Liquid-liquid equilibria of n-heptane, methanol and deep eutectic solvents composed of carboxylic acid and monocyclic terpenes

Driven by the absence of quaternary LLE data for systems containing deep eutectic solvents (DES), phase equilibrium data has been determined for systems composed of n-heptane, methanol, and hydrophobic DES. Three DES were studied; levulinic acid was used as hydrogen bond acceptor (HBA) and L-menthol, thymol and carvacrol were used as hydrogen bond donors (HBDs). It is a commonly used assumption that the ratio of HBA and HBD is the same as the initial DES composition and equal in both phases. However, it was shown that the ratio of HBD and HBA is not constant and changes with initial concentration of the eutectic mixture in the system. Carvacrol- and thymol-based systems show a larger miscibility gap than L-menthol-based systems. This is caused by stronger HBD-HBA interactions of thymol-levulinic acid and carvacrol-levulinic acid which keep a larger fraction of the DES constituents together in the lower phase than in the L-menthol-levulinic acid-based systems. The investigated systems are promising candidates for separation technologies where high solubility of target compounds is desired in both liquid phases, as in centrifugal partition chromatography, for example.