Spin-lattice relaxation studies on deep eutectic solvent/Choliniumtetrachloroferrate mixtures: Suitability of DES-based systems towards magnetic resonance imaging studies

Insight into the Molecular Structure, Interaction, and Dynamics of Aqueous Reline Deep Eutectic Solvent: A Nuclear Magnetic Resonance Investigation

Reline, which is composed of choline chloride (ChCl) and urea, is the first and most widely used deep eutectic solvent (DES) described by Abbot and co-workers. Due to the hygroscopic feature, traces of water are unavoidable, which significantly affect the physicochemical properties of reline. At present, the local structure of molecules and the impact from the presence of water are still the most significant questions in this field. Herein, reline and six aqueous dilutions with a controlled amount of water (from 3.2 to 50.0 wt %) were studied mainly by using a combination of nuclear magnetic resonance (NMR) techniques. According to 1D ³⁵Cl NMR, 1D ¹⁵N NMR, and 2D ¹H-¹⁵Cl heteronuclear Overhauser effect spectroscopy, we probed the interactions of urea···Cl^- and Ch⁺···Cl^- in pure reline, which gradually dissociated in the presence of water. Moreover, it was revealed that the dissociation rate altered when the water content reached 9.0 wt %, which is ascribed to the higher preference of hydration for Cl^- ion compared to other species in the system. Furthermore, selected cross peaks in ¹H-¹H correlation spectroscopy spectra were analyzed. Accordingly, an enhanced correlation was observed for urea···Ch⁺ at a lower water fraction within 9.0 wt %. When the water content increased to 24.9 wt %, the water solvation of Ch⁺ and urea was also observed in COSY spectra. The interaction of H₂O···Ch⁺ got continuously stronger when the water content increased from 24.9 to 50.0 wt %, while H₂O···urea got enhanced when the water content reached 33.3 wt % and then diminished gradually from 33.3 to 50.0 wt %. ¹H-¹H nuclear Overhauser effect spectroscopy and ¹H-¹H rotating frame Overhauser effect spectroscopy experiments were also conducted for dynamics investigation. The τ_c value for the species in 9.0 wt % aqueous reline is very close to τ_c^crit of 0.44 ns. For pure reline and the aqueous reline with a water fraction of less than 9.0 wt %, the τ_c value of the species is longer than 0.44 ns, while for the sample with water of 24.9 wt %, the τ_c value is much shorter than 0.44 ns. Based on our NMR study, we revealed that with the water amount increasing from 0 to 50.0 wt %, the species involved in the system behaved as the large molecules or molecules in viscous liquids transiting to the medium-sized molecules in nonviscous liquids and finally to small molecules in nonviscous liquids.