Physical properties, ion-ion interactions, and conformational states of ionic liquids with alkyl-phosphonate anions

We investigate the ionic conductivities, phase behaviors, conformational states, and interactions of three ionic liquids based on imidazolium cations and phosphonate anions with varying alkyl chain lengths. All three ionic liquids show high conductivities, with 1,3-dimethylimidazolium methyl-phosphonate [DiMIm(MeO)(H)PO2] being the most conductive (7.3 × 10(-3) S cm(-1) at 298 K). The high ionic conductivities are a result of the low glass-transition temperatures, Tg, which do not change significantly upon changing the cation and/or anion size. However, there is a slight dependence of the temperature behavior of the conductivity on the size of the ions, as seen from the fragility parameter (D) obtained from fits to the Vogel-Fulcher-Tammann equation. The molecular-level structure and interactions of the phosphonate anions were examined by Raman spectroscopy and first-principles calculations. The calculations identify two stable conformations for the methyl- and ethyl-phosphonate anions by rotation of the methyl and ethyl groups, respectively. The broad Raman signatures of the anions suggest the coexistence of two anion conformers in the ionic liquids and non-negligible cation-anion interactions, with a dependence on the position and shape of the bands of the cation species and the alkyl group of the anion.