Anomalous Diffusivity in Porous Solids: Levitation Effect

Fluids confined to zeolites and other porous solids exhibit many distinct properties. One such property is the diffusivity, which exhibits anomalous dependence on the size of the guest molecule confined to the pore. This is termed the levitation effect. A diffusivity maximum as a function of the diameter of the guest is seen. The diameter for which the guest has maximum diffusivity is seen to be associated with a minimum in the activation energy. The existence of similar behavior in other porous solids, framework flexibility, and effect of temperature are discussed. Experimental verification of the existence of the anomalous maximum is then discussed. Diffusion of n-hexane and its isomers in zeolite NaY are then discussed in detail. The reduction in the end-to-end length of n-hexane while passing through the 12-ring window and the reasons for the same are discussed. Section 3 discusses possible observations of size-dependent maximum in other condensed matter systems