Extent of adsorption

With the help of equiUbrium constants, the extent of adsorption can be predicted as a function of pH and solution variables (7,25,43). Based on this model, the partitioning of metal ions and of ligands (organic and inorganic anions between water and pelagic clays and suspended particles) can be explained. 

Surface Area. Overall catalyst surface area can be determined by the BET method mentioned eadier, but mote specific techniques are requited to determine a catalyst s active surface area. X-ray diffraction techniques can give data from which the average particle si2e and hence the active surface area may be calculated. Or, it may be necessary to find an appropriate gas or Hquid that will adsorb only on the active surface and to measure the extent of adsorption under controUed conditions. In some cases, it maybe possible to measure the products of reaction between a reactive adsorbent and the active site. Radioactively tagged materials are frequentiy usehil in this appHcation. Once a correlation has been estabHshed between either total or active surface area and catalyst performance (particulady activity), it may be possible to use the less costiy method for quaHty assurance purposes. 

If a surface is polar, its resulting electric field will induce a dipole moment in a molecule with no permanent dipole and, through this polarization, increase the extent of adsorption. Similarly, a molecule with a permanent dipole moment will polarize an otherwise nonpolar surface, thereby increasing the attraction. 

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 . 

A more complex but more versatile separation method is chromatography, a technique widely used in teaching, research, and industrial laboratories to separate all kinds of mixtures. This method takes advantage of differences in solubility and/or extent of adsorption on a solid surface. In gas-liquid chromatography, a mixture of volatile liquids and gases is introduced into one end of a heated glass tube. As little as one microliter (10-6 L) of sample may be used. The tube is packed with an inert solid whose surface is coated with a viscous... 

In the case of solutions, if we define the extent of adsorption in the same way, the equation... 

Many organic species are adsorbed on platinum at potentials ranging from approximately 0.1 to 0.7 V (RHE). At more positive potentials they desorb because of oxidation or of displacement from the surface by adsorbing oxygen. The extent of adsorption of the organic substances can be considerable, and they can take up as many as 70% of the surface sites. 

The temperature dependence of the extent of adsorption was not interpreted, except that the results were considered to be consistent with the magnetic measurements of Selwood (see Section II,C) which indicate that the number of carbon-metal bonds between adsorbed species and the surface increases threefold between 120°and 200°C due to extensive dissociative chemisorption. The authors proposed that two forms of chemisorbed benzene exist at the nickel surface, (i) an associatively adsorbed form which can be displaced by further benzene, and which may be w- or hexa-dissociatively adsorbed form that requires the presence of hydrogen to bring about its removal from the surface. 

These include electrostatic interaction between the particles and interaction of particles with the fluid governed by their wettability, morphology and density (17-19) the extent of adsorption of the polymer and its influence on the interaction of particles, the orientation or configuration of the adsorbed polymers (and surfactant when it is present) and resultant interaction of adsorbed layers the hydrodynamic state of the system and its influence on the interaction of floes themselves. 

The extent of adsorption is a function of temperature T, as implied by the term isotherm , so the construction of an isotherm graph should be performed within a thermostatted system. When adsorbing from solution, the value of 9 also depends on the solvent generally, if the solvent is polar, such as water or DMF, then the extent of adsorption is often seen to decrease because molecules of solvent will occupy sites on the substrate in preference to molecules of solute. 

Polychlorotrifluoroethylene (PCTFE) is ordinarily prepared by emulsion polymerization. A polymer suitable for thermal processing requires coagulation, extensive washing, and postpolymerization workup. Coagulation to provide a filterable and washable solid is a slow, difficult process and removal of surfactant is an important part of it. Complete removal may be extremely difficult depending on the extent of adsorption to the polymer particles. Consequently we set out to develop a suspension polymerization process, which would be surfactant-free and afford an easily isolated product requiring a minimum of postreaction workup. 

To determine which mechanism is operative, it is necessary to obtain information on the extent of adsorption of the polymer (adsorption isotherm), the effect of the adsorbed polymer on the... 

Some emphasis is given in the first two chapters to show that complex formation equilibria permit to predict quantitatively the extent of adsorption of H+, OH , of metal ions and ligands as a function of pH, solution variables and of surface characteristics. Although the surface chemistry of hydrous oxides is somewhat similar to that of reversible electrodes the charge development and sorption mechanism for oxides and other mineral surfaces are different. Charge development on hydrous oxides often results from coordinative interactions at the oxide surface. The surface coordinative model describes quantitatively how surface charge develops, and permits to incorporate the central features of the Electric Double Layer theory, above all the Gouy-Chapman diffuse double layer model. 

The net charge at the hydrous oxide surface is established by the proton balance (adsorption of H or OH" and their complexes at the interface and specifically bound cations or anions. This charge can be determined from an alkalimetric-acidimetric titration curve and from a measurement of the extent of adsorption of specifically adsorbed ions. Specifically adsorbed cations (anions) increase (decrease) the pH of the point of zero charge (pzc) or the isoelectric point but lower (raise) the pH of the zero net proton condition (pznpc). 

The Gibbs equation relates the extent of adsorption at an interface (reversible equilibrium) to the change in interfacial tension qualitatively, Eq. (4.3) predicts that a substance which reduces the surface (interfacial) tension [(Sy/8 In aj) < 0] will be adsorbed at the surface (interface). Electrolytes have the tendency to increase (slightly) y, but most organic molecules, especially surface active substances (long chain fatty acids, detergents, surfactants) decrease the surface tension (Fig. 4.1). Amphi-pathic molecules (which contain hydrophobic and hydrophilic groups) become oriented at the interface. 

The adsorption of fatty acids on the non-polar hydrophobic surface (Hg) is dominated by their hydrophobic properties. The extent of adsorption increases with increasing chain length. The following relationship of the free energy of adsorption, AGacis. and the number of C atoms, nc, of the fatty acids can be established ... 

If this is not the case, the adsorption behavior can be described by competitive adsorption. The extent of adsorption depends not only on the individual adsorption constants but also on the intrinsic kinetics of adsorption of the individual adsorbates and thus, the adsorption layer formed is influenced by the qualitative and quantitative composition of the complex mixture of adsorbable solutes full equilibrium is often not attained (Cosovic, 1990). 

Adsorption isotherm of (Aldrich) humic acid (HM) on 8-AI203 as a function of pH. Extent of adsorption was determined both by measurements of light absorption at 254 and 436 nm, respectively and by measurements of dissolved organic carbon (DOC) of the residual HM in solution (original concentration = 25 mg per liter). 

Electrocapiilary phenomena on Hg-electrode in presence and absence of an adsorbate (camphor). From a measurement of interfacial tension (a) (e.g., from droptime of a Hg-electrode) or of differential capacity (d) (e.g., by an a.c-method) as a function of the electrode potential (established by applying a fixed potential across tine Hg-electrolyte interface) one can calculate the extent of adsorption (b) (from (a) by the Gibbs Equation) and of the structure of the interface as a function of the surface potential. Figs, a, c and d are interconnected through the Lippmann Equations. 

In Figs. 2.10a and b experimental data on the shift in the titration curves were given and in Example 2.4 it was shown how these effects are quantified, and that the extent of adsorption can be determined from the displacement of the titration curve. 

The extent of adsorption of reactive elements, e.g., metal ions, to particles can be readily determined by titrating a particle suspension or a sample of natural water containing particles with a metal ion (Fig. 11,3a) or to inverse the titration, i.e., to titrate a dilute standard metal solution with particles (Fig. 11.3b). 

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