Study on the protonation equilibria of drug naproxen in different surfactant media

Naproxen is a drug with very low solubility in aqueous solution. In the present work, the acid-base equilibria for this drug was studied pH-potentiometrically in ethanol-water mixtures (25% v/v) at constant ionic strength of 0.15 mol dm−3 NaCl and the temperature of 25°C. The potentiometeric measurements were carried out in presence of three micelle-forming surfactants with different concentrations, namely sodium dodecylsulfate (anionic SDS), dodecyltrimethylammonium bromide (cationic DTAB) and polyethylene glycol dodecyl ether (non-ionic Brij 35). The results obtained indicate that, irrespective of the surfactant type, the protonation constant of the drug naproxen decreases with increasing concentrations of the surfactants.