Adsorption of anions

Unbelievable progress in sulfate adsorption studies took place during the single crystalhne era, especially after direct visudiza-tion of adlayer and phase transition in 2D srrlfate adlattice on Pt(lll). Partly similar observations vere reported for Irflll). In addition, quantitative agreement of sulfate smface coverage on Pt(lll) determined by radiotracer technique and resrrlts of thermodynamic analysis made Pt(l 11) a really famous electrode and inspired the studies of more general problem order-disorder in adlayer. It was a real shift to atomic level electrochemistry. 

Parallel events in the field of in situ IR spectroscopy (for a review of sulfate IR studies, see Ref. 26) resulted in a coupled shift to molecular level bi-sulfate anion was recognized in sulfate adlayer even in solutions with predominating sulfate. It was com-pletey new situation, when chemical equilibrium is affected by adsoibate-surface interactioa In usual terms of solution equilibria, the effect corresponds to increase of pKa from its bulk value (ca. 2) to 3.3-4.7 (pKa is potential-dependent). To agree this situation with bulk thermodynamics, one should simply use electrochemical potential instead of chemical. The phenomena of adsorption-induced protonation is relative to UPD, when adsoibate-surface interaction shifts redox equihbria. In more molecular terms, the species determined as bi-sidfate ions are probably interfacial ion pairs, i.e., the phenomenon can be considered as coadsorptioa This situation is screened in purely thermodynamic analysis, as excess surface protonation is hidden in Gibbs adsorptions of sulfate and H. However it becomes important for any further model consideration, as it can affect lateral interactions and the order in the adlayer. The excess adsorption-induced protonation of various anions is a very attractive field. In particular it is the only chance to explain why multicharged oxoanions can form complete mono-layers on platinum. 

An important detail consists in suppression of perchlorate reduction by sulfate and chloride in mixed solutions. For sulfate-containing systems, it is also important to remember that when chloride appears it suppresses sulfate adsorption and induces smoothing of ascending branch of adsorption vs. potential curves. In any case a search for other reference with weakly adsorbing anions is reasonable. 

Direct experimental results on anions Gibbs adsorption, even if obtained for poorly characterized dispersed platinum, are still of interest for comparison. The effect of potential on the difference of these values for some anions (in particular sulfate and perchlorate) 

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Fig. V-11. Electrocapillary curves (a) adsorption of anions (from Ref. 113) (b) absorption of cations (from Ref. 6) (c) electrocapillary curves for -pentanoic acid in QAN HCIO4. Solute activities from top to bottom are 0, 0.04761, 0.09096, 0.1666, and 0.500 (from Ref. 112).

The method of action of the polymers is thought to be encapsulation of drill cuttings and exposed shales on the borehole wall by the nonionic materials, and selective adsorption of anionic polymers on positively charged sites of exposed clays which limits the extent of possible swelling. The latter method appears to be tme particularly for certain anionic polymers because of the low concentrations that can be used to achieve shale protection (8). 

Also, the adsorption of anionic or neutral surfactants on chrysotile fibers in aqueous dispersions enhances fiber separation, with a concomitant increase of surface area (26). Such effects have not been reported for amphibole fibers. 

Conversely, the adsorption of anions makes the potential more negative on the metal side of the electrical double layer and this will tend to accelerate the rate of discharge of hydrogen ions. This effect has been observed for the sulphosalicylate ion and the benzoate ion . 

Some emphasis has been placed inthis Section on the nature of theel trified interface since it is apparent that adsorption at the interface between the metal and solution is a precursor to the electrochemical reactions that constitute corrosion in aqueous solution. The majority of studies of adsorption have been carried out using a mercury electrode (determination of surface tension us. potential, impedance us. potential, etc.) and this has lead to a grater understanding of the nature of the electrihed interface and of the forces that are responsible for adsorption of anions and cations from solution. Unfortunately, it is more difficult to study adsorption on clean solid metal surfaces (e.g. platinum), and the situation is even more complicated when the surface of the metal is filmed with solid oxide. Nevertheless, information obtained with the mercury electrode can be used to provide a qualitative interpretation of adsorption phenomenon in the corrosion of metals, and in order to emphasise the importance of adsorption phenomena some examples are outlined below. 

