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

The distance between ions and molecules in a solution of an ionic liquid is estimated. It is found that in the concentration range of 1–2 mol/L, the concentration dependence of the electrical conductivity should have a maximum associated with the formation of contact ion pairs in solution. The conductivity of concentrated solutions of 1-butyl-4-methylpyridinium tetrafluoroborate in dimethylformamide is measured in the temperature interval of 10–70°C and the density of these solutions is measured in the temperature interval of 10–60°C. The conductivity and the density are analyzed as a function of the temperature and the concentration. The density of solutions decreases linearly with increasing temperature, and the conductivity passes through a maximum as the concentration increases. As the temperature rises from 10 to 70°C, the concentration c_max that corresponds to the maximum conductivity κ_max increases from 1.258 to 1.825 mol/L. The general form of the conductivity dependences on the temperature and the concentration is obtained using the normalized values of conductivity (κ/κ_max) and concentration (c/c_max). In the κ/κ_max vs. c/c_max coordinates, all the values of normalized conductivity κ/κ_max fit a single curve. It is shown that for the concentration not exceeding ~1.0 mol/L, as the temperature rises, the conductivity κ increases in proportion to the limiting high-frequency conductivity of the solvent κ_∞. Based on the analysis of the κ vs. κ_∞ dependencies, the solvation numbers of ionic-liquid ions in dimethylformamide are shown to decrease from 2.89 to 1.09 as the concentration increases from ~0.1 to ~1.0 mol/L.