Insights into the structural features of deep eutectic solvents: the eutectic point as an unicum in their physical properties and the surface tension as a method for its determination

Deep Eutectic Solvents (DESs) are a class of solvents that are increasing their relevance thanks to their environmental-friendly characteristics, to their catalytic properties and to the complexity of their structural features. Formed by simply mixing two H-bond capable substances in solvent-free conditions, the network of interactions between them and between the molecules of the same species lead to a difficult crystal lattice organization, therefore to liquid systems at certain molar ratio of the formers. Solid-liquid phase equilibrium profiles can then be written in order to identify the eutectic point of the mixtures, the minimum in the melting points of the systems. In this work the structural features of a DES mixture at different molar fractions were analysed in order to underline eventual singularities at the eutectic point. Ionic conductivity, viscosity and surface tension measures of the liquids at different molar fractions were analysed. It emerged that the eutectic point is a unicum in the mobility of the charges (as emerged from ionic conductivity activation energies) that is more hampered at the eutectic; the surface tensions showed a minimum correspondent to the same eutectic, indicating a less-structured liquid at this point; viscosity on the contrary didn't show any singularity in the profiles. The surface tension was then applied as a method for the determination of the eutectic point of different DESs liquids; the technique revealed to be precious for the eutectic point determination of really low-melting point mixtures, taking in account the structural diversity of the eutectic underlined with all the measures.