At low surface coverage

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 . 

A variation of this approach has recently been provided by Lyakhov et al. [598] who, from measurements of water adsorption on CuS04 5 H20, on MgS04 7 H20, and on their respective dehydration products, discern a correlation between strengths of surface bonding and S—T behaviour. At low surface coverages, the mutual dipole—dipole repulsions in the adsorbed layer inhibit water loss, in part by a blocking action on loss of water of crystallization and in part by polarization effects which provide a... 

Fig. 28—Schematic representation of two extreme polymer conformations at the surface of the solid at low surface coverage S is the cross-sectional diameter of the polymer chain, and R is the radius of gyration of the molecule in the bulk [42].

Note that at low surface coverages where (1 + KaPa + KbPb + ) 1, the fraction of the sites occupied by each species will be proportional to its partial pressure. 

At low surface coverages (high SOHj.) only the RO--CdOH+ surface species is required to fit the data. For example, decreasing SOH. from 7.4 x 10 to 2.9 x 10 M increases the Cd(II) adsorption... 

It is clear, for the CdCl /a-Al O system at pH 7, that other reactions are not negligible in their contribution to the macroscopic proton coefficient at low surface coverage, when SO—CdOH"1" is the only postulated metal-containing surface species, the macroscopic proton coefficient is less than 2. Xp however, does approach 2.0 at low pH and high T (Figure 7). 

The X-ray photoelectron spectroscopy (XPS) identification of the products of xanthate sorption at the surface of galena by Laajalehto et al. (1993) showes that lead xanthate in the molecular form is the adsorbed entity. The distribution of xanthate at the surface is irregular and even at low surface coverage the formation of three dimensional aggregates of lead xanthate occurs. 

Bismuth promotion suppresses the hydrogen sorption on platinum (curves b-d). The peak at -0.05 V indicates the oxidation of adsorbed bismuth, which overlaps the OH adsorption on uncovered platinum surface sites 18). Bismuth adatoms are discharged in the low potential region (Bi°) and occupy three platinum sites at low surface coverages. The structure of the oxygen-containing species are (BiOH), (BiO), and [Bi(OH)J,d 19). 

Figure 6.17. Free energy diagram for oxygen reduction over Pt(lll) at low surface coverage at zero cell potential (U=0), at the equilibrium potential (zero overpotential) (U=1.23) and at the highest potential where all reaction steps are exothermic (adapted from [117]).

Parkinson, E.L., Ettelaie, R., Dickinson, E. (2005). Using self-consistent-field theory to understand enhanced steric stabilization by casein-like copolymers at low surface coverage in mixed protein layers. Biomacromolecules, 6, 3018-3029. 

A critical examination is made of various proposed explanations for the deviation of the adsorption isotherm from linearity at low surface coverage. 

Bridging forces, arising when a polymer binds to both surfaces, usually lead to an attraction at large separations. Bridging is only effective at low surface coverage. Only then do the polymer segments have a chance to find an adsorption site on the opposite surface. 

The persistence of interaction at low surface coverage could be explained by clustering, dipole-dipole interactions between adsorbed molecules at distances further apart than nearest-neighbor positions, or by changes in the electronic nature of the adsorbent surface induced by the adsorbed CO. At present, the latter explanation is favored because it is consistent with the fact that other gases (discussed in II, 3/) affect the position of the CO bands. 

Goethite EXAFS study of As(V) and Cr(VI) adsorption on goethite. Monodentate binding favored at low surface coverage of As(V), bidentate at high surface coverage Fendorf et al. (1997)... 

In the present paper, the previous analysis for moving suspensions is extended to allow reversible deposition by including the short-range Rom repulsion. The apparent surface reaction is then shown to be reversible and first order- in both directions and thus is analogous to gas adsorption at low surface coverage. Applications include solid-liquid separations, particle separation, and chromatography. 

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