| 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 howionic the RTILs are. The ionic nature of the RTILs is defined in this study as the molar conductivity ratio(Λimp/ΛNMR), calculated from the molar conductivity measured by the electrochemical impedance method(Λimp) and that estimated by use of pulse-field-gradient spin−echo NMR ionic self-diffusion coefficients andthe Nernst−Einstein relation (ΛNMR). This ratio is compared with solvatochromic polarity scales: anionicdonor ability (Lewis basicity), ET(30), hydrogen bond donor acidity (α), and dipolarity/polarizability (π*), aswell as NMR chemical shifts. The Λimp/ΛNMR well illustrates the degree of cation−anion aggregation in theRTILs at equilibrium, which can be explained by the effects of anionic donor and cationic acceptor abilitiesfor the RTILs having different anionic and cationic backbone structures with fixed counterparts, and by theinductive and dispersive forces for the various alkyl chain lengths in the cations. As a measure of the electrostaticinteraction of the RTILs, the effective ionic concentration (Ceff), which is a dominant parameter for theelectrostatic forces of the RTILs, was introduced as the product of Λimp/ΛNMR and the molar concentrationand was compared with some physical properties, such as reported normal boiling points and distillationrates, glass transition temperature, and viscosity. A decrease in Ceff of the RTILs is well correlated with thenormal boiling point and distillation rate, whereas the liquid-state dynamics is controlled by a subtle balancebetween the electrostatic and other intermolecular forces.
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