Simulations of hydrated Na+-zeolite 4A, incorporating a mobile zeolite framework, were performed at 298K for a range of hydrations. The sodium ions in Na(1) sites, located at the centers of the six-rings, were found to be virtually immobile for all hydrations. These ions could diffuse very slowly via knock-on events in which an ion was knocked out of position by a mobile ion and was immediately replaced. The sodium ions in Na(2) and Na(3) sites, associated with eight-rings and four-rings respectively, diffused between these sites with a self-diffusion coefficient ranging from 7 x 10-12m2/s in dehydrated crystal to about 100 x 10-12m2/s in fully hydrated material. The self-diffusion coefficient of the mobile ions increased with hydration, in agreement with the results of conductivity experiments. Although the mobile ions each made several elementary hops during the 0.5ns of the simulation, this time-scale was too short to be certain that the diffusion coefficients reflected intercavity diffusion rather than intracavity diffusion. It was shown that the sodium ions in the Na(2) and Na(3) sites were preferentially hydrated at low hydrations.

Molecular Dynamics Studies of Sodium Diffusion in Hydrated Na+-Zeolite-4A. D.A.Faux: Journal of Physical Chemistry B, 1998, 102[52], 10658-62