Heats of immersion of silica gels: Effects of pore narrowing

Abstract A microporous silica gel (Davison 03) and a mesoporous silica gel (Davison 59) were thermally treated in Vacuo in the temperature range 20–510 °C. The effects of thermal treatment on the water contents, nitrogen surface areas, average pore radius and heats of immersion in both water and methanol were investigated and discussed. The results obtained indicate that the heat of immersion of a microporous silica gel (Davison 03) in water is actually less than the heat of immersion of a mesoporous silica gel (Davison 59). Thus, besides other factors to be considered, pore narrowing can actually lead to a decrease in the heat of immersion of silica gel in water. However, the situation is reversed when methanol is used as the immersion liquid and, owing to the dispersion forces involved, pore narrowing could lead to an increase in the measured heat of immersion in methanol. Another contributing factor, namely the chemistry of the surface, i.e. the number of hydroxyl groups per unit surface area has also been considered. Preliminary studies are presented for the heat of immersion of the same solids in cyclohexane, where the forces involved are purely dispersion, but with such bulkier molecules another complicating factor should be introduced, namely the accessibility of the adsorbate molecules to micropores, and the availability of the surface for immersion.