Effect of large anions in thermal properties and cation-anion interaction strength of dicationic ionic liquids

The properties of an ionic liquid (IL) are directly related to the structure of its ions, which determine the nature of intermolecular cation-anion interactions. In this paper, we determined the thermal properties and cation-anion interaction strength of a series of dicationic imidazolium-based ILs containing alkyl spacer with eight methylene cation [BisOct(MIM)2] and organic larger anions (ibuprofenate, hippurate, ascorbate, N-triflate, and docusate). Results showed that thermal properties do not depend on the volume or molecular weight of the anion, but on its organic nature, which is, the more organic, the less stable. The cation-anion interaction strength was determined by electrospray ionization mass spectrometry using collision-induced experiments. Results showed that the larger the anion, the weaker the cation-anion interaction. This tendency can be understood considering that the higher the anion volume, the smaller the charge density and, therefore, the cation-anion interaction weakens. On the other hand, our results showed that cation-anion interaction strength is related to more negative sites of the anion surface electrostatic potential. The more negative the sites, the stronger the cation- anion interaction.