A combined experimental and theoretical study on the structures, interactions and volumetric properties of guanidinium-based ionic liquid mixtures

Abstract

Mixtures of ionic liquids (ILs) have shown their potential in both physical and chemical processes, regarded as alternatives to common ILs. In this work, four guanidinium-based ILs, 2-ethyl-1,1,3,3-tetramethylguanidinium ethyl sulfate ([TMG(C₂)][C₂OSO₃]) and bis(trifluoromethylsulfonyl)imide ([TMG(C₂)][NTf₂]), and 2,2-diethyl-1,1,3,3-tetramethylguanidinium ethyl sulfate ([TMG(C₂)₂][C₂OSO₃]) and bis(trifluoromethylsulfonyl)imide ([TMG(C₂)₂][NTf₂]), are employed to investigate the structures, interactions and properties of four systems of IL–IL binary mixtures, including [TMG(C₂)][C₂OSO₃]_x[NTf₂]_1−x, [TMG(C₂)]_x[TMG(C₂)₂]_1−x[C₂OSO₃], [TMG(C₂)]_x[TMG(C₂)₂]_1−x[NTf₂] and [TMG(C₂)₂][NTf₂]_x[C₂OSO₃]_1−x. Combining experiments with theory, the relationships among H-bond interactions, structures and volumetric properties have been revealed. ¹H NMR characterizations show the changes of H-bond interactions in the IL–IL mixtures in relation to composition, and DFT calculations reveal significant cation–anion interactions through the active hydrogen atom (N⁺–H) and the methyl groups in the cations with the anions in the manner of H⋯O and H⋯F. The ethyl group in the [C₂OSO₃]⁻ anion hardly forms interactions with other components. The size effect of the calculated system has been evaluated for the IL–IL clusters with 2, 4 and 8 ions. Different structures due to variation of cationic and anionic species have remarkable influence on the volumetric properties of the IL–IL mixtures. Negative excess molar volume (V^E_m) is found in [TMG(C₂)]_x[TMG(C₂)₂]_1−x[C₂OSO₃], and it is caused by the close packing of ions. Positive V^E_m values indicate that interaction loss occurs in the other three systems, where a linear arrangement or square packing of ions with low space utilization is found.