Liquid Structure Scenario of the Archetypal Supramolecular Deep Eutectic Solvent: Heptakis(2,6-di-O-methyl)--cyclodextrin/levulinic Acid

The concept of supramolecularsolvents has been recently introduced,and the extended liquid-state window accessible for mixtures of functionalizedcyclodextrins (CDs) with hydrogen bond (HB) donor species, e.g., levulinicacid, led to the debut of supramolecular deep eutectic solvents (SUPRA-DES).These solvents retain CD's inclusion ability and complementit with enhanced solvation effectiveness due to an extended HB network.However, so far, these promising features were not rationalized interms of a microscopic description, thus hindering a more completecapitalization. This is the first joint experimental and computationalstudy on the archetypal SUPRA-DES: heptakis(2,6-di-O-methyl)-beta-CD/levulinic acid (1:27). We used X-ray scatteringto probe CD's aggregation level and molecular dynamics simulationto determine the nature of interactions between SUPRA-DES components.We discover that CDs are homogeneously distributed in bulk and thatHB interactions, together with the electrostatic ones, play a majorrole in determining mutual interaction between components. However,dispersive forces act in synergy with HB to accomplish a fundamentaltask in hindering hydrophobic interactions between neighbor CDs andmaintaining the system homogeneity. The mechanism of mutual solvationof CD and levulinic acid is fully described, providing fundamentalindications on how to extend the spectrum of SUPRA-DES combinations.Overall, this study provides the key to interpreting structural organizationand solvation tunability in SUPRA-DES to extend the range of sustainableapplications for these new, unique solvents. X-ray scattering and molecular dynamicsprovide atomisticinsight into the structural organization of an archetypal supramoleculardeep eutectic solvent (SUPRA-DES) made of cyclodextrin and levulinicacid, providing ground for further sustainable applications.