Surface coverage, definition

It is useful to define the tenns coverage and monolayer for adsorbed layers, since different conventions are used in the literature. The surface coverage measures the two-dimensional density of adsorbates. The most connnon definition of coverage sets it to be equal to one monolayer (1 ML) when each two-dimensional surface unit cell of the unreconstructed substrate is occupied by one adsorbate (the adsorbate may be an atom or a molecule). Thus, an overlayer with a coverage of 1 ML has as many atoms (or molecules) as does the outennost single atomic layer of the substrate. 

Figure 6.2. (Top) Definitions of local electrophobic and local electrophilic behaviour for two reactions exhibiting global volcano-type behaviour (a) and global inverted-volcano-type behaviour (b). (Bottom) Corresponding variations in surface coverages of adsorbed electron donor (D) and electron acceptor (A) reactants. As shown in this chapter volcano-type behaviour corresponds in general to high reactant coverages, inverted-volcano-type behaviour corresponds in general to low reactant coverages.

Another approach to improve the stability of bonded phases at acidic pH is the nse of sterically hindered silanes, which use, for example, isopropyl groups as side chains, instead of methyl groups. Due to the somewhat lower surface coverage, this technique leaves a larger amount of silanols on the snrface than a high-qnality bonding with a dimethylsi-lane, but this is balanced against the definite improvement in hydrolytic stability at acidic pH. 

Dimensions, of surface irregularities, 1329 Dimers of water, 975 definition, 899, 902 surface coverage, 904 Dipole... 

We emphasize two natural limitations of the finite cluster model. It does not allow to make a statement about the dependence of essential parameters such as adsorption and transition energies on the level of surface coverage, and it does not account adequately for charge delocalization or surface relaxation phenomena. Further, it excludes by definition any information about the modification of the surface band structure as a consequence of the organic molecule adsorption. The following case study of 1-propanol on Si(001) - (2 x 1) is intended to clarify how these elements can be consistently incorporated into the description of the Si surface interaction with organic species. 

The surface coverage of an adsorbate is another important parameter in ordering. We shall use the common definition of coverage where one monolayer corresponds to one adsorbate atom or molecule for each unit cell of the clean, unreconstructed substrate surface. Thus, if an adsorbed undissociated carbon monoxide molecule bonds to alternating top-layer metal atoms exposed at the Ni(100) surface, we have a coverage of a half monolayer. 

The reason for these discrepancies is quite probably to be ascribed to a strong dependence of the activity coefficient of /, f(rs)irs, upon E and rs at the high surface coverages employed for the estimate of l. In particular, I values obtained from Ms versus E plots at constant Es are affected by the potential dependence of the activity coefficient at constant rs (see Eq. 21) conversely, the l values obtained from the dependence of crM upon rs at constant E are affected by the 7s-dependence of the activity coefficient at constant E. These different dependences may have opposite effects on the l values obtained on the basis of the two alternative thermodynamic definitions. This may also explain the anomalously high l values for bromide and chloride adsorption on polycrystalline Ag obtained by Schmidt and Stucki16 from Ms versus E plots at constant Es. For this reason, at high surface... 

A significant problem in surface complexation models is the definition of adsorption sites, The total number of proton-exchangeable sites can be determined by rapid tritium exchange with the oxide surface (25). Although surface equilibria are usually written in terms of one surface site, e.g. Equations 5, 6, 8, 9, adsorption isotherms for many ions show that the number of molecules adsorbed at maximum surface coverage (fmax) is less than the total number of surface sites. For example, uptake of Se(VI) and Cr(VI) ions on Fe(0H)3(am) at T ax 1/3 and 1/4 the total... 

A problem arises here in the definition of surface coverage since C" is not a single molecular weight, It cannot be equated with the fraction of occupied sites. Furthermore, It is assumed that two surface species are in equilibrium, which is not likely. 

Another idealized model is the localized adsorption, a rationale for the Langmuir isotherm. Now the surface is supposed to consist solely of identical distinct adsorption sites with the number concentration Ca in cm-2 the latter has a definite value for each particular adsorbent-adsorbate system. Let the fractional surface coverage be 6 = ca/Ca. In such a model, proportional to cg is ca/Ca(l - 6), the surface concentration divided by the concentration of free adsorption sites. Hence, strictly speaking, the corresponding dimensional constant fc c, now in cm3, is ... 

In goal definition, the scope and purpose of the LCA is defined. The functional unit and system boundaries are also established. The functional unit is the reference point to which environmental impacts are attributed. The choice of functional unit depends on the application of the LCA. For a process it could be a tonne of product or feedstock, or a tonne of impurity removed. For a commodity product the functional unit could be a tonne of product, but for specialty chemicals and most consumer products performance properties are more important and the choice can become complex. For example, performance of a paint would be related to its surface coverage and durability hence a possible functional unit for the paint might be the quantity required to cover a square meter of surface over a time span of 20 years. 

There are dense (monolayer) bonded layers and bonded layers with submonolayer coverages. By definition, the former have bonding density close to the maximal limit, the latter have surface coverage significantly lower than the unit, e.g. from zero up to 0.3. The most important property of chemically modified support with submonolayer coverage is the organization of bonded molecules throughout the surface. 

Although the use of ionizing radiation has contributed to a definite picture of certain chemisorptions and catalyses, in other cases it has run into difficulties that have not yet been resolved. With several oxides, for example, water or some other poison is required for radiation sensitivity, and the role of the poison has not been clarified. Because this behavior seems to be fairly widespread, it is important to understand it more fully, and considerable attention could well be given to irradiation of such oxides in a high state of purity and with varying degrees of surface coverage. 

There is no clear definition of what magnitude of enhancement entitles a system to be classified as a SERS-active system. In this review we will arbitrarily set the demarcation line for SERS at a 100-fold enhancement level. Any enhancement higher than that will be considered as SERS, while lower enhancements will be ignored. The reason for this limit is that it is within simple surface coverage effects (roughness factor) and trivial enhancements resulting from reflectivity of metal surfaces and possible orientation effects. " ... 

A problem arises here in the definition of surface coverage since "CM is... 

The second time dependent term in the coverage definition (4) is mostly determined by the processes that consume the chemisorbed species of the particles. The sum of the desorption rate and the rate of the bimolecular interaction determines the character of this type of the oscillations. Assuming that the bimolecular interaction is the dominant process in the surface chemical reaction, these oscillations are revealed in Figure 8. It is interesting to note that the amplitude of this type of the oscillations is constant with respect of time. 

Surface Coverage The ratio of the amount of adsorbed material to the monolayer capacity. The definition is the same for either of monolayer and multilayer adsorption. 

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