Adsorption of insoluble surfactants at the Au(111)/solution interface

Abstract

The adsorption of surfactants, which form insoluble monolayers on an aqueous substrate, onto a single crystal gold electrode have been described. Adsorption of this class of surfactants have been characterized using a combination of electrochemistry and Langmuir-Blodgett techniques. We have developed a technique to simultaneously measure the film pressure at the gas-solution (GS) interface and the film pressure of the surfactants that spread to the metal-solution (MS) interface. We have shown that surfactants such as octadecanol and stearic acid, which interact weakly with the metal surface, adsorb at an uncharged MS interface (at the potential of zero charge) and progressively desorb when the electrode surface is charged negatively. The electrode potential (charge density at the metal surface) influences the transfer of the surfactant from the GS interface to the MS interface. The transfer ratio is 1:1 at an uncharged MS interface, and is progressively reduced to zero when the MS interface is charged. We have employed 12-(9-anthroloxy) stearic acid, a surfactant dye molecule, to study the mechanism of potential induced desorption and adsorption of the film of insoluble molecules. With the help of electroreflectance spectroscopy and light scattering measurements, we have shown that if desorbed, the surfactant molecules form micelles (flakes or vesicles) that are trapped under the electrode surface. The micelles spontaneously spread back onto the electrode surface when the charge density at the metal approaches zero. The repeatable desorption and readsorption involve micellisation of the film at negative potentials and spontaneous spreading of the micelles to reform the monolayer at potentials close to pzc.