Adsorption and Affinity Energy Distribution

This model assumes that the surface is covered by two types of sites and that a different Moreau model applies to each type of sites, considered as homogeneous and acting independently. The equation of this isotherm is 

This model was used by Gritti and Guiochon to account for the behavior of propranolol on several Cig-bonded silica adsorbents, from methanol/water solutions. Similar results were obtained for Kromasil-Cig [101,102], XTerra-Cjs [84], Symmetry-Ci8 [103]. When the mobile phase contains a high concentration of a monovalent salt, the adsorption follows bi-Langmuir isotherm behavior [101]. In the absence of salt, at low concentrations of a monovalent salt or with di- (e.g., phthalate, succinate, naphthalene sulfonate) or tri-valent salts (e.g., citrate), the isotherm data are best modeled by a bi-Moreau isotherm [91,104]. 

The heterogeneity of the surface of the adsorbents used in chromatography has two important consequences. First, adsorption isotherm data do not fit well to the simple isotherm models designed for homogeneous srufaces, i.e., the Langmuir 

The study of heterogeneous surfaces has now become an important part of adsorption studies. It has been shown that, for gas-solid equilibria, the experimental or apparent or global isotherm is related to the AED by the following equation 

Since the adsorption energy, e, is related to the adsorption equilibrium constant, K, by the relationship 

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In Chapter 7 general kinetics of electrode reactions is presented with kinetic parameters such as stoichiometric number, reaction order, and activation energy. In most cases the affinity of reactions is distributed in multiple steps rather than in a single particular rate step. Chapter 8 discusses the kinetics of electron transfer reactions across the electrode interfaces. Electron transfer proceeds through a quantum mechanical tunneling from an occupied electron level to a vacant electron level. Complexation and adsorption of redox particles influence the rate of electron transfer by shifting the electron level of redox particles. Chapter 9 discusses the kinetics of ion transfer reactions which are based upon activation processes of Boltzmann particles. 

In Eqs. (56), is the adsorption affinity at infinite temperature e and a are the mean and square root of variance of energy, respectively and 5 is the heterogeneity parameter related to the spread of the energy distribution. 

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