The effect of seed preparation on the chirality of the secondary nuclei

Previous secondary nucleation experiments were conducted with chiral NaClO 3 crystal seeds prepared by “regular” cooling (1.6°C/h). In these experiments seeds were used prepared by “slow” cooling (0.1°C/day) and by evaporation. In all cases, at low supercoolings almost all the nucleated crystals have the same chirality as the seed. Above a certain supercooling, however, crystals of both chiralities are obtained. This transition is consistent with the embryos coagulation secondary nucleation mechanism, and it was found to be a strong function of the way the crystals are prepared. The transition supercoolings vary from 2.5–4°C for seeds prepared by evaporation, to 6.8–9°C by “regular” cooling and to 10.5–11°C by “slow” cooling. ESEM observations of the seed surface link the above variations to variations in the surface roughness and the bulk crystal strain. These findings have implication for the separation of chiral compounds by crystallization.