(78h) Competitive Adsorption of Linear Alkanes on Nanoporous Carbon: Density Functional Theory and Molecular Dynamics Simulation

Competitive adsorption of linear alkanes in microporous materials has been observed and studied in experiments and molecular simulation. While it is well known that larger molecules have higher affinity to attractive pore surfaces, the entropy penalty of confining flexible chain molecules will also critically influence the partitioning of molecules in a mixture. As understanding experiments and even molecular simulations of multicomponent systems can be challenging at the molecular scale, a theoretical model that accounts for both effects accurately is thus invaluable to predict the pore selectivity for different components. Our earlier work in graphite pores has qualitatively shown the transition of partitioning coefficients for longer alkanes as pressure increases by employing inhomogeneous statistical associating fluid theory (iSAFT). The entropic effect due to confinement is mainly accounted by the chain connectivity in iSAFT, however, the external potential is not physical enough given that most external potential models (e.g. Steele 10-4-3 potential) are defined for spherical molecules. Although Lorentz-Berthelot rule is often used in many density functional theory applications, the real interaction energy still needs to be well understood.

In this work, we take the molecular dynamics (MD) simulation of alkanes on graphite pore system as a reference to quantify the attraction between graphite surface and alkanes. Using inputs from MD simulation within iSAFT, we then study the competitive adsorption among alkanes in graphite pores. The significance for predicting the composition of shale gas in nanoscale pores is further discussed.