Separation of a group of N-phenylpyrazole derivatives by micellar electrokinetic chromatography: application to the determination of solute-micelle association constants and estimation of the hydrophobicity

Micellar electrokinetic chromatography (MEKC) was applied to the separation of a group of N-phenylpyrazole derivatives. Sodium dodecyl sulfate (SDS) as micellar system and 2-(N-cyclohexylamino)ethanesulfonic acid (CHES) as separation buffer (pH 10) were employed in the absence and presence of different percentages of medium chain alcohols (n-propanol or n-butanol). The separation of multicomponent mixtures of the solutes studied enabled the rapid determination of their retention factors which, in turn, allowed the study of the separation selectivity of compounds and the determination of their solute-micelle association constants (from the linear variation of the retention factors as a function of the total surfactant concentration in the separation buffer). Separation selectivity was studied according to the elution range and number of solutes separated in all the electrolyte solutions employed (45 micellar phases). The effect of the buffer concentration (0.05, 0.08 and 0.10 M), the alcohol nature (n-propanol or n-butanol) and the alcohol percentage (1, 3 or 5%) of the values obtained for the solute-micelle association constants was also studied. The best separation (12 solutes) was performed when a 0.08 M CHES buffer, pH 10, 0.02 M SDS modified by 5% n-butanol was used. The possibilities of using MEKC for evaluating the hydrophobicity of compounds was investigated through the study of the correlation between the logarithm of the retention factors of N-phenylpyrazole derivatives and their logarithm of the octanol-water distribution coefficients estimated by high performance liquid chromatography (HPLC).