Methane at the gas/water interface: Molecular simulations of surface adsorption and second surface virial coefficients

Abstract

Liquid surface tensions are moderated by the adsorption of species like surfactants and gases at interfaces. In order to disentangle the competing polar and nonpolar interactions that oppose/favor adsorption at a gas/water interface and gain molecular insights into the role of inter-solute interactions on moderating the surface tension, we report here molecular simulations of purely nonpolar methane adsorption onto aqueous surfaces over a broad range of temperatures (0–50 °C) and bulk gas pressures (up to ~750 bar). As expected, positive methane adsorption at the interface significantly lowers the surface tension with increasing pressure, although this effect is not without bound as plateaus/minima in the surface tension are observed near 500 bar. The excess surface adsorption of methane directly observed from simulation agrees quantitatively with those determined from the surface tension following Gibbs adsorption isotherm, giving confidence in the thermodynamic consistency of the simulation results and Gibbs’ interpretation of the effect of solute adsorption. From the simulation results we were able to evaluate surface adsorption coefficients and second surface virial coefficients for methane’s sitting at the interface. The second virial coefficients determined from simulation were found to be slightly more attractive than those predicted from statistical thermodynamic integrals for two-dimensional gases, suggesting water could help drive methane interactions in the interface. Generalizing the statistical thermodynamic integral for the second surface virial coefficient to that for a gas confined to a pseudo-two-dimensional slit, however, near quantitative agreement with the simulation results is observed, indicating water plays effectively no role. Comparisons of the simulation results with experiment are generally favorable, although the simulations appear to under predict net adsorption likely as a result of the neglect of methane polarization interactions.