Van der Waals adsorption

Van der Waal s Adsorption. Van der Waal s adsorption involves forces which are of the order of magnitude of the forces between the molecules... 

Chemisorption and Physisorption. One classification of adsorption phenomena is based on the adsorption energy the energy of the adsorbate-surface interaction. In this classification there are two basic types of adsorption chemisorption (an abbreviation of chemical adsorption) and physisorption (an abbreviation of physical adsorption). In chemisorption the chemical attractive forces of adsorption are acting between surface and adsorbate (usually covalent bonds). Thus, there is a chemical combination between the substrate and the adsorbate where electrons are shared and/or transferred. New electronic configurations are formed by this sharing of electrons. In physisorption the physical forces of adsorption, van der Waals or pure electrostatic forces, operate between the surface and the adsorbate there is no electron transfer and no electron sharing. 

Dietrich, S., (1991), Fluid interfaces - wetting, critical adsorption, van der Waals tails, and the concept of the effective interface potential , in Taub, H., Torzo, G., Lauter, HJ. and Fain, S.C., (eds), Phase. Transitions in Surface Films 2, NATO Advanced Science Series, Physics, Vol. 267, 391-423. 

Furthermore, adsorption at basic pH, where the surface is weakly charged, was found to be low but not negligible. To explain this adsorption, Van der Waals... 

Adsorption refers to the interaction between polymer molecules and the solid surface. This interaction causes polymer molecules to be bound to the surface of the solid, mainly by physical adsorption, van der Waals forces, and hydrogen bonding. Essentially, the polymer occupies surface adsorption sites. Adsorption depends on the surface area exposed to the polymer solution, and it is the only mechanism that removes polymer from the bulk solution if a free solid powder, such as silica sand or latex beads, is introduced into the bulk solution and stirred until equilibrium is reached. 

There is always some degree of adsorption of a gas or vapor at the solid-gas interface for vapors at pressures approaching the saturation pressure, the amount of adsorption can be quite large and may approach or exceed the point of monolayer formation. This type of adsorption, that of vapors near their saturation pressure, is called physical adsorption-, the forces responsible for it are similar in nature to those acting in condensation processes in general and may be somewhat loosely termed van der Waals forces, discussed in Chapter VII. The very large volume of literature associated with this subject is covered in some detail in Chapter XVII. 

All gases below their critical temperature tend to adsorb as a result of general van der Waals interactions with the solid surface. In this case of physical adsorption, as it is called, interest centers on the size and nature of adsorbent-adsorbate interactions and on those between adsorbate molecules. There is concern about the degree of heterogeneity of the surface and with the extent to which adsorbed molecules possess translational and internal degrees of freedom. 

The quantity zoi will depend very much on whether adsorption sites are close enough for neighboring adsorbate molecules to develop their normal van der Waals attraction if, for example, zu is taken to be about one-fourth of the energy of vaporization [16], would be 2.5 for a liquid obeying Trouton s rule and at its normal boiling point. The critical pressure P, that is, the pressure corresponding to 0 = 0.5 with 0 = 4, will depend on both Q and T. A way of expressing this follows, with the use of the definitions of Eqs. XVII-42 and XVII-43 [17] ... 

It must be remembered that, in general, the constants a and b of the van der Waals equation depend on volume and on temperature. Thus a number of variants are possible, and some of these and the corresponding adsorption isotherms are given in Table XVII-2. All of them lead to rather complex adsorption equations, but the general appearance of the family of isotherms from any one of them is as illustrated in Fig. XVII-11. The dotted line in the figure represents the presumed actual course of that particular isotherm and corresponds to a two-dimensional condensation from gas to liquid. Notice the general similarity to the plots of the Langmuir plus the lateral interaction equation shown in Fig. XVII-4. 

One may choose 6(Q,P,T) such that the integral equation can be inverted to give f Q) from the observed isotherm. Hobson [150] chose a local isotherm function that was essentially a stylized van der Waals form with a linear low-pressure region followed by a vertical step tod = 1. Sips [151] showed that Eq. XVII-127 could be converted to a standard transform if the Langmuir adsorption model was used. One writes... 

The second general cause of a variable heat of adsorption is that of adsorbate-adsorbate interaction. In physical adsorption, the effect usually appears as a lateral attraction, ascribable to van der Waals forces acting between adsorbate molecules. A simple treatment led to Eq. XVII-53. 

