Carboxylic acid DES: thermodynamical and structural characterization

The quest for sustainable chemistry is fortunately an unstoppable process, and a new class of biodegradable compounds, the Deep Eutectic Solvents (DES) [1,2], has recently been proposed as inexpensive and “greener” alternative to flank the family of solvents currently considered the “pillar” of solution green chemistry—ionic liquids. Such a new class is composed of an eutectic mixture of solids, usually the quaternary ammonium salt choline chloride, and a hydrogen-donor (HBD) Brønsted base, where the extensive charge delocalization resulting from the interactions between the anion and the non-symmetrical HBD moiety is responsible for the often very large decrease of the mixture solidification temperature, as compared to the individual components (up to 200 °C). Though DES share most of ionic liquids physical properties, like negligible vapor pressure, ionic conductivity, high solvating power for a wide variety of substances (but they can dissolve metal oxides), they are chemically distinct from conventional cation–anion paired IL, since they can contain various anionic and/or cationic species and their being in the liquid state depends largely on the molar fractions of the mixtures. Furthermore, most DES are inherently atoxic, since their components are benign molecules obtained from natural sources and even food grade ingredients, like organic acids, sugars, polyalcohols, amines, and amino acids [3]. Despite the undeniable value of these compounds, their structures and structure-property relations have not been fully characterized yet [4]. In this study, we report the density, viscosity and structure of some choline chloride—carboxylic acid mixtures at different compositions, using the Energy Dispersive variant of X-Ray Diffraction (EDXD), and interpret experimental data with molecular dynamics simulations.