(484a) Accurate and Robust Classical Density Functional Theory for the Adsorption of Fluids Into Nanoporous Materials

Classical density functional theory for square-well and multi-yukawa simple and chain fluids was implemented in two dimensions and used to describe supercritical adsorption of the model fluids into cylindrical pores of various size and wall-fluid interaction. The implementation uses Fast Fourier Transform (FFT) to integrate the convolution integrals appearing during the calculation of the nonlocal terms, which significantly increases computing performance. Pair attractive interactions are taken into account using the Fourier transform of the direct correlation function (DCF), calculated from the first-order mean spherical approximation (FMSA) theory [1,2].

The theory was tested against Monte-Carlo simulation data for adsorption of the model fluids into narrow cylindrical pores. Because of the confinement effects in the pore, the density of the absorbed fluid is significantly different from the bulk fluid, which constitutes a challenge for the usual theories based on the expansion of the thermodynamics around the bulk fluid density. The overcome the difficulty we proposed and tested reference fluid density functional approach of Gillespie et al [3] and an approach with a functional that mimics energy route thermodynamics. The approaches tend to improve predictions of the density profiles and adsorption isotherms for almost all the pores studied.

The case of the adsorption of chain fluids is more interesting. Confinement limits number of possible conformations of the chains, which are consequently depleted near the walls and repelled out of the narrow pores. Developed density functional theory manages to capture these depletion effects, but because of accumulations of different approximations adopted in the theory, the agreement with simulation is more qualitative than quantitative. This seems to be mainly due to inaccurate inhomogeneous contact value, used to account for chain connectivity. Inclusion of the contact value greatly improves predictions for the adsorption isotherms, while the density profiles are improved only at low densities.