Adsorbate surface coverage

In order to discuss the nature of the interaction between an adsorbed molecule and a surface it is important that the surface coverage be less than one monolayer since in multimolecular adsorption and capillary condensation the spectrum of the adsorbate molecule perturbed by interaction with other adsorbate molecules may mask the spectrum of the adsorbate molecule perturbed by interaction with the adsorbent. Surface coverages may be determined by obtaining an adsorption isotherm with the adsorbate... 

The work reported here was designed to address the issue of adsorbate surface coverage in the effect on SERS of UPD Pb on Ag electrodes in aqueous chloride and bromide media using interfacial H20 species as the probe molecule. No studies have been reported on the effect of UPD layers on the SERS of interfacial solvent molecules previously. However, the solvent is an ideal choice for such studies, because it will always remain in intimate contact with the electrode surface. Moreover, the SERS of interfacial H20 has been characterized quite extensively in aqueous halide media (18-29) and allows the possible influence of anion on the response of the system to be assessed. 

Changes in bond strength with variation of coordinative unsaturation of surface atoms are very similar to the effects of coadsorption [11]. When two adsorbate atoms share a bond with the same surface atoms, the surface atom effectively has an increased coordination number. Therefore the adatom bond strength is weaker than when there is no sharing between metal surface atoms. The heat of adsorption generally decreases at high adsorbate surface coverage as a result of this effect. 

In these equations 6 is the adsorbate surface coverage calculated from surface pressure data by means of the Gibbs adsorption equation, x, Xg are the mole fractions of the adsorbate and solvent respectively in the bulk solution, a is the activity of the adsorbate in the bulk solution, II(= 7 — 7) is the experimental surface pressure of the adsorbed film, 7 is the surface tension of the test solution, 7° is the value of 7 of the pure solvent, R is the gas constant and T is the temperature. 

The dependence of the adsorbate surface coverage 9 on the concentration of the adsorbate species in the solution ca is described by the adsorption isotherm. In the simplest case, that is, adsorption of a species with a single adsorption state on a structurally well-defined surface (e.g. a liquid or single-crystalline electrode), the adsorption can be usually described by the phenomenological Frumkin isotherm... 

A characteristic feature of isotherms determined for homogeneous adsorbents should be a linear region at low adsorbate surface coverages. In this region Henry s law is applicable ... 

The Jovanovic Equation. In the kinetic derivation of the Langmuir equation the rate of desorption is proportional to the adsorbate surface coverage and independent of the equilibrium bulk gas pressure. Jovanovic derived a general equation for monolayer adsorption by taking account of the mechanical contact between the adsorbed and bulk phases. After a complex kinetic derivation he obtained the equation ... 

Very little work has been published on this subject, although it is well known that multilayer adsorption may begin at low adsorbate surface coverages. The majority of treatments have adopted the patchwise heterogeneity model or have not accounted for adsorbate self-interactions and extended the analysis by using a multilayer adsorption isotherm as the local isotherm in the integral equation. The effects of multilayer adsorption in the patchwise heterogeneity analysis with a monolayer local isotherm have been demonstrated by van... 

The apparent independence of CT and water reduction can be attributed to a low adsorbed surface coverage of CT on the electrode. The amount of CT adsorbed on the electrode siuface was determined from an Anson analysis of early time CA profiles for blank and CT containing solutions (22). The charge transferred during the first 0.1 seconds of CA profiles for blank and CT solutions is shown in Figure 3. For the time span shown in Figure 3, virtually all the current was due to electrical double-layer charging and to reduction of compounds adsorbed on the electrode surface before the start of the experiment. 

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