First Evidence of Cyclodextrin Inclusion Complexes in a Deep Eutectic Solvent

Supramolecular host-guest inclusion complexes based on cyclodextrins (CDs) are generally studied in aqueous solutions or more rarely in the presence of a cosolvent. In this study, we investigate for the first time the ability of CDs to retain their host-guest properties in deep eutectic solvents (DESs). Five cyclodextrins were solubilized in mixtures of choline chloride:urea (ChCl:U), and these new solutions were characterized by measuring their viscosity and density as a function of temperature. The heat of dissolution of β-CD in ChCl:U was measured by calorimetry. The dissolution process is exothermic meaning that the interaction between ChCl:U and CD is favorable (enthalpy of dissolution of -23.3 J/g). The ability of CDs to form inclusion complexes with various guests in ChCl:U was demonstrated using two different methods: UV-visible spectroscopy and static headspace gas chromatography. On one hand, the experimental data obtained from the complexation between methyl orange and CDs follows the theoretical fitted curved for a 1:1 inclusion complex. On the other hand, the observed volatility reductions of four volatile organic compounds in different mixtures of ChCl:U:CD were correlated to the CD/guest formation constant values determined in water. These results will certainly contribute to enlarging the applications of CDs in the DES field.