Lennard-Jones mixtures in slit-like pores: a comparison of simulation and density-functional theory

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Abstract
The adsorption of binary Lennard-Jones mixtures in narrow slit-like pores is studied theoretically and by simulation. The model parameters are chosen to correspond to Ar-Kr mixtures in a carbon pore. The theoretical approach is based upon application of the Meister-Kroll-Groot version of density-functional theory, while simulation studies are carried out by using a constant-temperature molecular-dynamics method. In order to determine the state of the bulk fluid in equilibrium with the simulated system, chemical potentials of both components are determined during simulation runs using the particle-insertion method. The investigations are performed at three reduced temperatures T* = 2, 1·5 and 1·0, taking the argon-argon potential depth as reference. At the lowest temperature we concentrated on the gas-liquid transition in the pore. It is found that the theory considered provides a good description of the fluid structure inside the pore and that it also reproduces the phase behaviour of the system observed during simulation runs reasonably well. The differences between theoretical results and pseudo-experimental data can be summarized as follows. For a pore width of 5 (in units of the molecular diameter of argon), the theory slightly overestimates the critical temperatures inside pores, predicting at T* = 1·5 capillary condensation for all bulk fluids having a mole fraction of argon less than 0·09, whereas the computer simulations performed at that temperature yield smooth adsorption isotherms for all bulk-fluid compositions. At the lowest investigated temperature, the theory leads to slightly lower bulk-fluid densities for the occurrence of capillary condensation.