Kinetics of Capillary Condensation of Water in Mesoporous Carbon: Nucleation and Meniscus Growth

Abstract

To examine the mechanisms of nucleation and subsequent meniscus growth in capillary condensation of water vapor in graphitic mesopores that occurs very slowly close to the saturation, we measured the kinetic curves of water uptake in four kinds of mesoporous carbons as a function of temperature. Plotting the amount of uptake against the square root of time gave typical Fickian curves. The plots produced a nonzero intercept with time axis, and for the mesoporous carbons without micropores, the effective value of the intercept increased with an increase in pore size. This suggests that in the initial stage of capillary condensation the number of mesopores with liquid plugs of water at the pore entrances increases with time. In addition, the dependence of the rate constant on pore size strongly suggests that the meniscus growth of water in the graphitic mesopores is controlled by the gradient of chemical potential between the saturated water vapor surrounding the mesoporous carbons and the vapor–liquid equilibrium for the confined water. The rate of water uptake tended to decline rapidly as the temperature approached 0 °C. For the mesoporous carbon with micropores the dependence of the rate constant on pore size and on temperature was different from that without micropores, indicating that the mechanisms of nucleation and meniscus growth are different.