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Chemistry

Adsorption of Amphiphillic Surfactants onto a Au(111)/Electrolyte Interface

Dan Bizzotto
We are using an innovative and novel approach to study adsorption of organic surfactants at gold surfaces. This research has broad practical applications in corrosion inhibition, solid state technology, chemically modified electrodes in modeling the behavior of biological membranes under the influence of an electric field. A Langmuir Trough is used to prepare monomolecular films (monolayers) of long-chain, aliphatic, amphiphillic molecules (fatty acids, phospholipids) on aqueous surfaces. The molecules in the monolayer can be aligned and their concentration controlled using the Langmuir Trough.
The gold surface can then be coated with well characterized monolayer y the traditional dipping technique or by the in-situ "horizontal touching" technique. In horizontal touching, the gold surface is placed parallel to and just touching the surface of the surfactant covered Langmuir trough in a hanging meniscus configuration. The monolayer coated electrode can be lifted of the surface with the adsorbed monolayer intact. The gold surface with the monolayer attached can be touched to the surface of surfactant on the trough to adsorb another layer of the surfactant onto the gold electrode.
This approach allows us to control the properties of the monolayer at the air/solution interface while we simultaneously probe the properties of the monolayer adsorbed at the metal/solution interface electrochemically using cyclic voltammetry, differential capacity, and chronocoulometry. We will assess the effectiveness of the oriented monolayer in blocking electron and ion transport processes (thereby inhibiting corrosion) at the metal/solution interface. Recently we have been able to use UV-Vis spectroelectrochemical techniques such as electroreflectance and fluorescence as well as light scattering to assist in determining the mechanisms of adsorption and desorption of the multiple layers of amphiphillic molecules. Some of the molecules used are shown below.
The adsorption of octadecanol onto Au(111) has been determined and the main results are presented below. This is a three dimensional representation of the film pressure at the metal/solution interface(MS) as a function of the film pressure at the gas/solution interface(GS) and the electrode potential. The straight line at E=Epzc represents a transfer ratio of unity, or MS = GS, a potential where a complete transfer from the GS to the MS interface is achieved. The influence of potential on the adsorption of octadecanol onto a Au(111) surface is quite dramatic. The surfactant will adsorb most strongly at the potential of zero charge, i.e. an uncharged gold surface. Any charging of the metal will decrease the efficiency of the transfer to the MS interface.