Bulk and Interfacial Properties of the Alkane + Nitrogen System

Abstract

Molecular dynamics simulations were performed to investigate the bulk and interfacial properties of the alkane + N2 system at geological conditions. n-C10 + N2, n-C16 + N2, and branched C16 + N2 systems are mainly considered in this study. The simulation results were compared to theoretical modeling using the VT-PPR78 equation of state and the density gradient theory, and the results are in good agreement with the experimental findings. The density profiles of alkanes varied monotonically across the interfaces, but N2 molecules are found to enrich the interfaces. The solubility of N2 in the alkane-rich phase increases with temperature, likely due to entropic factors. This solubility also increases with pressure, likely due to energetic factors. There was no significant effect of the alkane size Nm and the chain branching on the solubilities of N2 in the alkane-rich phase. We observed a direct correlation between the solubility of N2 in the alkane-rich phase and the swelling of the alkane-rich phase. The interfacial tension (IFT) of the alkane + N2 system decreases with pressure, and this decrease is less marked at higher temperatures. For this system, the IFT decreases with temperature. Also, these IFTs increase with Nm and decrease with chain branching. Furthermore, the surface excess of N2 changes nonmonotonically as a function of pressure in the alkane + N2 system. The fact that the IFTs decreased with pressure might be explained by the positive surface excess of N2. The surface excess of N2 decreases with temperature. This explains why the decrease in the IFTs with pressure is less marked at higher temperatures. There was no significant effect of Nm and chain branching on the surface excess of N2.