ADSORPTION OF PERFLUORINATED SURFACTANTS ON GOLD ELECTRODES .2. BEHAVIOR OF IONIC COMPOUNDS

The capacitance of a gold rotating disc electrode has been measured in Na2SO4 electrolyte containing ionic perfluorinated surfactants and co mpared to that obtained with non-ionic compounds. It is shown that the adsorption mode of perfluorinated surfactants and the interactions be tween metal and molecules strongly depend on the hydrophilic head of t he molecules. The adsorption isotherm obtained with the cationic compo und (F1098) leads to the conclusion that the molecules are strongly ad sorbed in a flat orientation over the whole potential domain. With the anionic compound (F1176) the results are consistent with an adsorptio n mode occurring solely at potentials higher than the zero charge pote ntial and generating a hydrophilic bilayer of molecules in the upright orientation, sensitive to the hydrodynamic flow. A comparison with th e behaviour of non-ionic compounds indicates that the hydrophilic head influences the strength of the metal-molecules bondings.