Evaluation of water in silica pores using differential scanning calorimetry

Abstract

The state of water confined in the pores of commercial silica gels has been investigated as a function of the water content using differential scanning calorimetry (DSC). There were at least two kinds of water present: freezable pore water and nonfreezable pore water. The amounts of these types of water were precisely determined as a function of total water content using the temperature dependence of the heat of melting of pore ice due to the melting temperature distribution. In all the silica gels, an endothermic peak of freezable pore water appeared after the nonfreezable pore water content was saturated, and then an endothermic peak of bulk water overflowing the silica gel appeared after both freezable and nonfreezable pore water content was saturated. The thicknesses of the layers of nonfreezable pore water were estimated to be 0.4–0.8 nm, which corresponds to two or three monolayers of water. Considering the densities of the various types of water, the pore volumes were also calculated from the DSC curves and were found to be in good agreement with those obtained using nitrogen gas adsorption-desorption. By using the average densities of water, the densities of freezable and nonfreezable pore water were estimated to be similar to those of bulk water and bulk ice, respectively.