Retardation of adsorption

Micelles. It is still a matter of debate whether the dissociation of surfactant molecules from micelles would lead to retardation of adsorption. It appears very unlikely that the time scale involved would be longer than a second. On the other hand, some surfactants have a very small solubility in water (e.g., phospholipids), and they are often present in small solid lumps or as vesicles. In such a case, it may take a very long time before surfactant molecules have reached the interface. 

From the theoretical point of view a similarity exists between electrostatic retardation of ion transport and coagulation retardation, known as slow coagulation (Fuchs, 1934). Both phenomena arise from electrostatic repulsion caused by the existence of the diffuse part of the DL. In the slow coagulation theory, the electric field if the DL is derived from the Gouy-Chapman model (cf. Chapter 2). This model does not consider a deviation of the diffuse layer from equilibrium. Initially, the same simplification was used by Dukhin et al. (1973) in describing the DL effect on the electrostatic retardation of adsorption. 

Electrostatic retardation of adsorption kinetics of surfactant anions is expected in the case... 

The calculation of the flow of surfactant anions to the surface of a bubble can be performed under the condition of electrostatic retardation of adsorption kinetics. It follows from Eq. (7.30),... 

The limiting stage is the overcoming of the electric double layer, with electrostatic retardation of adsorption, the value of j is given by Eq. (7.36). 

Now the condition under which, electrostatic retardation of adsorption kinetics of surfactant anions appears at the main part of the siuface, is determined. Substituting the estimate for T from Eq. (9.34) into (7.26) and (7.29) yields the following condition. 

An estimate of the total desorption flow from the surface of a strongly retarded region in the neighbourhood of the rear pole of the bubble is derived as follows. When electrostatic retardation of adsorption-desorption kinetics does not exists, the results of Chapter 8 [Eq. (8.145)] can be applied. For ionic surfactant, the equation for surface tension variation somewhat differs from that for non-ionic surfactant. With regard to these differences, the following estimate of desorption flow results. 

At a given electrostatic retardation of adsorption-desorption kinetics of the surfactants, its approximate anion flux to the bubble surface is... 

The kinetics of this reaction, which can also be regarded as an erosion reaction, shows die effects of adsorption of the reaction product in retarding the reaction rate. The path of this reaction involves the adsorption of an oxygen atom donated by a carbon dioxide molecule on die surface of the coke to leave a carbon monoxide molecule in the gas phase. 

The low yields of 6,6 -disubstituted-2,2 -bipyridincs recorded in Table I are probably the result of steric retardation of the adsorption of 2-substituted pyridines. This view is supported by the observation that 2-methylpyridine is a much weaker poison for catalytic hydrogenations than pyridine. On the other hand, the quinolines so far examined (Table II) are more reactive but with these compounds the steric effect of the fused benzene ring could be partly compensated by the additional stabilization of the adsorbed species, since the loss of resonance energy accompanying the localization of one 71-electron would be smaller in a quinoline than in a pyridine derivative. 

From the results of other authors should be mentioned the observation of a similar effect, e.g. in the oxidation of olefins on nickel oxide (118), where the retardation of the reaction of 1-butene by cis-2-butene was greater than the effect of 1-butene on the reaction of m-2-butene the ratio of the adsorption coefficients Kcia h/Kwas 1.45. In a study on hydrogenation over C03O4 it was reported (109) that the reactivities of ethylene and propylene were nearly the same (1.17 in favor of propylene), when measured separately, whereas the ratio of adsorption coefficients was 8.4 in favor of ethylene. This led in the competitive arrangement to preferential hydrogenation of ethylene. A similar phenomenon occurs in the catalytic reduction of nitric oxide and sulfur dioxide by carbon monoxide (120a). 

On the surface of metal electrodes, one also hnds almost always some kind or other of adsorbed oxygen or phase oxide layer produced by interaction with the surrounding air (air-oxidized electrodes). The adsorption of foreign matter on an electrode surface as a rule leads to a lower catalytic activity. In some cases this effect may be very pronounced. For instance, the adsorption of mercury ions, arsenic compounds, or carbon monoxide on platinum electrodes leads to a strong decrease (and sometimes total suppression) of their catalytic activity toward many reactions. These substances then are spoken of as catalyst poisons. The reasons for retardation of a reaction by such poisons most often reside in an adsorptive displacement of the reaction components from the electrode surface by adsorption of the foreign species. 

