Effects of Alkyl Chain Length and Solvents on Thermodynamic Dissociation Constants of the Ionic Liquids with One Carboxyl Group in the Alkyl Chain of Imidazolium Cations

Thermodynamic dissociation constants of the Bronsted acidic ionic liquids (ILs) are important for their catalytic and separation applications. In this work, a series of imidazolium bromides with one carboxylic acid substitute group in their alkyl chain ([{(CH2)(n)COOH}mim]Br, n = 1,3,5,7) have been synthesized, and their dissociation constants (pK(a)) at different ionic strengths have been determined in aqueous and aqueous organic solvents at 0.1 mole fraction (x) of ethanol, glycol, iso-propanol, and dimethyl sulfoxide by potentiometric titrations at 298.2 K. The standard thermodynamic dissociation constants (pK(a)(T)) of the ILs in these solvents were calculated from the extended Debye-Huckel equation. It was found that the pK(a) values increased with the increase of ionic strength of the media and of the addition of organic solvent in water. The pK(a)(T) values also increased with the increase of the alkyl chain length of cations of the ILs. In addition, the effect of solvent nature on pK(a)(T) values is interpreted from solvation of the dissociation components and their Gibbs energy of transfer from water to aqueous organic solutions.