Mixing of menthol-based hydrophobic deep eutectic solvents as a novel method to tune their properties

Due to the limited number and water-immiscibility of hydrophobic deep eutectic solvents (hDESs), new methods of expanding the repertoire and tuning the properties of these solvents are required. Here, menthol was combined with organic acids or alcohols to produce nine hDESs, all of which were freely miscible with each other. Three of the hDESs, vis. ML, MB, and MT (containing menthol combined with levulinic acid, 1,3-butanediol, and tert-butanol, respectively) were selected as the least to most non-polar solvents. Two and three of these hDESs were mixed at equal volume ratios to produce three binary mixtures (namely, LB, BT, and LT) and one ternary mixture (LBT), respectively, via simplex centroid mixture design. The properties of these mixtures as extraction solvents were characterized using Ginko biloba leaves (GBLs) as a model plant. Mixtures containing ML exhibited high capability for extracting phenolic compounds and flavonoids, comparable to that of methanol and 70% methanol. They provided wide extraction coverage with reliability, as evidenced by the high numbers of variables with coefficients of variation within 30%. The liquid chromatography-mass spectrometry-based profiles of 47 secondary metabolites showed that MB and BT presented similar results to methanol and 70% methanol based on heatmap and hierarchical cluster analysis. This study shows that combining the menthol-based hDESs furnishes properties that differ from those of the individual components; in this way, the repertoire of hDESs can be expanded, and therefore, the hDESs and their mixtures can be used as green task-specific extraction solvents as substitutes for volatile organic solvents. (C) 2019 Elsevier B.V. All rights reserved.