Hydrophobicity of cationic solutes measured by electrokinetic chromatography with cationic microemulsions.

The effect of the structure of employed surfactants on the selectivity of microemulsion electrokinetic chromatography (MEEKC) was investigated. Both their ionic groups and their hydrocarbon chain lengths did not affect the selectivity for neutral solutes. However, for cationic solutes, ionic interactions between catonic solutes and anionic surfactants both in the aqueous phase and in the microemulsion phase were observed. Based on these results, determination of the hydrophobicity of cationic solutes was done using MEEKC with cationic microemulsions. The obtained migration index (MI), which had been proposed as a novel hydrophobicity scale measured by MEEKC (Ishihama, Y.; et al. Anal. Chem. 1995, 67, 1588), correlated with conventional hydrophobicity scales for a water/1-octanol system, a liposome system, and an HPLC system using a phospholipid-immobilized artificial membrane (IAM) column. Quantitative structure-activity relationship studies on a protein binding affinity using MI as well as the hydrophobicity scales using the liposome and the IAM column provided better results than those using the water/1-octanol system.