Adsorption of Chain Molecules in Slit-Shaped Pores: Development of a SAFT-FMT-DFT Approach

Density functional theory (DFT) is commonly used to treat the adsorption of molecules in carbon and silica pores of various geometries. In this work, we develop a DFT with an accurate molecular-based equation of state to calculate thermodynamic properties using fundamental measure theory (FMT), which is a rigorous approach for the treatment of homogeneous and nonhomogeneous hard-sphere fluids. A theoretical framework results with adsorbing molecules treated as hard-sphere chains with square-well attractive interactions. The Mansoori–Carnahan–Starling–Leland and Carnahan–Starling–Boublik equations of state are used for the hard-sphere interactions, and a version of the statistical associating fluid theory for potentials of variable range (SAFT-VR) is used to describe the square-well fluid. First- and second-order perturbative attractive terms are included in the theory. Theoretical predictions are in good agreement with published results for Monte Carlo simulations of the adsorption of chain molecules. In an extension to published studies, the impact of potentials of increasing complexity are compared using square-well and Lennard–Jones wall potentials. For more realistic adsorption of molecules of varying chain length, full slit-shaped pores are modeled using a 10–4 wall potential. The new approach provides accurate predictions of adsorption of chain molecules in model systems. It should be useful for predicting adsorption on flat surfaces and in slit-shaped pores as well as for analyzing experimental data.