Inhibition, by anions

The conditions under which oxide films are protective on these metals in relation to inhibition by anions may be characterised in terms of three important properties of the oxide film (see also Sections 1.4 and 1.5) ... 

Nature of the metal surface The critical concentration of an anion required to inhibit the corrosion of iron may increase with increasing surface roughness. Thus, Brasher and Mercer" showed that the minimum concentration of benzoate required to protect a grit-blasted steel surface was about 100 times greater than that required to protect an abraded surface. However, surface preparation had little effect on the critical inhibitive concentrations for chromate" or nitrite " The time of exposure of the iron surface to air after preparation and before immersion may also affect the ease of inhibition by anions. There is evidence """ that the inhibition by anions occurs more readily as the time of pre-exposure to air increases. Similarly, if an iron specimen is immersed for some time in a protective solution of an inhibitive anion, it may then be transferred without loss of inhibition to a solution of the anion containing much less than the critical inhibitive concentration . ... 

When aluminium is immersed in water, the air-formed oxide film of amorphous 7-alumina initially thickens (at a faster rate than in air) and then an outer layer of crystalline hydrated alumina forms, which eventually tends to stifle the reaction In near-neutral air-saturated solutions, the corrosion of aluminium is generally inhibited by anions which are inhibitive for iron, e.g. chromate, benzoate, phosphate, acetate. Inhibition also occurs in solutions containing sulphate or nitrate ions, which are aggressive towards iron. Aggressive anions for aluminium include the halide ions F ,... 

Equation (1) is generally used to estimate the rate constant, kin the micellar pseudophase, but for inhibited bimolecular reactions it provides an indirect method for estimation of otherwise inaccessible rate constants in water. Oxidation of a ferrocene to the corresponding ferricinium ion by Fe3 + is speeded by anionic micelles of SDS and inhibited by cationic micelles of cetyltrimethylammonium bromide or nitrate (Bunton and Cerichelli, 1980). The variation of the rate constants with [surfactant] fits the quantitative treatment described on p. 225. Oxidation of ferrocene by ferricyanide ion in water is too fast to be easily followed kinetically, but the reaction is strongly inhibited by anionic micelles of SDS which bind ferrocene, but exclude ferricyanide ion. Thus reaction occurs essentially quantitatively in the aqueous pseudophase, and the overall rate depends upon the rate constant in water and the distribution of ferrocene between water and the micelles. It is easy therefore to calculate the rate constant in water from this micellar inhibition. 

Bimolecular E2 reactions involving OH " in aqueous solution are speeded by cationic and inhibited by anionic micelles (Minch et al., 1975) whereas spontaneous SN reactions are generally inhibited strongly by cationic micelles and less strongly by anionic micelles it is therefore relatively easy to observe micellar control of product formation. 

Two new lines of evidence suggest that proton abstraction comes first. A careful study of both and H isotope effects supports the carbanion intermediate, as does the strong inhibition by anions of 3-nitropropionate and 3-nitro-2-hydroxypropionate. To provide a good electron sink the carboxylate group adjacent to the proton that is removed by fumarate hydratase must either be actually protonated in the enzyme-substrate (ES) complex or paired with and hydrogen bonded to a positively charged group. 

Figure 52-3 Formation and secretion of thyroid hormone. Iodine transport into the gland is inhibited by anions such as thiocyanate (SCN ), perchlorate (C 0 ), and pertechnetate (TcO ). The oxidation and organic binding of iodide to thyroglobuiin are blocked by thioureas, sulfonamides, and high concentrations of iodide. (Modified from Berger S, Quinn JLThyroid function. ln Tietz NW, ed. Fundamentals of clinical chemistry, 2nd ed. Philadelphia WB Saunders Co, 1976 585.)...

The X-ray studies of LADH have shown (107) that the anion Pt(CN)4 binds at the same site on the enzyme as the pyrophosphate group of the coenzyme. Arginine-47 is involved in this general anion binding site. The relation between the anion binding site, Arg-47, the catal5d ic zinc atom and the reactive Cys-46 is shown in Fig. 16. Early kinetic studies (1) and optical rotatory experiments (232,233) showed that LADH was inhibited by anions like halides, perchlorate, and thiosulfate and that this inhibition was coenzyme-competitive. Recent kinetic studies... 

