The hydrotropic solubilization of riboflavin by nicotinamide-mechanistics studies

The mechanism of hydrotropic solubilization of slightly soluble solutes was studied using a model system, the riboflavin-nicotinamide system. By dissolving riboflavin in concentrated aqueous solutions of nicotinamide, great increases in the solubility of riboflavin were achieved. Concentrations of 2.0 M nicotinamide produced a 36-fold increase in riboflavin solubility at 30$\sp\circ$C. Complexation as a hydrotropic mechanism was disproved in this work by studying the effect of nicotinamide on riboflavin fluorescence. Complexation causes the fluorescence of the complexed molecule to be quenched. Nicotinamide in concentrations ranging from $7.0 \times 10\sp{-8}$ M to 2.0 M was unable to quench the fluorescence of riboflavin ranging in concentration from $7.0 \times 10\sp{-7}$ M to $1.0 \times 10\sp{-4}$ M. Therefore, complexation was rejected as a possible mechanism for complexation. The self-association of nicotinamide in aqueous solution was characterized using light-scattering monophotometry and vapor pressure osmometry. From light-scattering data, it was determined that nicotinamide associates primarily as dimers and trimers with a dimerization constant of 9.99 L/mol and a trimerization constant of 13.1 L/mol. Monomers, dimers and trimers together account for 98% of all species in solution. Higher order aggregates are present and can be characterized by an average aggregation number of 4.37. Vapor pressure osmometry confirmed the self-association of nicotinamide; however, the calculated association constants were 0.203 L/mol and 14.1 L/mol for dimerization and trimerization, respectively. The solubilization effect was studied in other solvents and was found to occur only in solvents capable of both hydrogen-bond donation and acceptance. The hydrotropic phenomenon was thus extended to other solvents and termed solvotropy . Additionally, temperature effects were studied with the conclusion that as temperature increases, self-association of the hydrotrope decreases and its solubilizing capacity diminishes. These effects were confirmed using urea, another hydrotropic agent. It is proposed that self-association of the solvotrope only occurs in solvents capable of accepting and donating hydrogens, and that the self-association is required for the solvotropic phenomenon