Hydrophobic, Highly Conductive Ambient-Temperature Molten Salts

TLDR

Imidazolium molten salts are known, but those with hydrophilic anions are water‑soluble. The authors synthesized low‑melting, hydrophobic 1,3‑dialkyl imidazolium ionic liquids, characterized them by NMR, elemental analysis, and physical property measurements, and examined how alkyl substituents and photophysical probes affect density, viscosity, conductivity, and solvent behavior. The salts exhibit viscosities as low as 35 cP and conductivities up to 9.6 mS cm⁻¹, making them promising solvents for electrochemical, photovoltaic, and synthetic uses.

Abstract

New, hydrophobic ionic liquids with low melting points (<−30 °C to ambient temperature) have been synthesized and investigated, based on 1,3-dialkyl imidazolium cations and hydrophobic anions. Other imidazolium molten salts with hydrophilic anions and thus water-soluble are also described. The molten salts were characterized by NMR and elemental analysis. Their density, melting point, viscosity, conductivity, refractive index, electrochemical window, thermal stability, and miscibility with water and organic solvents were determined. Their density, melting point, viscosity, conductivity, refractive index, electrochemical window, thermal stability, and miscibility with water and organic solvents were determined. The influence of the alkyl substituents in 1, 2, 3, and 4(5)-positions on these properties was scrutinized. Viscosities as low as 35 cP (for 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)amide (bis(triflyl)amide) and trifluoroacetate) and conductivities as high as 9.6 mS/cm were obtained. Photophysical probe studies were carried out to establish more precisely the solvent properties of 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)amide). The hydrophobic molten salts are promising solvents for electrochemical, photovoltaic, and synthetic applications.