From Salt in Solution to Solely Ions: Solvation of Methyl Viologen in Deep Eutectic Solvents and Ionic Liquids

Abstract

Solvation and transport properties of methlyviologen dichloride (MVCl₂) in 1:2, 1:4 and 1:4 molar mixtures of choline chloride (ChCl) and ethylene glycol (EG), including the deep eutectic solvent (DES) ethaline (1:2 mixture), were studied through the application of the hole theory to the measured physical property measurements, cyclic voltammetry, and Raman spectroscopy. The ChCl:EG mixtures were compared to the ionic liquid (IL), 1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl) imide, [PYR₁₃][TFSI] and ChTFSI:EG mixtures with the same molar ratios in order to understand the impact of the anion and the hydrogen bond donor. Exchanging the chloride anion of ethaline with TFSI is found to increase the fluidity of the solvent and promote stronger solute-solvent interactions. Raman spectroscopy suggests methylviologen is strongly solvated by EG in ChTFSI:EG solutions and interstitially accommodated in holes in ChCl:EG solutions and [PYR₁₃][TFSI]. Complex solvents such as ILs and DESs are regarded as “designer solvents” and it is demonstrated here that the physical properties and solvation characteristics of these fluids strongly depend on the choice of the anion as well as hydrogen bonding.