Ionization state and ion migration mechanism of room temperature molten dialkylimidazolium fluorohydrogenates

The ionization state of room temperature molten salts, alkylimidazolium fluorohydrogenates (RMIm-(HF)(2.3)F: R = alkyl group, M =methyl group), was evaluated from the observed diffusion coefficient and viscosity, using the Stokes-Einstein relation. Assuming that the dissociation degree of the salt is acceptable for representation of the ionic state of the molten salts, the larger the cation size with elongation of the alkyl chain was, the higher the dissociation degree of the salt. Further, we proposed that an idea of the "degree of ordering of cations and anions" was more suitable to represent the ionization state without solvent species. On the basis of this idea, the smaller the cation size of RMIm(HF)(2.3)F salt was, the higher the ordering of the ion, indicating formation of domain particles of aggregated ions as a unit of mobile species such as A(AX)(m)(+) and X(AX)(n)(-) for A(+)X(-) salt. It was found that highly ordered particles, with large numbers for m and n, showed a high diffusion coefficient.