The Electrochemical Surface Forces Apparatus: The Effect of Surface Roughness, Electrostatic Surface Potentials, and Anodic Oxide Growth on Interaction Forces, and Friction between Dissimilar Surfaces in Aqueous Solutions

Abstract

We present a newly designed electrochemical surface forces apparatus (EC-SFA) that allows control and measurement of surface potentials and interfacial electrochemical reactions with simultaneous measurement of normal interaction forces (with nN resolution), friction forces (with μN resolution), and distances (with Å resolution) between apposing surfaces. We describe three applications of the developed EC-SFA and discuss the wide-range of potential other applications. In particular, we describe measurements of (1) force-distance profiles between smooth and rough gold surfaces and apposing self-assembled monolayer-covered smooth mica surfaces; (2) the effective changing thickness of anodically growing oxide layers with Å-accuracy on rough and smooth surfaces; and (3) friction forces evolving at a metal-ceramic contact, all as a function of the applied electrochemical potential. Interaction forces between atomically smooth surfaces are well-described using DLVO theory and the Hogg-Healy-Fuerstenau approximation for electric double layer interactions between dissimilar surfaces, which unintuitively predicts the possibility of attractive double layer forces between dissimilar surfaces whose surface potentials have similar sign, and repulsive forces between surfaces whose surface potentials have opposite sign. Surface roughness of the gold electrodes leads to an additional exponentially repulsive force in the force-distance profiles that is qualitatively well described by an extended DLVO model that includes repulsive hydration and steric forces. Comparing the measured thickness of the anodic gold oxide layer and the charge consumed for generating this layer allowed the identification of its chemical structure as a hydrated Au(OH)(3) phase formed at the gold surface at high positive potentials. The EC-SFA allows, for the first time, one to look at complex long-term transient effects of dynamic processes (e.g., relaxation times), which are also reflected in friction forces while tuning electrochemical surface potentials.

The Intersection of Interfacial Forces and Electrochemical Reactions

“…11 As shown in Figure 11a, the adhesion decreases dramatically as the surface roughness on the gold surface increases (Au-1 to Au-3). In this case, the large decrease in adhesion is a result of the decreased contact area between the interfaces.…”

“…DLVO theory was found to do an excellent job of predicting interaction forces when the mica−gold separation distances were greater than about one Debye length, 3 and several subsequent studies of interfacial forces under external potential control reached similar conclusions. 5,11,13,14 Further, the magnitudes of the double layer forces were found to saturate at electrochemical potentials that are large in magnitude: increasing the (negative) magnitude of the applied potential from −600 to −700 mV resulted in no measurable change in the magnitude of the measured double layer repulsion ( Figure 4b). 3 The presence of a force saturation regimewhere large increases in surface potentials lead to marginal increases in double layer forcesis also predicted by DLVO theory ( Figure 4a).…”

“…3 These deviations were attributed primarily to surface roughness effects 3 in agreement with subsequent electrochemical AFM and SFA measurements 5,11,13−15 however, later work also demonstrated the importance of specific ion−surface adsorption effects in causing short-range deviations from the predictions of DLVO theory. 5,11,14,15 For example, recent work by Valtiner et al 5 shows that both the saturation of long-range double layer forces and the magnitudes and ranges of short-range deviations from DLVO Figure 5. (a) Schematic of the experimental geometry of the surfaces and potential-measuring electrode surfaces in the SFA used for measuring the dissolution of one of the surfaces (the lower quartz surface in this case) induced by the close proximity (overlap of electric double layers) of another surface (the upper mica surface in this case) having a more negative surface potential, V. This electrochemical potential difference can be natural to the two surfaces (depending on the solution conditions) or externally applied, as illustrated in Figure 6a.…”

“…5 Several other studies also indicate that ion−surface binding/ condensation effects, i.e., the increased buildup of surface bound ions, play significant roles in modifying interaction and especially short-range and adhesion forces between two surfaces. 5,11,14,15 Indeed, in 2001, Wang and Bard had already used AFM with in situ electrochemical control of polycrystalline gold substrates to reach the conclusion that ion binding and condensation effects were a major cause of deviations in force measurements from the Gouy−Chapman−Stern model of electric double layers utilized in DLVO theory. 15 In general, these short-range deviations from DLVO theory can be accounted for by using empirical models incorporating exponentially decaying steric repulsion terms superimposed with DLVO theory, 5,11 for example, one distinct term accounting for the compression of asperities (rough surfaces) and another accounting for the compression of surface-bound ions and/or solvent molecules.…”

“…A clear manifestation of such a phenomenon can be observed when the fluid is sheared between two flat surfaces such as mica. To perform such an experiment, Valtiner et al 11 modified a recently developed 3D sensor actuator 8 and installed it with an electrochemical cell in the SFA (Figure 12a). In this new setup, the Ag/AgCl reference electrode was inserted through the actuator and its extremity was immersed in a small bath of electrolyte solution.…”