Molecular dynamics characterization of the water-methane, ethane, and propane gas mixture interfaces

The co-existing natural gas and water bulk phases arise in a wide range of technological and environmental processes. The liquid-gas mixture is separated by an interface which plays a crucial role in mass transport across the phases. In this work, we use the molecular dynamics (MD) technique to investigate the molecular organization, solubility, density, and compositions of natural gas-water interfaces. We apply the NP(N)AT ensemble which is an appropriate statistical methodology to dodge the defects of the conventional ensembles in surface physics, and ultimately, to characterize the interfacial thermodynamics and mechanics. High interfacial density, excess, and radial pair distribution function of the gas components in order of propane, ethane, and methane, respectively, suggest the interfacial adsorption as per favorable interactions with a dense hydrogen bonding network near the surface in the liquid water phase. It is also found that the gas components solubility is negligible. Nevertheless, methane molecules present in natural gas can further dissolve in water, comparing to pure methane. We lastly conclude a heterogeneous formation of structure II hydrate from the adsorption and composition results. Moreover, we systematically increase the pressure from 1 MPa to 50 MPa and the temperature from 273.15 K to 303.15 K to calculate the interfacial tension using the mechanical approach. We observe a decrease in the interfacial tension along with an increase in both pressure and temperature. Given the remarkable hydrocarbon adsorption acting as a surfactant, this interfacial tension attenuation is more highlighted in a natural gas-water system compared to the pure liquid-vapor water or water-pure methane systems at the same temperature and pressure. We employ MD combined with fundamental thermodynamics to predict the interfacial tension via its independent relations with pressure and temperature which agrees with the classical scaling laws, namely, the Eotvos rule. The corresponding molecular mechanisms captured by the microscopic and macroscopic properties at the interfacial regions prospectively demonstrate a sensitivity to both temperature and pressure, which contributes to the developing understanding and applications of the imperative water-natural gas interface. (C) 2019 Elsevier Ltd. All rights reserved.