Thermal dynamics of lithium salt mixtures of ionic liquid in water by PGSE NMR spectroscopy

Abstract

The dynamics of ionic liquid (ILs)–H₂O–Li⁺ salt mixtures have been investigated with the help of ¹H NMR and pulsed gradient spin echo (PGSE) NMR spectroscopy. For this purpose two imidazolium-based ILs are selected such as 1-butyl-3-methylimidazolium bromide ([BMIM]Br) and 1-octyl-3-methylimidazolium bromide ([OMIM]Br); and the selected lithium salts are lithium chloride (LiCl) and lithium percholorate (LiClO₄). The acidity of the C2 proton of ionic liquids has been studied by the help of ¹H NMR spectroscopy and follows the order C2_{[OMIM]Br–H2O–LiCl} > C2_{[BMIM]Br–H2O–LiCl} > C2_{[BMIM]Br–H2O–LiClO4} > C2_{[OMIM]Br–H2O–LiClO4}. It has been explained on the basis of the H-bonding interactions between the C2 proton of the ionic liquids with both the Br⁻ and water molecules. The strength of the micellization of ionic liquids also contributed to the observed acidity order and has been discussed. The translational and rotational dynamics of ¹H NMR active molecules (ionic liquids and water molecules) has been discussed by applying the Stokes–Einstein (SE) and Stokes–Einstein–Debye (SED) equations, respectively. The dynamics of [OMIM]Br decrease with the increase in the composition of ionic liquids and follow the hydrodynamic theory. However, the deviation induced by the lithium salts on the translational dynamics of water molecules is very small (<1 ns) to be probed by the studied NMR time scale. On the other hand, its rotational dynamics show good correlation with the bulk viscosity of mixtures. Finally, the activation theory, validity of the SE equation, the frictional coefficient in correlation with the microviscosity phenomenon, and the hole formation theory have been explored in ionic liquid–H₂O–Li⁺ salt mixtures.