Crystallization and time-dependent changes in betaine-urea-water natural deep eutectic solvents

The ability to predict eutectic compositions resulting in a liquid (natural) deep eutectic solvent [(NA)DES] at room temperature or below widens their applicability range. This study examines the effect of the components' molar ratio and water content on the stability and supramolecular network of betaine-urea-water (BUW) NADES. It also explores time-dependent changes in the solvents and possible methods to determine the stability. After being prepared at elevated temperatures (≥ 70 ˚C), some BUW crystallized upon cooling or seeding with pre-existing crystals, creating orthorhombic betaine hydrate crystals. BUW ratios yielding liquid NADES were determined by proton nuclear magnetic resonance (1H NMR), identifying a solid-liquid boundary. Moreover, BUW conductivity decreased continually over time, suggesting a dynamic state of the solvents. Our results indicate that betaine crystallization depends on water and urea concentrations, supporting the reported synergistic effect. Additionally, this work demonstrates 1H NMR as a method to predict BUW ratios forming liquid NADES.