Physicochemical Properties of 1,3-Dialkylimidazolium Fluorohydrogenate Room-Temperature Molten Salts

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Abstract
Reaction of some N-alkylimidazolium chlorides with anhydrous hydrogen fluoride (HF) gave nonvolatile room temperature molten salts (room temperature ionic liquids), RMImF⋅2.3HF where RMIm=1,3­dimethylimidazoliumRMIm=1,3­dimethylimidazolium (DMIm), 1-ethyl-3-methylimidazolium (EMIm), 1-methyl-3-propylimidazolium, 1-butyl-3-methylimidazolium, 1-methyl-3-pentylimidazolium, and 1-hexyl-3-methylimidazolium. Vacuum stable salts at room temperature exhibited similar stoichiometry regardless of the type of cation. In the differential scanning calorimetry (DSC) curve, DMImF⋅2.3HF exhibited both the freezing and melting on the cooling and heating process, respectively, while EMImF⋅2.3HF showed the glass transition on the cooling process and devitrification and melting on the heating process. The other salts show only the glass transition on the DSC curves. High specific conductivities, 110 and 100 mS cm−1,100 mS cm−1, were observed at 298 K for DMImF⋅2.3HF and EMImF⋅2.3HF, respectively. Introduction of the longer alkyl side chains to the imidazolium cation increased the viscosity and decreased the conductivity. These salts were stable in air and did not etch a Pyrex glass container at ambient conditions. The dissociation pressures of HF from the salts were negligibly small at ambient condition. The electrochemical windows of these salts was about 3 V. © 2003 The Electrochemical Society. All rights reserved.