Determination of the distribution constants of aromatic compounds and steroids in biphasic micellar phosphonium ionic liquid/aqueous buffer systems by capillary electrokinetic chromatography

• Analyte distribution constants between aqueous and phosphonium ionic liquid phases.
• First time use of CMC determination of cationic micelle based on system peaks in CE.
• CMC determination by system peaks provide good results but is time demanding.
• Higher distribution constants were obtained using [P14444]OAc than [P14444]Cl.
• The retention mechanism covers other than hydrophobic interaction.

The distribution constants of some analytes, closely connected to the petrochemical industry, between an aqueous phase and a phosphonium ionic liquid phase, were determined by ionic liquid micellar electrokinetic chromatography (MEKC). The phosphonium ionic liquids studied were the water-soluble tributyl(tetradecyl)phosphonium with chloride or acetate as the counter ion. The retention factors were calculated and used for determination of the distribution constants. For calculating the retention factors the electrophoretic mobilities of the ionic liquids were required, thus, we adopted the iterative process, based on a homologous series of alkyl benzoates. Calculation of the distribution constants required information on the phase-ratio of the systems. For this the critical micelle concentrations (CMC) of the ionic liquids were needed. The CMCs were calculated using a method based on PeakMaster simulations, using the electrophoretic mobilities of system peaks. The resulting distribution constants for the neutral analytes between the ionic liquid and the aqueous (buffer) phase were compared with octanol-water partitioning coefficients. The results indicate that there are other factors affecting the distribution of analytes between phases, than just simple hydrophobic interactions.