Water-induced modifications in the physicochemical and structural properties of betaine and choline chloride – ethylene glycol deep eutectic solvents

Deep eutectic solvents (DESs) have attracted considerable attention as sustainable alternative solvents in a wide range of applications. A key property of these mixtures is their viscosity, which can, for some systems, be too high for practical applications. Adding water lowers the viscosity by replacing hydrogen bonds between the components, with those involving water. Thus, a key factor in various applications is determining the optimal water content that maintains the solvent's ability to dissolve the substances of interest while sufficiently reducing its viscosity. However, some discrepancies are often observed in the reported viscosities of DESs, upon water addition . Herein, we studied the physicochemical properties and the structure of anhydrous and aqueous betaine/choline chloride – ethylene glycol-based DESs through an experimental and molecular dynamics (MD) combined approach. The DESs were prepared either by incorporating water as a starting component or after the DES formation, for comparison. Solvatochromic parameters, diffusion, density, and shear viscosity were assessed. The density and viscosity measurements were carried out in the temperature range 293–323 K. All properties were found to be independent of the preparation route. Experimental density and viscosity data of the anhydrous DES at room temperature were then used to calibrate CHARMM force fields, which were subsequently employed in MD simulations of the solvents with different water contents. Our results show that the addition of one molar equivalent of water produces a reduction in viscosity comparable to raising the temperature of the anhydrous DES by 5–10 K. Notably, choline chloride – ethylene glycol (1:3) exhibits a higher density and viscosity than ethaline (1:2) at higher water contents, indicating greater tolerance to water addition. This crossover behavior is attributed to the retention of water molecules within the excess ethylene glycol, which limits the hydration of the choline cation, as indicated by MD. Thus, while increasing the additional molar ratio of ETG reduces the viscosity of the DES, relative to ethaline, it also enhances the system's tolerance to water, reflecting in a higher activation energy for viscous flow at high water contents.