Site densities, rate determining steps adsorption

Table 2 lists the two parameters n and Qx necessary to describe the model as determined with columns differing by the density of immobilized polyclonal antibody. As previously described, from the variation of the column capacity one can evaluate the contribution to the transport to the binding sites (I/nmt = 0.040) and calculate the effective adsorption rate constant ka. The results agree with those obtained from frontal analysis. The value of the apparent adsorption rate constant k is close to the value of Aa for experiments carried out both at high flow rates and with an immunoadsorbent column of low capacity 22). In this case, the rate-controlling step is the biospecific interaction. 

A fraction of the number of atoms incorporated into the step edge at a cathodic overpotential is supplied by direct transfer from the solution. The rate of this direct transfer reaction is determined by the frequencies of deposition and dissolution, Eqs. (1) and (2), applied to the step sites st, i.e., sites adjacent to the step edge. We can define cathodic, anodic, and exchange current densities for these sites by analogy to the adsorption sites [see Eq. (7)]. Then, with v >t = DT / mon, the partial propagation rate Vr>r due to direct transfer will be given by... 

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. 

---