Molecular Aggregation of L-isoleucine in Aqueous Solution and its Impact on the Determination of Solubility and Nucleation Kinetics

Comparison between the solubility of L-Isoleucine as measured using gravimetric and dissolution methods reveals significant differences which is consistent with the presence of solute aggregation in solution. Calculation of the critical aggregation concentration confirms this analysis revealing this concentration to lie between the two measured solubility values suggesting the existence of a stability zone roughly defined by the temperature/concentration range of 40°C/33g/L to 75°C/42g/L in which a mixture of free and aggregated molecules under the slurried conditions appear to exist. Dynamic light scattering result reveals that the aggregate size lies within the range of 40-170 nm. The potential impact of relying on the measurement of solubility using gravimetric data based on solution isolation of the slurried state for such systems is highlighted through a comparison of the values of representative crystallization parameters such as metastable zone width (MSZW) and nucleation kinetics as determined using solubilities derived from both methodologies. Reduction in solution pH in the aggregated molecular state is consistent with the aggregates being formed from neutral species suggesting, in turn, that this compound might crystallize via a 2-step nucleation process. This research suggests that caution should be exercised when using solubility data derived from gravimetric measurements particularly for amphiphilic molecules where solute ordering in solution might be expected.