ИСТИНА

Deep eutectic solvents (DES) are solutions of Lewis or Bronsted acids and bases with compositions close to eutectic. DES is finding more and more potential applications in chemical industry due to the combination of a number of unique properties. The main disadvantage of DES is their high hygroscopicity. Small changes in composition lead to sharp changes in their basic physicochemical properties: density, viscosity, electrical conductivity, crystallization temperatures. When using DES in technology, it is important to control all parameters of solvents. In this regard, it is necessary to have data on the range of stability of DES in relation to moisture and to know the equilibrium compositions of three-component mixtures “DES - water” under given conditions. The objects of this work were four systems based on choline chloride, which are most widespread in chemical technology along with other DES: the binary system "water - choline chloride", and three quasi-binary sections with a molar ratio of 2:1 of the first component related to second in the systems: "urea (1) - choline chloride (2) - water", "ethylene glycol (1) - choline chloride (2) - water", "glycerol (1) - choline chloride (2) - water". Using the isopiestic method, the hygroscopic points of DES - water solutions were determined in the relative humidity range from 4.5 to 86% at the temperature 22 ± 0.5 °C. Saturated vapor pressure of water was additionally assessed under the assumption of an ideal gas model. It was found that at low humidities, crystals were obtained in equilibrium with DES - water solutions; which contradicts the widespread information in the literature that anhydrous DES systems have eutectic temperatures below room temperature. Figure 1 shows solutions of DES with water that were subjected to slow evaporation of water in a desiccator with dried potassium hydroxide. In systems based on glycerol and ethylene glycol, crystals of choline chloride were observed, and in the system with urea, crystals were obtained that differed in geometry from choline chloride and needle-shaped urea crystals. The latter result requires further detailed study by spectral and thermal analysis methods. In the current study the compositions of saturated solutions that are in equilibrium with crystal phases at 22 ± 0.5 °C were also determined. The work was supported by the Ministry of Science and Higher Education of the Russian Federation (State Assignment No. 075-01252-22-03 dated 26.10.2022).