Adsorption of Surface-Active Substances at Mercury-Solution Interfaces

Abstract

A review is given of adsorption of surface-active substances at mercury-solution interfaces, especially with regard to the structure of adsorbed layers. The difficulty in studying adsorption equilibrium at a mercury electrode surface exists in the fact that the amount adsorbed depends on the electrode potential as well as the bulk concentration of organic substances. It is generally accepted that both the Langmuir isotherm and the Gibbs isotherm are valid for adsorption of organic substances. The dependence of the amount adsorbed on the electrode potential has been explained quantitatively by Frumkin and Butler independently. The physical picture of the latter theory, regarding adsorbed molecules as dielectrics with a permanent dipole and a given polarizabillty, may be easier for chemists to follow, but no less important is the fundamental expression for charge density of the former theory. A linear relationship which is usually assumed to hold between the surface coverage and the double layer capacity should be tested by further experiments. Some conclusions on adsorption rate and mechanism of adsorption can be derived from the frequency dependence behavior of the heights of adsorption-desorption peaks, which appear on a differential capacity-potential curve. The dependence of peak potential on concen-tration has been successfully explained. The rate constant of the adsorption process has also been estimated by Lorenz for aliphatic alcohols and acids of low molecular weight, while the formation of a condensed adsorbed layer has been pointed out for higher homologues of those. The frequency effect observed for those substances has been interpreted semiquantitatively by assuming that a relatively slow association process may take place between adsorbed molecules after a rapid adsorption process. 4.3μF/cm2, recently obtained by Laitinen and his co-worker for a monolayer of palmitic acid on a mercury-coated platinum electrode, would be reasonable for the differential capacity value of a monolayer film of higher fatty acids. In comparison to this value, 1.8 μF/cm2 observed with nearly saturated solutions of higher aliphatic alcohols and acids, might indicate the formation of a bimolecular adsorbed layer. There seems to remain some ambiguity for the orientation of neutral molecules at an electrode surface, but this problem may be solved in the case of long chain surfactant anions, which have been lately studied in detail by Eda. With solutions of sodium dodecyl sulfate in sodium sulfate, four peaks are observed on the differential capacity-potential curve. The two peaks which appear at the most positive or negative potential may correspond to the adsorption-desorption reaction in an ordinary sense. The relatively small peak which appears on the positive branch near the electrocapillary maximum may be attributed to the reorientation of the adsorbed anions ; and therefore a bimolecular adsorbed layer may be formed on the more positively charged mercury surface than this peak potential. The other small peak on the negative branch may correspond to the phase separation, which means the transformation of a gaseous phaseinto a condensed phase.