Characterization of nonfreezable pore water in mesoporous silica by thermoporometry

The freezing/melting behavior of water confined in mesopores was evaluated by differential scanning calorimetry (DSC) using micropore-free SBA-15 materials with different pore sizes as model materials. We determined the mesoporous structure (pore size distribution, specific surface area, and pore volume) by using Ar gas adsorption/desorption measurements, and investigated the thickness dependence of nonfreezable pore water (t(nf)) on pore size. The t(nf) value was calculated as the difference between the pore radius, calculated from an Ar adsorption isotherm using the nonlocal density functional theory (NLDFT) analysis, and the radius of ice crystals that formed in the mesopores during the DSC measurement. Several studies have reported t(nf) values ranging from 0.35 to 1.05 nm depending on the evaluation method, whereas the t(nf) value estimated for the SBA-15 used in this work was approximately 0.7 nm. The difference between the reported t(nf) values and the value obtained in this work is mainly due to an underestimation of the pore diameter by the method based on the classical Kelvin equation. After appropriate correction of the pore diameter, the reported t(nf) value agreed with our results.