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

Abstract

Molecular dynamics of a model guest tetrahedral molecule AX₄ 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_AXat which the maximum in D_R is seen decreases.