Surface interaction, effect adsorbates

The application of surface-enhanced Raman spectroscopy (SERS) for monitoring redox and other processes at metal-solution interfaces is illustrated by means of some recent results obtained in our laboratory. The detection of adsorbed species present at outer- as well as inner-sphere reaction sites is noted. The influence of surface interaction effects on the SER spectra of adsorbed redox couples is discussed with a view towards utilizing the frequency-potential dependence of oxidation-state sensitive vibrational modes as a criterion of reactant-surface electronic coupling effects. Illustrative data are presented for Ru(NH3)63+/2+ adsorbed electrostatically to chloride-coated silver, and Fe(CN)63 /" bound to gold electrodes the latter couple appears to be valence delocalized under some conditions. The use of coupled SERS-rotating disk voltammetry measurements to examine the kinetics and mechanisms of irreversible and multistep electrochemical reactions is also discussed. Examples given are the outer- and inner-sphere one-electron reductions of Co(III) and Cr(III) complexes at silver, and the oxidation of carbon monoxide and iodide at gold electrodes. 

Surface Interaction Effects Upon Adsorbed Redox Couples Valence-Trapped Versus Valence-Delocalized Cases... 

It should be emphasized that the value of tf resulting from use of (1.49) or (1.50) applies to a particular value of n,. Because of the joint effects of the energetic non-uniformity of the adsorbent surface and the interaction of adsorbate molecules in the adsorbed film itself, the heat of adsorption in general varies significantly with the amount adsorbed. It is therefore essential to repeat the calculation of (f for a succession of values of n, and thereby obtain the curve of against n,. 

Henry s law corresponds physically to the situation in which the adsorbed phase is so dilute that there is neither competition for surface sites nor any significant interaction between adsorbed molecules. At higher concentrations both of these effects become important and the form of the isotherm becomes more complex. The isotherms have been classified into five different types (9) (Eig. 4). Isotherms for a microporous adsorbent are generally of type I the more complex forms are associated with multilayer adsorption and capillary condensation. 

Trace contaminants are also significant at charged solid surfaces, affecting both the charging process and the surface conductivity. In ambient air atmospheres their effect is often determined by interaction with adsorbed water vapor, whose dominant concentration may be sufficiently large to form a monolayer. Topical antistatic agents for solids typically rely on interaction with adsorbed water and can lose effectiveness at low relative humidity (4-2.1). 

Blocking of reaction sites The interaction of adsorbed inhibitors with surface metal atoms may prevent these metal atoms from participating in either the anodic or cathodic reactions of corrosion. This simple blocking effect decreases the number of surface metal atoms at which these reactions can occur, and hence the rates of these reactions, in proportion to the extent of adsorption. The mechanisms of the reactions are not affected and the Tafel slopes of the polarisation curves remain unchanged. Behaviour of this type has been observed for iron in sulphuric acid solutions containing 2,6-dimethyl quinoline, /3-naphthoquinoline , or aliphatic sulphides . 

In the following paper, the possibility of equilibration of the primarily adsorbed portions of polymer was analyzed [20]. The surface coupling constant (k0) was introduced to characterize the polymer-surface interaction. The constant k0 includes an electrostatic interaction term, thus being k0 > 1 for polyelectrolytes and k0 1 for neutral polymers. It was found that, theoretically, the adsorption characteristics do not depend on the equilibration processes for k0 > 1. In contrast, for neutral polymers (k0 < 1), the difference between the equilibrium and non-equilibrium modes could be considerable. As more polymer is adsorbed, excluded-volume effects will swell out the loops of the adsorbate, so that the mutual reorientation of the polymer chains occurs. 

The influence of water can be included by adding water molecules to the DFT calculation. Whereas the interaction with water will be discussed in more detail later, in short, the water interaction will be most important for adsorbates that easily form hydrogen bonds, react with water, or form strong ionic bonds to the surface. For other adsorbates, such as H, the effect of water is negligible [Jerkiewicz, 1998 Roudgar and Gross, 2005]. 

The key parameters of the electronic structure of these surfaces are summarized in Table 9.3. The calculated rf-band vacancy of Pt shows no appreciable increase. Instead, there is a shght charge transfer from Co to Pt, which may be attributable to the difference in electronegativity of Pt and Co, in apparent contradiction with the substantial increase in Pt band vacancy previously reported [Mukerjee et al., 1995]. What does change systematically across these surfaces is the J-band center (s ) of Pt, which, as Fig. 9.12 demonstrates, systematically affects the reactivity of the surfaces. This correlation is consistent with the previous successes [Greeley et al., 2002 Mavrikakis et al., 1998] of the band model in describing the reactivity of various bimetallic surfaces and the effect of strain. Compressive strain lowers s, which, in turn, leads to weaker adsorbate-surface interaction, whereas expansive strain has the opposite effect. 

In above sections the main attention has been paid to adsorption-caused change in electrophysical characteristics of semiconductor adsorbent caused by surface charging effects. However, as it was mentioned in section 1.6, the change in electrophysical characteristics of such adsorbents can be caused by other mechanisms, e.g. by direct interaction of absorbate with the surface defects provided (as in the case of oxide adsorbents) by superstoichiometric atoms of metals and oxygen... 

The effects of calcium on polymer-solvent and polymer-surface interactions are dependent on polymer ionicity a maximum intrinsic viscosity and a minimum adsorption density as a function of polymer ionicity are obtained. For xanthan, on the other hand, no influence of specific polymer-calcium interaction is detected either on solution or on adsorption properties, and the increase in adsorption due to calcium addition is mainly due to reduction in electrostatic repulsion. The maximum adsorption density of xanthan is also found to be independent of the nature of the adsorbent surface, and the value is close to that calculated for a closely-packed monolayer of aligned molecules. 

The electron spin resonance (ESR) technique has been extensively used to study paramagnetic species that exist on various solid surfaces. These species may be supported metal ions, surface defects, or adsorbed molecules, ions, etc. Of course, each surface entity must have one or more unpaired electrons. In addition, other factors such as spin-spin interactions, the crystal field interaction, and the relaxation time will have a significant effect upon the spectrum. The extent of information obtainable from ESR data varies from a simple confirmation that an unknown paramagnetic species is present to a detailed description of the bonding and orientation of the surface complex. Of particular importance to the catalytic chemist... 

Temkin s isotherm can describe the effects of surface heterogeneity or of surface modification on adsorption but we should also take into account the lateral interactions between adsorbed molecules. For the adsorption of simple... 

In adsorption, the solvent always plays a double role, affecting both lateral interactions between the adsorbate molecules and determining the effective interaction between the surface and the adsorbate. For polymers, this means that they adsorb strongly from some solvents, whereas from others they do not at all. As a consequence, mixed solvents can give rise to an adsorption/desorption transition the polymer is desorbed by a so-called displacer. 

Even complicated, charged macromolecules like proteins can be succesfully displaced. As an example we give in Figure 6 the displacement isotherm for human plasma albumin from silica by morpholine (21). Of course, in this case where charge effects and a variety of segment/surface interactions play a role, our simple Equation 5 does not apply. Nevertheless, for practical work it is important to realize that most macromolecules, often thought to be irreversibly adsorbed, can be removed completely from the adsorbent surface by the concerted action of a large number of small molecules. 

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