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

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 (VEm) is found in [TMG(C2)]x[TMG(C2)2]1−x[C2OSO3], and it is caused by the close packing of ions. Positive VEm 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.