Adsorption and distribution of gas molecules at the (CH4 + CO2)-water interface: insights from analysis of intrinsic interfacial structure

The interface between CH4 + CO2 mixture and water is of significant importance in the context of CO2 sequestration through gas hydrate formation. Understanding the structural and dynamical properties of this interface can provide insight into the nature of interaction between the gas mixture and the surface of water. In this work, classical molecular dynamics simulations were applied to study the (CH4 + CO2)-water interface. To elucidate the effect of capillary wave fluctuations on the properties of the interface, we examined the intrinsic interfacial structure by identifying the true set of molecules present at the surface of water. The intrinsic density profiles of gas molecules revealed that CO2 molecules preferentially adsorb at the (CH4 + CO2)-water interface and that adsorption of CH4 + CO2 mixture became less selective with an increase in temperature. The study also showed that the distribution of CO2 molecules near the interface is significantly more non-uniform compared to that of CH4. The density of CO2 above the wells of the rough interface was found to be considerably greater than that above the humps, while such non-uniformity in distribution was less pronounced in the case of CH4. The differences observed in the distribution of CH4 and CO2 adsorbed at the interface are explained on the basis of strength of gas-water interactions. The time scale of dynamics of the water surface and that of the gas molecules were examined which showed the time period of fluctuations at the water surface to be significantly longer than the diffusion time of gas molecules above the surface. This allows the gas molecules to rapidly diffuse above the relatively less mobile water surface to attain energetically favourable distributions at every instant. Capillary waves at the surface of water affect the distribution of CH4 and CO2 at the gas-water interface in significantly different ways. [Display omitted] •Molecular dynamics simulations are applied to study (CH4 + CO2)-water interface.•Layer of adsorbed gas becomes richer in CO2 as temperature decreases.•Interfacial roughness causes non-uniform distribution of adsorbed gas.•Non-uniformity is more pronounced in the case of CO2 compared to that of CH4.•Non-uniformity is explained in terms of gas-water interaction at the rough surface.