Kinetics of contracting geometry-type reactions in the solid state: implications from the thermally induced transformation processes of α-oxalic acid dihydrate.

This study focuses on the physico-geometrical constraints of the kinetics of the thermal decomposition of solids as exemplified by the thermal dehydration of α-oxalic acid dihydrate and the subsequent thermally induced sublimation/decomposition of the as-produced anhydride using the samples of crystalline particles (CPs) and a single crystal (SC) form. The CP and SC samples possess approximately similar geometrical figures with different sizes. The shapes of the original dihydrate and the as-produced anhydride from thermal dehydration are practically congruent. Therefore, proper evaluations of the current kinetic understanding of contracting geometry-type reactions were expected by the comparisons of the kinetic behaviors among different sample forms and thermally induced processes. The kinetic analysis of the thermal dehydration process revealed that the consecutive physico-geometrical processes comprised of an induction period, a surface reaction, and a phase boundary-controlled reaction, where distinguishable differences in the rate behavior were observed between the CP and SC samples for the surface reaction. On the other hand, the thermally induced sublimation/decomposition of the anhydride was described as an ideal single-step geometry contraction process, for which the CP and SC samples exhibited the same rate variation behavior under isothermal conditions. However, the sublimation/decomposition processes of the CP and SC samples were characterized by the different Arrhenius parameters, in which the compensative changes in the apparent activation energy and preexponential factor were apparent. Implications for the kinetic modeling of the solid-state reactions and the interpretation of kinetic results were obtained from the results of the comparative kinetic study for different sample forms and thermally induced processes.