Variation anormale de la vitesse de decomposition d'un solide—I

Experimental study and Monte-Carlo simulation of the nucleation and growth processes during the dehydration of Li2SO4·H2O single crystals

A kinetic model for the dehydration of lithium sulfate monohydrate is proposed in order to account for experimental data obtained on single crystals by thermogravimetry at 80 degrees C under fixed water vapour pressure, and by optical microscopy. This model is based on the assumptions of Mampel's model, the nucleation takes place randomly at the surface of the solid and is followed by isotropic growth toward the centre of the crystal. Calculated rates dalpha/dt are obtained by means of Monte-Carlo simulations and compared to the experimental ones, which leads to the determination of two kinetic constants: the areic frequency of nucleation (in number of nuclei m(-2) s(-1)) and the areic reactivity of growth (in mol m(-2) s(-1)).

A kinetic study of the dehydrations of the alums KCr(SO4)2. 12H2O and KAl(SO4)2. 12H2O

A kinetic study of the dehydrations of the alums KCr(SO4)2. 12H2O and KAl(SO4)2. 12H2O in water vapour atmospheres is reported. The product-yield-time data are described by the Avrami-Erofe’ev (n= 2) and contracting cube equations. The observed kinetic behaviour differed from expectation for this reaction, which is known to proceed through the growth of three-dimensional nuclei. The retention of the volatile product (water) in the vicinity of the reactant solid influences the course of interface generation and development during the early stages of dehydration. Later there is a ‘first order’ approach to the establishment of equilibrium between water vapour and the solid reactant/product mixture. This pattern of kinetic behaviour is confirmed by a parallel thermal analysis (DSC) study in dry nitrogen. Evidence was obtained that an appreciable quantity of water was retained by the solids throughout these reactions. This is consistent with the view that water, temporarily retained at the active reactant-product interface promotes the difficult recrystallization step. Photoacoustic studies on partly thermally dehydrated samples of KCr(SO4)2. 12H2O showed the formation of a temporary water depleted surface layer that readily rehydrated either by adsorption from the atmosphere or by facile intracrystalline water diffusion. All the present observations can be explained by a reaction mechanism based on two main features: (i) the dehydration reaction is initiated across the total surface but is limited in extent and cannot be considered topotactic; (ii) the rate determining step for the subsequent unlimited advance of dehydration into the solid bulk is product crystallization, possibly topotactic, and this confers apparent topotacticity to the whole processes.