For example, reactions between anions and neutral molecules are typically speeded by cationic and inhibited by anionic, micelles, whereas reactions between cations and neutral molecules are typically speeded by anionic and inhibited by cationic, micelles. However charge effects are not all important, because reactions between non-ionic reactants are often speeded by micelles, and rates of some ionic reactions are affected by non-ionic micelles. Some examples of these rate effects are in Table 1 [1-6]. 

This evidence of the uptake of inhibitive anions into oxide films forms the basis of the chemical or pore plugging theory of inhibition, associated originally with Evans etal. In this theory the rdle of the inhibitive anion is to promote the repair of weak points or pores in the oxide film, where corrosion has started, by reacting with dissolving iron cations to form insoluble products of separate phase, which plug the gaps. These insoluble products may contain the inhibitive anion either as a salt, e.g. phosphate, or a basic salt, e.g. azeiate , or as an insoluble oxide, e.g. CrjOj from chromate Precipitation of such solid products is favoured if the pH in the region of the pores does not become acid. Thus, on the basis of this theory, inhibition by anions such as phosphate, borate, silicate and carbonate, is enhanced by their buffer properties which serve to prevent a fall in pH in the anodic areas. Since ferric salts are usually more insoluble than... 

The bromination of phenyl n-pentyl ether is more para-selective in anionic micelles than it is in water. This contrasts with the lower para-selectivity of nitration of bromobenzene in the cationic micelles formed by dissolving lauric acid in 95% H2S04. It is not clear whether these effects are due to substrate orientation or to micelle-induced changes in the selectivity parameter for electrophilic aromatic substitution. The rates of solvolysis of alkyl p-trimethyl-ammoniumbenzenesulphonate trifluoromethanesulphonates (42) are strongly inhibited by anionic micelles of sodium lauryl sulphate or sodium dodecanoate. In water, homomicelles of (42) or sodium dodecanoate micelles, undergo inversion of stereochemistry, but in sodium lauryl sulphate 22% retention of... 

In their study of the emulsifier effect on the lipolytic activity, Mozaffar et al. [28] foxmd that the activity of the extracellular lipase of Pythium ultimum was more affected by the surfactant structure than by the surface area of the oil droplets. Replacing linear alkylbenzene sulfonate (LAS) by alpha olefin sulfonate or alkyl ether sulfate severely impairs the activity of Lipolase [16]. Other lipases are strongly inhibited by LAS, but not by sodium dodecyl sulfate (SDS) and sodium laurate, or by Aerosol OT and by cetyl trimethyl ammonium bromide (CTAB) but not by SDS. At low concentration, some nonionic surfactants favor and others impair the lipolytic activity [27]. Another lipase has been reported to be inhibited by anionic surfactants [29]. 

Hartley [13] formulated a sign rule to explain shifts of indicator equilibria in the presence of anionic or cationic surfactant micelles, to which a corollary has been added for kinetic systems which says reactions involving anions are catalysed by cationic micelles and inhibited by anionic ones. Reactions involving cations are catalysed by anionic micelles and inhibited by cationic ones... [14]. These concepts have been illustrated on many occasions, as we will see below. The acid-catalysed hydrolysis of benzylideneaniline to benzaldehyde is... 

Copper. Little work has been carried out on the mechanism of inhibition by anions of copper corrosion in neutral solutions. Inhibition occurs in solutions containing chromate, benzoate, or nitrite ions. Chloride and sulfide ions are aggressive, and there is some evidence that chloride ions can be taken up into the cuprous oxide film on copper to replace oxide ions and create cuprous ion vacancies that permit easier diffusion of cuprous ions through the film, thus increasing the corrosion rate. 

It is known that, with certain electron acceptors, the activity of sulfite oxidase is sensitive to inhibition by anions, such as phosphate (see Cohen and Fridovich, 1971). In keeping with this finding, Kessler and Rajagopalan (1974) reported anion effects on the form of the Mo(V) EPR spectra of sulfite oxidase. Phosphate, thiocyanate, fluoride, and sulfate produced particularly marked effects relative to, e.g., chloride or nitrate. These changes were seen in experiments at pH 7 but not at pH 9. Unfortunately, however, the spectral changes were not analyzed in detail. Their interpretation might be complicated by the superimposition of pH effects (Section 7.4) and anion effects. Phosphate buffers are susceptible to substantial decreases in apparent pH on freezing (Williams-Smith et a/., 1977), a factor not appreciated at the time the work was done. Clearly, further work on the various spectra from sulfite oxidase is required. ... 

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