Nanometer-scale probing of potential-dependent electrostatic forces, adhesion, and interfacial friction at the electrode/electrolyte interface

The atomic force microscope (AFM) was used to examine the influence of an applied electrochemical potential on the interfacial properties of the electrode/electrolyte interface. Measurements of electrostatic force, adhesion, and friction coefficient were performed at two different electrode surfaces: glassy carbon and a thin film of sulfonate-derivatized poly(aniline) (SPANi). At the carbon electrode, changes in electrostatic force between probe and substrate exhibited a potential-dependent transition from repulsive to attractive values at potentials negative and positive of the potential of zero charge (E(pzc)). Simultaneous measurements of tip-substrate adhesion and friction coefficient showed a change from low to high values over the same potential range, suggesting a common mechanism dominated by the electrostatic force. Measurement of these same properties at a SPANi-coated electrode also displayed a potential-dependent response. The electrostatic force and the adhesion tracked with the oxidation state of the initially neutral film. However, the friction coefficient appeared insensitive to the charge state of the polymer. A calculation of the forces between probe and substrate using DLVO theory accurately reflected the measured force curves as well as the change in adhesive force as a function of surface charge. Consideration of the forces that determine the friction coefficient suggested that the influence of electrostatic interactions was strongly dependent upon the geometry of the tip-sample contact and the presence of microgaps between the tip and the substrate over which electrostatic forces could operate. The absence of potential-dependent friction at the SPANi/electrolyte interface reflected a compliant substrate, which gave rise to a predominantly adhesive tip/sample contact.