Conformational analysis of n-butane in zeolite NaCaA: Temperature and concentration dependence

Investigations on n-butane sorbed in zeolite NaCaA have been carried out at several temperatures and sorbate concentrations in order to obtain the conformational properties. The configurational-bias Monte Carlo method has been employed. Results are compared with pure or unconfined fluid at the same density as the confined fluid. This provides insight into the role of the host on the conformational properties of n-butane. Surprisingly, unlike what is observed in silicalite or faujasite, the influence of zeolite on n-butane conformation is dependent on sorbate concentration. Effect of zeolite A is to decrease gauche conformer population at low sorbate concentrations and enhance it at high sorbate concentrations as compared to pure or unconfined fluid at the same density. It is shown that the conformational properties of n-butane in zeolite A are determined largely by the proximity of n-butane to the inner surface of alpha-cage and that steric factors have a relatively minor role. This is in contrast to silicalite where it is known that the steric factors play a major role. Such a proximity necessitates the curvature of the n-butane to be similar to that of the inner surface of the alpha-cage resulting in an enhancement of gauche conformer population. It is shown that sorbate-zeolite as well as sorbate-sorbate dispersion interactions are responsible for the proximity of the n-butane to the inner surface of the alpha-cage. Absence of sorbate-zeolite dispersion interactions gives rise to an increase in the gauche conformer population. The radial distribution function suggests that the confined fluid is more structured than the pure fluid at the same density indicating the strong influence the host has on the fluid structure.