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.
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.
The mechanical properties of finite-length (5,0)/(8,0) single-walled carbon nanotube (SWCNT) heterojunctions with manipulated topological defects are investigated using molecular dynamics simulation calculations. The results show that the mechanical properties and deformation behavior of SWCNT heterojunctions are mainly affected not only by the diameter of the thinner segment of the SWCNT heterojunction but also by the orientation of the heptagon-heptagon (7-7) pair in the junction region. Moreover, the orientation of the 7-7 pair strongly affects those properties in the compression loading than those in tensile loading. Finally, it is found that the location of buckling deformation in the heterojunctions is dependent on the orientation of the 7-7 pair in the compression.
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