Effect of Au nanotube size on molecular behavior of water/ethanol mixtures

Abstract

The molecular behavior of water/ethanol mixtures of different weight fractions inside Au nanotubes of different radii were investigated by molecular dynamics simulations. Three different weight fractions of water/ethanol (25/75, 50/50, and 75/25) and radii of Au nanotubes (13, 22, and 31.1 Å) were considered in order to understand the effects of Au nanotube size and water/ethanol fraction on the structural and dynamical behaviors of the water and ethanol molecules. The density profiles show two shell arrangements inside the Au nanotubes because water molecules prefer to adsorb onto the wall of the Au nanotube. According to the density distribution, the space inside the Au nanotubes can be divided into three regions, those of contact, transition, and bulk regions, in order from the interior wall surface to the nanotube center. The bulk region has a lower local weight fraction compared to the system water/ethanol weight fraction. Meanwhile, the local water/ethanol weight fraction in the contact region is higher than that of the system. When the system water/ethanol weight fraction becomes higher, the local water/ethanol weight fraction also becomes higher. In the transition and bulk regions, diffusion coefficients for water and ethanol molecules become higher due to the weak interaction with Au atoms. The values of diffusion coefficients for water molecules in the contact regions are much lower than for those in other regions and are similar for different system water/ethanol weight fractions due to the strong interaction with Au atoms. When the radius of the Au nanotube becomes larger, the values of local weight fraction inside the larger radius Au nanotube become higher than those inside small radius Au nanotubes because the ratio of water number to the nanotube inner surface area becomes higher. In addition, water inside a larger radius Au nanotube has a shorter water–water hydrogen bond lifetime (H-bond) in the contact region because the smaller curvature causes a weaker interaction with Au atoms.