Weak physical adsorption

The AG° values of acetone adsorption decrease slightly in the sequence H2O,Me0H, NM. They are indicative of a weak physical adsorption at the Hg/solution interface. It is also evident that the Gibbs energy of adsorption is enhanced by the electric field, particularly at the point of adsorption maximum. Small values of AG , similar to those determined at the solution/air interface, attest to the absence of specific interactions cf acetone with the mercury surface (which is opposite to the TU adsorption case). Hence, the solute-solvent interaction in the solution is an important factor in the adsorption of acetone, as shown for the zero charge on the Hg electrode in Fig. 11. 

Our data indicate that ammonia acts as a mild inhibitor for hydrogen absorption in Ni-containing AB5 alloys. The measured heat of adsorption of NH3 on Ni is about -11 kcal/mol NH3, suggesting weak, physical adsorption. Since Ni is viewed to be a catalyst for the hydriding reaction, weak physical adsorption of NH3 at these Ni sites would retard the reaction and promote a broadening of the reaction front as shown in Figures 12 and 13. 

Direct bonds between substrate and adsorbate are loosely divided into weak physical adsorption (physisorption), and stronger chemical bonding (chemisorption). We are here focusing on chemisorption cases, where strong substrate-adsorbate interactions make it reasonable to first consider the direct interactions between the adsorbate and the substrate. In physisorption, this interaction is likely to be competing with the interactions between neighbouring adsorbates which may be of similar strength. 

For describing weak physical adsorption from solution, which is commonly the case for nonpolar and weakly polar organic molecules, the BET equation is useful once it is modified to account for adsorption from solution rather than from the gas phase. Nonpolar organics have low solubility in water and are adsorbed onto soil solids from very dilute solutions. This would be analogous to restricting gas phase adsorption to very low pressure, P, of the oiganic vapor. The BET isotherm (equation 10.25) is essentially linear for P/Pq 1 and small values of c (c < 1), taking the form ... 

Thus, the interfacial friction can be evaluated from measurement of AT and A/. This procedure has been applied to a number of systems in which weak physical adsorption occurs, such as the adsorption of Xe, Kr, N2 on Au, and of H2O and CeH on Ag [34-38]. In all the above cases slippage was observed, and the ratio of the coefficient of sliding friction to the mass density was of the order x/Amf = (10 - 10 )s As an example, the frictional stress acting on the monolayer Xe film sliding on a Ag(lll) surface at a velocity v = 1 cms F = xv, equals about 10Nm [40]. It is much smaller than typical shear stresses involved in sliding of a steel block on a steel surface under boundary lubrication condition (Eq. 6), which is of order 10 Nm 2 [39]. 

Moderate amounts of acidic pesticides were adsorbed to organic soil colloids, such as are present in muck soils, (51,143,147,175, 179) and to charcoal (53, 57,147,182,183). For both adsorption depended upon pH, being greater under acid conditions where the pesticides were adsorbed in the molecular form. The compounds were readily desorbed from the adsorbents with water (51, 57). Adsorption probably occurred through hydrogen bonding or weak physical adsorption. 

The choice of solvent is also important, as the enthalpy measured is essentially that of the displacement of the solvent from the filler surface by the probe molecule. For acid-base characterisation a non-polar solvent such as n-heptane is ideal. However, poor solubility of the desired probe molecule may demand the use of a polar solvent, when this is the case the enthalpy of interaction of the solvent with the filler must be considered when interpreting the results. If the probes in question are only capable of weak physical adsorption, (i.e., not via hydrogen bonding), and are only soluble in highly polar solvents such as tetrahydrofuran and dimethyl formamide, situations can arise where apparently no adsorption occurs. This is due to the solvent having equal or stronger interaction with the substrate than the probe. In such cases the value of performing FMC experiments should be questioned and the use of more soluble model compounds considered. If chemical adsorption of the probe occurs the polarity of the solvent is somewhat less critical, provided the solubility of reaction products is not enhanced. 

In gas-solid chromatography, the solute molecules interact with the surface of solid adsorbents through relatively weak physical adsorption forces. Such weak forces are desirable, because the adsorption process must be... 

The immediate site of the adsorbent-adsorbate interaction is presumably that between adjacent atoms of the respective species. This is certainly true in chemisorption, where actual chemical bond formation is the rule, and is largely true in the case of physical adsorption, with the possible exception of multilayer formation, which can be viewed as a consequence of weak, long-range force helds. Another possible exception would be the case of molecules where some electron delocalization is present, as with aromatic ring systems. 

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. 

Strong adsorbate-substrate forces lead to chemisorption, in which a chemical bond is fomied. By contrast, weak forces result inphysisorption, as one calls non-chemical physical adsorption. 

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. 

The attention of this article is focused on physical adsorption, which involves relatively weak intermolecular forces, because most commercial appHcations of adsorption rely on this phenomenon alone. Chemisorption is discussed only briefly in some sections on specific appHcations. 

