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(C-2)][C2OSO3]) and bis(trifluoromethylsulfonyl)imide ([TMG(C-2)][NTf2]), and 2,2-diethyl-1,1,3,3-tetramethylguanidinium ethyl sulfate ([TMG(C-2)(2)][C2OSO3]) and bis(trifluoromethylsulfonyl)imide ([TMG(C-2)(2)][NTf2]), are employed to investigate the structures, interactions and properties of four systems of IL-IL binary mixtures, including [TMG(C-2)][C2OSO3](x)[NTf2](1-x), [TMG(C-2)](x)[TMG(C-2)(2)](1-x)[C2OSO3], [TMG(C-2)](x)[TMG(C-2)(2)](1-x)[NTf2] and [TMG(C-2)(2)][NTf2](x)[C2OSO3](1-x). Combining experiments with theory, the relationships among H-bond interactions, structures and volumetric properties have been revealed. H-1 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 HMIDLINE HORIZONTAL ELLIPSISO and HMIDLINE HORIZONTAL ELLIPSISF. 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(C-2)](x)[TMG(C-2)(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.