288001 Energetic Origins of Metastable Zone, Induction Supersaturation, and Implications towards Nucleation
For decades, the metastable zone in solution crystallization – a supersaturated region of the phase diagram in which no significant nucleation of crystals occurs – has been viewed as a kinetic phenomenon that originates from the strong dependency of the rate of nucleation on solute concentration. Here we show that, for several different compounds, when the system is slowly supersaturated, the observed metastability limit (MZL) becomes independent of both the rate of change of supersaturation and the time required to reach MZL, a result that suggests an MZL that in fact has energetic origins.
Our experiments are based on the crystallization of a variety of compounds from aqueous solutions. In these studies we employed an evaporation-based microfluidic crystallization platform, in which several small (5 μL) droplets of aqueous solutions were subjected to slow, regulated evaporation. By exploring the metastability limit exhibited by a large number of droplets, we show that the observed probability distribution of nucleation is well described by the classical descriptions of the work required to produce a critical cluster. From this probability distribution we extract an induction supersaturation that occurs when the energy barrier to nucleation has a magnitude of 1kT, the product of Boltzmann’s constant and the absolute temperature. This induction supersaturation is determined by the energetics of cluster formation and can be regarded as the limiting lower boundary of metastability. This surprising result raises important concerns regarding the current interpretation of the processes responsible for the nucleation of crystals from solution.