Brillouin and NMR spectroscopic studies of aqueous dilutions of malicine: Determining the dilution range for transition from a water-in-DES system to a DES-in-water one

Brillouin and NMR spectroscopy were used to explore transitions in the structuring of liquid binary mixtures composed of water and a deep eutectic solvent (DES) known as malicine and composed of malic acid (MA) and choline chloride (ChCl). Malicine/H2O binary mixtures were studied for DES contents ranging from 100 to 40 wt%. Data representation of sound propagation velocities - obtained from Brillouin spectroscopy - versus DES content exhibited two well-differentiated linear regimes above and below ca. 70 DES wt% while transitioning from a water-in-DES system to a DES-in-water one along with dilution. Two linear regimes intersecting at ca. 70 DES wt% were also observed when representing the self-diffusion coefficients versus malicine contents, with MA and ChCl - and even water - exhibiting nearly identical self-diffusion coefficients for contents above 70 wt%, that diverged for contents below 70 wt%. Interestingly, further insights about this behaviour were provided by spin-lattice relaxation time of MA, also revealing the occurrence of hydrophobic hydration around non-polar moieties as in other aqueous binary mixtures of non-ionic H-bond co-solvents - e.g., DMSO:H2O or dioxane: water. (C) 2019 Elsevier B.V. All rights reserved.