Forces causing adsorption

Adsorption forces cause a-methyl-(3-isopropyl stilbene to be arranged in such a way that the substituents at the ethylenic bond deviate from the metal surface. The reaction under consideration consists of the two consecutive one-electron steps resulting in the formation of the dianion. The dianion accepts two protons, from the metal side. Therefore, regardless of the configuration of the initial stilbene, the reduction proceeds with a predominant formation of the erythro product (Scheme 6-1). 

In the past, polyvalent anions such as polyphosphonic acids could not be analyzed by ion chromatography, because their retention times increase drastically with increasing charge number. Strong adsorptive forces, caused by the organic skeleton of these com-... 

Whenever the concentration of a species at the interface is greater than can be accounted for by electrostatic interactions, we speak of specific adsorption. It is usually caused by chemical interactions between the adsorbate and the electrode, and is then denoted as chemisorption. In some cases adsorption is caused by weaker interactions such as van der Waals forces we then speak of physisorption. Of course, the solvent is always present at the interface so the interaction of a species with the electrode has to be greater than that of the solvent if it is to be adsorbed on the electrode surface. Adsorption involves a partial desolvation. Cations tend to have a firmer solvation sheath than anions, and are therefore less likely to be adsorbed. 

Of course this need not necessarily be the case, as in any mechanism where species are attached to an anode, changes in their electronic make-up, resulting from hydrogen replacement by fluorine, might be expected to cause variations in the adsorption forces with possible desorption into the electrolyte as a consequence. 

Gas-adsorption processes Involve the selective concentration (adsorption) of one or more components (adsorbates) of a gas (or vapor) at the surface of a microporous solid (adsorbent) The attractive forces causing the adsorption are generally weaker than those of chemical bonds and are such that, by Increasing the temperature of the adsorbent or reducing an adsorbate s partial pressure, the adsorbate can be desorbed The desorption step Is quite Important in the overall process First, desorption allows recovery of adsorbates In those separations where they are valuable, and second, It permits reuse of the adsorbent for further cycles ... 

If the forces causing adsorption were known with a sufficiently high degree of accuracy and if their dependence on the distance of the adsorbed atom or molecule from the adsorbing surface were also exactly known, it would, theoretically, be possible to calculate the change in potential energy accompanying the act of adsorption. 

The influence of the repulsion forces is often neglected or accounted for by subtracting a fixed percentage, say 40%, from the adsorption energy as calculated with the attraction forces only (18). When adsorption is caused by van der Waals forces only, the contribution of the repulsion... 

In all these electrostatic considerations the surface of the ionic crystal was idealized, as described in Sec. IV,2, as if it were cut by means of an ideally sharp razor blade. Our lack of knowledge of the structural deviations of the surface arrangements with respect to the structure inside the crystal renders it impossible for us to make any quantitative or semiquantitative statements regarding the actual adsorption energies caused by electrostatic forces. We can only say that in most ionic crystals negative ions i.e., halide ions or oxide ions, tend to form the outside (adsorbing) surface. We shall have an opportunity (see, for example, Secs. V,5 and VI,5) to revert to this phenomenon. 

Of particular interest is the spectrum at the bottom of Figure 12. In this case the penetration depth is computed to be on the order of approx. 50 nm. Furthermore, considering that the field strength at the interface drops off exponentially, one could expect a possible adsorption effect. Figure 13 speculates on how the alcohol molecules should be arranged on the quartz surface. Unfortunately, however, the spectrum of the reflection plate was predominant here and the possible occurance of new bands - caused by adsorption forces - could therefore not be detected. 

Solid surfaces may accommodate and orient molecules at distances close to molecular bonds and reaction rates are influenced by physical order in the adsorbed layer. Deposition of small molecules on a crystal surface under appropriate temperature and pressure conditions produces ordered molecular monolayers and multilayers. These structures result from the balance of the forces causing adsorption imposed by the surface and the forces between neighboring adsorbed species. Under such conditions, certain reactant... 

This experiment is concerned with the multilayer physical adsorption of a gas (the adsorbate) on a high-area solid (the adsorbent). Since such adsorption is caused by forces very similar to those that cause the condensation of a gas to a bulk liquid, appreciable adsorption occurs only at temperatures near the boiling point of the adsorbate. The adsorption of N2 gas on a high-area solid will be studied at 77.4 K (the boihng point of liquid nitrogen), and the surface area of the solid will be obtained. 

The specific surface area of a ceramic powder can be measured by gas adsorption. Gas adsorption processes may be classified as physical or chemical, depending on the nature of atomic forces involved. Chemical adsorption (e.g., H2O and AI2O3) is caused by chemical reaction at the surface. Physical adsorption (e.g., N2 on AI2O3) is caused by molecular interaction forces and is important only at a temperature below the critical temperature of the gas. With physical adsorption the heat erf adsorption is on the same order of magnitude as that for liquefaction of the gas. Because the adsorption forces are weak and similar to liquefaction, the capillarity of the pore structure effects the adsorbed amount. The quantity of gas adsorbed in the monolayer allows the calculation of the specific surface area. The monolayer capacity (V ,) must be determined when a second layer is forming before the first layer is complete. Theories to describe the adsorption process are based on simplified models of gas adsorption and of the solid surface and pore structure. 

When the mobile phase washes over the sample spot and starts on down the paper, it will begin to siphon when its level gets below the solvent level in the solvent tank. This will cause the solvent to move too fast for the desired separation equilibrium to be attained, and streaking will occur. If a small-diameter rod is placed so the entire width of the paper makes contact with it, then a considerable adsorption force is developed between the solvent and the rod. This is sufficient to stop the siphoning action, and the solvent moves down the paper by capillary action alone. 

Much study has been given to exploring the relative roles of physical and chemical forces in adsorption phenomena.1,2 Physical adsorption is caused by forces similar to those that cause vapor molecules to condense to a liquid the adsorption of nitrogen on carbon at low temperatures is an example. In physical adsorption, the attachment is weak adsorbed nitrogen will leave the carbon surface if a vacuum is applied or if the temperature is slightly elevated. 

Non-equilibrium Surface Forces Caused by Dynamic Adsorption Layers... 

Rather high electrolyte concentrations characteristic of natural and waste waters, substantially weaken non-equilibrium surface forces of the diffusion - electric nature but scarcely affect nonequilibrium surface forces caused by the dynamic adsorption layer of nonionic surfactant (Dukhin 1981). Therefore, we consider such forces are important and their mechanism deserves special attention. The flotation of tiny particles is, nevertheless, possible if the distance between the bubble and particle surfaces becomes smaller than a critical value h j-. The film of thickness h r is thiimed, becomes unstable, and collapses if long-range attractive forces exist between the particle and the bubble, drawing them together. 

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