Behavior of Ra in groundwater. The general behavior of Ra has been examined under laboratory conditions and in various environments (see Osmond and Cowart 1992). A major goal of field studies of Ra isotopes have aimed at obtaining bulk, in situ values of adsorption rates and so the retardation factors. Note that Ba serves as a very close chemical analogue to Ra but is typically 10 times more abundant, and so its behavior is related to that of Ra. 

There are various parameters and assumptions defining radionuclide behavior that are frequently part of model descriptions that require constraints. While these must generally be determined for each particular site, laboratory experiments must also be conducted to further define the range of possibilities and the operation of particular mechanisms. These include the reversibility of adsorption, the relative rates of radionuclide leaching, the rates of irreversible incorporation of sorbed nuclides, and the rates of precipitation when concentrations are above Th or U mineral solubility limits. A key issue is whether the recoil rates of radionuclides can be clearly related to the release rates of Rn the models are most useful for providing precise values for parameters such as retardation factors, and many values rely on a reliable value for the recoil fluxes, and this is always obtained from Rn groundwater activities. These values are only as well constrained as this assumption, which therefore must be bolstered by clearer evidence. 

Krishnaswami S, Graustein WC, Turekian KK, Dowd F (1982) Radium, thorium, and radioactive lead isotopes in groundwaters application to the in-situ determination of adsorption-desorption rate constants and retardation factors. Water Resour Res 6 1663-1675 Krishnaswami S, Bhushan R, Baskaran M (1991) Radium isotopes and Rn in shallow brines, Kharaghoda (India). Chem Geol (Isot Geosci) 87 125-136 Kronfeld J, Vogel JC, Talma AS (1994) A new explanation for extreme " U/ U disequilibria in a dolomitic aquifer. Earth Planet Sci Lett 123 81-93... 

Whiskers are sometimes formed in solutions with high concentrations of surface-active substances. These are long single crystals, growing in only one direction, while growth in the remaining directions is retarded by adsorption of surface-active substances. Whiskers are characterized by quite... 

Equation (6.4.4) is valid when the coverage of the electrolyte-membrane interface is small. At higher concentrations of transferred ion, the ion transfer is retarded by adsorption on the opposite interface, so that the dependence of G0 on c is characterized by a curve with a maximum, as has been demonstrated experimentally. 

In the case of porous electrodes, diffusion in the pores can retard the rate of adsorption, in particular at low methanol concentrations. This could be the reason why at a concentration of 10-2 M mainly COH is formed independently of 6 [14]. 

Some active materials are carriers for drugs (drug delivery systems), some have immobilized peptides to enable cell adhesion or migration, some are degradable by hydrolysis or by specific enzyme action. Some contain bioactive agents (e.g., heparin, thrombomodulin) to prevent coagulation or platelet activation while others incorporate bioactive groups to enhance osteo-conduction. Many include polyethylene oxide to retard protein adsorption and this is perhaps the closest we have come to a kind of inertness. 

Leaching and desorption of As from its associated mineral surfaces such as iron, aluminum and manganese oxides under the influence of the aquifer complex geochemistry, largely take part in its transport from sediment to aquifer pore-water. Adsorption has widely been considered as the retardation of As transport (Smedley 2003). 

Figure 6.6 Demonstration of the retardation of Pt adsorption on alumina by high ionic strength (a) 1 1 electrolytes ...

The movement of the analyte is an essential feature of separation techniques and it is possible to define in general terms the forces that cause such movement (Figure 3.1). If a force is applied to a molecule, its movement will be impeded by a retarding force of some sort. This may be as simple as the frictional effect of moving past the solvent molecules or it may be the effect of adsorption to a solid phase. In many methods the strength of the force used is not important but the variations in the resulting net force for different molecules provide the basis for the separation. In some cases, however, the intensity of the force applied is important and in ultracentrifugal techniques not only can separation be achieved but various physical constants for the molecule can also be determined, e.g. relative molecular mass or diffusion coefficient. 

Girvin et al. [358] evaluated the release of PCBs from electrical substation soils contaminated with transformer fluids. They observed that there are two phases to the uptake and release of PCBs with these soils. The initial phase is a rapid, labile phase that is followed by a slower, nonlabile phase. The labile phase occurs at a scale of hours to days while the nonlabile phase releases over weeks and months. Girvin et al. [422] also reviewed the effects of adsorption on the mobility of PCBs and their transport. In an example presented for a hexa-chlorobiphenyl, these authors noted that the PCB isomer would have a retardation factor Rf of 1400 for the particular case given. This means that the ground-... 

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