Spread insoluble cationic polysoap monolayers: II. Specific effect of counterion binding on surface density pressure and potential at collapse point

Abstract

The selective binding of SCN − ions by an insoluble cationic polysoap monolayer spread on binary mixtures of 1 1 electrolytes KSCN and KX(X = F, Cl, Br, I, NO 3, OH, ClO 4) has been studied at constant ionic strength. The surface potential and pressure of the monolayers has been measured. The selective binding of SCN − as well as the collapse surface density vary with the electrolyte mixture composition and are assigned to the counterion/polyion association (CIPIA) or “ion-pair” formation. The degrees of association of the various counterions have been deduced. It is concluded that the counterions may penetrate inside the polysoap monolayer. When all the sites of the polyion are neutralized by one counterion (SCN −), the monolayer behavior is that of an electrode selective to this counterion. A linear variation of the surface potential with log [SCN −] is observed which verifies Nernst law.