydrophobic substrate, we apply the instantaneous interface construction [A. P. Willard and D. Chandler, J. Phys. Chem. B, 114, 1954 (2010)] to examine the similarity between a water-vapor interface and a water-hydrophobic surface interface. The intrinsic interface refers to molecular structure in terms of distances from the instantaneous interface. We show that attractive interactions between a hydrophobic surface and water affect capillary wave fluctuations of the instantaneous liquid interface, but these attractive interactions have essentially no effect on the intrinsic interface. Further, the intrinsic interface of liquid water and a hydrophobic substrate differs little from that of water and its vapor.The same is not true, we show, for an interface between water and a hydrophilic substrate. In that case, strong directional substrate-water interactions disrupt the liquid-vapor-like interfacial hydrogen bonding network.Comment: 6 pages, 5 figure

Characterizing heterogeneous dynamics at hydrated electrode surfaces

In models of Pt 111 and Pt 100 surfaces in water, motions of molecules in the first hydration layer are spatially and temporally correlated. To interpret these collective motions, we apply quantitative measures of dynamic heterogeneity that are standard tools for considering glassy systems. Specifically, we carry out an analysis in terms of mobility fields and distributions of persistence times and exchange times. In so doing, we show that dynamics in these systems is facilitated by transient disorder in frustrated two-dimensional hydrogen bonding networks. The frustration is the result of unfavorable geometry imposed by strong metal-water bonding. The geometry depends upon the structure of the underlying metal surface. Dynamic heterogeneity of water on the Pt 111 surface is therefore qualitatively different than that for water on the Pt 100 surface. In both cases, statistics of this adlayer dynamic heterogeneity responds asymmetrically to applied voltage.Comment: 6 page, 4 figure

Water exchange at a hydrated platinum electrode is rare and collective

We use molecular dynamics simulations to study the exchange kinetics of water molecules at a model metal electrode surface -- exchange between water molecules in the bulk liquid and water molecules bound to the metal. This process is a rare event, with a mean residence time of a bound water of about 40 ns for the model we consider. With analysis borrowed from the techniques of rare-event sampling, we show how this exchange or desorption is controlled by (1) reorganization of the hydrogen bond network within the adlayer of bound water molecules, and by (2) interfacial density fluctuations of the bulk liquid adjacent to the adlayer. We define collective coordinates that describe the desorption mechanism. Spatial and temporal correlations associated with a single event extend over nanometers and tens of picoseconds.Comment: 10 pages, 9 figure

Solvation at Aqueous Metal Electrodes

We present a study of the solvation properties of model aqueous electrode interfaces. The exposed electrodes we study strongly bind water and have closed packed crystalline surfaces, which template an ordered water adlayer adjacent to the interface. We find that these ordered water structures facilitate collective responses in the presence of solutes that are correlated over large lengthscales and across long timescales. Specifically, we show that the liquid water adjacent to the ordered adlayers forms a soft, liquid-vapor-like interface with concomitant manifestations of hydrophobicity. Temporal defects in the adlayer configurations create a dynamic heterogeneity in the degree to which different regions of the interface attract hydrophobic species. The structure and heterogeneous dynamics of the adlayer defects depend upon the geometry of the underlying ordered metal surface. For both 100 and 111 surfaces, the dynamical heterogeneity relaxes on times longer than nanoseconds. Along with analyzing time scales associated with these effects, we highlight implications for electrolysis and the particular catalytic efficiency of platinum.Comment: 9 pages, 8 figure