Solvation by aqueous solutions of imidazole-based ionic liquids: 2- A comparison between alkyl and alkoxy side-chains

Abstract

Binary mixtures of ionic liquids (ILs) and molecular solvents are employed for practical reasons, e.g., reduction of solution viscosity and overcoming solubility/miscibility problems. We obtain information on solvation in these media from solvent properties, e.g., empirical polarity ET(probe) calculated from the spectra of solvatochromic probes. To evaluate the effect on solvation of the IL cation side-chain we studied the solvatochromic response of the polarity probe WB (2,6-dichloro-4-(2,4,6-triphenylpyridinium-1-yl)phenolate) dissolved in the following aqueous ILs: 1-R-3-methylimidazoliumAcO, where R = 1-heptyl, C7MeImAcO; and 3,6-dioxa-(1-heptyl), C5O2MeImAcO; AcO = acetate; in the temperature range 15–60 °C. The relative acidity of the imidazolium ring hydrogens bears on solvation. Consequently, we extended our study to aqueous solutions of 1-R′-1,2-dimethylimidazoliumAcO, R' = 1-butyl (C4Me2ImAcO) and methoxyethyl (C3OMe2ImAcO). Our results showed that the expected effect of the side-chain ether oxygen on ET(WB) is impaired by formation of intramolecular hydrogen bonding, e.g., with hydrogens of the imidazolium ring. We corroborated this conclusion by 1H NMR data, and theoretical calculations (molecular dynamics simulations; quantum chemistry calculations). Introducing second ether oxygen (C5O2MeImAcO) and replacing C2-H by C2-CH3 in the imidazolium ring (C3OMe2ImAcO) increased the difference in polarity between ILs with alkoxy and alkyl side-chains. For C7MeImAcO and C5O2MeImAcO the dependence of ET(WB) on the water mole fraction was treated according to a model that includes solvation by the complex solvent (IL⋅⋅⋅water); the latter is the most efficient solvent species present because it forms more hydrogen-bonds with the phenolate oxygen of WB.