ABSTRACT
The adsorption of insoluble surfactants spread as a monolayer at the gas-solution (G–S) interface onto electrified metal - solution (M – S) interfaces is described. Surfactants used include octadecanol, pentadecyl-pyridine, 12-AS, andthelipids dioleyl phosphatidylcholine (DOPC) and dimyristoyl phosphatidylcholine (DMPC). The adsorption of surfactants that do not display any specific interaction with the metal surfaces used [Au(111) and Hg] reveal that the transfer ofthe molecules from the G – S interface to the M–S interface is strongly dependent on potential. The transfer ratio between the two interfaces is 1 : 1 at an uncharged electrode surface and decreases to zero at sufficiently negative charge densities. A comparison of the adsorption of DOPC on Au(111) and Hg is described. Surfactants with head groups that specifically interact with gold (i. e., pyridine) were also studied. This type of adsorption (chemisorption) displayed stable low capacitance layers, and in one case, spontaneous formation of a bilayer from a monolayer was observed. In all cases, the surfactants were repeatably desorbed and readsorbed at negative potentials. The fate of the desorbed surfactants was investigated using in situ spectroelectro-chemical techniques. The mechanism of the adsorption-desorption process involves formation of the aggregates near the electrode surface at negative potentials. At less negative potentials, the aggregates are contact adsorbed onto the electrode surface and hemiaggregates are formed that then spread to form the adsorbed layer. Confocal fluorescence images illustrate the adsorption-desorption process. The potential controlled spreading from hemi-micelles to an adsorbed layer as observed by atomic force microscopy (AFM) for the adsorption of sodium dodecylsulfate (SDS) onto Au(111) is described.
