Structural properties and stability of the Betaine-Urea natural deep eutectic solvent

This work focuses on the stability and supramolecular structure of the betaine-urea-water (B:U:W) natural deep eutectic solvent. Solutions spanning a range of molar ratios of betaine, urea, and water were prepared, varying the temperature and preparation times, and were analyzed by attenuated total reflection Fourier-transform infrared spectroscopy and Nuclear Magnetic Resonance. Density Functional Theory and the Natural Bond Orbital analysis were employed to obtain the most stable conformations for each mixture. The experimental results show that, in non-anhydrous conditions, betaine:urea (1:1), a minimum of two moles of water are needed to form a metastable transparent liquid, and a minimum of three moles of water is required to have a stable NADES. Comparison of the 13C NMR spectra of B:U:W 1:1:2 and 1:1:3 shows for the latter that the carbonyl groups of betaine and urea form stronger hydrogen bonds with water, and that the CH3 group of betaine becomes more deprotected by the addition of the extra water molecule, making 1:1:3 a more stable solution. Our experimental and computational results show that water is of crucial importance to the NADES supramolecular structure and stability. A better understanding of the structural characteristics of NADES can lead to better envisage applications for these green solvents.