Adsorbates can physisorb onto a surface into a shallow potential well, typically 0.25 eV or less [25]. In physisorption, or physical adsorption, the electronic structure of the system is barely perturbed by the interaction, and the physisorbed species are held onto a surface by weak van der Waals forces. This attractive force is due to charge fiuctuations in the surface and adsorbed molecules, such as mutually induced dipole moments. Because of the weak nature of this interaction, the equilibrium distance at which physisorbed molecules reside above a surface is relatively large, of the order of 3 A or so. Physisorbed species can be induced to remain adsorbed for a long period of time if the sample temperature is held sufficiently low. Thus, most studies of physisorption are carried out with the sample cooled by liquid nitrogen or helium. 

The saturation coverage during chemisorption on a clean transition-metal surface is controlled by the fonnation of a chemical bond at a specific site [5] and not necessarily by the area of the molecule. In addition, in this case, the heat of chemisorption of the first monolayer is substantially higher than for the second and subsequent layers where adsorption is via weaker van der Waals interactions. Chemisorption is often usefLil for measuring the area of a specific component of a multi-component surface, for example, the area of small metal particles adsorbed onto a high-surface-area support [6], but not for measuring the total area of the sample. Surface areas measured using this method are specific to the molecule that chemisorbs on the surface. Carbon monoxide titration is therefore often used to define the number of sites available on a supported metal catalyst. In order to measure the total surface area, adsorbates must be selected that interact relatively weakly with the substrate so that the area occupied by each adsorbent is dominated by intennolecular interactions and the area occupied by each molecule is approximately defined by van der Waals radii. This... 

If we knew the variation m A as a fiinction of coverage 0, this would be the equation for the isothenn. Typically the energy for physical adsorption in the first layer, -A E, when adsorption is predominantly tlnongh van der Waals interactions, is of the order of lO/rJ where T is the temperature and /rthe Boltzmann constant, so that, according to equation (B1.26.6), the first layer condenses at a pressure given by PIPq. 10... 

Emmett P H and Brunauer S 1937 The use of low temperature van der Waals adsorption isotherms in determining the surface area of iron synthetic ammonia catalysts J. Am. Chem. See. 59 1553-64... 

In such an experiment the material actually adsorbed by the solid (the adsorbent) is termed the adsorbate, in contradistinction to the adsorptive which is the the general term for the material in the gas phase which is capable of being adsorbed. The adsorption is brought about by the forces acting between the solid and the molecules of the gas. These forces are of two main kinds—physical and chemical—and they give rise to physical (or van der Waals ) adsorption, and chemisorption respectively. The nature of the physical forces will be dealt with in the next section meanwhile it is convenient to note that they are the same in nature as the van der Waals forces which bring about the condensation of a vapour to the liquid state. 

Fig. 3.24 Test of the tensile strength hysteresis of hysteresis (Everett and Burgess ). TjT, is plotted against — Tq/Po where is the critical temperature and p.. the critical pressure, of the bulk adsorptive Tq is the tensile strength calculated from the lower closure point of the hysteresis loop. C), benzene O. xenon , 2-2 dimethyl benzene . nitrogen , 2,2,4-trimethylpentane , carbon dioxide 4 n-hexane. The lowest line was calculated from the van der Waals equation, the middle line from the van der Waals equation as modified by Guggenheim, and the upper line from the Berthelot equation. (Courtesy Everett.)...

Forces of Adsorption. Adsorption may be classified as chemisorption or physical adsorption, depending on the nature of the surface forces. In physical adsorption the forces are relatively weak, involving mainly van der Waals (induced dipole—induced dipole) interactions, supplemented in many cases by electrostatic contributions from field gradient—dipole or —quadmpole interactions. By contrast, in chemisorption there is significant electron transfer, equivalent to the formation of a chemical bond between the sorbate and the soHd surface. Such interactions are both stronger and more specific than the forces of physical adsorption and are obviously limited to monolayer coverage. The differences in the general features of physical and chemisorption systems (Table 1) can be understood on the basis of this difference in the nature of the surface forces. 

Adsorption on a nonpolar surface such as pure siUca or an unoxidized carbon is dominated by van der Waals forces. The affinity sequence on such a surface generally follows the sequence of molecular weights since the polarizabiUty, which is the main factor governing the magnitude of the van der Waals interaction energy, is itself roughly proportional to the molecular weight. 

The size of particles removed by such filters is less than the size of the passages. The mechanism of removal includes adsorption (qv) of the impurities at the interface between the media and the water either by specific chemical or van der Waals attractions or by electrostatic interaction when the medium particles have surface charges opposite to those on the impurities to be removed. 

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