Molecular dynamics studies of hydrated zeolite 4A

Molecular dynamics (MD) simulations of hydrated zeolite 4A incorporating a mobile zeolite framework have been performed at 298 K for a range of hydration over a time scale of 0.5 us. The hydration energy of the water is found to be 57 kJ/mol of water at full hydration, in excellent agreement with previous MD simulations. The water molecules are found to locate at preferred sites on the inside of the alpha-cages with additional water located within the volume of the alpha-cages. The latter are more mobile. The self-diffusion coefficient of the water molecules contained in the alpha-cages is found to increase from 2.7 x 10(-10) m(2)/s with 56 water molecules per unit cell to 6.8 x 10(-10) m(2)/s with 168 water molecules per unit cell. The diffusion coefficient is found to decrease close to full hydration, namely 224 water molecules per unit cell, due to the blocking effect of water molecules in the fully hydrated cages. The MD self-diffusion coefficient at full hydration is found to be approximately 3 times the experimental value, with evidence that a longer simulation will yield an improved agreement with experiment. The water self-diffusion coefficients are approximately 1 order of magnitude larger than the self-diffusion coefficients of the sodium ions, suggesting that the sodium ions are relatively strongly bonded to the zeolite cage and their primarly effect is to hinder the motion of water between alpha-cages.