Adsorption and Desorption of Water in Two-Dimensional Hexagonal Mesoporous Silica with Different Pore Dimensions

We have investigated the adsorption and desorption of water in two-dimensional hexagonal mesoporous silica (MPS) with three different pore dimensions. All the adsorption–desorption isotherms exhibited type V behavior with significant hysteresis. Hysteresis was observed in the transient region between the layer adsorption state and the pore filling state, where capillary condensation and evaporation occurred. When the relative humidity is changed in steps from the onset point of capillary condensation and capillary evaporation, the relaxation curves of water uptake and release were measured at 298 K using a gravimetric method. For all MPSs, the relaxation rates of water uptake and release increased as the magnitude of the stepwise change in relative humidity increased. The relaxation curves of some MPSs could be well fitted by the exact solution of the Fickian diffusion equation at a small stepwise change in relative humidity. On the other hand, the adsorbed mass is proportional to the square root of time at large stepwise changes in relative humidity. The relationship between the mass flux of water uptake or release and the difference in chemical potential between the initial and final states of moist gases depends on the structural properties of the mesopores.