Impact on Physical Properties of Chloroaluminate Species Distribution in 1-Ethyl-3-methylimidazolium chloride (EMIC)–Aluminum Chloride (AlCl₃) System

Abstract

The chloroaluminate species distribution of the EMIC–AlCl₃ system was modeled using thermodynamic data in temperatures ranging from 90 °C to 110 °C and 0 to 1 mole fraction of AlCl₃. A thermodynamic model was developed considering the activity (a_i) of species and equilibrium constant (K) for the polymeric reactions. The model species distribution results were compared with the available literature data. The main anion species existing from 0 to 0.5 mole fractions of AlCl₃ ($X_{{{\text{AlCl}}_{3} }}$) are Cl⁻ and AlCl₄⁻, and Al₂Cl₇⁻_, Al₃Cl₁₀⁻ and Al₄Cl₁₃⁻ species exist at higher concentrations of AlCl₃. At all temperatures, the distribution peak maxima of AlCl₄⁻, Al₂Cl₇⁻, Al₃Cl₁₀⁻, and Al₄Cl₁₃⁻ occur at 0.5, 0.67, 0.75, and 0.8 mole fractions of AlCl_3, respectively. With the increase in temperature, the mole fractions of Al₂Cl₇⁻, Al₃Cl₁₀⁻, and Al₄Cl₁₃⁻ peak maxima decrease. These species distribution data were used to analyze the change in ionic melts' physical properties, such as electrical conductivity, diffusion coefficient, density, and kinematic viscosity. Electrical conductivity data showed that when AlCl₄⁻ mole fraction declined from 0.50 to 0.07, conductivity decreased from 5.58 to 3.94 S/m. The concentration of AlCl₄⁻ species was found to be critical in regulating the electrolyte's electrical conductivity. Diffusion coefficient data increased from 1.3 × 10⁻¹¹ to 2.16 × 10⁻¹¹ m²/s² with $X_{{{\text{AlCl}}_{3} }}$ depicting the importance of Al₂Cl₇⁻ concentration for better diffusion coefficient in the electrolyte. Al₂Cl₇⁻ was also found significant for obtaining higher density. Finally, reduced kinematic viscosity was obtained at lower AlCl₄⁻ concentration due to the diminishing effect of strong anion–cation interaction between AlCl₄⁻ and EMIC⁺.