Low-temperature molecular motions in a deep eutectic solvent choline chloride/urea studied by spin-probe EPR

Deep eutectic solvents (DESs) are eutectic mixtures of substances with the melting points considerably lower than those for their individual components. The study of various properties of DESs is currently attracting great attention due to the possibilities of their use as solvents in a wide variety of technological fields. In this work, it is shown using a choline chloride/urea mixture (1:2) that the method of pulsed spin-probe EPR can be used to study low-temperature stochastic molecular librations in DESs. A peculiarity of this type of motion is its sensitivity to the nanostructure of the substance under study. The studies carried out in this work showed that the choline chloride/urea mixture (1:2) exhibits a glass transition considerably broadened in temperature and starting below macroscopic glass transition. This broadening indicates that its nanostructure is rather loosely packed.