Surface coverage, organic compounds

Direct measurements on metals such as iron, nickel and stainless steel have shown that adsorption occurs from acid solutions of inhibitors such as iodide ions, carbon monoxide and organic compounds such as amines , thioureas , sulphoxides , sulphidesand mer-captans. These studies have shown that the efficiency of inhibition (expressed as the relative reduction in corrosion rate) can be qualitatively related to the amount of adsorbed inhibitor on the metal surface. However, no detailed quantitative correlation has yet been achieved between these parameters. There is some evidence that adsorption of inhibitor species at low surface coverage d (for complete surface coverage 0=1) may be more effective in producing inhibition than adsorption at high surface coverage. In particular, the adsorption of polyvinyl pyridine on iron in hydrochloric acid at 0 < 0 -1 monolayer has been found to produce an 80% reduction in corrosion rate . 

Adsorption is, of course, of major importance in the inhibition of corrosion by organic compounds (adsorption inhibitors) that have the ability to adsorb strongly on the metal surface, thus impeding the dissolution reaction and reducing the corrosion rate. It follows that the coverage of a metal surface by adsorbed inhibitor can be evaluated from the relationship... 

Relatively little is known about the speciation of organic compounds, and organic reductants in particular, when adsorbed to metal oxides. It is known that surface coverage is higher for bidentate organic ligands, such as catechol and salicylate, than... 

Adsorbed organic compounds do not necessarily occupy surface sites in a random fashion. Favorable hydrophobic interactions, for example, may cause some organic compounds to adsorb in groups, enhancing overall surface coverage. Electrostatic repulsion between charged adsorbate molecules, in contrast, may space molecules on the surface apart from one another. 

Adsorptive accumulation — Organic substances which exhibit -> surface activity and electroactivity can be electrochemically analyzed by adsorptive accumulation on the surface of a an electrode, e.g., mercury electrode, followed by the reduction, or oxidation of the adsorbate using -> voltammetry [i,ii]. Also, the adsorption of highly stable and inert -> complexes of metal ions with surface-active organic ligands is utilized for the determination of trace metals [iii]. In all these methods the maximum voltammetric response is linearly proportional to the surface concentration of the adsorbed analyte at the end of the accumulation period [iv]. In the majority of cases, the adsorption on mercury can be described by the -> Frumkin isotherm /icx=o = 0exp(ad)/(1- 9), where f is the adsorption constant, cx=o is the concentration of the dissolved compound at the electrode surface, 6 = T/rmax is the surface coverage, T is the surface concentration of the adsorbed compound, rmax is the maximum surface concentration and a is the Frumkin... 

A rate constant and activation energy of organic compounds chemisorption on the oxides surface in the most cases depend on the surface coverage. It may he connected with different heterogeneity types of oxide surfaces. The reasons for oxides surface heterogeneity are as follows. 

Organic compounds commonly interact with silica and other oxides surface at high temperatures. Ratio of surface site number of the types that undergo reaction (nt) to the total number of the sites present before (no) is the relative surface coverage... 

One of the main problems of organic compound reactions proceeding on the heterogeneous oxide surfaces is possible dependence of their mechanism on the surface coverage. For instance, this deduction follows immediately from different variance of distribution functions of oxide surfaces on the donor and acceptor properties of the active sites [147-148], which determines their nucleophilic and electrophilic abilities to interact with organic compounds from the gas or liquid phase. 

Thus the thermodynamic characteristics of adsorption at small coverage of different classes organic compounds determined by gas chromatography show that surface of ful-lerene molecular crystals and surface of graphitized carbon black have essentially different adsorption properties. On adsorption on fullerene crystals the electron-acceptor and electron-donor properties of fullerene molecules are manifested. Adsorption data on fullerenes Ceo nd C70 show that properties of fullerene Ceo a-nd C70 molecules arranged in surface layer of crystals are different. 

Susceptibility of a particular organic compound towards oxidation depends upon both surface coverage and upon rates of electron transfer within the surface precursor complex. Some organic compounds (such as phthalate)... 

Equations of this type can describe both the physical dipole/dipole interactions and the subsequent desorption from the film. Since ka and k have different temperature coefficients, increasing temperature can lead to either increased, decreased or unchanged surface coverage. Provided that a critical minimum surface is maintained, wear and friction can be controlled. But once 0 falls below this critical value, believed to be approx 0.5, friction and wear will rise. The adsorption of dilinoleic acid [4], a series of organic sulphur compounds [5] and a ZDDP (zinc dialkyldithiophosphate) [6] has been described in these terms. 

The sorption properties of the above carbonaceous materials were examined by means of gas chromatography [420], which allows quantitative characterization of sorbent-sorbate interactions at a small coverage of sorbent surface. Table 17.3 presents the specific retention volumes of various organic compounds measured for different carbons. 

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