(200i) Molecular Simulation Study of Water Uptake in Mesoporous Silica Thin Film

Canonical ensemble molecular dynamics (NVT-MD) simulations were performed to investigate the kinetics of water uptake into an atomic model of mesoporous silica thin film. Adsorption/desorption of water in mesoporous silica exhibits phase-change-like behavior in adsorption/desorption processes with a pronounced hysteresis loop which is attributed to capillary condensation and evaporation. It is well-known that physical and thermodynamic properties of liquids confined or adsorbed in nanopores shows significant differ from that of bulk phase. These difference are due to the fact that the liquid-liquid and liquid-pore wall interactions are not negligible, because of characteristic scale length of the system is comparable with range of intermolecular interactions. In particular, there are strong interactions due to hydrogen bonding between water and surface functional groups of silica (silanol group) in the silica-water system. Therefore, the behavior of water transport near the silica surface should be complicated. In order to clarify the water adsorption in mesoporous silica and to design new adsorbents of water, it is important to understand the kinetics of water transport in the mesoporous silica at a molecular level. The objectives of this study were to elucidate the kinetics of water uptake into the mesoporous silica from a molecular point of view.

In this sturdy, mesoporous silica thin film was modeled as a thin film of α-quartz crystal with a finite cylindrical pore normal to the film surfaces. The crystal structure was sliced by two parallel planes (top and bottom planes) and a finite cylindrical pore was aligned normal to the two parallel planes. Consequently, the stable water adsorption layer and the water meniscus can be formed. The effect of pore radius on the adsorption properties was assessed using two models of mesoporous silica thin films having different pore radius of 1.38 nm and film thickness of 7.93 nm. Initially, two water liquid thin films of N/2 molecules, which obtained by NVT-MD calculation at 300 K, were placed on the upper and lower surfaces.

The calculation results showed that the kinetics of water uptake depended on the number of water molecules and there were two different transport mechanisms in the pore. One was diffusion of water along the inner surface of the pore, and the other was capillary rise of liquid water accompanying the formation of meniscus at the pore entrance. The latter could be expressed by the continuum dynamics. The dynamical properties of water molecules vicinity of silica surface were affected by the state of adsorption. We also discussed the molecular pictures of water uptake on mesoporous silica thin films.