Esin-Markov effect

Gouy—Chapman theory and involves coulombic and possibly specific interactions due to weak electron orbital overlapping. The amount of specifically adsorbed ions at the electrode generally varies linearly with the charge at the electrode and logarithmically with the ion concentration in the solution. Further evidence of specific adsorption of ions at electrodes is the Esin—Markov effect, i.e. the shift in the pzc due to specific adsorption of ions [6]. 

The degree of specific adsorption should vary with electrolyte concentration, just as there should be a change in the point of zero charge due to specific adsorption of charges. This is the Esin-Markov effect, expressed by the Esin-Markov coefficient, / ... 

Cu(l-10) An adsorption process is apparent between -0.290 and -0.40 V (Fig. 1) which consumes 0.050 mC/cm2. As shown in Fig. 8 the peaks shift -0.061 V/decade with chloride concentration which reflects the Esin-Markov effect. At potentials below the -0.310 V STM reveals the (110) terraces to be elongated with steps faceted in the <100> direction, i.e. orthogonal to the close packed <110> of the metal lattice. As the potential is moved toward more positive potentials a faceting transition occurs where the chloride covered terraces undergo a reconstruction, as shown Fig. 9. 

Another indicator of specific adsorption of charged species is the Esin-Markov effect, which is manifested by a shift in the PZC with a change in electrolyte concentration (33). Table 13.3.2 provides data compiled by Grahame (2). The magnitude of the shift is usually linear with the logarithm of electrolyte activity, and the slope of the linear plot is the Esin-Markov coefficient for the condition of = 0. Similar results are obtained at nonzero, but constant, electrode charge densities hence the Esin-Markov coefficient can be written generally as... 

Esin-Markov Effect The change in zero point of charge of a species that occurs when the electrolyte can become specifically adsorbed. In the presence of indifferent electrolytes, the zero point of charge is a constant. 

Equation (19) relates the dependence of the pzc on the concentration of the electrolyte solution in the presence of specific adsorption (F 0 when au = 0) and its variation with the electrode charge. The dependence of the pzc of a mercury electrode on the logarithm of KI concentration was used for the first time for studying the iodide specific adsorption in [17] and later was named the Esin-Markov effect. As follows from the model theories of the electric double layer (see Sect. 3.2), the limiting slope of the aforementioned dependence should tend to the value —RT/kF, where the coefficient X(0 < X < 1) characterizes the discrete nature of the charge of specifically adsorbed anions. 

In principle, in the case of nonspecific adsorption the nature of the electrode metal should play a secondary role therefore, the fundamental relationships found for this type of adsorption in the case of mercury should be of general validity, independent of the nature of the metal. In reality, the experimental results obtained with mercury, the methods for the evaluation of these experimental data, and the theoretical efforts to interpret them served as very important basis for the study of any kind of electrode. Some of these, for instance, methods of Hurwitz and of Parsons and coworkers and the Esin-Markov effect, were mentioned in the previous section. 

Cations adsorption is of interest for interpretation and prediction of pzc dependences on salt concentration, considered in a general form in Ref. 102. Experimental data for solutions of various anionic composition (Fig. 6c) demonstrate no pronounced slope difference for anions of essentially different adsorption behavior. All slopes are very low (even lower than expected in the absence of Esin-Markov effect studied earlier for similar systems.) The decrease of slope can result from two contributions (decrease of cations adsorption with potential and displacement of hydrogen with increase of anion concentration). This result means that the straightforward interpretation of Esin-Markov coefficients for platinum metals (if any) should take into accoimt that these values can be underestimated. 

The potential of uncharged surface, measured vs a reference electrode with constant potential, varies linearly with the logarithm of concentration (activity) of the electrolyte [3]. This phenomenon, which is the criterion for the specific adsorption to take place, is known in literature as Esin-Markov effect. It was discovered by the well-known Russian electrochemist from Sverdlovsk Prof. Oleg Anatolievich Esin and his young student Boris Fedorovich Markov. 

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 effect of ionic strength on the charging curves can be quantihed in a form of the Esin-Markov coefficient ... 

The theory of the AV-A behavior developed by L.M.B. (2) was based on earlier calculations by Mingins and Pethica (M.P.) (9) from their experimental work on monolayers of SODS at the A—W interface. Recently these authors (10) reported a numerical error in their earlier work their conclusions question the model of the ionized monolayers used by L.M.B. (2) to explain the A V-A curves. The so-called Esin-Markov coefficient for adsorbed ions at the charged mercury/aqueous electrolyte has received considerable attention (11, 12, 13) particularly since it clearly demonstrates the discrete-ion effect. Its counterpart at ionized monolayers may be defined by the differential expression... 

Currently no adequate quantitative theory of the discrete-ion potentials for adsorbed counterions at ionized monolayers exists although work on this problem is in progress. These potentials are more difficult to determine than those for the mercury/electrolyte interface because the non-aqueous phase is a dielectric medium and the distribution of counterions in the monolayer region is more complicated. However the physical nature of discrete-ion potentials for the adsorbed counterions can be described qualitatively. This paper investigates the experimental evidence for the discrete-ion effect at ionized monolayers by testing our model on the results of Mingins and Pethica (9, 10) for SODS. The simultaneous use of the Esin-Markov coefficient (Equation 3) and the surface potential AV as functions of A at the same electrolyte concentration c yields the specific adsorption potentials for both types of adsorbed Na+ ions—bound and mobile. Two parameters which need to be chosen are the density of sites available to the adsorbed mobile Na+ ions and the capacity per unit area of the monolayer region. The present work illustrates the value... 

This model allows one to explain such qualitative features of the phenomenon as the drastic (exponential) dependence of F on the electrode charge, as well as the existence of the aforementioned two regions in the F(c) variation. However, a more detailed experimental study has revealed serious deviations from the predictions of Eq. (72). For many ions, one can observe the effect of Esin and Markov [248], a stronger shift of the p.z.c. with the variation of the bulk concentration than it is... 

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