A kinetic and mechanistic study has been undertaken of the nucleation and growth reaction that is the second of the two consecutive rate processes that occur during the dehydration of
d
lithium potassium tartrate monohydrate. Electron microscopic examinations of the cleaved surfaces of partly reacted crystals show the development of three-dimensional nuclei that are composed of small crystals of the anhydrous product and above 450 K there is evidence of intranuclear melting. Consistent with this model, the second reaction obeys the Avrami-Erofe’ev equation {[ — ln (1 — α)]
1/2
=
kt
}. Overall rates of the dehydrations of single crystals and of crushed powder samples were closely similar. The activation energy for dehydration was 150-160 kJ mol
-1
for both first (reported in part I, preceding paper) and second reactions and for both single crystal and crushed powder reactants. The addition of product crystallites to the reactant reduced sharply, or eliminated, the induction period to the nucleation and growth process. From consideration of the kinetic characteristics, together with the textural changes observed microscopically, we conclude that the following mechanism very satisfactorily accounts for our results. The first reaction proceeds to the dehydration of all crystal surfaces, representing water losses from a layer
ca
. 10 µm thickness. This deceleratory process occurs initially in a structure resembling that of the reactant but later the increasing water site vacancy concentration results in increasing reactant disorder and possibly includes fusion of the outer layer. When the first reaction water evolution has slowed, recrystallization to the structure of the anhydrous product occurs at a limited number of sites to generate germ nuclei that effectively act as seed crystals for nucleus growth. During the second reaction the reactant—product contact interface is identified as a zone of diffusive water loss, similar to that described for the first reaction. Here, however, the product crystallites promote reorganization of dehydrated material, thereby opening channels for water escape and continually exposing new hydrate surfaces at which dehydration continues. This product recrystallization enables advance of the nucleus interface to be maintained, so that rates of both first and second reactions are subject to control by diffusive loss of water from an active boundary of the reactant. Product reorganization removes the inhibiting character of accumulated product layer by introducing escape channels for water loss so that interface advance continues and, although spasmodic, this migrates forward at a constant average linear rate. The work is of interest because kinetic measurements have been obtained for both of the consecutive rate processes that contribute to the overall reaction. The controls of both are shown to be closely similar. The reaction model proposed here provides insight into the structure of the dehydration interface and the mechanism of water release.
