Effect of water on the local structure and phase behavior of imidazolium-based protic ionic liquids.

We report on the effect of water on local structure and phase behavior of two protic ionic liquids, C2HImTFSI and C2HImTfO. Raman and infrared spectroscopy are employed to investigate the local coordination state. We find that water interacts weakly with TFSI(-) while more specifically with TfO(-) through the -SO3 group. Additionally, we observe that upon addition of water the -NH stretching frequency does not change in C2HImTFSI, while it red-shifts in C2HImTfO, indicative of different hydrogen bonding configurations. Supported by the appearance of some additional features in the 800-1000 cm(-1) frequency range where ring out-of-plane bending (γ) modes are found, we hypothesize that in C2HImTFSI water interacts only with the cation coordinating to the ring C(2)H and the N(3)H sites, while it interacts with both cation and anion in C2HImTfO forming hydrogen bonds that involve the cationic N-H site as well as the anionic -SO3 group. These different local structures also reflect in the phase behavior investigated by DSC, which reveals a more homogeneous solution when water is added to C2HImTfO, as compared to H2O/C2HImTFSI mixtures. Finally we report that the addition of water also significantly affects both Tm and Tg.