Structural and Dynamic Properties of Water near Monolayer-Protected Gold Clusters with Various Alkanethiol Tail Groups

Molecular dynamics simulations are performed to investigate the structural and dynamic properties of water molecules close to clean gold nanoclusters and four different Monolayer Protected Clusters (MPCs) comprising gold nanoclusters and alkanethiol surfactants with methyl, carboxyl, amine and hydroxyl tail group. The effects of these tail groups on the local structure of water are quantified by the analysis of the reduced density profiles, the average number of hydrogen bonds, and the water orientation distribution. Moreover, the dynamic properties of the water molecules are evaluated by means of diffusion coefficients and residence time. The simulation results indicate that water molecules close to clean gold nanoclusters and nonpolar methyl MPCs form a two-shelled structure in which the molecules in the first shell prefer lying on the surface of the nanocluster or methyl MPCs. The existence of interfacial hydrogen bonds between the water molecules and the tail group of MPCs results in a weakening of the water-water hydrogen bond network. Moreover, the presence of the two water shells constrains the motion of the water molecules close to the clean nanocluster and nonpolar MPC. As a result, the residence time of the water molecules adjacent to the clean nanocluster and nonpolar MPC are significantly longer than those of the molecules close to the three polar MPCs.