Spontaneous Transition toward Chirality in the NaClO3 Crystallization in Boiling Solutions

The crystallization of NaClO3 from supersaturated boiling solutions leads to a strong bias of enantiomorphic crystals of the same chiral sign, which in the range of the experimental errors cannot be distinguished from that ora homochiral crystal mixture. The crystallization reactor is a closed system but with a temperature gradient between the walls of the reactor and the air/liquid interface that entails an intense recycling of the subcritical nuclei formed during the induction period of the primary nucleation in the bulk. During this period, the evolution of the population of subcritical nuclei takes place without any other noticeable crystal growth process. In these experimental conditions, the fast evolution of a myriad of supercritical nuclei and the immediate separation of the crystals strongly suggest that the evolution toward homochirality should occur during the primary nucleation process. The formation of stationary chiral compositions in the closed system by recycling through the irreversible step of the primary nucleation is discussed in relation to the chiral recognition in the primary nucleation process and the role of the heterogenity created by the phase transition and the nonuniform temperature distribution.