Evaporation rates of water contained within porous silica particles

We have used a gravimetric technique to measure the rate of evaporation of water contained within the pores of powdered silica particles. The experimental conditions are such that the rate is directly proportional to the water vapour pressure. For silica powders of different mean pore sizes, the reduced water evaporation rates are in agreement with the reduction of vapour pressure predicted according to the Kelvin equation. For a particular silica powder, evaporation rates determined from samples with different water contents show the variation in effective pore radius as a function of pore filling volume. Experiments in which a partially water filled powder is overlaid with a dry powder show that the achievement of a homogeneous distribution of water throughout the powder sample takes several hours.