The problem of ion adsorption models

In the presented theories of electrostatic retardation, very simple models are used to enable an analytical solution of the different problems and to clarify the physics of the mechanisms. The objective of further work is of course the generalisation of models with respect to the adsorption isotherm, content of background electrolyte, and ion transport properties. 

As adsorption isotherm the equation of Stem-Martynov (1979) was used for simplification (cf. Chapter 2). This isotherm describes the surfactant adsorption only in a restricted concentration interval. A much more advantageous model of adsorption site dissociationbinding model was developed by Koopal (1993) and could be used preferentially. 

The importance of electrostatic retardation increases with the surface potential, i.e. with the adsorption surfactant molecules. Especially in some practical systems of high background electrolyte, only at densely packed adsorption layers the electrostatic retardation will set in. This state of adsorption has not been taken into consideration so far. With increasing background electrolyte concentration counterions build the Stem layer. The charge of the adsorption layer is compensated partially by the diffuse layer and the Stem layer (Eq. 2.5) which decrease with the increased amount of counterions in the Stem layer. Simultaneously, the Stem potential is lowered and the electrostatic retardation becomes less effective. This aspect was discussed already by Kretzschmar et al. (1980). Consequently, the electrostatic retardation can exist in NaCl solution while it can disappear under certain conditions in CaCl2 solutions. 

The Stem-Martynov isotherm does not take into account the intermolecular interaction of adsorbed molecules, as it is considered in the Frumkin isotherm (cf Eq. (2.43)). The classical version of the Framkin isotherm was derived for nonionic surfactant adsorption layers. The incorporation of electrostatic interaction was proposed by Borwankar Wasan (1986, 1988). 

On the basis of this generalised adsorption isotherm Borwankar Wasan developed a macro-kinetic approach of ionics adsorption kinetics taking into account the interaction between adsorbed ions within the adsorption layer. On the other hand, the deviation of the diffuse part of the DL from the equilibrium state was ignored in their models. 

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