A comparison of the structure and dynamics of liquid water at hydrophobic and hydrophilic surfaces—a molecular dynamics simulation study

The structure and dynamics of liquid water at the interface with three solid surfaces has been investigated via molecular dynamics simulation. The three surfaces include a flat hydrophobic surface, an atomically rough hydrophobic surface, and a contrasting, hydrophilic, fully hydroxylated silica surface. The results of analysis show that, as expected, the solvent near each of the two hydrophobic surfaces behaves essentially equivalently, with loss of hydrogen bonding at the interface. For the hydroxylated surface, surface–solvent hydrogen bonding is stronger than interactions in the bulk solvent, with the nearest solvent layer interacting specifically with up to three surface hydroxyl groups. Nevertheless, distinct structural perturbation of the solvent extends in every case no more than about 10 Å from the surface, and the perturbation is only strong in the immediate solvation layer. Furthermore, the corresponding dynamical perturbation of the solvent, as measured by the diffusion rates and reorientation times in comparison to the bulk, is always relatively small. For the hydrophilic case, it is largest, but even here it is less than a factor of 5 at the immediate interface and less than a factor of 2 in the second hydration layer. The residence time for solvent at the interface is found to be insensitive to the hydrophilicity of the surface. Calculated nuclear magnetic resonance (NMR) order parameters for the solvent are found to reflect solvent orientational ordering, but are shown not to be distinctive of the nature of that order.