Prediction of electrophoretic mobilities. Part 2.† Effect of acid dissociation constant on the intrinsic mobilities of aliphatic carboxylates and amines

Electrophoretic mobility is the most important parameter governing the separation of solutes in capillary zone electrophoresis (CZE). Classical theoretical models for the prediction of absolute mobilities consider only the effect of hydrodynamic friction. Application of these models to literature mobility data demonstrates that they are not appropriate for small organic solutes. Accurate prediction of absolute mobilities of small ions requires an additional frictional term, the dielectric friction. Dielectric friction results from the work necessary to orientate the solvent dipoles in response to the solute charge. This study investigates whether the pK value of a charged functional group can be used as a relative measure for the dielectric friction. Using the pKa as a measure of dielectric friction of weak acids, the absolute mobility of aliphatic carboxylates is given by: