Interfacial Segregation, Structure, and Diffusion ofn-Alkane Mixture Films Adsorbed on Smooth and Rough Gold Surfaces

Abstract

The compositional, structural, and diffusional properties of equal-weight mixtures of liquid n-hexane and n-hexadecane films supported on atomically smooth and rough gold surfaces are explored with the use of temperature-controlled, canonical, molecular dynamics simulations at 315 K. Preferential adsorption of n-hexadecane molecules is found in the region next to the solid surface. The selective molecular segregation effect is much stronger at the smooth Au(111) surface, where it is accompanied by pronounced layering exhibited in the density profile of the adsorbed film normal to the surface, which shows the formation of two well-defined interfacial liquid layers. For both the smooth and rough solid surfaces, in the region of the first layer located about 5 Å from the solid surface, the alkane molecules order with their axis oriented parallel to the adsorbing surface, thus maximizing the attractive physisorption energy. The orientational ordering is accompanied by the formation of ordered domains in the first liquid-film layer, with the molecules in each domain lying parallel to each other. The interfacial domain ordering is frustrated in the alkane mixture film adsorbed on the rough surface. Reduced diffusive motion is found for molecules located near the solid surfaces. However, in the region of the first adsorbed layer of the liquid film adsorbed on the smooth surface, the rate of diffusion, particularly of the hexane molecules, is larger than at the rough surface. It is found that the smaller, hexane, molecules diffuse through channels at the boundaries between the ordered domains formed by the hexadecane molecules. For the rough surface case, where the domain ordering is frustrated, such interdomain boundary channels do not form and consequently the domain boundary-assisted diffusion of the smaller molecules is not operative there, resulting in a smaller diffusion rate.