Formation of low-density water clusters in the silicalite-1 cage: A molecular dynamics study

A series of molecular dynamics simulations have been performed to examine changes to the structural and dynamical properties of water molecules in silicalite-1 as a function of temperature and loading. The ab initio fitted silicalite-1/water potential which is newly developed(18) and the BJH flexible water/water potential(15) have been employed. The water loading was varied from I to 8 water molecules per intersection, equivalent to 8-64 molecules per simulation cube. The simulations have been carried out at 298 and 393 K. The results show that the water structure inside the silicalite-1 cages changes dramatically as a function of loading. We found that the probability of water molecules residing in a straight channel is always higher than that of residing in the sinusoidal channels. Under high loading, the observed clusters form a structure similar to that of pure water. We call it a "low-density cluster" for the following reasons: (i) The cluster consists of five water molecules (four in the first hydration shell of the central one) which is consistent with that of pure water. (ii) However, molecules in the cluster are not coordinated together via hydrogen bonds. The radius of the first hydration shell of 3.35 Angstrom is 0.5 Angstrom longer than that of pure water. (iii) Molecules in the cluster are less flexible than those of pure water. In terms of dynamical properties, for low loadings, a preferential diffusion path is observed along the center of the channel tube. The water molecules were detected to diffuse closer to the surface when the concentration was higher than six molecules per intersection. The diffusion coefficient of water decreases when the concentration increases. The D values for all concentrations at 393 K are higher than those at 298 K. The temperature dependence almost disappears at a loading of eight water molecules per intersection. In addition, the anisotropic diffusion is less pronounced for water in silicalite-1 in comparison to that of nonpolar molecules.