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 compared to that obtained with non-ionic compounds. It is shown that the adsorption mode of perfluorinated surfactants and the interactions between metal and molecules strongly depend on the hydrophilic head of the molecules. The adsorption isotherm obtained with the cationic compound (F1098) leads to the conclusion that the molecules are strongly adsorbed in a flat orientation over the whole potential domain. With the anionic compound (F1176) the results are consistent with an adsorption mode occurring solely at potentials higher than the zero charge potential and generating a hydrophilic bilayer of molecules in the upright orientation, sensitive to the hydrodynamic flow. A comparison with the behaviour of non-ionic compounds indicates that the hydrophilic head influences the strength of the metal-molecules bondings.