Characterization of Surfactant Selectivity in Micellar Electrokinetic Chromatography

Abstract

The solvation parameter model is used to delineate the contribution of individual intermolecular interactions to the retention properties of seven common surfactants used in micellar electrokinetic chromatography. Buffer composition and concentration, pH, temperature, and voltage had only a small influence on selectivity for sodium cholate micelles in the normal experimental range for separations. Surfactant concentration was found to affect retention by changing the phase ratio without significantly changing selectivity while the addition of organic solvent in low concentration (up to 10% v/v) has an influence on the solvophobic properties of the buffer (lowers cohesion and reduces its capacity as a hydrogen-bond acid) in a manner which seems to depend very little on solvent identity. The bile salt surfactants, sodium cholate, sodium deoxycholate, sodium taurocholate, and sodium deoxytaurocholate, are quite similar as a group, exhibiting modest differences in their cohesion and hydrogen-bond acidity. Sodium dodecyl sulfate has complementary properties to the bile salt surfactants; it is less cohesive and a weaker hydrogen-bond base and a stronger hydrogen-bond acid. The sodium salt of N-dodeconyl-N-methyltaurine is a stronger hydrogen-bond base than the support buffer and weak hydrogen-bond acid; again properties that set it apart from the bile salts and sodium dodecyl sulfate for the separation of hydrogen-bonding solutes. Hexamethyltrimethylammonium bromide is the least cohesive and the strongest hydrogen-bond base of the surfactants studied but its relatively small migration window may make it less useful in practice for complex separations. Lithium perfluorooctanesulfonate has rather unique selectivity (high cohesion, significant capacity for dipole-type interactions and strong hydrogen-bond acidity) that would make it a good choice with the other surfactants for general selectivity optimization in methods development.