THE SOLID-STATE DEHYDRATION OF D-LITHIUM POTASSIUM TARTRATE MONOHYDRATE IS COMPLETED IN 2 RATE-PROCESSES .2. THE NUCLEATION AND GROWTH 2ND REACTION AND DEHYDRATION MECHANISM

A kinetic and mechanistic study has been undertaken of the nucleation and growth reaction that is the second of the two consecutive rate pro cesses that occur during the dehydration of d lithium potassium tartra te monohydrate, Electron microscopic examinations of the cleaved surfa ces of partly reacted crystals show the development of three-dimension al 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 equati on {[-In (1 - alpha)]1/2 = kt}. Overall rates of the dehydrations of s ingle 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 bo th single crystal and crushed powder reactants. rhe addition of produc t crystallites to the reactant reduced sharply, or eliminated. the ind uction period to the nucleation and growth process. From consideration of the kinetic characteristics, together with the textural changes ob served microscopically, we conclude that the following mechanism very satisfactorily accounts for our results. The first reaction proceeds t o the dehydration of all crystal surfaces, representing water losses f rom a layer ca. 10 mum thickness. This deceleratory process occurs ini tially in a structure resembling that of the reactant but later the in creasing water site vacancy concentration results in increasing reacta nt disorder and possibly includes fusion of the outer layer. When the first reaction water evolution has slowed, recrystallization to the st ructure of the anhydrous product occurs at a limited number of sites t o generate germ nuclei that effectively act as seed crystals for nucle us growth. During the second reaction the reactant- product contact in terface is identified as a zone of diffusive water loss, similar to th at described for the first reaction. Here, however, the product crysta llites promote reorganization of dehydrated material, thereby opening channels for water escape and continually exposing new hydrate surface s at which dehydration continues. This product recrystallization enabl es advance of the nucleus interface to be maintained, so that rates of both first and second reactions are subject to control by diffusive l oss of water from an active boundary of the reactant. Product reorgani zation removes the inhibiting character of accumulated product layer b y introducing escape channels for water loss so that interface advance continues and, although spasmodic, this migrates forward at a constan t average linear rate. The work is of interest because kinetic measure ments have been obtained for both of the consecutive rate processes th at contribute to the overall reaction. The controls of both are shown to be closely similar. The reaction model proposed here provides insig ht into the structure of the dehydration interface and the mechanism o f water release.