Adsorption isotherm electrical effects

The interfacial tension decreases with increasing amount of surface potential. The reason is the increased interfacial excess of counterions in the electric double layer. In accordance with the Gibbs adsorption isotherms, the interfacial tension must decrease with increasing interfacial excess. At charged interfaces ions have an effect similarly to surfactants at liquid surfaces. 

This is a kind of "differential adsorption isotherm", relaying how strongly the counterion charge increases with the surface charge. Figures 3.45 and 3.46 are illustrations of a Esin-Markov analysis for Agl in KNOg. For this system it was Eilso found that p Increases from Li to Cs this is a typical specific effect, caused by the increased non-electric adsorption in this direction. At the same time, the cj°(pAg) curves for different salt concentrations are wider apart for CsNOg than... 

The adsorption of either ions or neutral molecules on the electrode surface depends on qn, i.e., on the apphed electric potential. Correspondingly, the electric field at the electrochemical interface is an additional free-energy contribution that either favors or restricts the adsorption of species on the electrode from the ionic conducting phase. A variety of adsorption isotherms has been proposed to account for the behavior of different electrochemical systems. Among them are the Langmuir, Frumkin, and Temkin isotherms [2]. Frumkin and Temkin isotherms, at variance with the Langmuir one, include effects such as adsorbate—adsorbate or adsorbate—surface interactions. 

Before starting with dynamic effects at a liquid interface, the equilibrium state of adsorption is described and adsorption isotherms as basic requirements for theories of adsorption dynamics are reviewed. Chapter 2 presents the transfer from thermodynamics to macro-kinetics of adsorption. As Chapter 7 deals with the peculiarities of ionic siu-factant adsorption and introduces some properties of electric double layers. 

The definition of Gibbs elasticity given by Eq. (19) corresponds to an instantaneous (Aft t ) dilatation of the adsorption layer (that contributes to o ) without affecting the diffuse layer and o. The dependence of o on Ty for nonionic surfactants is the same as the dependence of o on Ty for ionic surfactants, cf Eqs (7) and (19). Equations (8) and (20) then show that the expressions for Eq in Table 2 are valid for both nonionic and ionic siufactants. The effect of the surface electric potential on the Gibbs elasticity Eq of an ionic adsorption monolayer is implicit, through the equilibrium siufactant adsorption T y which depends on the electric properties of the interface. To illustrate this let us consider the case of Langmuir adsorption isotherm for an ionic surfactant (17) ... 

Although the final symmetry cannot be deduced only from the spectroscopic data, it is important to stress that the spectroscopic data supply information on the strength of the chemical bonding [48] and on the effect of the applied electric field [55, 56]. Moreover, in situ STM data have been reported only for a full monolayer, but not for submonolayer coverage. The in situ FTIR is sensitive also in the submonolayer regime. By appropriate calibration (through the full monolayer as determined by in situ STM), it is possible to determine the adsorption isotherm, even for metals for which it is very difficult to obtain capacitance data. 

A weak influence on Ylvw has also the change (of reasonable values) in the parameters of the surfactant adsorption layers in the film [263]. A certain decrease in its value can be attributed to the screening of van der Waals interactions by the change in the double electric layer in the presence of electrolyte [257,258], At idi2 > 4 this effect can be accounted for if in Eq. (3.89) b = 0 [258]. If all correction are introduced in the calculation of Ylvw, the accuracy of the theoretical fl(Ii) isotherm increases from 1 to 15% in the thickness interval studied. 

Interpretations of the logarithmic laws have been based on the adsorption of reactive species, the effects of electric fields developed across oxide layers, quantum-mechanical tunnelling of electrons through the thin scales, progressive blocking of low-resistance diffusion paths, non-isothermal conditions in the oxide layer, and nucleation and growth processes. A concise summary of these theories has been given by Kofstad. ... 

The meticulous study of the effects of adsorption of organic compounds on the electrocapillary curves of mercury, which was accomplished by Frumkin at the Karpov Institute, led to the discovery of the famous isotherm that now bears his name (the Frumkin isotherm [7]). Whereas the Langmuir isotherm was based on the somewhat naive assumption that adsorbed species did not interact, Frumkin showed that such interactions were often very strong indeed. In addition, Frumkin combined his results with an electrical model of two capacitors in parallel, which later became the prototype for many multistate models. 

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