In-depth study of binary ethanol-triacetin mixtures in relation to their excellent solubilization power.

A significant synergistic effect is observed when solubilizing curcumin or tetrahydrocurcumin in binary mixtures of ethanol and triacetin. The present work deals with a detailed investigation of the solvent system by means of COSMO-RS-based calculations, dynamic light scattering, small-and-wide-angle X-ray scattering, and dielectric relaxation spectroscopy. Theoretical calculations lead to the conclusion that the enhanced solubility is not primarily the result of an interaction optimum between individual surface charge densities. Scattering experiments also exclude the formation of mesoscopic structures as the main reason. However, dielectric relaxation spectra suggest that in the concentration range of 0.3 ≤ x (triacetin) ≤ 0.6, ethanol molecules are released from their living polymer ethanol network and can interact with triacetin on a molecular level. The mesoscopic aggregation, thus, decreases. The concentration range of the ethanol-triacetin complexes has a significant overlap with the range of maximum solubility of (tetrahydro)curcumin. Nevertheless, despite detailed investigations, the exact origin of the solubilization power of the solvent remains speculative.