Influence of alkanethiol self-assembled monolayers with various tail groups on structural and dynamic properties of...

Molecular dynamics simulations are performed to investigate the structural and dynamic properties of a water layer lying on a clean Au(111) surface and on alkanethiol self-assembled monolayers (SAMs) with three different tail groups: methyl, carboxyl, and hydroxyl. The effects of these functional groups on the local structure of the water are quantified by analyzing the reduced density profiles of the oxygen and hydrogen atoms, the average number of hydrogen bonds, and the distribution of the O-H bond angle, respectively. Meanwhile, the dynamic properties of the water layer are evaluated by analyzing the diffusion coefficients of the water molecules in the xy-plane and z-direction. The simulation results indicate that in both the hydrophobic and the hydrophilic alkanethiol SAMs, the formation of a two-layer water structure is suppressed. And the water molecules can approach the SAMs composed of hydroxyl tails most closely and SAMs composed of methyl tails furthest. Due to the existence of hydrogen bonds between water molecules and hydrophilic alkanethiol SAMs, the distribution of water molecules is more uniform than that in the hydrophobic interface. Meanwhile, the water-water hydrogen bond network weakens. Furthermore, the mobility of the water molecules in the hydrophilic interface is reduced more significantly than in the hydrophobic interface. The results developed in this study yield detailed insights into the microscopic interfacial phenomena.