Retention mechanism of sulfonamides in micellar reversed phase liquid chromatography

A mechanism of retention of sulfonamides in the micellar liquid chromatography, based on similar models verified for other compounds, was proposed. The magnitudes of hydrophobic and electrostatic interactions of sulfonamides were estimated for different concentrations of surfactant in the mobile phase and at varied pH. With increasing SDS concentrations and at a constant pH, a decrease in the hydrophobic interaction intensity was noted. When pH is varied, the values of the constant, describing the hydrophobicity of the sulfonamides, remained at an approximately same level (taking into account the standard deviation values). Greater differences were observed in the constants related to the electrostatic interactions informing about the increase or decrease in retention, following from ionic interactions. For each sulfonamide, the molar fraction of the ionic fractions present in the mobile phase was calculated from the formula derived, which permitted determination of the type of electrostatic interactions, varying with the eluent’s changing pH. On the basis of the chromatographic data collected, it was established that retention of sulfonamides in the SDS micellar system depends on the hydrophobic and electrostatic interactions. For varied pH values, the hydrophobic interactions of sulfonamides did not change significantly, all changes in the retention of the sulfonamides were attributed to changes in the electrostatic interactions.