Surface Forces at Electrified Interfaces

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CTS-9907687T.K. VanderlickPrinceton UniversitySummaryThe aim of this proposal is to use direct force measurements to investigate the structure and properties of electrified interfaces. Such interfaces are, of course, of premier importance in the far-reaching and significant field of electrochemistry. In addition, however, electrochemical based forces play significant roles in biology (e.g. establishing the conformation and function of biomolecules), in physical chemistry (e.g., swelling of clays), in engineering (e.g., stabilization of colloids and emulsions), and in many industrial processes.While the electrode/electrode interface is one of the most complex solid/liquid interfaces, its structure can be effectively interrogated through the measurement of surface forces. Such interrogations can take advantage of the range and strength of electrostatic interactions, as well as the ability to modify interactions by changing the applied potential to the electrochemically active surfaces. Forces acting between surfaces reflect the structural details of the surfaces themselves as well as that of the medium trapped between them.The primary tool for the proposed investigations is an "electrochemical surface forces apparatus." This new device extends the unique capabilities of the surface forces apparatus to systems in which one, or both, interacting surfaces is metallic and under applied potential control. The proposed work is divided along three themes providing opportunities for a range of significant scientific and technological advances.The first theme of research is to use force profiling to elucidate the basic structure of electrode/electrolyte interfaces. The objective is to measure, as a function of separation, the forces of interaction between surfaces where one, or both, are typical electrode materials, namely gold (Au), silver (Ag) or platinum (Pt). Forces will be measured as a function of the potential applied to the surfaces and also the nature of the electrolyte. The role of specific adsorption of anions will be explicitly investigated. The most basic experiment will involve interactions between molecularly smooth mica and one electrochemically active surface, acting as a working electrode. Specific objectives include establishing the potential of zero charge for the various electrochemical systems; testing DLVO predictions for interactions between dissimilar surfaces; establishing the limits of application of DLVO theory, and probing the short-range structure of electrode/electrolyte interfaces.The second theme of research is to study the adhesion and associated deformations of electrified surfaces. The framework of contact mechanics will be used to study the loading and unloading behavior of electrified surfaces under potential control. One system of investigation will be mica in contact with gold. The electrochemical nature of gold surfaces (three distinct potential regimes: Au/AuOH/AuO) provides a rich foundation for these investigations.The third theme of research focuses on a variety of electrode modifications schemes for selective and active control of surface forces. The goal is to demonstrate how certain molecular adsorbates or coating formed on electrodes can serve to alter their interactions with other surfaces in useful and novel ways. Systems for study include ionizable self-assembled monolayers which respond to pH changes; self-assembled monolayers that release attached molecules when electrical potentials are applied the underlying metal substrate; and adsorbed surfactant monolayers which undergo structural changes with applied potential. Alteration of electrode surfaces by underpotential deposition of metals will be also examined.