Sulfonamide Partitioning to Aqueous Cationic Micellar Systems

Abstract

Advances in analytical chemistry have resulted in a growing body of literature showing measurable concentrations of pharmaceuticals in both drinking and wastewater. Removal of such chemicals is typically inefficient and often poorly characterized. To characterize one such method of removal (micellar enhanced ultrafiltration, (MEUF)), interactions of a cetyl trimethylammonium bromide (CTABr) surfactant and sulfonamide antibiotics were examined by NMR and semi-equilibrium dialysis (SED). The locus and orientation of binding in a micelle was established for seven sulfonamides by 1H NMR, and it was found that hydrophilic sulfonamides showed weak coordination with the micelle, whereas hydrophobic sulfonamides penetrated into the micellar interior with coordination of the SO2NH group to the charged surface layer. Binding constants were determined by 1H NMR and showed apparent order of magnitude differences between nuclei. Several compounds were unable to be characterized in this manner due to low change in chemical shift with addition of CTABr. SED was performed as an alternative method to determine binding constants. Values determined in this manner were higher than those determined by 1H NMR. Binding constants were converted into changes in Gibbs free energy and used to evaluate and, where necessary, modify the orientation and locus proposed by 1H NMR. Attempts are made to correlate binding constants with octanol-water partition coefficients to determine if a free energy relationship can be derived. Characterization of these systems may allow for a predictive methodology to determine the MEUF removal efficiencies of new sulfonamide and surfactant combinations. It is also hoped that this work may be generalized to predict MEUF efficiency for a wide range of contaminants that might be found in wastewater.