Adsorption force

As indicated in the previous section, the adsorption of a gas by a solid is the outcome of the forces of attraction between the individual molecules of the gas and the atoms or ions composing the solid. These forces have been studied theoretically over a number of decades, and though impressive advances have been made in recent years these remain more in the nature of refinements than of fundamental changes in the ideas themselves. And since. 

The forces which bring about adsorption always include dispersion forces, which are attractive, together with short-range repulsive forces. In addition, there will be electrostatic (coulombic) forces if either the solid or the gas is polar in nature. Dispersion forces derive their name from the close connection between their origin and the cause of optical dispersion. First 

An expression for the short-range repulsive force (which arises from the interpenetration of the electron clouds of the two atoms) can also be derived from quantum-mechanical considerations as 

Several relations have been devised for the calculation of the parameter C from the molecular properties of two atoms A and B. One of the best known is that of Kirkwood and Miiller  

To apply these various equations to the adsorption of a gas on a solid, it is necessary to consider the interaction of the surface layers of a solid composed of atoms (or ions) of a substance Y, say, with isolated molecules of gas X. The individual interactions of each atom in gas molecule X with each atom of the solid Y have to be added up to obtain the potential 0(z) of a single molecule of the gas with reference to the solid  

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If we consider the case of a gas in adsorption equilibrium with a surface, there must be no net free energy change on transporting a small amount from one region to the other. Therefore, since the potential represents the work done by the adsorption forces when adsorbate is brought up to a distance x from the surface, there must be a compensating compressional increase in the free energy of the adsorbate. Thus... 

Fig. 3.10 Contributions to the lowering of chemical potential of the condensed liquid in a capillary, arising from adsorption forces (c) and meniscus curvature (Ap). The chemical potential of the free liquid is , and that of the capillary condensed liquid is (= ) z is the distance from the capillary wall. (After Everett. )...

Adsorption Forces. Coulomb s law allows calculations of the electrostatic potential resulting from a charge distribution, and of the potential energy of interaction between different charge distributions. Various elaborate computations are possible to calculate the potential energy of interaction between point charges, distributed charges, etc. See reference 2 for a detailed introduction. 

Activated carbon has the strongest physical adsorption forces or the highest volume of adsorbing porosity of any material known to mankind. 

Gas Phase Adsorption - This is a condensation process where the adsorption forces condense the molecules from the bulk phase within the pores of the activated carbon. The driving force for adsorption is the ratio of the partial pressure and the vapour pressure of the compound. 

Silica gel and aluminium oxide layers are highly active stationary phases with large surface areas which can, for example, — on heating — directly dehydrate, degrade and, in the presence of oxygen, oxidize substances in the layer This effect is brought about by acidic silanol groups [93] or is based on the adsorption forces (proton acceptor or donor effects, dipole interactions etc) The traces of iron in the adsorbent can also catalyze some reactions In the case of testosterone and other d -3-ketosteroids stable and quantifiable fluorescent products are formed on layers of basic aluminium oxide [176,195]... 

Grahame introdnced the idea that electrostatic and chemical adsorption of ions are different in character. In the former, the adsorption forces are weak, and the ions are not deformed dnring adsorption and continne to participate in thermal motion. Their distance of closest approach to the electrode surface is called the outer Helmholtz plane (coordinate x, potential /2, charge of the diffuse EDL part When the more intense (and localized) chemical forces are operative, the ions are deformed, undergo partial dehydration, and lose mobility. The centers of the specifically adsorbed ions constituting the charge are at the inner Helmholtz plane with the potential /i and coordinate JCj < Xj. 

Adsorptive Force Affecting Underground Liquid Movement. 697... 

Both adsorptive and capillary forces play an important part in soil-liquid interaction (see Figure 18.3). This is very important for unsaturated soil. The total force (i.e., the sum of capillary force and adsorptive force) is termed the matrix potential, which has a negative gage pressure relative to the external gas pressure on the soil water (more often the gage pressure is referred to as the atmospheric pressure). 

In addition to the nonelectrostatic adsorptive force, there is an image force between a dipole and a metal, which will be present whenever charged or dipolar particles in a medium of one dielectric constant are near a region of another dielectric constant. If the metal is treated as an ideal conductor, the image-force contribution to the energy of a dipole in the electrolyte is proportional to p2j z3, where z is the distance of the dipole from the plane boundary of the metal (considered ideal, with no surface structure), and to 1 + cos2 0. This ideal term is, of course, the same for all metals. If... 

Adsorption effluent treatment, 9 432 Adsorption equilibrium, 1 591—594 Adsorption forces, 1 583-584 gas adsorption, 1 619-621 Adsorption free energy, contributions to, 24 139... 

Water present in and moving through the unsaturated zone is subjected to several rules of physics in addition to those influencing the water below the water table. The presence of retained moisture above the water table is due to adsorptive forces between the water molecules and soil particles, in addition to surface tension of the water surface. 

These bonding interactions just described are frequently considered to be representative of the major types of forces which exist between species, although there is some disagreement about their nature and magnitude involved. It is probable that combined or hybrid forces come into play in real material interactions. It is also probable that multiple types of attractive and repulsive adsorptive forces are operative [158,160-162]. 

Adsorption of nonionic compounds on subsurface solid phases is subject to a series of mechanisms such as protonation, water bridging, cation bridging, ligand exchange, hydrogen bonding, and van der Waals interactions. Hasset and Banwart (1989) consider that the sorption of nonpolar organics by soils is due to enthalpy-related and entropy-related adsorption forces. 

Enthalpy-related adsorption forces include the following processes ... 

A kinetic estimate is also possible because at equilibrium and in absence of adsorption forces the rate of condensation per unit surface p/... 

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