Ion-association adsorption of water-soluble porphyrin at a liquid-liquid interface and an external electric field effect on the adsorption

By using total internal-reflection fluorescence (TIRE) spectroscopy it was shown that an unprotonated form of water-soluble porphyrin (5,10,15,20-tetraphenyl-21 H, 23 H-porphinetetrasulfonic acid, TPPS) is preferentially adsorbed over the diprotonated one by the toluene-water or di(2-ethylhexyl)phthalate (DEHP)-water interface in the presence of a cationic surfactant (cetyltrimethylammonium bromide, CTAB). The pH dependencies of the TIRF intensity were satisfactorily analyzed by a simple adsorption model, including the protonation equilibria in the bulk aqueous phase and the adsorption equilibrium at the interface of TPPS. Further, it was found that applying an electric field across the DEHP-water interface significantly changed the ion-association adsorption equilibria. This result was explained in terms of the electrophoresis of TPPS in the interfacial region.