Our approach relies on the existence of a boundary for the MZW in nucleation from solutions. We measure this limit of a given compound by introducing small droplets of solution at various initial conditions into a micro-device, in which crystallization is driven by different rates of evaporation of the solvent. The droplets are observed periodically using an automated imaging system and the nucleation time for any crystals formed is recorded. From this information, we determine the limit of MZW at which the solution crystallizes. We measured this limit for wide range of model compounds in our earlier studies [1].
In this work, we propose a way of estimating nucleation kinetics from this limit of MZW using the classical nucleation theory (CNT). Since the rate of evaporation of solvent (and thus the rate of generation of supersaturation) is very slow, we consider the droplet to be at an equilibrium steady state at all points of time and examine the evolution of the ‘quasi-equilibrium cluster size distribution' in the droplet. The limit of the metastable zone can then be interpreted as the minimum driving force required for the formation of a ‘critical nucleus' in the droplet. CNT expresses the nucleation kinetics in terms of a kinetic prefactor and the surface energy between the solution and the nucleus. We estimate the pre-factor by considering how rapidly the cluster size distribution evolves to populate a single nucleus and we calculate the surface energy from the supersaturation at which the droplet nucleates. We compare our estimates of kinetics with the information available in literature for various systems and discuss the role of rate of supersaturation in the methods used for the measurement of nucleation kinetics.
References:
[1] Guangwen He, Venkateswarlu Bhamidi, Reginald B. H. Tan, Paul J. A. Kenis and Charles F. Zukoski, Cryst. Growth & Design, 6 (2006), 1175