A molecular dynamics study on transport properties and structure at the liquid–vapor interfaces of alkanes

Abstract

In this paper, self-diffusion and structure in the vicinity of liquid–vapor interfaces of several species of n-alkanes are examined using molecular dynamics simulations. The united atom NERD force field was utilized to model the alkane molecules, and the obtained properties were validated by the reported experimental results. Surface tension was calculated based on the averaged pressures, and tail corrections were implemented based on the molecular virial formulae. Present surface tension results for butane, hexane and decane are in good agreement with experimental and other simulated values. Molecular orientation and chain conformation in the interface region were examined by observing the ordering parameter and radius of gyration for decane and tetracosane. In the interface region, molecules are preferentially oriented parallel to the interface and shape of the molecules is slightly flattened in the z-direction for both decane and tetracosane. With an increase in temperature, molecules are preferentially less parallel to the interface. The Einstein relation modified for confined regions was used to obtain the self-diffusion coefficient according to migration of molecules parallel to the interface. Self-diffusion coefficient increases in the interface region for both decane and tetracosane. This increase in self-diffusion coefficient has a correlation with decrease in density. The dependency of self-diffusion coefficient on density is nonlinear in the liquid and vapor side of the interface region and is linear in between the above two regions. Influence of chain length of the alkane molecules has been studied by comparing the physical and structural quantities of decane, tetracosane and hexatriacontane at the same reduced temperature. At the same reduced temperature, self-diffusion coefficient decreases with an increase in chain length in the interface region as well as in the bulk liquid region. All three alkanes show similar tendency of ordering and chain conformation at the interface at the same reduced temperature.