Lecture 4 The Interfacial Region of Electrode.ppt

2. Experimentally it is observed that specific adsorp-tion occurs more with anions than with cations. This is in agreement with chemical models of the interfacial region. Why?a 3. The degree of adsorption depends on electrolyte concentration. The degree of coverage of a surface by specific adsorption of ions can be described by monolayer adsorption isotherms. Three types of isotherm are generally considered Adsorption isotherms: (a) Langmuir; (b) Temkin; (c) Frumkin Langmuir isotherm the fraction of coverage the energetic coefficient of proportionality the activity of species i in bulk solution Temkin isotherm This considers that the adsorption energy is a function of the degree of coverage according to the excess of species i a parameter that treats the interaction energy between the adsorbed species g Frumkin isotherm or the maximum surface excess Special: when g = 0 and putting ?i/?s = ?, the Langmuir isotherm is obtained Special: the Temkin isotherm is a special case of the Frumkin isotherm when ?i/?s = 0.5 Content 3.1 Introduction 3.2 The electrolyte double layer 3.3 Double layer models 3.4 Specific adsorption 3.5 The solid metallic electrode 3.6 The semiconductor electrode 3.7 Electrokinetic phenomena and colloids For a metal, the occupation of the electronic levels close to EF is given more correctly by the expression f = 1/[1 + exp (E - EF)/kBT] the probability of occupation of a level of energy E Fermi Level Boltzmann Constant when E = EF, f = 0.50 when E = EF+ kBT , f = 0.27 when E = EF - kBT , f = 0.73 The density of states occupied by electrons in a metal in the region of the Fermi level, EF, at T = 0, T1, and T2, where T2 T1. By convention, for a metal, it is said that only electrons with energies within kBT of EF can be transferred. The interfacial structure of a solid electrode depends on various factors. Variation of the electrostatic potentials with distance from a metallic electrode, (a) Classical representation; (b) The ‘jellium’(胶体) model.