Specific Electrical Conductivity of Concentrated Solutions of 1-Butyl-4-Methylpyridinium Tetrafluoroborate in Dimethylformamide

The distance between ions and molecules of ionic liquid in solution is estimated. It is found that in the concentration range of 1–2 mol/l, a maximum should be expected on the concentration dependence of specific electrical conductivity (EC), since contact ion pairs are formed in the solution. Specific EC of concentrated solutions of 1-butyl-4-methylpyridinium tetrafluoroborate in dimethylformamide was measured in the temperature range of 10–70°C, and the density of these solutions was measured in the temperature range of 10–60°C. The dependences of specific EC and density on temperature and concentration are analyzed. The density of solutions decreases linearly with increasing temperature, and specific EC passes through a maximum with increasing concentration. With an increase in temperature from 10 to 70°C, the concentration cmax corresponding to the maximum EC κmax increases from 1.258 to 1.825 mol/l. To generalize the temperature and concentration dependences of the specific EC, the normalized EC κ/κmax and normalized concentration c/cmax were used. In the coordinates κ/κmax – c/cmax, all values of the normalized EC κ/κmax fit on to a single curve. It is shown that at concentrations not exceeding ~1.0 mol/l, with increasing temperature, the specific EC κ increases in direct proportion to the limiting high-frequency electrical conductivity of the solvent κ∞. Based on the analysis of the κ – κ∞ dependencies, the solvation numbers of ionic liquid ions in dimethylformamide were determined, which decrease from 2.89 to 1.09 with an increase in concentration from ~0.1 to ~1.0 mol/l.