Effective extraction of harmine by menthol/anise alcohol-based natural deep eutectic solvents

Natural deep eutectic solvents composed of two or more natural products are now regarded as green and sustainable alternatives to the conventional solvents and ionic liquids because of the significant advantages of low cost, easy preparation and hypotoxicity. In this work, three kinds of new hydrophobic natural deep eutectic solvents based on natural perfumes, DL-menthol and anise alcohol were prepared and used to extract harmine from aqueous phase. Differences of raw material molar ratios in the preparation lead to the differences in hydrogen bonding of the result products. Hydrogen bonding is the main driving force underlying the extraction, which can be demonstrated by the thermodynamic analysis. The natural deep eutectic solvent with the molar ratio of DL-menthol to anise alcohol at 1 : 1 exhibits high extraction ability for harmine and the optimal extraction conditions are 5.0 min of extraction time, pH 8.0 and 50 : 1 of phase volume ratio (volume of water phase : volume of extractant phase), respectively. Furthermore, the suggested natural deep eutectic solvent can be easily recycled by adjusting pH of the aqueous phase and can be reused at least five times via back extraction. These results indicate that the perfume-based natural deep eutectic solvents are ideal extraction solvents for effectively extracting bioactive compounds.