Bulk and interfacial properties of methane-heavy hydrocarbon mixtures

Methane-heavy hydrocarbons are typical mixtures in natural gas extraction, purification and production processes. However, the microscopic behavior of methane diffusion, dissolution and adsorption within heavy hydrocarbon liquids has not been solved yet. Therefore, the bulk and interfacial characteristics of methane-heavy hydrocarbon mixtures were systematically analyzed. And the real and rippled interfaces were identified using a new method. The results indicate that density of heavy hydrocarbon-rich phase decreases with decreasing chain length and increasing pressure, while that of the methane-rich phase increases instead. When methane molecules dissolve into bulk liquids, heavy hydrocarbon chains stretch and the intermolecular separation distances increase. Additionally, the interaction energy between the two molecules is the main reason for system swelling. The Gibbs relative adsorption of methane improves until reaching the saturation limit as pressure adds. The intrinsic interface analyses show that the strong adsorption of methane mainly exists in the first surface layer in dilute solutions, which becomes more pronounced with increasing methane fraction. For heavy hydrocarbons with shorter chains, surface layers are narrower and their separation distances are reduced. Furthermore, methane molecules prefer staying at the outer edge of surface layers and are nearer to gas phase than heavy hydrocarbons by approximately 0.2–0.7 Å.