Unraveling the Wettability of Gold Surfaces: A Reactive Molecular Dynamics and Experimental Approach

While early experimental work established that Au is intrinsically hydrophilic, meaning that the bare metal-water interface is strongly wetting in the absence of adsorbed contaminants, a knowledge gap between computational predictions and experimental measurements persists. Classical nonreactive force fields lack the accuracy to model this interface correctly, predicting contact angles that range from complete wetting to artificial hydrophobicity depending entirely on arbitrary interaction parameters or incompatible optimization objectives. To bridge this computational-experimental gap, we developed a ReaxFF reactive force field for the Au-water interface, combining established bulk parameter sets with solid-liquid interactions trained strictly against van der Waals-corrected DFT data. A benchmark of nine nonreactive pairwise potentials revealed that nearly all predict complete droplet spreading, rendering macroscopic contact angles insufficient for model validation. By contrast, our ReaxFF/ACKS2 model, which corrects the spurious water-to-Au charge transfer produced by the default EEM and QEq charge equilibration schemes, resolved finite contact angles of 14.0° for Au(111), 18.1° for Au(001), and 24.1° for Au(110). Complementing these simulations, goniometry on freshly cleaned polycrystalline Au yielded contact angles near 30° that gradually increased to 75° over time in ambient air. By accounting for airborne contamination, the reactive model and the experimental data are brought into quantitative consistency, bracketing the intrinsic wettability of Au from the clean-surface and contaminated-surface sides.