Adsorption and ion-pairing interactions of phospholipids in the system of two immiscible electrolyte solutions: Part III: The behaviour of a lecithin layer at the water/1,2-dichloroethane interface at interfacial potential differences lower than the zero-

The behaviour of l-α-lecithin (DPPC) was studied at the water/1,2-dichloroethane interface within a range of interfacial potential differences below the zero-charge potential difference, using surface tension and voltammetric measurements. The general five-step model, established in Parts I and II of this series, has been confirmed. The surface pressure of DPPC adsorbed at the interface exhibits a maximum in acidic media close to the zero-charge potential difference, Epzc, of the base electrolytes. It is demonstrated that, in contrast to the potentials higher than Epzc, where the change in the surface pressure is due to desorption of the protonated form of DPPC, the observed change in the surface pressure at interfacial potential differences below Epzc can be explained by alterations in the concentrations of the adsorbed zwitter-ionic and protonated forms of DPPC governed by the general acid/base equilibrium.