Surface stress and electrocapillarity of solid electrodes

There are two fundamental excess thermodynamic parameters that characterize a surface, the surface free energy and the surface stress. The surface fre e energy is the reversible work per unit area to form new surface while mai ntaining a constant equilibrium density of surface atoms. The surface stres s is the reversible work per unit area required to form new surface by elas tic deformation of a preexisting surface, and thus the atom density is alte red. For a fluid surface the surface free energy is equal to the surface st ress, but for a solid this is in general not true. We develop thermodynamic arguments that describe proper interpretations of wafer curvature experime nts that are typically used in electrocapillarity experiments of solid elec trodes. Additionally, we consider stress evolution during underpotential de position. The sources of stress relate to electrocapillarity differences be tween overlayer and substrate, interface stress, and coherency stress. Expe rimental results are presented for the systems Pb2+/Au(111), Pb2+/ Ag(111), and Ag+/Au(111). We show how it is possible to use the experimental data t o extract results for the interface stresses in each of these systems. The following values of interface stress were determined: for the incommensurat e Pb/Au(111) interface, 1.76 +/- 0.04 N/m; for the incommensurate Pb/Ag(111 ) interface, 0.9 +/- 0.04 N/m; and for the coherent Ag/Au(111);interface, - 0.08 +/- 0.04 N/m. Finally, we employ the thermodynamic arguments developed to consider two important problems in the electrocapillarity of solids. Th e first is a comparison of the magnitude of the change in surface free ener gy and surface stress that result from pure double-layer effects. The secon d is the potential-induced 23 x root3 <->(111) reconstruction that occurs o n Au surfaces. Here, we calculate the difference in surface stress between the reconstructed and unreconstructed surface, obtaining -0.43 N/m, which c ompares favorably with recently published experimental results.