Efflorescence kinetics of sodium carbonate decahydrate: a universal description as a function of temperature, degree of reaction, and water vapor pressure

Abstract

The efflorescence of sodium carbonate decahydrate (SC-DH) required to form its monohydrate (SC-MH) was systematically studied under isothermal and linear nonisothermal conditions at different atmospheric water vapor pressures (p(H₂O)) using a humidity-controlled thermogravimetry instrument equipped with a cooling circulator. The universal kinetic description at various temperatures (T) and p(H₂O) values was evaluated using the extended kinetic equation with an accommodation function (AF) comprising p(H₂O) and the equilibrium pressure of the reaction (P_eq(T)). By optimizing two exponents in the AF, all kinetic data were universally described in terms of the isoconversional kinetic relationship examined at individual degrees of reaction (α). This enabled the examination of the isothermal kinetic relationship and the parameterization of the contribution of the self-generated water vapor, allowing the incorporation of kinetic data recorded in a stream of dry N₂ into the universal kinetic description as a function of T, α, and p(H₂O). The results indicated that the reaction is physico-geometrically controlled by the surface reaction at the hemispherical top surface of SC-DH particles and subsequent advancement of the reaction interface toward the center and bottom of these particles, where the interfacial process is regulated by an elementary step of the consumption of H₂O vacancies to form the SC-MH building unit. The apparent activation energy (E_a) of ∼178 kJ mol⁻¹ was determined using the extended kinetic approach considering the effect of p(H₂O) correlated with the intrinsic E_a of the Arrhenius-type temperature dependence (∼63 kJ mol⁻¹) by subtracting the contribution of the temperature dependence of P_eq(T) in the AF.