Acidic characterization of polycarboxyl imidazolium ionic liquids

The acid-dissociation constant pK(a) of different polycarboxyl imidazolium ionic liquids (ILs) has been determined by potentiometric titration method at 25 degrees C, and the values are in the range of 1.43 to 1.92, which are close to that of oxalic acid, but much smaller than those of succinic acid and acetic acid. The stronger acidity of polycarboxyl imidazolium ILs results from the strong electron withdrawing effect of positively charged imidazolium ring. It is also demonstrated that the acidity of the ILs is enhanced as the number of carboxylic groups appended to cations increases from two to three. The acidity of the ILs also depends on properties of the anions, and decreases in the order HSO4- > NO3- > PF6- > H2PO4- > Cl-, Br- > CF3CO2- > BF4-> CF3SO3-. The Bronsted acidity of the polycarboxyl imidazolium ILs has also been studied by IR technique using pyridine as the spectroscopic probe.