Melting of ice in porous solids

Abstract

Differential scanning calorimetry (d.s.c.) and nuclear magnetic resonance (n.m.r.) techniques were used to study the melting of ice in porous solids. At low water contents (less than about three monolayers) no freezing or melting was observed. As the water content was increased, a single melting peak was observed as the pores filled. The amplitude of this peak reached a constant value when the pores were completely filled, and then at higher water contents a second peak was observed increasing in amplitude as more water was added. The lower melting point was characteristic of the pore water and this melting point decreased with decresing pore radius. The higher melting point (0°C) was that of the non-pore, bulk, water. The pore volume determined calorimetrically agreed with the quoted pore volume. A coefficient of 0.9 for the linear correlation between melting point depression (ΔT) and the reciprocal radius (1/r) confirmed the applicability of the Kelvin equation. It is concluded that the observed properties of water in pores are essentially bulk properties at distances of more than about 10 Å from the surface, and influenced more by the Kelvin effect, i.e., capillarity, than by the surface of the silica. D.s.c. can be used as a rapid method to characterise these silicas. Two simple measurements on silica samples containing water allow an estimation of the surface area, pore volume and an average pore size to be made.