The Forces that Cause Adsorption

As we shall sec in Secs. V,7,8 and VI,1, the forces that cause adsorption on metal surfaces are exercised by the adsorbent body as a whole, rather than by the constituent atoms. It is, therefore, important to know the... 

The force of repulsion, as we have seen, is due to the charges on the surface. Inherent in any body is a natural force that tends to bind particles together. This force is exactly the same as the force that causes adsorption of particles to an adsorbing surface. This is caused by the imbalance of atomic forces on the surface. 

The forces that cause adsorption at the liquid-gas phase are primarily non-bonding van der Waals forces and hydrophobic interactions. The adsorbed amount increases with the concentration of a substance in the Hquid phase and decreases with temperature increase. The action is described by the Gibbs adsorption isotherm. ... 

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. 

The Theories of adhesion discussed in the articles Adsorption theory of adhesion, Diffusion theory of adhesion, Eiectrostatic theory of adhesion and Mechanicai theory of adhesion aim to describe the forces that cause the adhesive and substrate to adhere the rheological theory is concerned with explanation of the values obtained for adhesion measured by destructive Tests of adhesion. 

The composition profile is approximated by a step profile, with a uniform composition xf in the surface layer (0bulk phase x, at z>L. It is assumed that the total amount of liquid can be divided into two parts with the first constituting the homogeneous bulk phase (mole numbers in it n° = til -I- 2) and the remainder standing under the influence of the forces emanating from the solid surface causing adsorption (mole numbers, referred to unit mass of adsorbent, = n, -i- 2 the superscript a referring to adsorption) [17]. Simple mass balance considerations lead to the following expressions [12] ... 

Colloidal particles are attracted and held on planar surfaces, or are repelled by such surfaces, by the same forces that cause attraction or repulsion between two colloidal particles. Some of the basic principles have been reviewed by Jirgensons and Strau-manis (Ref. 53, p. 101.). The adsorption of colloidal silica on alumina and colloidal alumina on silica was shown by Her.(402) to occur at about pH 4. Once the ionic charge on the surface has been covered by a single layer of colloidal particles of opposite charge, the covered surface then bears the charge of the adsorbed particles and no further adsorption occurs. 

Catalysts are highly porous particles and adsorption occurs primarily on the internal surface of the particles. The attractive forces that hold the molecule to the surface of the solid are the same that cause vapors to condense van der Waals forces). The adsorption process is classified as either physical or chemical. The basic difference between physical and chemical adsorption is the manner in which the molecule is bonded to the catalyst. In physical adsorption the molecule is bonded to the catalyst by weak forces of intermolecular cohesion. The chemical nature of the adsorbed species remains unchanged therefore, physical adsorption is a readily reversible process. In chemical adsorption a much stronger bond is formed between the molecule and the catalyst surface. A sharing or exchange of electrons takes place—as happens in a chemical bond. Chemical adsorption is not easily reversible,... 

Adsorption processes may be classified as physicid or chemical, depending on the nature of the forces involved. Physical adsorption, also termed van der Waals adsorption, is caused by molecular interaction forces the formation of a physically adsorbed layer may be likened to the condensation of a vapor to form a liquid. Ihis type of adsorption is only of importance at temperatures below the critical temperature for tbie gas. Not only is the heat of physical adsorption of the same order of magnitude as that of liquefaction, but physic ly adsorbed layers behave in many respects like two dimensional liquids. 

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. 

The weakness of the adsorbent-adsorbate forces will cause the uptake at low relative pressures to be small but once a molecule has become adsorbed, the adsorbate-adsorbate forces will promote the adsorption of further molecules—a cooperative process—so that the isotherms will become convex to the pressure axis. 

The colloid probe technique was first applied to the investigation of surfactant adsorption by Rutland and Senden [83]. They investigated the effect of a nonionic surfactant petakis(oxyethylene) dodecyl ether at various concentrations for a silica-silica system. In the absence of surfactant they observed a repulsive interaction at small separation, which inhibited adhesive contact. For a concentration of 2 X 10 M they found a normalized adhesive force of 19 mN/m, which is small compared to similar measurements with SEA and is probably caused by sufactant adsorption s disrupting the hydration force. The adhesive force decreased with time, suggesting that the hydrophobic attraction was being screened by further surfactant adsorption. Thus the authors concluded that adsorption occurs through... 

The existence of multiple peaks for molecular desorption has been attributed to lateral interactions among adsorbed species 62-64). As discussed previously, adsorption onto the surface lattice may occur preferentially in next nearest neighbor sites to form p(2 x 2) structures. Even at low coverages, attractive forces may cause adatoms to occupy next nearest neighbor positions, so that clusters of adsorbate form which have local twofold periodicity 65) with respect to the surface. Such effects are entirely consistent with the perturbations of the surface electronic wave functions due to adsorption 66-68) which show that these binding sites represent the... 

The adsorption of mixed surfactants at the air—water interface (monolayer formation) is mechanistically very similar to mixed micelle formation. The mixed monolayer is oriented so that the surfactant hydrophilic groups are adjacent to each other. The hydrophobic groups are removed from the aqueous environment and are in contact with other hydrophobic groups or air. Therefore, the forces tending to cause monolayers to form are similar to those causing micelles to form and the thermodynamics and interactions between surfactants are similar in the two aggregation processes. 

So far we have had an overview of the forces involved in the adsorption process for ions in solution. However, we have not yet said anything about how to determine these forces or how to describe the adsoibed state of the ion. Physical quantities such as pressure, volume, temperature, and amount of substance describe the conditions in which a particular material exists that is, they describe the state of a material. These quantities are interrelated and one cannot be changed without causing a change in one or more of the others. The mathematical relationship among these physical quantities is called the equation of state of the system. Well-known examples of equations of states for gases are the ideal gas (PV = nRT) and the virial [P = RT(n/V) + RTB2T n/vf +. ..] equations of state. 

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