Glycerol Ethers as Hydrotropes and Their Use to Enhance the Solubility of Phenolic Acids in Water

The use of glycerol ethers (with alkyl side chains ranging from one to six methyl groups) as hydrotropes to enhance the solubility of gallic and syringic acids in water was here studied. These compounds were selected due to their biological and industrial applications and for serving as model molecules for lignin solubilization. The results obtained were compared against traditional cosolvents, demonstrating the exceptional hydrotropic ability of glycerol ethers. Setschenow constants show that the hydrophobicities of both solute and hydrotrope play an important role in the solubility enhancement by hydrotropy, shedding light into its molecular mechanism. The solubility curves of gallic acid and syringic acid in the aqueous glycerol ether solutions were fitted using a recently proposed statistical thermodynamics-based model. This allowed the estimation of solute recovery from hydrotropic solution by using water as the antisolvent. Unlike what is usually claimed it is here shown that in some conditions it is impossible to recover the solute by simply adding water. This analysis paves the way for a rational design and selection of hydrotropes, in which both solubility enhancement and solute recovery are critical parameters to be taken into account.