Calculation of the Molar Concentrations of Ions in the Molten System AlCl3–1-Butyl-3-Methylimidazolium Chloride

Chloroaluminate ionic liquids are the most promising electrolytes for the low-temperature electrolysis of aluminum and in aluminum-ion batteries. An important problem in this area is the determination of the ionic composition and the ion concentrations in an ionic liquid. The low-temperature AlCl3–1-butyl-3-methylimidazolium chloride melt is studied in the acidic range of aluminum chloride concentrations (at the molar fractions of aluminum chloride from 0.5 to 0.67). The cycle of measurements of the ionic liquid density is carried out by a dilatometric method in a wide temperature range (from 0 to 100°C) in order to calculate the molar concentrations of ions in the electrolyte under study. The molar volumes of the ionic liquid are calculated from the experimental values of density. The isotherms and polytherms of the density and molar volume of the ionic liquid are linear. The ionic liquid density decreases and the molar volume increases with increasing temperature. The density increases and the molar volume of the ionic liquid decreases with increasing molar fraction of aluminum chloride in the melt due to an increase in the concentration of the heavier anion ( $${\text{A}}{{{\text{l}}}_{{\text{2}}}}{\text{Cl}}_{{\text{7}}}^{-}$$ ) compared to that of the $${\text{AlCl}}_{{\text{4}}}^{-}$$ anion. The ionic liquid under study is presented as a mixture of two salts: 1-butyl-3-methylimidazolium– $${\text{AlCl}}_{{\text{4}}}^{-}$$ (ionic liquid at an aluminum chloride molar fraction of 0.5) and 1-butyl-3-methylimidazolium– $${\text{A}}{{{\text{l}}}_{{\text{2}}}}{\text{Cl}}_{{\text{7}}}^{-}$$ (ionic liquid at an aluminum chloride molar fraction of 0.67). The rule of additive addition of densities and molar volumes is proved for mixtures of these salts. The molar concentrations of ions present in the mixture are calculated. The dependences of the concentration of each ion on the temperature and the molar fraction of aluminum chloride can be described by linear functions. The molar concentrations of each type of ions decrease with increasing temperature due to an increase in the molar volume.