Behavior of water molecules near monolayer-protected clusters with different terminal segments of ligand.

Molecular dynamics simulations are performed to investigate the behavior of water molecules near gold monolayer protected clusters (MPCs) with two different types of surfactant, HS(CH(2))(5)(OCH(2)CH(2))(2)COOH (type1) and HS(CH(2))(11)COOH (type2). The effects of the different moieties of the two ligands on the local structure of the water molecules are quantified by means of the reduced density profiles of oxygen and hydrogen atoms, and the hydrogen bond statistics. The adsorption characteristics of water molecules are evaluated by means of their residence time near the MPCs. The results show that the hydrophilic oligo (ethylene glycol) segment increases the number of water molecules, which penetrate the protective layer of MPC. As a result, the inter-water hydrogen bond network in the protective layer of type1 MPC is stronger than that in the protective layer of the type2 MPC. It is shown that the presence of interfacial hydrogen bonds increases the adsorption of water molecules near the MPCs and therefore constrains the motion of MPCs. As a result, the residence time of the water molecules adjacent to the type1 MPC is longer than that of the molecules adjacent to the type2 MPC.