Electrosorption measurement methods

The processes classified in the third group are of primary importance in elucidating the significance of electric variables in electrosorption and in the double layer structure at solid electrodes. These processes encompass interactions of ionic components of supporting electrolytes with electrode surfaces and adsorption of some organic molecules such as saturated carboxylic acids and their derivatives (except for formic acid). The species that are concerned here are weakly adsorbed on platinum and rhodium electrodes and their heat of adsorption is well below 20 kcal/mole (25). Due to the reversibility and significant mobility of such weakly adsorbed ions or molecules, the application of the i n situ methods for the surface concentration measurements is more appropriate than that of the vacuum... 

In this review, we will consider the adsorption of a single species coadsorption phenomena will not be considered, since it is generally impossible to divide the flow of charge among several species. We will present the thermodynamics on which the concept of the electrosorption valency is based, discuss methods by which it can be measured, and explain its relation to the dipole moment and to partial charge transfer. The latter can be explained within an extension of the Anderson-Newns model for adsorption, which is useful for a semi-quantitative treatment of electrochemical adsorption. Our review of concepts and methods will be concluded by a survey of experimental data on thiol monolayers, which nowadays are adsorbates of particular interest. 

Electrochemistry shares many concepts with surface science, and for the last two decades there has been an exchange of methods and ideas between these two neighboring disciplines. However, the electrosorption valency has no equivalent in surface science, since experiments at the solid/gas or solid/vacuum interface cannot be performed at constant potential. However, for low coverages, and near the potential of zero charge, the electrosorption valency can be related to the dipole moment of the adsorbate, which can be measured both in surface science and, though with greater difficulty, also in electrochemistry. In the following, we point out the relation between these two quantities. 

Fig. 8J Electrochemical method for the measurement of the electrosorption of benzene (2 ixM in 0.5 M H SO ) on platinized platinum. = 1.0 Vis. The area A yields the charged consumed in oxidizing the benzene initially adsorbed on the surface. B and C represent the change in charge associated with the formation and the reduction of the oxide, respectively, resulting from the adsorption of benzene. Reprinted with permission from Duic, Bockris and Gileadi, Electrochim. Acta, 13, 1915, (1968). Copyright 1968, Pergamon Press.

An additional method for the determination of adsorption on solid electrodes by capacitance measurements, based on the theory of electrosorption developed by Frumkin, is discussed in Section 22.2. 

Although the electronic conductivity of an interphase that is present on an electrode can be related to various optoelectronic properties that are also measurable with spectroscopic techniques, the direct measurement of surface conductivities is not a spectroelectrochemical method. It is nevertheless a surface sensitive method that provides results closely related to those of other methods discussed in this book. Data on the electrosorption of alcohols on gold electrodes [45] or the electrode potential dependent conductivity of intrinsically conducting polymers [46] have been obtained with in situ surface conductivity measurements. Figure 4.4 shows the electrical resistance of a poly(2-propylaniline) film measured in situ under experimental conditions suppressing any influence of solution phase conduction. The influence of... 

The main problem of the in situ radio-tracer study of electrosorption phenomena originates from the very nature of the method, because the radiation measured consists of two main parts. The first one is that coming from the solution phase or from the solution layer contacting the electrode. The second radiation component is the radiation coming from the adsorption... 

Quantitative measurements of the coverage by Br d on Pt(lll) surface were obtained by purely electrochemical methods, as described in detail in Ref. [59]. Briefly, by utilizing the ring-shielding properties of the RRDE, Sect. 4.1.2.2, it was possible to determine the potential-dependent surface coverage by bromide and its electrosorption valence (y) on Pt(lll). The electrosorption valence... 

On Pt, this amount has been shown to agree with the true surface area measured by means of the BET method, providing a simple and convenient means for evaluating in situ the true surface area of the electrode or knowing the effective area after a fraction of the electrode surface has been covered with some inert substance, e.g., as in electrosorption of organics. 

The electrosorption of organic compounds as determined by the electrocapillary method and from measurements of differential capacity, has been reviewed in Volume III of this series by Frumkin and Damaskin. Their discussion applies primarily to adsorption... 

The experimental methods for determination of the potential of zero charge have been reviewed by Swinkels, Conway, and by Argade and Gileadi. Methods of measurement of electrosorption of organic compounds on solid electrodes have been discussed recently by Gileadi. ... 

The sensitivity of such methods depends on the ratio between the volume of the solution and the surface area of the electrode. A typical electrochemical cell has about 10 cm of solution per cm of surface area. In view of the very small amounts of material needed to form a monolayer, there is a need to increase the sensitivity of this type of measurement, by using a high-surface-area electrode and low concentration in solution. The surface-to-volume ratio can also be increased by using porous electrodes or by rolling up an electrode in a minimum volume of solution. Care must be taken, however, to ensure that potential control and uniformity are maintained in this type of measurement, remembering that the extent of electrosorption is potential dependent. 

Whenever the lateral interaction in an adlayer is weak, we observe a linear dependence of I R/R on coverage 6 (or on the average film thickness d). Such a relation, which simply implies that the film dielectric constants, ej, are independent of 0 (Eq. 29), is conveniently used for a precise determination of coverages and hence of adsorption isotherms. Especially in cases like halide adsorption where the evaluation of surface excesses from charge measurements is hampered by an unknown electrosorption valency (unless a very elaborate concentration dependence study is made " ), the spectroscopic method of determining coverages can indeed be advantageous. Adzic et have... 

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