Benzene oligocarboxylic acids: Characterization by isocratic, linear gradient, and frontal anion-exchange chromatography

A series of nine benzene oligocarboxylic acids is characterized using anion-exchange chromalography to determine the steric mass-action affinity parameters: the characteristic charge, the equilibrium ion-exchange constant (and the corresponding Gibbs free energy of ion-exchange), and steric factor. Isocratic elution under various concentrations of sodium chloride in the mobile phase provides the characteristic charge and the ion-exchange constant. The former agrees closely with the number of carboxylic acid groups in the sample, and the latter seems to be strongly influenced by the relative positions of the carboxylic acid groups on the benzene ring. Analysis of all samples under a linear salt gradient provides retention times that follow the same order as the ion-exchange constants determined isocratically. Using an appropriate model, along with the isocratically determined characteristic charge and ion-exchange constant, the linear gradient retention times are predicted theoretically and agree well with the experimental times. Frontal experiments led to the calculation of the characteristic charge and the steric factor. The former agrees well with the value determined isocratically. For most samples, the steric factor increases slowly with the characteristic charge.