Hydrophobic eutectic solvents: Thermophysical study and application in removal of pharmaceutical products from water

Exposure to persistent contaminants presents a risk to humans and animals. Specifically, the occurrence of drugs in aqueous ecosystems is related to the increase in antimicrobial resistance. Several methods to remove them have been studied but most have cost and toxicity drawbacks. Thus, the study of new eco-sustainable procedures is needed as the liquid–liquid extraction with hydrophobic solvents (ESs). In this study, we prepared six ESs mixtures with thymol (Thy) and octanoic acid (Oct) or decanoic acid (Dec) and evaluated their extractive ability in aqueous mixtures of quercetin (Q), nitrofurantoin (NF), or tetracycline (TC). In addition, we measured, correlated, and modelled both the solid–liquid equilibria of the two systems and seven thermophysical properties of the mixtures. From these calculations, we obtained the melting enthalpies of the eutectic points, isobaric thermal expansibilities, intermolecular free lengths, orientational dipolar parameters, thermodynamic properties of the surface, critical temperatures, and activation energies of viscous flow. The perturbed chain statistical associating fluid theory was found to be suitable for predicting the thermodynamic behaviour of these systems. Next, we determined the solubility of the three drugs both in water and in the eutectic mixtures, as well as the efficiency extraction (EE) of the latter. We obtained the highest extraction efficiency for the equimolar [Thy:Oct] mixture, which was the most polar solvent. The values were: EEQ=97.8%, EENF=84.9%, and EETC=95.0%. Further, performing the procedure with three cycles increased these values above 99%. The results showed the highest hydrophobicity for quercetin and the lowest for nitrofurantoin.