Adsorption isotherm for flexible molecules in random porous media. Can we regard the system as a binary mixture?

Abstract

The adsorption isotherm for repulsive n-alkanes in a random porous medium has been determined by computer simulation. The porous medium or matrix is obtained by quenching an equilibrium configuration of repulsive spheres. Keeping fixed this configuration of the matrix particles, Grand-Canonical Monte Carlo simulations were used to determine the adsorption isotherm of repulsive n-butane and n-hexane models in the random porous medium. In addition to that, canonical Monte Carlo simulations were performed for the equilibrium binary mixture in which the spherical and n-alkane particles are allowed to move. Chemical potential of the n-alkane molecule in the binary mixture is computed by using the test particle method. We found that, at the same chemical potential, the structure of the n-alkane in the binary mixture and in the quenched medium differs at high densities. However, at the same chemical potential, densities of the n-alkane in the random porous medium and in the binary mixture are quite similar. A consequence of that is that adsorption isotherms can be computed if the properties of the binary mixture are known. We have recently proposed an equation of state for mixtures of hard n-alkane molecules which contains as a particular case the sphere+n-alkane mixture. The chemical potential of the n-alkane in the mixture can be computed analytically and we used this result for determining adsorption isotherms. Excellent agreement with simulation is found. The results of this work suggest that the knowledge of the properties of the binary mixture can be very useful for obtaining adsorption isotherms in random porous media. Finally, the effect of attractive forces on the properties of alkanes in binary mixtures and confined in a random porous medium was also studied. In general, the presence of attractive forces reduces the similarity between the properties of alkanes in these two systems.