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 nucle ation rate, the rate of crystal growth by two-dimensional nucleation a nd the induction time in unseeded precipitation in the presence of add itives. According to the theory, the additives behave as active centre s for nucleation and do not alter the surface free energy and edge fre e energy of the nuclei by adsorption on them, because of the short lif etime of the nuclei and/or their small surface area. The surface free energy and edge free energy of the nuclei formed on the additives, how ever, can be different from the surface free energy and edge free ener gy of the nuclei on the original active centres in heterogeneous nucle ation. Induction times in unseeded barium sulphate precipitation are m easured as a function of the concentration of an additive, a co-polyme r of maleic acid and vinyl sulphonic acid, and interpreted by the prop osed theory. It is concluded that this theory describes fairly well th e obtained experimental data, which show a stimulation of the precipit ation process at very low additive concentrations and a change-over in to retardation at an additive concentration of almost-equal-to 0.005 p pm.