Water sorption on composite “silica modified by calcium nitrate”

Composite SWS-8L was synthesized by impregnating a mesoporous silica KSK with a saturated aqueous solution of Ca(NO3)2. Its equilibrium with water vapor was studied by measuring sorption isobars and isosters at T = 30–150 °C. The water sorption increased up to 0.2–0.3 g/g due to the presence of the salt inside the silica pores. The water uptake was not a simple addition of those by the bulk salt and the silica, because the confined salt changed its properties. The isosteric sorption heat decreased from 52 to 47 kJ/mol when the water uptake increased from 0.07 to 0.23 g/g. The X-ray diffraction study revealed no formation of crystalline dihydrate in the confined state during the hydration process that can be caused by the dihydrate dissolution inside the pores. For approximation of the sorption equilibrium, a simple analytical equation based on the Polanyi principle of temperature invariance was suggested.Sorption kinetics was studied by the Isothermal Differential Step method at T = 50 and 70 °C and the pressure range 10–35 mbar. Confinement of calcium nitrate to the silica pores significantly hastened the sorption with respect to the bulk salt. The hydration of the confined salt was limited by the intraparticle water diffusion instead of intrinsic chemical transformation of the bulk salt.Comparison of the new composite adsorbent with those based on CaCl2, MgSO4 and LiBr confined to the same host matrix showed that SWS-8L can be envisaging further development for adsorptive air conditioning driven by low temperature heat.