Structure and Mass Transport Characteristics at the Intrinsic Liquid-Vapor Interfaces of Alkanes

In this paper, an instantaneous interface definition has been used to study the intrinsic structure and self-diffusion coefficient in the vicinity of the liquid-vapor interfaces of decane and tetracosane at three different temperatures using molecular dynamics simulations, and the results have been compared with those obtained on the basis of the conventional Gibbs dividing surface (time- and space-averaged interface). The alkane molecules were modeled using the united atom NERD force field. Partial layered structures of alkane molecules at the liquid-vapor interface are observed as a pinned structure of alkane liquids based on the intrinsic interface. This kind of characteristic has not been observed in the density profiles obtained based on the Gibbs dividing surface. By examining the orientation order parameter and radius of gyration of the alkane molecules, it was observed that the alkane molecules were preferentially oriented to be more parallel to the intrinsic interface than to the Gibbs dividing surface, and the shape of the alkane molecules is slightly changed in the vicinity of the liquid-vapor interfaces. The self-diffusion coefficient parallel to the intrinsic interface was examined using the Green-Kubo relation, where the projection of the velocity in the parallel direction to the local intrinsic interface is used in the velocity correlation function. It was found that the self-diffusion coefficient in the direction parallel to the intrinsic interface changes as the position approaches the interface in a more obvious manner as compared with the self-diffusion coefficient obtained with respect to the Gibbs dividing surface. These results suggest that the use of an instantaneous interface definition allowed us to capture sharp variations in transport properties which are originating due to steeper structure at the liquid-vapor interfaces.