The perspective of cooperative hydrotropy on the solubility in aqueous solutions of cyrene

Cyrene is an emerging bio-based green solvent that has been shown to have the ability to increase the solubility of hydrophobic substances in water. Even though the water-Cyrene system is an attractive solvent, its applications are hampered by difficulties in the understanding of its solvation mechanism, caused by a delicate chemical equilibrium established between water and Cyrene. This work aims to rationalize the solvent capability of the water-Cyrene system and to establish the mechanisms of solvation through which hydrophobic solutes are dissolved in it. Using the cooperative model of hydrotropy, it is shown that hydrotropy is the solubilization mechanism of hydrophobic solutes in the water-Cyrene system, in most of its concentration range. Furthermore, the ketone form of Cyrene is revealed to be the principal hydrotrope of the system, with the diol form acting as a hydrotrope only at low Cyrene concentration. The parameters of the cooperative model, namely the number of hydrotrope molecules aggregated around the solute and the maximum solubility increase, are shown to be correlated with the hydrophobicity of the solutes quantified by their octanol-water partition coefficient. This result not only supports recent studies on the mechanism of hydrotropy but also adds a predictive ability to the cooperative model, which is then explored to successfully predict the solubility curves of phthalic acid, aspirin, gallic acid and vanillin in water-Cyrene mixtures.