(5bt) Fluctuations of Water near Extended Hydrophobic and Hydrophilic Surfaces

The primary goal of this work is to study water density fluctuations in probe volumes of different shapes and sizes, both in the bulk as well as near various hydrophobic and hydrophilic surfaces. We introduce a formalism involving a coarse-grained density that allows us to readily combine umbrella sampling with available molecular dynamics simulation packages. It has been established, both theoretically and using simulations, that the presence of a purely repulsive extended hydrophobic surface induces drying in its vicinity, resulting in the formation of a vapor-liquid interface. Recent reports in the literature have called attention to an apparent discrepancy between the theory and simulations on addition of weak dispersive attractions between the extended hydrophobic surface and water, which results in the disappearance of drying. However, an examination of the fluctuations of water near the attractive surface reveals that the probability of density depletion near the surface is significantly larger than that in bulk, akin to density fluctuations at the water-vapor interface. Hence, there is no discrepancy between theory and simulation and the absence of drying is simply the result of the interface been drawn to the hydrophobic surface in the presence of attractions. As the attraction between the hydrophobic surface and water increases, we expect the adhesion of the interface to the surface to get stronger, resulting in the probability of density depletion getting smaller. Near a model hydrophobic surface with attractions that are typical of alkanes, water density depletion is easier than in the bulk for up to 3 molecular diameters. In contrast, fluctuations near a model hydrophilic surface are very similar to that in the bulk.