How Ionic Are Room-Temperature Ionic Liquids? An Indicator of the Physicochemical Properties

Abstract

Room-temperature ionic liquids (RTILs) are liquids consisting entirely of ions, and their important properties, e.g., negligible vapor pressure, are considered to result from the ionic nature. However, we do not know how ionic the RTILs are. The ionic nature of the RTILs is defined in this study as the molar conductivity ratio (Λ_imp/Λ_NMR), calculated from the molar conductivity measured by the electrochemical impedance method (Λ_imp) and that estimated by use of pulse-field-gradient spin−echo NMR ionic self-diffusion coefficients and the Nernst−Einstein relation (Λ_NMR). This ratio is compared with solvatochromic polarity scales: anionic donor ability (Lewis basicity), E_T(30), hydrogen bond donor acidity (α), and dipolarity/polarizability (π*), as well as NMR chemical shifts. The Λ_imp/Λ_NMR well illustrates the degree of cation−anion aggregation in the RTILs at equilibrium, which can be explained by the effects of anionic donor and cationic acceptor abilities for the RTILs having different anionic and cationic backbone structures with fixed counterparts, and by the inductive and dispersive forces for the various alkyl chain lengths in the cations. As a measure of the electrostatic interaction of the RTILs, the effective ionic concentration (C_eff), which is a dominant parameter for the electrostatic forces of the RTILs, was introduced as the product of Λ_imp/Λ_NMR and the molar concentration and was compared with some physical properties, such as reported normal boiling points and distillation rates, glass transition temperature, and viscosity. A decrease in C_eff of the RTILs is well correlated with the normal boiling point and distillation rate, whereas the liquid-state dynamics is controlled by a subtle balance between the electrostatic and other intermolecular forces.