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Molecular dynamics simulations of hydrotropic solubilization and self-aggregation of nicotinamide.

Hydrotropy is a phenomenon where the presence of a large quantity of one solute enhances the solubility of another solute. The mechanism of this phenomenon remains elusive and a topic of debate. This study employed molecular dynamics simulation to investigate the hydrotropic mechanism of a model system consisting of a hydrotropic agent, nicotinamide (NA), a poorly water-soluble solute, PG-300995 (PG), and water. Our study demonstrates that NA and PG undergo significant aggregation in the aqueous solution, a result correlating closely to the self-aggregation of NA under the same conditions. The correlations are found both structurally and dynamically, suggesting that the self-aggregation of NA may be a prerequisite, or at least a major contributor, to its hydrotropic effects. The self-aggregation of NA allows the segregation of the hydrophobic solute from water, a key step to ease the energy increase to the system. Energetic evidences directly show that the hydrotropic solubilization is favored in the presence of NA aggregation. These results are in strong support of the molecular aggregation hypothesis for hydrotropic solubilization. Additionally, it is found that the restoration of water-water HBs from the interference of the NA and PG molecules plays an important role for the aggregation. The HBs between the solute and the hydrotrope may contribute, but is not vital, to the aggregation and hence the hydrotropic effects. The dynamic data confirm that the aggregates, while remain in liquid state, are much more active dynamically than a pure NA amorphous/liquid phase under the same temperature and pressure. By equilibrating an NA amorphous agglomerate with water, it is found that the aggregation state, rather than an NA-water two phase system, is the equilibrium state of the NA + water system.