Influence of History of Solution in Crystal Nucleation of Fenoxycarb: Kinetics and Mechanisms

Nearly 1800 induction time experiments have been performed on crystal nucleation of fenoxycarb in isopropanol to investigate the influence of solution pretreatment. For each preheating temperature and preheating time, at least 80 experiments were performed to obtain statistically valid results. The relationship between the inverse of the induction time and the preheating time can be reasonably described as an exponential decay having time constants ranging up to days depending on the temperature. This dependence on the preheating temperature corresponds to an activation energy of more than 200 kJ/mol. Given sufficiently long preheating time and high temperature, the solution appears to reach a steady-state where the "memory" effect has disappeared. Density functional theory modeling suggests that the molecular packing in the crystal lattice is not the thermodynamically stable configuration at the level of simple dimers in solution, while modeling of the first solvation shell reveals that solute aggregation must exist in solution due to the low solvent-to-solute molecular ratio. It is thus hypothesized that the dissolution of crystalline material at first leaves molecular assemblies in solution that retain features of the crystalline structure which facilitates subsequent nucleation. However, the longer the solution is kept at a temperature above the saturation temperature and the higher the temperature, the more these assemblies disintegrate and transform into molecular structures less suited to form critical nuclei.