Sorption Properties of Calcium Nitrate Dispersed in Silica Gel: The Effect of Pore Size
Статья (Full article),
Russian Journal of Physical Chemistry A
, E-ISSN: 1531-863X
||Crystalline materials; Differential scanning calorimetry; Hysteresis; Nanostructured materials; Pore size; Silica gel; Sorption; Synthesis (chemical); Vapor pressure; X ray diffraction analysis
Novosibirsk State University, Novosibirsk, 630090, ul. Pirogova 2, Russian Federation
Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk, 630090, pr. Akademika Lavrent'eva 5, Russian Federation
New composite Ca(NO3)2-in-silica gel mesopore sorbents of water were synthesized and analyzed. Sorption isobars at a water vapor pressure of 17 mbar and various sizes of silica gel pores (6, 10, 12, and 15 nm) were measured over a temperature range of 30 to 160 °C. It was demonstrated that the composite sorbs water by three mechanisms: (a) heterogeneous adsorption on silica gel, (b) the formation of salt dihydrate, and (c) formation of salt aqueous solution in silica gel pores. The modification of silica gel with the salt increased significantly its sorption capacity for water (from 0.04-0.06 to 0.2-0.3 g/g). The sorption properties of the composite were found to deviate from those predicted by a linear combination of the properties of silica gel and bulk calcium nitrate. The temperature of conversion of the water-free salt into its crystalline dihydrate shifts increases with decreasing size of the pores in the host matrix. Near this transition, a sorption hysteresis was observed, which narrowed with increasing dispersity of Ca(NO3)2. The properties of a solution in the pores were found to be identical to those of the corresponding bulk solution, with no hysteresis observed. Measurements with the use of differential solvation, X-ray diffraction, and differential scanning calorimetry demonstrated that the properties of the salt change due to its being dispersed into nanosized grains rather than due to its interaction with the matrix.