Molecular-level insights into inherent heterogeneity of maline deep eutectic system

The nature of heterogeneity in maline deep eutectic system (DES) is characterised by inspecting the properties of the mixture at DES composition (1:1 choline chloride/malonic acid) and various mole fractions of the components at the vicinity of DES. Steady-state and time-resolved emission, and fluorescence anisotropy of suitable fluorescent probes in different choline chloride-malonic acid mixtures are used to describe the heterogeneity in various mixtures. Herein, we establish that nanodomains consisting of different molecule-ion clusters render DES' heterogeneous nature. The intermolecular interaction and the formation of molecule-ion structures are studied by spectroscopic technique including 1H NMR, 1H-1H-nuclear Overhauser effect, NMR diffusometry and mass spectrometry. It is demonstrated that the DES mixture comprises of a particular relative population of two different domains: a less polar domain (LP, 34%) and a more polar domain (MP, 66%). The physical properties of these domains strongly depend on the molar ratio of choline chloride and malonic acid. The experimental findings are further supported with theoretical predictions.