Influence of hydrophilic surface specificity on the structural properties of confined water

The influence of chemical specificity of hydrophilic surfaces on the structure of confined water in the subnanometer regime is investigated using grand canonical Monte Carlo simulations. The structural variations for water confined between hydroxylated silica surfaces are contrasted with water confined between mica surfaces. Although both surfaces are hydrophilic, our study shows that hydration of potassium ions on the mica surface has a strong influence on the water structure and solvation force response of confined water. In contrast to the disrupted hydrogen bond network observed for water confined between mica surfaces, water between silica surfaces retains its hydrogen bond network displaying bulklike structural features down to surface separations as small as 0.45 nm. Hydrogen bonding of an invariant contact water layer with the surface silanol groups aids in maintaining a constant number of hydrogen bonds per water molecule for the silica surfaces. As a consequence, water depletion and rearrangement upon decreasing confinement is a strong function of the hydrophilic surface specificity, particularly at smaller separations. An oscillatory solvation force response is only observed for water confined between silica surfaces, and bulklike features are observed for both surfaces above a surface separation of about 1.2 nm. We evaluate and contrast the water density, dipole moment distributions, pair correlation functions, and solvation forces as a function of the surface separation.