Liquid structure scenario of the archetypal supramolecular deep eutectic solvent: Heptakis(2,6-di-O-methyl)--cyclodextrin/levulinic acid.

The concept of supramolecular solvents has been recently introduced and the extended liquid state window accessible for mixtures of functionalised cyclodextrins (CD) with hydrogen bond (HB) donor species, e.g. levulinic acid, led to the debut of supramolecular deep eutectic solvents (SUPRA-DES). These solvents retain CD's inclusion ability and complement it with enhanced solvation effectiveness due to an extended HB network. However, so far these promising features were not rationalised in terms of a microscopic description, thus hindering a more complete capitalization. This is the first joint experimental and computational study on the archetypal SUPRA-DES: Heptakis(2,6-di-O-methyl)-?-CD/levulinic acid (1:27). We used X-ray scattering to probe CD's aggregation level and Molecular Dynamics simulation to determine the nature of interactions between SUPRA-DES components. We discover that CDs are homogeneously distributed in the bulk and that HB interactions, together with electrostatic ones, play a major role in determining mutual interaction between components. However, dispersive forces act in synergy with HB to accomplish a fundamental task in hindering hydrophobic interactions between neighbour CDs and maintaining the system homogeneous. The mechanism of mutual solvation of CD and levulinic acid is fully described, providing fundamental indications on how to extend the spectrum of SUPRA-DES combinations. Overall, this study provides the key to interpret structural organization and solvation tunability in SUPRA-DES, to extend the range of sustainable applications for these new, unique solvents.