The kinetics and mechanism of water evolution from molten dl lithium potassium tartrate monohydrate

Abstract

A kinetic and microscopic investigation of the thermal dehydration of dl lithium potassium tartrate monohydrate is reported and the reaction mechanism discussed. This work forms part of a more comprehensive study concerned with the influence of reactant structure on the reactivity and the mechanism of chemical change. The other hydrated reactants with which this salt will be compared contain the d and meso forms of the tartrate anion and crystallize with different structures. dl lithium potassium tartrate monohydrate lost the single molecule of water of crystallization in one predominantly deceleratory process that was studied between 350-460 K. Reaction was accompanied by melting to yield a residual glassy anhydrous product that was amorphous to X-ray diffraction. An initial, relatively rapid release of water (6%) was followed by a deceleratory process that led to a zero-order reaction (that, in crystals, extended between 18% and 80% ) before completion by an approximately first-order stage. Dehydrations of crushed powder reactant samples differed from single crystals in being relatively more rapid (an eight-fold increase); the deceleratory process was long and the zero-order process shorter (50-85%). The activation energy for dehydrations of crystal and of powder was 330 + 30 kJ mol -1 . This pattern of kinetic behaviour was not in accordance with expectation for a homogeneous reaction, the rate was not directly related to reactant concentration terms. Alternative analyses of the obedience of data to rate expressions applicable to solid state reactions were equally unsuccessful. Our mechanistic interpretation of the rate data, therefore, considered a priori the factors expected to participate in the control of water evolution from the melt. It is concluded that the vitreous or molten phase is not homogeneous and, therefore, behaviour is different from reactions in an isotropic fluid or in a solid. Two models are proposed to explain our observations. In the two phase equilibrium mechanism it is assumed that the reactant particles are composed of two phases, zones of hydrate are embedded in dehydrated material that retains a constant but small proportion of water. (These phases participate in an equilibrium analogous to that of liquid/vapour.) The surface boundary layer model envisages the initial development of a peripheral barrier zone through which the constant rate of water diffusion is rate controlling. This class of reaction, proceeding in a fluid but the absence of added solvent, has received relatively little attention. The present discussion is intended to identify the characteristic behaviour and draw attention to the necessity to consider such mechanisms in discussions of reactions of solids where there is the possibility of melt participation.