Molecular dynamics investigation on n-alkane-air/water interfaces

Abstract

Alkanes are the important molecules in crude oil that have applications in different fields. In this project, molecular dynamics simulations have been applied to investigate the n-alkane-air and n-alkane-water interfaces. The surface tension, diffusion coefficients, order parameters, radial distribution function, density profiles and radius of gyration of alkanes in the bulk and on the interfaces with air/water were calculated and compared. It was found that with the n-alkane chain length increasing, the surface tension of the interface system increases. However, with the temperature increasing, the surface tension of the alkanes decreases. The alkanes on the interface can diffuse faster than in the bulk in both kinds of interface systems. While n-alkane diffusion rate in the bulk of interface systems is faster than in the pure alkane systems, the radius of gyration of alkanes in the bulk, on the interface and in the pure alkane systems is consistent. Decomposing the diffusion coefficients into the lateral direction and z-direction, the lateral diffusion rate of alkanes on the interface is about 2–10 times faster than in the z-direction in both kinds of interface systems. Although molecules prefer to pack randomly in the bulk, they are more orderly packed on the interface. The result can help to explain the interface effect in water/air-alkane interface systems. The project also demonstrates that using NAMD program and CHARMM forcfield can predict reasonable structure and properties of n-alkane-air/water systems.