Surface free energy history dependence

A solid, by definition, is a portion of matter that is rigid and resists stress. Although the surface of a solid must, in principle, be characterized by surface free energy, it is evident that the usual methods of capillarity are not very useful since they depend on measurements of equilibrium surface properties given by Laplace s equation (Eq. II-7). Since a solid deforms in an elastic manner, its shape will be determined more by its past history than by surface tension forces. 

However, G) is not a true thermodynamic quantity, because it does not express the equilibrium instead it depends on the history of the solid surface formation, as we see above. So far, we have considered only the isotropic solids. If, however, the solid is anisotropic, such crystals can respond differently in different directions when increasing the surface area. This property increases the number of required equations twofold, and the matter is rather complex because it is unclear which shape the anisotropic crystal would prefer for a given volume, if its total surface free energy is minimized this subject is beyond the scope of this book. 

Surface reaction steps involving adsorbed (chemisorbed, to be accurate) hydrogen, such as H adsorption, desorption, H-H combination, and surface-bulk transfer, play a determining role in the H cathodic reactions. The HAR and the HER most often share a common step of H electroadsorption from protons or wafer, and only a study of the overall mechanism of these reactions makes it possible to predict the conditions in which the H uptake under the surface can increase. The problem of analyzing all the data on H entry rate is that this rate, even in a pure metal, depends on many variables the nature of the metal, its thermal-mechanical history, the surface conditions (especially on iron, surface states are not easily reproducible due to the difficulty of removing oxide films on the electrodes), composition of the electrolyte, cathodic current density or electrode potential, temperature, etc. The determining factors in the kinetics of the H cathodic reactions on bare metal surfaces are the cathodic overpotential and the surface parameters, which are the density of sites for H adsorption and the free energy of adsorbed H, both dependent on the structure and the chemical composition of the surface. 

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