Surface Tension and Adsorption at the Vapor-Liquid Interface in a Methane-Ethane System

Molecular dynamics is used to calculate the vapor-liquid equilibrium and liquid-vapor surface tension for a methane-ethane system. Good agreement of the parachor value for ethane between the molecular model and experimental data is shown for the 203-253 K range of temperatures and pressures up to 60 atm. The dependence of the surface tension of the mixture on pressure in the range of 4-40 atm at a temperature of 213 K shows a drop in both the surface tension and the difference in densities between the liquid and vapor as the pressure rises and approaches the critical locus. Approximating the density profiles obtained for the same conditions, it is concluded that the width of the interphase boundary also grows. The amount of methane adsorbed on the surface of the liquid film is calculated. The dependence of the molar adsorption of methane on the difference between the densities of the components in the liquid and gas phases is obtained, along with its analytical expression in the context of the Gibbs theory. Features of the approach that is used include no need for approximations of the ideal gas or the ideal solution, and the use of only experimentally obtained data as input. The resulting values of methane adsorption are in good agreement with the derived analytical dependence.