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 C2H and the
N3H 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.