α,ω-alkyl dicarboxylic acids: Characterization by isocratic anion-exchange chromatography

Abstract

A homologous series of 7 α,ω-alkyl dicarboxylic acids is characterized using isocratic anion-exchange chromatography at different mobile phase salt concentrations to determine the characteristic charge and the equilibrium ion-exchange constant (and the corresponding Gibbs free energy of ion-exchange). The characteristic charge agrees closely with the number of carboxylic acid groups in the samples, namely 2, and the ion-exchange constant seemed to remain almost unaffected by the number of methylene groups in the sample. Succinic acid, having 2 methylene groups between the terminal carboxylic acid groups, always produces 2 chromatographic peaks despite the proven chemical purity of the sample. The characteristic charge calculated for both peaks of succinic acid is approximately 2, as is that of the other homologues. The Gibbs free energy of ion-exchange calculated for the weakly retained peak of succinic acid is similar to that of the other samples, whereas that of the strongly retained peak is lower by 2.1 kJ/mol. The 2 peaks of succinic acid are explained in terms of an equilibrium between two conformers in solution, one binding the solution counterions tightly and the other loosely.