Characterization of the solvation properties of surfactants by solvatochromic indicators

Kamlet-Taft solvatochromic scales have been used for the characterization of three different micellar electrokinetic chromatography (MEKC) systems, providing information about the polarity parameters,of both the aqueous phase and the micellar phase of each system. The surfactants studied are sodium cholate (SC), which belongs to the family of bile salts, lithium perfluoro-n-octanesulfonate (LPFOS), a fluoro-surfactant, and n-hexadecyl-trimethylammonium bromide (HTAB), a cationic surfactant. Sodium n-dodecyl sulfate, the most commonly used surfactant in MEKC, was already studied in a previous work. The solvatochromic measurements allow the determination of the critical micellar concentration (cmc) of the surfactants and also the partition coefficients of the solvatochromic indicators in each system. Both cmc and partition coefficients have been also determined through MEKC experiments, and good agreement is observed between the two methods. The results of the solvatochromic study indicate that LPFOS is the surfactant that has properties more similar to those of the aqueous phase, whereas SC is the most hydrogen-bond basic and acidic surfactant, and HTAB has the lowest hydrogen-bond acidity.