Determining binding of sulfonamide antibiotics to CTABr micelles using semi-equilibrium dialysis

• Micelle binding constants determined by semi-equilibrium dialysis.
• Sulfonamide antibiotics with negative charge bind cationic CTABr micelles.
• ​Binding constants were better correlated with Ka2 values than with KOW values.
• Micelle interactions predict effectiveness of micelle-enhanced ultrafiltration (MEUF).

Membrane filtration processes such as ultrafiltration are increasingly employed in industrial and municipal wastewater treatment. However, these filtration processes are usually poor for removing low molecular weight organic and inorganic contaminants. Micellar enhanced ultrafiltration (MEUF), where contaminants first partition into micellar phases prior to removal by ultrafiltration, has been shown to be an effective removal technique for a variety of trace contaminants in wastewater. To characterize or predict the removal efficiency of MEUF, it is important to have a quantitative measure of the extent to which a species is able to bind to a micelle as well as to develop an understanding of the dynamics of the binding interactions.Herein, we report micellar binding constants for a series of sulfonamide antibiotics (sulfadoxine, sulfathiazole, sulfamethoxazole, sulfamerazine, sulfadiazine, sulfamethazine, sulfacetamide, sulfaguanidine, and sulfanilamide) with cetyltrimethylammonium bromide (CTABr) micelles, as determined using a semi-equilibrium dialysis method. Sulfadoxine, sulfathiazole and sulfamethoxazole exhibited the strongest tendency to bind to CTABr micelles, indicating that they possessed greater potential for removal via MEUF than did sulfanilamide or sulfaguanidine, which displayed little affinity for binding with CTABr micelles.Linear free energy plots of Log KB (in CTABr) versus Log KOW and pKa2 values indicated that there was a weak correlation between binding and Log KOW and a stronger correlation with pKa2 for these compounds, likely reflecting the stronger binding of ionizable sulfonamide molecules in the anionic form by the cationic micelles.