van der Waals Effects in Interfacial Water on the Cl- and H-Terminated Si(111) Surfaces

The properties of interfacial water on the Cl- and H-terminated Si(111) surfaces are reinvestigated using a DFT-based first-principles approach, able to describe van der Waals effects. Although van der Waals interactions have significant quantitative effects, they do not qualitatively change the conclusions of previous first principles studies, where van der Waals effects were not taken into account or at most quite approximately described: the interaction of water with both substrates is found to be rather weak, although bonding with the Cl-terminated Si(111) surface is relatively stronger than with the H-terminated one, due to both the larger van der Waals interactions and the presence of H bonds and electrostatic contributions. Therefore, according to a molecular picture for attributing the hydrophilic/hydrophobic character, both surfaces should be considered as hydrophobic. However, for water oil the Cl-terminated Si(111) surface, the estimated macroscopic contact angle (36 degrees), which is relatively small, in qualitative agreement with the experimental findings, is compatible with a weakly wettable character of this substrate, thus suggesting that sometimes a macroscopic description (based oil the measured contact angle) and a microscopic one (based on the theoretical estimate of interaction strengths) can lead to a different assignment of the hydrophobic/hydrophilic character of a given substrate.