Acid–Base Property of N-Methylimidazolium-Based Protic Ionic Liquids Depending on Anion

Abstract

Proton-donating and ionization properties of several protic ionic liquids (PILs) made from N-methylimidazole (Mim) and a series of acids (HA) have been assessed by means of potentiometric and calorimetric titrations. With regard to strong acids, bis(trifluoromethanesulfonyl) amide (Tf(2)NH) and trifluoromethanesulfonic acid (TfOH), it was elucidated that the two equimolar mixtures with Mim almost consist of ionic species, HMim(+) and A(-), and the proton transfer equilibrium corresponding to autoprotolysis in ordinary molecular liquids was established. The respective autoprotolysis constants were successfully evaluated, which indicate the proton-donating abilities of TfOH and Tf(2)NH in the respective PILs are similar. In the case of trifluoroacetic acid, the proton-donating ability of CF(3)COOH is much weaker than those of TfOH and Tf(2)NH, while ions are predominant species. On the other hand, with regard to formic acid and acetic acid, protons of these acids are suggested not to transfer to Mim sufficiently. From calorimetric titrations, about half of Mim is estimated to be proton-attached at most in the CH(3)COOH-Mim equimolar mixture. In such a mixture, hydrogen-bonding adducts formation has been suggested. The autoprotolysis constants of the present PILs show a good linear correlation with dissociation constants of the constituent acids in an aqueous phase.

Enhanced NH3 capture by imidazolium‐based protic ionic liquids with different anions and cation substituents

“…Solubility of NH 3 in ILs with different anions was measured at 313 K and the results were as shown in Figs and . For protic ILs, the order of NH 3 capacity was [Bim][NTf 2 ] > [Bim][SCN] > [Bim][NO 3 ] and the sequence was contrary to the pK a values of the corresponding acids of the anions: HNTf 2 (–13∼–14.1) < HSCN (–1.85) < HNO 3 (–1.38) , , , . In the case of conventional ILs with [Bmim] + and [Bmmim] + cations, similar trends in NH 3 solubility in ILs were found: [NTf 2 ] ‐ > [DCA] ‐ > [SCN] ‐ .…”

Effects of a proton‐conducting ionic liquid on secondary ion formation in time‐of‐flight secondary ion mass spectrometry

“…As previously described, PILs are formed through proton transfer from a Brønsted acid HA to a Brønsted base B as follows: where HB + is a conjugate acid of the base B, and A − is a conjugate base of the acid HA. Because of the proton‐donating and ‐accepting abilities of HB + and A − , PILs are regarded as amphoteric solvents and undergo autoprotolysis; consequently, the following equilibrium holds in PILs: …”

Hydrogen bond in imidazolium based protic and aprotic ionic liquids

“…Although the ΔpK a is better as the solvent parameter for not only the inter-ionic interactions in PIL but also various static/dynamic macroscopic properties of PIL, the auto-protolysis constant K s could be more suitable for representing the acid-base property as they are determined on the basis of directly measured pH in PIL [54][55][56]. Although the ΔpK a is better as the solvent parameter for not only the inter-ionic interactions in PIL but also various static/dynamic macroscopic properties of PIL, the auto-protolysis constant K s could be more suitable for representing the acid-base property as they are determined on the basis of directly measured pH in PIL [54][55][56].…”