(387f) Theoretical And Experimental Analysis Of Charge Transport And Associated Surface Charge Densities In Thin-Film Modified Electrodes

The phenomenon of charge transfer at the nano-scale is fundamental to a number of applications in sensor technology, electro-catalysis, photonics and solar photo-conversion. Surface charge densities at solid-liquid interfaces are also critical to determining adhesion properties and biocompatibility of materials, stability of colloidal systems and liquid flow. Molecularly structured surfaces, like those of self-assembled monolayers on gold, provide a convenient, flexible and simple platform to tailor and study the interfacial properties and charge transfer characteristics of metal-electrolyte interfaces. We present here a method to experimentally determine the surface charge densities at the Stern layer of a gold-monolayer-electrolyte system, as a function of applied potential at the metal surface. We also characterize the charge transfer processes that occur in a monolayer, in the absence of Faradaic reactions. Exact analytical expressions for the Stern layer charge densities are obtained from the solution of the Poisson equation that characterizes the electrostatic behavior of the monolayer-electrolyte interface. While solving the Poisson equation, we allow for charge transfer within the monolayer and discreteness of charge effects. The unknown parameters in the analytical expression for Stern charge density are obtained from measured impedance values by using an integral form of the charge transport equation. The continuum charge density distributions thus obtained demonstrate the existence of quantum mechanical tunneling effects in the monolayer film. We describe the effects that bulk electrolyte and monolayer properties have on the charge transport behavior. Finally, we demonstrate the application of energy conservation to account for the change in free energy of the Stern layer with applied potential. Estimates of the change in free energy allow for a description of the various adsorption events that occur at the monolayer-electrolyte interface.