Structural origin of proton mobility in a protic ionic liquid/imidazole mixture: insights from computational and experimental results

The structure, dynamics, and phase behavior of a binary mixture based on the protic ionic liquid 1-ethylimidazolium bis(trifluoromethanesulfonyl) imide (C(2)HImTFSI) and imidazole are investigated by H-1 NMR spectroscopy, vibrational spectroscopy, diffusion NMR, calorimetric measurements, and molecular dynamics simulations. Particular attention is given to the nature of the H-bonds established and the consequent occurrence of the Grotthuss mechanism of proton transfer. We find that due to their structural similarity, the imidazolium cation and the imidazole molecule behave as interchangeable and competing sites of interaction for the TFSI anion. All investigated properties, that is the phase behavior, strength of ion-ion and ion-imidazole interactions, number of specific H-bonds, density, and self-diffusivity, are composition dependent and show trend changes at mole fractions of imidazole (w) approximately equal to 0.2 and 0.5. Beyond chi = 0.8 imidazole is not miscible in C2HImTFSI at room temperature. We find that at the equimolar composition (chi approximate to 0.5) a structural transition occurs from an ionic network mainly stabilized by coulombic forces to a mixed phase held together by site specific H-bonds. The same composition also marks a steeper decrease in density and increase in diffusivity, resulting from the preference of imidazole molecules to H-bond to each other in a chain-like manner. As a result of these structural features the Grotthuss mechanism of proton transfer is less favored at the equimolar composition where H-bonds are too stable. By contrast, the Grotthuss mechanism is more pronounced in the low concentration range where imidazole acts as a base pulling the proton of the imidazolium cation. At high imidazole concentrations the contribution from the vehicular mechanism dominates.