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 (Λ_imp/Λ_NMR 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 ¹H 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.