Hydrogen Bond Formation. This faciUtates adsorption if the mineral and the adsorbate have any of the highly electronegative elements S,0,N,F, and hydrogen. A weak (physical) bond is estabflshed between the sohd wall and the reagent through the alignment of the cited elements. 

Surface areas are deterrnined routinely and exactiy from measurements of the amount of physically adsorbed, physisorbed, nitrogen. Physical adsorption is a process akin to condensation the adsorbed molecules interact weakly with the surface and multilayers form. The standard interpretation of nitrogen adsorption data is based on the BET model (45), which accounts for multilayer adsorption. From a measured adsorption isotherm and the known area of an adsorbed N2 molecule, taken to be 0.162 nm, the surface area of the soHd is calculated (see Adsorption). 

Most adsorption processes are exothermic (AH is negative). Adsorption processes involving nonspecific interactions are referred to as physical adsorption, a relatively weak, reversible interaction. Processes with stronger interactions (electron transfer) are termed chemisorption. Chemisorption is often irreversible and has higher heat of adsorption than physical adsorption. Most dispersants function by chemisorption, in contrast to surfactants, which... 

It has been outlined by several authors that the single macromolecule may be irreversibly bound because of the large number of weakly interacting segments. The first papers on the construction of polymer-coated silica adsorbents involved the physical adsorption of water-soluble polymers. Polyethylene oxides [28, 29] and poly-/V-vinylpyrrolidone [30] are examples of the stationary phases of this type. 

The (unbalanced) forces acting in surface layers are diverse in their intensities and character. Physical (van der Waals) forces between molecules are weak and give rise to slight energy effects (up to 20kJ/mol). These forces decrease slowly with increasing distance (i.e., they operate within a relatively wide region) and are responsible for weak physical (often multiplayer) adsorption. 

It is most convenient to explain catalysis using an example. We have chosen a hydrogenation catalysed by nickel in the metallic state. According to the schematic of Fig. 3.1 the first step in the actual catalysis is adsorption . It is useful to distinguish physisorption and chemisorption . In the former case weak, physical forces and in the latter case relatively strong, chemical forces play a role. When the molecules adsorb at an active site physisorption or chemisorption can occur. In catalysis often physisorption followed by chemisorption is the start of the catalytic cycle. This can be understood from Fig. 3.2, which illustrates the adsorption of hydrogen on a nickel surface. 

Usually adsorption, i.e. binding of foreign particles to the surface of a solid body, is distinguished as physical and chemical the difference lying in the type of adsorbate - adsorbent interaction. Physical adsorption is assumed to be a surface binding caused by polarization dipole-dipole Van-der-Vaals interaction whereas chemical adsorption, as any chemical interaction, stems from covalent forces with plausible involvement of electrostatic interaction. In contrast to chemisorption in which, as it has been already mentioned, an absorbed particle and adsorbent itself become a unified quantum mechanical system, the physical absorption only leads to a weak perturbation of the lattice of a solid body. 

The fact of a transfer of an electron from an absorbed particle to adsorbent [25] is widely considered as a criterion to differentiate between various forms of adsorption. Yet, as it has been already mentioned in previous section, there is a neutral form of chemisorption, i.e. weak binding formed without changing the surface charge state which only affects the dipole component of the work function. On the other hand, in several cases the physical adsorption can result in electron transitions in solids. Indeed, apart from formation of a double layer, changing the work function of adsorbent [26] the formation of surface dipoles accompanying physical adsorption can bring free charge carriers to substan-... 

The adsorption action of activated carbon may be explained in terms of the surface tension (or energy per unit surface area) exhibited by the activated particles whose specific surface area is very large. The molecules on the surface of the particles are subjected to unbalanced forces due to unsatisfied bonds and this is responsible for the attachment of other molecules to the surface. The attractive forces are, however, relatively weak and short range, and are called Van der Waals forces, and the adsorption process under these conditions is termed as a physical adsorption (physisorption) process. In this case, the adsorbed molecules are readily desorbed from the surface. Adsorption resulting from chemical interaction with surface molecules is termed as chemisorption. In contrast to the physical process described for the adsorption on carbon, the chemisorption process is characterized by stronger forces and irreversibility. It may, however, be mentioned that many adsorption phenomena involve both physical and chemical processes. They are, therefore, not easily classified, and the general term, sorption, is used to designate the mechanism of the process. 

Ethylene adsorption at room temperature is rapid and reversible. Even after prolonged exposure to the catalyst, the ethylene is recoverable as such by brief evacuation (10). The isotherms are nonlinear and show some evidence of saturation at 0.5-0.6 cm3/gm, a value roughly five times that of the type I hydrogen. Since the adsorption is quite weak, it would seem that this adsorption is, in part, physical adsorption. To investigate this possibility, adsorption of ethylene (boiling point — 104°C) was compared to that of ethane (boiling point — 89°C) (IS). By traditional criteria physical adsorption of ethane should be greater than that of ethylene, and the comparison of the relative adsorption should let us assay what fraction of the ethylene adsorption is physical. 

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