Effect of water on the transport properties of protic and aprotic imidazolium ionic liquids – an analysis of self-diffusivity, conductivity, and proton exchange mechanism

Abstract

In this paper we report on the transport properties of protic and aprotic ionic liquids of the imidazolium cation (C2C1Im(+) or C2HIm(+)) and the TFSI(-) or TfO(-) anion as a function of added water. We observe that the self-diffusion coefficient of the ionic species increases upon addition of water, and that the cation diffuses faster than the anion in the entire water concentration range investigated. We also observe that the overall increase of anionic and cationic diffusion coefficients is significant for C2HImTfO while it is rather weak for C2C1ImTFSI, the former being more hydrophilic. Moreover, the difference between cationic and anionic self-diffusivity specifically depends on the structure of the ionic liquid's ions. The degree of ion-ion association has been investigated by comparing the molar conductivity obtained by impedance measurements with the molar conductivity calculated from NMR data using the Nernst-Einstein equation. Our data indicate that the ions are partly dissociated (Λimp/ΛNMR in the range 0.45-0.75) but also that the degree of association decreases in the order C2HImTfO > C2HImTFSI ≈ C2C1ImTfO > C2C1ImTFSI. From these results, it seems that water finds different sites of interaction in the protic and aprotic ionic liquids, with a strong preference for hydrogen bonding to the -NH group (when available) and a stronger affinity to the TfO anion as compared to the TFSI. For the protic ionic liquids, the analysis of (1)H NMR chemical shifts (upon addition of H2O and D2O, respectively) indicates a water-cation interaction of hydrogen bonding nature. In addition, we could probe proton exchange between the -NH group and deuterated water for the protic cation, which occurs at a significantly faster rate if associated with the TfO anion as compared to the TFSI.

Effect of Water on the Local Structure and Phase Behavior of Imidazolium-Based Protic Ionic Liquids

“…In that study, we hypothesized a specific interaction through the -NH group of the imidazolium ring, and observed stronger perturbations in the systems based on the TfO anion as compared to those based on TFSI. 5 Our results also suggested that the addition of water can affect the 'ionicity' of the PIL, an important aspect with respect to the use of PILs as electrolytes.…”

“…6 in reference [ 5]). These findings indicate a molecular vicinity of water to the imidazolium cation as well as an NH- 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 and C 4 C 1 ImTFSI.…”

“…7, 5 The molar volume (V m ) can be calculated using molar mass (M w ) and density (ρ) as follows: for C 2 HImTFSI, V m = M ρ =377. 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Table 3: Observed infrared modes of C 2 HImTfO and C 2 HImTFSI at room temperature and their assignments according to the literature.…”

Basicity Characterization of Imidazolyl Ionic Liquids and Their Application for Biomass Dissolution

“…Ionic liquids (ILs), generally defined as organic salts that melt below 100 ∘ C, have been closely studied because they are designable, thermally stable, nonvolatile, and capable of dissolving even polymeric compounds [5][6][7]. Such properties have led to an explosion in their applications as green, designable solvents for a variety of processes linked to green chemistry and clean technology [8][9][10]. The introduction of the specific functional group in ionic liquid anions or cations is to form functional ILs [11,12].…”

Study the Effect of Various Operating Parameters of Proton Exchange Membrane

“…Among them, the Proton Exchange Membrane Fuel Cell (PEMFC), which is focus of this paper, is described by the use of a polymer electrolyte membrane [5][6][7][8][9].…”