Migration behavior of cationic solutes in micellar electrokinetic capillary chromatography

Abstract

A phenomenological approach is presented to describe the migration of cationic solutes in micellar electrokinetic capillary chromatography (MECC). The migration behavior of an organic base is complicated by the presence of an acid-base equilibrium, the ion-pairing formation between the conjugated acid of the base and the monomer surfactants, and the interactions of both the base and its conjugated acid with the micellar pseudophase. An equation was derived that allows the calculation of the migration factor of a cationic solute in MECC with anionic micelles. Two limiting cases were considered: first the cationic solute completely associates with the anionic surfactant (ion-pair formation constant, KIP, approaches infinity), and therefore there is no free charged species in the solution; second, the KIP = 0 and the free conjugated acid, BH+ migrates in the aqueous bulk solvent at its own electrophoretic velocity. An estimate for the ion-pair formation constant between cationic solutes and free surfactant can be obtained by using the model.

Application of Retention Factors in Affinity Electrokinetic Chromatography and Capillary Electrophoresis

“…Using a phenomenological approach and electrophoretic mobilities, rather than linear velocities, as done by Terabe and coworkers, relations between the analyte migration behavior and the distribution coefficient can be derived. 17,18 The effective electrophoretic mobility of an analyte, μA, will be determined by the fraction of time spent in each of the two phases and may be described by…”

“…(4), changes in the viscosity of the background electrolyte and possible complexation with surfactant monomers due to addition of the pseudo-stationary phase have to be considered and corrected for. 17,[28][29][30][31] Usually the analyte and marker molecule concentrations used in EKC are kept low in order to comply with the assumption that the mobility of the pseudo-stationary phase is not affected by interaction with these molecules. The presence of a solute in the pseudo-stationary phase, however, may affect the structural properties of the phase, and thus both the solute distribution and electrophoretic migration.…”

Determination of the Critical Micelle Concentration of Cationic Surfactants by Capillary Electrophoresis

“…The effective electrophoretic mobility of a solute in MEKC using cationic surfactant can be specifically defined by [15,17,18] …”

Microemulsion electrokinetic chromatography of drugs varying in charge and hydrophobicity Part II: Strategies for optimization of separation

“…Electrostatic interactions (repulsion for negative ions and attraction for positive ions) will, in addition to other interactions, play a role. Since ionized solutes have their own electrophoretic mobility, the migration time observed will be the result of both partitioning between the oil droplets and the water phase and their electrophoretic behavior [3].…”