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

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Abstract

We investigate the effect of water on the transport properties of protic and aprotic ionic liquids of imidazolium.

Abstract

In this paper we report on the transport properties of protic and aprotic ionic liquids of the imidazolium cation (C ₂C ₁Im ⁺ or C ₂HIm ⁺) 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 C ₂HImTfO while it is rather weak for C ₂C ₁ImTFSI, 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 C ₂HImTfO > C ₂HImTFSI ≈ C ₂C ₁ImTfO > C ₂C ₁ImTFSI. 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 H ₂O and D ₂O, 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.

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Ionic-liquid materials for the electrochemical challenges of the future.

Michel Armand, Frank Endres, Douglas MacFarlane … (2009)

Ionic liquids are room-temperature molten salts, composed mostly of organic ions that may undergo almost unlimited structural variations. This review covers the newest aspects of ionic liquids in applications where their ion conductivity is exploited; as electrochemical solvents for metal/semiconductor electrodeposition, and as batteries and fuel cells where conventional media, organic solvents (in batteries) or water (in polymer-electrolyte-membrane fuel cells), fail. Biology and biomimetic processes in ionic liquids are also discussed. In these decidedly different materials, some enzymes show activity that is not exhibited in more traditional systems, creating huge potential for bioinspired catalysis and biofuel cells. Our goal in this review is to survey the recent key developments and issues within ionic-liquid research in these areas. As well as informing materials scientists, we hope to generate interest in the wider community and encourage others to make use of ionic liquids in tackling scientific challenges.