Molecular Clustering of Fenoxycarb and Salicylic Acid in Organic Solvents and Relation to Crystal Nucleation

Molecular clustering of fenoxycarb and salicylic acid in organic solvents has been investigated by dynamic light scattering. The cluster size has been determined as a function of time in solutions of different concentrations, including supersaturated conditions. The experiments reveal the presence of nanometer-sized clusters from the very beginning in all solutions at all solute concentrations. The cluster size increases systematically with increasing concentration and with increasing time over a time period of days. The cluster size at an equal mole fraction of each solute in different solvents increases with decreasing solubility. In the saturated solution, the cluster size for each solute is roughly independent of the solvent but is smaller for salicylic acid compared to fenoxycarb. Most importantly, it is found that the cluster size in the supersaturated solution at x/x* = 1.05 correlates with the nucleation behavior of each compound in different solvents, i.e., the larger the clusters, the easier the nucleation. Across the two solutes and the different solvents, the cluster size at equal supersaturation (x/x* = 1.05) decreases proportionally to increasing interfacial energy. The results of this work indicate that the underlying reason why nucleation becomes more difficult as the solute-solvent interaction becomes stronger is not primarily related to ease of desolvation but is rather due to the fact that strong solvation leads to smaller prenucleation clusters, and nucleation is more difficult the smaller the clusters are.