Heats of water sorption on silica gel containing CaCl2 and LiBr

The heats of sorption of water vapor on the calcium chloride (or lithium bromide)-porous matrix (KSK and KSM silica gels and MSM-41 sorbent) composite sorbents and on pure silica gels were measured by calorimetry. It was shown that samples based on KSK mesoporous silica gel can feature (a) adsorption on the surface of silica gel (pure or modified with salt ions) with a heat effect of Q = 76-84 kJ/mol (at sorption values w < 1.0 wt %), (b) formation of lower crystal hydrates of the salt with Q = 66-72 kJ/mol (at 1 wt % < w < 9-14 wt %), and (c) vapor absorption with Q = 50-54 kJ/mol by a salt solution in pores (at w > 9-14 wt %). Sorption mechanisms (a) and (c) were also observed for samples based on KSM microporous silica gel and MSM-41. The possibility of the formation of solid salt hydrates in micropores was discussed. It was revealed that the heats of sorption measured in the regions of formation of hydrates and solutions are close to those obtained earlier by the sorption method. It was shown that the introduction of a hygroscopic salt into silica gel pores substantially increases the sorption capacity of the composite system compared to the initial host matrix.