EFFECT OF ADDITIVES ON NUCLEATION RATE, CRYSTAL-GROWTH RATE AND INDUCTION TIME IN PRECIPITATION

A theory is proposed and general expressions are derived for the nucleation rate, the rate of crystal growth by two-dimensional nucleation and the induction time in unseeded precipitation in the presence of additives. According to the theory, the additives behave as active centres for nucleation and do not alter the surface free energy and edge free energy of the nuclei by adsorption on them, because of the short lifetime of the nuclei and/or their small surface area. The surface free energy and edge free energy of the nuclei formed on the additives, however, can be different from the surface free energy and edge free energy of the nuclei on the original active centres in heterogeneous nucleation. Induction times in unseeded barium sulphate precipitation are measured as a function of the concentration of an additive, a co-polymer of maleic acid and vinyl sulphonic acid, and interpreted by the proposed theory. It is concluded that this theory describes fairly well the obtained experimental data, which show a stimulation of the precipitation process at very low additive concentrations and a change-over into retardation at an additive concentration of almost-equal-to 0.005 ppm.