Dynamic measurements of surface stress for the detection of metastable states at the onset of two-dimensional condensation

Abstract

Time-dependent changes of surface stress and changes of the specific surface energy of sputtered gold with a weak (111) fiber texture due to the adsorption of uracil from aqueous solutions were measured by Koesters laser interferometry and coulometry. The transients were induced by potential steps from a negative electrode potential at which uracil is not adsorbed into the potential range of stability of a condensed layer of uracil. Both methods yielded qualitatively similar results ascribed to contributions from the double layer only. Quantitative differences are attributed to excess non-equilibrium stresses which change during times of the order 0.1 s. This occurs only at potentials positive to the limit of stability of the condensed layer of the adsorbed uracil. This effect is explained by a relatively slow formation of the condensed layer. From the small mass changes obtained with the quartz frequency balance it is concluded that uracil replaces adsorbed water. Reliable measurements of mass changes from frequency changes and of changes of surface stress from strain are possible only with samples which are not heavily prestressed.