Adsorption–Desorption and Transport of Water in Two-Dimensional Hexagonal Mesoporous Silica

Abstract

The adsorption–desorption isotherms and relaxation rates of water in Zr-doped two-dimensional hexagonal mesoporous silica with pore diameters of 3.8 nm were measured at 283–298 K using a gravimetric method. A type V adsorption–desorption isotherm with significant hysteresis was obtained. The adsorption layer was formed on the pore surface at low relative pressure of water vapor, capillary condensation was initiated at around 0.4–0.6 relative humidity, and then pore filling was completed near the saturated vapor pressure. The relaxation rate of water uptake in the capillary condensation state was much lower than that in the layer-adsorption state or pore-filling state. The isotherms and relaxation rates during desorption had a similar trend. Effects of the hydrophilicity of mesopores and temperature on the adsorption–desorption kinetics suggest that the water transport mechanism in mesopores includes activated processes. In the capillary condensation and evaporation states, the relaxation curve could be estimated by the Fickian diffusion equation at a small stepwise change in relative humidity. The relaxation curves at large stepwise change in relative humidity suggest that the transport mechanism could be liquid water flow due to capillary action.