Molecular dynamics studies of sodium diffusion in hydrated Na+-zeolite-4A

Molecular dynamics simulations of hydrated Na+-zeolite-4A incorporating a mobile zeolite framework have been performed at 298 K for a range of hydration. The sodium ions in Na(I) sites, located at the centers of the six-rings, are found to be virtually immobile at all hydrations. These ions may diffuse very slowly by knock-on events in which an ion is knocked out of position by a mobile ion and immediately replaced. The sodium ions in Na(2) and Na(3) sites, associated with the eight-rings and four-rings respectively, diffuse between these sites with a self-diffusion coefficient ranging from 7 x 10(-12) m(2)/s in the dehydrated crystal to about 100 x 10(-12) m(2)/s in the fully hydrated material. The self-diffusion coefficient of the mobile ions increases with hydration, in agreement with the results of conductivity experiments. Although the mobile ions each make several elementary hops during the 0.5 ns of the simulation, this time scale is still too short to be certain that the diffusion coefficients reflect intercavity diffusion rather than intracavity diffusion. Finally, it is shown that the sodium ions in the Na(2) and Na(3) sites are preferentially hydrated at low hydration.