Can Polar Surfaces Be Hydrophobic? Molecular Simulation Study of Model Silica Surfaces

Solvation of charged species in water is often associated with favorable free energy of hydration dominated by the favorable electrostatic interactions between water molecules and the solute. It is therefore, expected that water near a surface containing such entities will display behavior characteristic of hydrophilic surfaces. In our study probing the details of water behavior in the vicinity of model silica surfaces, we find that this picture is not always true [1]. We characterized the behavior of water near molecularly flat silica surfaces with varying partial charges. The behavior of water is quantified by various parameters such as water structure, vicinal water density, water density fluctuations and contact angle. In the vicinity of uncharged silica surface water displays the behavior characteristic of a hydrophobic surface and near highly charged silica surface the behavior is hydrophilic. Interestingly, for intermediate charges water behavior is not hydrophilic but displays a hydrophobic character. This suggests that polar surfaces can be hydrophobic. In our paper, we further elaborate on this and discuss the origins of this behavior. We also explore the differences in the hydrophobic and “polar-hydrophobic” surfaces. This has implications in fundamental understanding of hydrophobicity, and has relevance to wide ranging phenomena such as protein folding, design of self-cleaning surfaces, and heterogeneous ice nucleation in clouds.

[1] Nicolas Giovambattista et al.,“Enhanced surface hydrophobicity by coupling of surface polarity and topography”, Proc. Natl. Acad. Sci., 106:15181 (2009)