Levitation effect: Size-dependent maximum in rotational diffusion in confined systems

Abstract

Molecular dynamics of a model guest tetrahedral molecule AX(4) with differing bond lengths l(AX) for A-X bonds have been carried out in a sphere with different surface roughness. The rotational-diffusion coefficient D(R) shows a maximum for a particular value of l(AX). We show that this corresponds to the distance at which the interaction of the guest with the atoms of the host is most favorable. Although, the intensity of the maximum decreases with increase in the roughness of the confining surface, it is seen that the maximum exists even for a reasonably high degree of roughness. The observed maximum arises from the minimum in the torque on the tetrahedral molecule from its interaction with the confining medium due to mutual cancellation of forces. Activation energy for rotation is seen to be also a minimum for the bond length for which D(R) is a maximum. These results suggest that there is a maximum in the rotational-diffusion coefficient when the rotating molecule is confined to a sphere of comparable size similar to the maximum in translational diffusion coefficient seen in porous solids (the levitation effect) [Yashonath and Santikary J. Phys. Chem. 98, 6368 (1994)]. On increase in the roughness of the sphere surface, the value of l(AX) at which the maximum in D(R) is seen decreases.

Diffusion in Nanoporous Phases: Size Dependence and Levitation Effect

“…Since we are discussing polyatomic systems, it is relevant to ask whether a size dependent maximum exists in the rotational diffusion coefficient, D r , as well. Recent studies by Sharma et al 166 attempted to study rotational motion of model tetrahedral molecules (similar to methane) confined to a model sphere. The bond length of the central atom to the atom at the periphery was varied to obtain tetrahedral molecules of different sizes.…”