“Catalyzed” nucleation: A critical review of proposed theories

In industrial-scale crystallization, secondary nucleation is the most important type of nucleation. In most cases, this phenomenon can be adequately explained as the result of mechanical forces that break existing crystallites apart, the strength of the effect being a smooth function of supersaturation and stirring vigor. However, in some special cases, crystallites of other polymorphs appear and/or no nucleation occurs except at a catastrophic rate above a supersaturation threshold, mimicking primary nucleation. In this paper, we review published theories for this latter type of secondary nucleation, for which we find contradicting explanations in the literature. We divide these explanations into two broad classes, depending on whether the secondary nucleation is mediated through local deviations of the bulk thermodynamic state variables (indirect effects) or directly through the interaction energy of the surface and growth units (direct effects). We argue that theoretical explanations of the second type are insufficient and while we can find no conclusive and theoretically consistent explanation for this well-established experimental phenomenon, it is hoped that this review will stimulate further research into this puzzle.