Mixing states of imidazolium-based ionic liquid, [C4mim][TFSI], with cycloethers studied by SANS, IR, and NMR experiments and MD simulations

Abstract

The mixing states of an imidazolium-based ionic liquid (IL), 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C₄mim][TFSI]), with cycloethers, tetrahydrofuran (THF), 1,4-dioxane (1,4-DIO), and 1,3-dioxane (1,3-DIO), have been clarified on the meso- and microscopic scales using small-angle neutron scattering (SANS), IR, and NMR experiments and molecular dynamics (MD) simulations. SANS profiles of [C₄mim][TFSI]–THF-d₈ and –1,4-DIO-d₈ solutions at various mole fractions x_ML of molecular liquid (ML) have shown that [C₄mim][TFSI] is heterogeneously mixed with THF and 1,4-DIO on the mesoscopic scale, to a high extent in the case of the latter solution. In fact, [C₄mim][TFSI] and 1,4-DIO are not miscible with each other above the 1,4-DIO mole fraction x_1,4-DIO of 0.903, whereas the IL can be mixed with THF over the entire range of THF mole fraction x_THF. The results of IR and ¹H and ¹³C NMR measurements and MD simulations showed that cycloether molecules are more strongly hydrogen-bonded with the imidazolium ring H atoms in the order of THF > 1,3-DIO > 1,4-DIO. Although 1,4-DIO and 1,3-DIO molecules are structural isomers, our results point out that 1,4-DIO cannot be strongly hydrogen-bonded with the ring H atoms. The solvation of [TFSI]⁻ by cycloethers through the dipole–dipole interaction promotes hydrogen bonding between the ring H atoms and cycloethers. Thus, 1,4-DIO with the lowest dipole moment cannot easily eliminate [TFSI]⁻ from the imidazolium ring. This results in the weakest hydrogen bonds of 1,4-DIO with the ring H atoms. 2D-NMR of ¹H{¹H} rotating-frame nuclear Overhauser effect spectroscopy (ROESY) showed the interaction of the three cycloethers with the butyl group of [C₄mim]⁺. 1,4-DIO mainly interacts with the butyl group by the dispersion force, whereas THF interacts with the IL by both hydrogen bonding and dispersion force. This leads to the higher heterogeneity of the 1,4-DIO solutions compared to the THF solutions.