Ricerc@Sapienza

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.