Advances in the development of novel green liquids: thymol/water, thymol/urea and thymol/phenylacetic acid as innovative hydrophobic natural deep eutectic solvents

Deep Eutectic Solvents (DESs) are increasing their relevance thanks to their ecologically favourable properties, their interesting structural features and their catalytic properties. In this paper we present three novel hydrophobic DESs mixtures prepared by mixing thymol with water, with urea and with phenylacetic acid. These natural DESs represent important advances in the development of green DESs liquids: Thymol/Water mixture is the first binary, water-based DES with hydrophobic properties; in Thymol/Urea DES, urea and its chelating properties were put in a water-separable phase; finally, Thymol/Phenylacetic acid DES is a mixture of two hydrogen-bond donor molecules showing hydrophobic and slightly acidic properties. The hydrogen bond properties within the different mixtures were investigated via DFT, while ab initio molecular dynamics results showed a substantial interaction between thymol and water in the Thymol/Water DES. The investigated DESs show excellent extraction properties of heavy metal salts and phenols from aqueous phases.