Theoretical study on the structure and electronic properties of alkylimidazolium iodide ionic liquids: the effect of alkyl chain length

Abstract

On the basis of quantum chemical calculations, the effect of the alkyl chain length upon the cation–anion interaction energies of 1-alkyl-3-methylimidazolium iodide ([C_nmim]I, n = 2–8) ionic liquids is studied. A number of energetically favorable conformations are considered for each cation–anion combination. In the most stable geometries for the [C_nmim]I ion pairs, the iodide anion locates above the imidazolium plane and has close contact with the C2–H15 of [C_nmim]⁺ cation. This preferential location of iodide around the imidazolium is analyzed and explained by the natural bond orbital (NBO) and the molecular electrostatic potential (MESP) methods. The magnitude of the interaction energy decreases as a function of increasing alkyl chain length. Correlation of the cation–anion interaction with thermophysical properties such as density, surface tension, viscosity, and melting point is further discussed. The density and surface tension of the studied ionic liquids are found to be fitted well in a linear fashion with the interaction energy values, whereas a close relationship between the interaction energies and the viscosity is not found. It seems the viscosity of an ionic liquid is not only determined by the cation–anion interaction, but also by the van der Waals interactions between the alkyl chains on the imidazolium cations and by the π–π stacking interactions between the imidazolium cations. Moreover, a linear correlation is observed between the interaction energy and the experimental melting point for ionic liquids containing cations with short alkyl chains, n = 2–4. For long alkyl chains, the melting point is governed by the van der Waals interactions between the alkyl chains on neighboring cations, which are not included in the calculated interaction energies. The present results can provide some useful information for understanding the cation–anion interactions that lead to the physicochemical properties of ionic liquids.