Coarse-grained theoretical modeling and molecular simulations of nitrogen + n-alkanes: (n-pentane, n-hexane, n-heptane, n-octane)

Abstract

The variation of the interfacial tension (IFT) with the pressure and molecular chain length as well as the impact of the interfacial adsorption of nitrogen on IFT are described for four binary systems composed by n-alkanes (n-pentane, n-hexane, n-heptane, n-octane) pressurized by nitrogen.Calculations are based on the Square Gradient Theory (SGT) applied to the Statistical Associated Fluid Theory equation of state and Coarse-Grained Molecular Dynamics (MD) simulations. The molecular descriptors of pure fluids (chain length, repulsive/attractive exponents and size/energy scale) and the interfacial parameters involve in SGT are fully obtained by applying the corresponding states principia and scaling arguments. The approach used here only needs the critical temperature, acentric factor and a single data of the saturate liquid density of pure fluids, which implies that it is entirely predictive for mixtures.According to our results, the interfacial tensions of the binary mixtures decrease as the pressure increases. Particularly, the reduction of IFT with increasing pressure at isothermal conditions evidences adsorption of nitrogen at the n-alkane surface, a behavior that was clearly corroborated by analyzing the trend of concentration profiles and Gibbs adsorption isotherms. Results based on SGT and MD revel that as the molecular chain length increases (from n-pentane to n-octane), the surface activity increases and the interface width decreases, implying thus that the IFT of the mixture increases.