Figure 7. Adsorption of an electronegative species from the gas phase onto a metal surface generates a dipolar layer due to electron transfer from the metal to the species. Adsorption of anions onto an electrode simulates the situation when the positive charge on the metal compensates for the adsorbed negative charge (zero diffuse-layer charge), and not when the charge on the metal is zero.

The effect of surface deformation in the Helmholtz layer should also be involved in Eq. (35). In consideration of specific adsorption of anions, such effects can be expressed by the potential gradient Lj y, z, as follows,... 

Some research groups also tried to build up bi- and multilayers of latex particles by using self-assembly techniques [92-94,97]. Either the alternate adsorption technique outlined in Figure 14 was used, in which cationic and anionic particles are successively adsorbed, or a slightly modified version of successive adsorption of anionic particles and a cationic poly-... 

The enhanced adsorption of anions and other substances that occurs at increasingly positive potentials causes a gradual displacement of water (or other solvent) molecules from the electrolyte layer next to the electrode. This leads to a markedly slower increase in the rate of oxygen evolution from water molecules and facilitates a further change of potential in the positive direction. As a result, conditions arise that are favorable for reactions involving the adsorbed species themselves (Fig. 15.9). In particular, adsorbed anions are discharged forming adsorbed radicals ... 

Stamenkovic V, Arenz M, Lucas C, Gallagher M, Ross PN, Markovic NM. 2003. Surface chemistry on bimetallic alloy surfaces adsorption of anions and oxidation of CO on Pt3Sn(lll). J Am Chem Soc 125 2736-2745. 

Teliska M, Murthi VS, Mukeqee S, Ramaker DE. 2007. Site-specific vs specific adsorption of anions on Pt and Pt-based alloys. J Phys Chem C 111 9267-9274. 

Uchida H, Ikeda N, Watanahe M. 1997. Electrochemical quartz crystal microhalance study of copper ad-atoms on gold electrodes. Part II. Further discussion on the specific adsorption of anions from the solutions. J Electroanal Chem 424 5-12. 

It is assumed that the quantity Cc is not a function of the electrolyte concentration c, and changes only with the charge cr, while Cd depends both on o and on c, according to the diffuse layer theory (see below). The validity of this relationship is a necessary condition for the case where the adsorption of ions in the double layer is purely electrostatic in nature. Experiments have demonstrated that the concept of the electrical double layer without specific adsorption is applicable to a very limited number of systems. Specific adsorption apparently does not occur in LiF, NaF and KF solutions (except at high concentrations, where anomalous phenomena occur). At potentials that are appropriately more negative than Epzc, where adsorption of anions is absent, no specific adsorption occurs for the salts of... 

Specific adsorption occurs, i.e. ions enter the compact layer, in a considerable majority of cases. The most obvious result of specific adsorption is a decrease and shift in the maximum of the electrocapillary curve to negative values because of adsorption of anions (see Fig. 4.2) and to positive values for the adsorption of cations. A layer of ions is formed at the interface only when specific adsorption occurs. 

Davis, J.A and Leckie, J.O., Surface ionization and complexation at the oxide/water interface, II adsorption of anions, J. Colloid Interface Sci., 74, 32-43, 1980. 

The adsorption of anionic polyacrylamides on sand, SiC and kaolinite is significantly increased in the presence of calcium by (i) reduction of electrostatic... 

Trogus, F., Sophany, T., Schechter, R.S., Wade, W.H. "Static and Dynamic Adsorption of Anionic and Nontonic Surfactants," SPE paper 6004, 1976 SPE Annual Technical Conference and Exhibition, New Orleans, October 3-6. 

These experimental results demonstrate that the specific adsorption of anions leads to a significant decrease in junction stability, as illustrated in particular in the potential regions of disordered anion adsorption, as well as in the fluctuations in the plateau currents at potentials of the 2D-ordered anion adlayers. The instability of the conductance plateaus were statistically analyzed, by determining the standard... 

The electrolyte effect for the adsorption of anionic surfactants which leads to an enhancement of soil removal is valid only for low water hardness, i.e. low concentrations of calcium ions. High concentrations of calcium ions can lead to a precipitation of calcium surfactant salts and reduce the concentration of active molecules. Therefore, for many anionic surfactants the washing performance decreases with lower temperatures in the presence of calcium ions. This effect can be compensated by the addition of complexing agents or ion exchangers. 

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