Alkanediol-based deep eutectic solvents for isolation of terpenoids from citrus essential oil: Experimental evaluation and COSMO-RS studies

Current liquid-liquid extraction techniques for fractionation of essential oils rely on hazardous volatile organic compounds to concentrate target solutes for further food, fine chemical and medical applications. Herein, biorenewable deep eutectic solvents based on various alkanediols were assessed as environmentally friendly alternatives to separate bioactive terpenoids from terpenes for essential oil downstream processing. In particular, DES composed of choline chloride as hydrogen bond acceptor and alkanediols including 1,3-butanediol, 2,3-butanediol, 1,2-propanediol or 1,3-propanediol as hydrogen bond donors at 1:2 ratio were used for fractionation of citrus essential oil model mixtures and real crude orange essential oil at mild conditions. The effect of the structure of alkanediols (i.e. length of hydrocarbon chain and the position of hydroxyl group) on the essential oil extraction performance provided by the different DES was explored in terms of experimental liquid-liquid equilibrium, distribution coefficients and selectivities. Additionally, the experimentally measured thermodynamic parameters were compared against those estimated using COnductor-like Screening MOdel for Real Solvents as a tool for solvent selection. Lastly, a further thermodynamic understanding of the behavior of the systems was performed by analysis of the computed sigma profiles, activity coefficients, excess enthalpies and molecular interactions between the terpenic hydrocarbon solutes and the solvents. Overall, the proposed alkanediol-based DES are feasible solvents for essential oil fractionation, arising as promising candidates to develop sustainable essential oil purification processes through selective isolation of terpenoid compounds.