Hydrophobic-hydrophilic interactions of water with alkanethiolate chains from first-principles calculations

The interaction of water with alkanethiolate chains is studied from first principles. A detailed analysis is performed by optimizing the structure of small water clusters, one-dimensional water chains, and ordered and disordered thin water layers adsorbed on hydroxyl(OH)- and methyl(CH3)-terminated alkanethiol monolayers. The hydrophilic/hydrophobic character of these two different substrates is investigated by means of an energetic analysis combined with hydrogen-bond counting. On the hydrophilic OH-terminated alkanethiol surface, zigzag, one-dimensional water chains and disordered thin water layers are the energetically favored structures. The ab initio results can be used to determine the optimal value of the empirical parameters characterizing a suitable force field to be used in classic molecular dynamics simulations.