Nucleation and growth processes occurring during the dehydration of certain alums: the generation, the development and the function of the reaction interface

Abstract

Recent advances in microscope techniques have been used in a reinvestigation of the kinetics of the formation and the growth of nuclei developed during dehydration of cleaved (111) faces of relatively perfect crystals of the alums KAl(SO$_4$)$_2\cdot$12H$_2$O and KCr(SO$_4$)$_2\cdot$12H$_2$O, and solid solutions of five intermediate compositions, KAl$_x$Cr$_{1-x}$(SO$_4$)$_2\cdot$12H$_2$O. There was observational evidence that nuclei are usually produced at sites of identifiable surface imperfection and that such sites are not randomly distributed across the reactant surfaces. Nucleation is a deceleratory process and, during the later stages, the continued generation of new reaction sites was associated with surface cracking. The rates of interface advance are constant at the boundaries of established nuclei, though this stability sometimes followed an initial period of either more or less rapid growth. The implications of these observations are discussed for the formulation of yield-time kinetic expressions for solid state decomposition. The activation energies determined for the constant rate of interface advance during dehydrations of chrome alum and the solid solutions were between 70 and 93 kJ mol$^{-1}$, with no systematic dependence on composition discerned, and the value for aluminium alum was greater (108 kJ mol$^{-1}$). Textural examinations of partially reacted surfaces give evidence that exposure to vacuum results in modification, attributable to water loss, of all the reactant surface and that dehydration is not restricted to the interface comprising the reactant-product contact zone. It is believed that each growth nucleus arises as a consequence of a local recrystallization and this is promoted at the interface by water temporarily retained within cracks and pores of the reactant and between the finely divided product particles. We also conclude that local surface stresses exert a more significant control upon these reactions than has hitherto been generally realized. This model contrasts with many mechanistic features of the accepted theory of the subject. Rates of nucleus growth in the five solid solutions (mixed alums) were less than those of the pure end members, a minimum reactivity was found at the composition 2Al:1Cr. There were no significant changes in activation energy with composition and we discuss the significance of the Arrhenius parameters calculated for these reactions.