Polyelectrolyte adsorption on mercury surfaces. Differential capacity in the presence of poly 2- and 4-vinyl pyridines

Abstract

The differential capacity of the double layer on a mercury water interface in the presence of poly-vinyl pyridine (2 P.V.P.) and of poly 4-vinyl pyridine (4 P.V.P.) was measured. The supporting electrolyte in most cases was 0.1 HNO 3 or 0.09 NaNO 3 + 0.01 HNO 3. The nature of the monovalent cation is of no or very little importance. Adsorption peaks at potentials −0.030 v. and −0.130 v. relative to a 1 N calomel electrode were observed in the case of 4 P.V.P. and 2 P.V.P., respectively, 4 P.V.P. giving the higher peak. Thus, in spite of its higher polarity and water solubility, 4 P.V.P. is more surface active. In both cases, however, full coverage is approached at very low bulk concentrations. The phenomenon is discussed in the light of the orientation abilities of both polymers in the surface. Surface tension values as calculated from the differential capacity curves decrease linearly with concentration at small concentrations when equilibrium is not yet reached. They remain constant at intermediate concentrations and start increasing again at the highest measured polymer concentrations. From the time-dependent results at low polymer concentrations, the adsorption kinetics can be obtained. The equilibrium values, on the other hand, indicate that the adsorbed polymer molecules form a surface phase which may be a hundred or more angstrom units thick. Surface activity of the counter anion facilitates the adsorption of the polybase at positive potentials, and vice versa, the polybase enhances adsorption of the counterion at negative potentials. This was proved by exchanging NO 3 − by the more surface-active Cl − and Br −.