Metastability of supersaturated solution and nucleation

A certain time may elapse for the occurrence of a first nucleation after a solution is made supersaturated. Thus, the supersaturated solution looks as if it could be in a metastable state though in a limited time. Such metastability of a supersaturated solution has been evaluated (rather technically) by the magnitude of the metastable zone width (MSZW), i.e., the supercooling at which a first nucleation event is detected when supersaturation is increased by cooling, or the induction time, i.e., the time elapsed until a first nucleation event is detected at a constant temperature. The larger these values are, the higher themetastability is. Surprisingly, however, these two nucleation-related quantities are not clearly understood. There is still much confusion. This chapter provides an overview of recent understandings of the MSZW and induction time with focus on the problems of (1) stochastic and deterministic aspects, (2) time-dependent change of solution structure, (3) role of nucleation, and (4) relevance to the operation of an industrial batch crystallizer.