Neutron scattering and molecular dynamics evidence for levitation effect in nanopores

Neutron scattering measurements and molecular dynamics simulations have been carried out on the three isomers of pentane (neopentane (neo), isopentane (iso), and n-pentane (n-)) adsorbed in zeolite NaY. The results show that the self-diffusivity of these isomers follow the order Ds(neo)>Ds(iso)>Ds(n-), suggesting that the larger the cross section perpendicular to the molecular axis of the isomer, the higher the self-diffusivity. This counterintuitive result provides the first direct experimental evidence in support of the mutual cancellation of forces on the diffusant leading to a diffusivity maximum and is often referred to as the levitation effect. We also provide a direct confirmation of the experimental observations by Kemball (Adv. Catal. 1950, 2, 233) by calculating the entropy.