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

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 (H 2 O)) using a humidity-controlled thermogravimetry instrument equipped with a cooling circulator. The universal kinetic description at various temperatures ( T ) and p (H 2 O) values was evaluated using the extended kinetic equation with an accommodation function (AF) comprising p (H 2 O) 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 N 2 into the universal kinetic description as a function of T , α , and p (H 2 O). 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 H 2 O 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 (H 2 O) 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.