Hydration Kinetics in Inorganic Salt–Water Vapor Systems: A Case of Lithium Sulfate

A kinetic theory and its application to the hydration reaction of inorganic salts were investigated with the aim of developing a universal description over a range of temperatures (T) and partial pressures of water vapor (p(H₂O)). The hydration reaction of lithium sulfate anhydride (LS-AH) to form its monohydrate was selected as a model reaction and systematically traced at different T and p(H₂O) values using a humidity-controlled thermogravimetry. The hydration process exhibited an induction period (IP) at a constant temperature. Subsequently, a sigmoidal mass gain process was observed, which was attributed to a consecutive surface reaction (SR) and phase boundary-controlled reaction (PBR). The reaction rates of the IP and mass gain process exhibited a reduction and an increase, respectively, with increasing T and p(H₂O) values. A conventional kinetic analysis that did not consider the effect of p(H₂O) revealed several issues with the universal kinetic description. These issues were addressed in steps with the development of an extended kinetic equation, which was formulated by incorporating an accommodation function into the fundamental kinetic equation. Consequently, the IP and mass gain processes of the LS-AH hydration were universally described over a range of T and p(H₂O) values, and intrinsic kinetic parameters with reasonable physicochemical significance were determined. Furthermore, this approach was extended to the physico-geometrical kinetic modeling, based on the consecutive IP–SR–PBR model.