Thermoporometry

Abstract

The texture of porous materials with pore diameters larger than about 2 nm can be characterized by a calorimetric investigation of the liquid-solid state transition of a capillary condensate saturating these materials. The method, called thermoporometry, is based on the thermodynamic conditions of the liquid-solid transformation of the capillary condensate inside the porous material [l, 21. Thermoporometry is mainly used for the determination of pore-size distribution. Upon solidification and melting of a condensate that totally saturates the porous material, the phase change takes place inside the pores and no material transport occurs. This is not the case in most adsorption experiments. The resulting freezing-point depression of the confined probe molecules and the thermal energy released or absorbed are measured as a hnction of the temperature [3]. The thermal analysis of these transformations for condensates such as water or benzene leads to the pore-size distribution through the "solidification curve". The pore shape is related to the hysteresis phenomenon observed between the solidification and melting curves, allowing a shape factor to be calculated. Thermoporometry has the following advantages:(1) it gives the actual size of the cavities instead of indicating the size of the (2) it can be performed on nonrigid materials, and (3) it can be used for the study of materials in the medium in which they are opening, applied.Thermoporometry can be used also for studying materials in which the pore diameters are in the range of approximately 2-30 nm (i.e., in the mesopore range).