Adsorption on a Solid Surface

Whatever the nature of the forces holding a solid together, it can be regarded as producing a field of force around each ion, atom, or molecule. At the surface of the solid, these forces can not suddenly disappear and thus reach out in space beyond the surface of the solid. Due to these unsaturated and unbalanced forces, the solid has a tendency to attract and retain on its surface molecules and ions of other substances with which it comes into contact. Thus, when a solid surface comes in contact with a gas or a liquid, the concentration of the gas or liquid is always greater on the surface of the solid than in the bulk gas or liquid phase. The process by which this surface excess is created is called adsorption. The balance of the forces is partially restored by the adsorption of the gas or the liquid on the surface of the solid. The substance attached to the surface is called adsorbate, and the substance to which it is attached is known as the adsorbent. 

Adsorption of a gas on a solid or liquid surface is a spontaneous process. It is, therefore, accompanied by a decrease in free energy of the system. Furthermore, the gaseous molecules in the adsorbed state have fewer degrees of freedom than in the gaseous state. This results in a decrease in entropy during adsorption. Using 

Depending upon the nature of the forces involved, the adsorption is of two types physical or van der Walls adsorption, and chemisorption or chemical adsorption. In the case of physical adsorption, the adsorbate is bound to the surface by relatively weak van der Walls forces identical with molecular forces of cohesion that are involved in the condensation of vapors into liquids. Chemisorption, on the other hand, involves exchange or sharing of electrons between the adsorbate molecules and the surface of the adsorbent, resulting in a chemical reaction. The bond formed between the adsorbate and the adsorbent is essentially a chemical bond and is thus much stronger than in physical adsorption. 

The type of adsorption that takes place in an adsorbate-adsorbent system depends upon the reactivity of the surface, the nature of the adsorbate, the nature of the adsorbent, and the temperature of adsorption. For example, the adsorption of oxygen on active carbon is physical adsorption to a large extent at temperatures below -100°C, and it is chemisorption at room temperature and above. When it is not certain that the process of adsorption is either physisorption or chemisorption, or when both are occurring in appreciable proportions, then it is preferable to use a less committal term sorption. 

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A more complex but more versatile separation method is chromatography, a technique widely used in teaching, research, and industrial laboratories to separate all kinds of mixtures. This method takes advantage of differences in solubility and/or extent of adsorption on a solid surface. In gas-liquid chromatography, a mixture of volatile liquids and gases is introduced into one end of a heated glass tube. As little as one microliter (10-6 L) of sample may be used. The tube is packed with an inert solid whose surface is coated with a viscous... 

Practical applications of surfactants usually involve some manner of surfactant adsorption on a solid surface. This adsorption is always associated with a decrease in free-surface energy, the magnitude of which must be determined indirectly. The force with which the adsorbate is held on the adsorbent may be roughly classified as physical, ionic, or chemical. Physical adsorption is a weak attraction caused primarily by van der Waals forces. Ionic adsorption occurs between charged sites on the substrate and oppositely charged surfactant ions, and is usually a strong attractive force. The term chemisorption is applied when the adsorbate is joined to the adsorbent by covalent bonds or forces of comparable strength. 

Let v5 be the number or moles of solute molecules adsorbed per area of surface. The subscript on v refers to the case of adsorption on a solid surface as contrasted with v used in the case of protein-ligand equilibria 4-47 49). Let [A] be the equilibrium solute concentration. 

We extend our description to adsorption at the solid-liquid interface. For many systems we can use the same models as for gas adsorption on a solid surface, we only have to replace the pressure P by the concentration c. The adsorption of macromolecules to surfaces is briefly discussed in Section 10.3.2. For macromolecules desorption is often negligible and thermodynamic equilibrium is only reached after a very long time, if at all. 

The apparent rate constant in (2.10), which is obtained by multiplying a true rate constant kc and the square root of an equilibrium constant, Keq, can show a law of dependence on temperature different from the simple Arrhenius law. In some cases, even a negative temperature dependence can be observed. Moreover, if both mechanisms (2.6) and (2.7)-(2.8) are active in parallel, the observed reaction rate is the sum of the single rates, and an effective reaction order variable from 1 /2 to 1 can be observed with respect to reactant A. Variable and fractionary reaction orders can be also encountered in heterogeneous catalytic reactions as a consequence of the adsorption on a solid surface [6],... 

At one time Taylor considered7 that if adsorption on a solid surface would catalyse the interconversion of ortho and para hydrogen, it was an indication that chemisorption with dissociation into atoms occurred the temperature at which such interconversion took place would (if this were true) have been some indication of the activation energy for chemisorption. No doubt chemisorption does promote this spin isomerization but it is now known that adsorption of the van der Waals type can result in speedy attainment of the equilibrium mixture of ortho and para hydrogen, provided that the surfaces are of some paramagnetic substance the close proximity to a paramagnetic surface can catalyse the interconversion,8 without dissociation of the adsorbed molecules. 

Adsorption on a solid surface is the process of a species present in a gas or liquid phase adhering to the surface of the solid [46,104,105], This adsorption occurs due to molecular interactions between the adsorbing species and the solid. If adsorption is characterized by relatively weak interactions, such as those typical of van der Waals forces, the process is called physisorption. Because such weak forces occur between all molecules, physisorption is typically reversible and will occur at any surface when the normalized concentration of the adsorbing species is sufficiently high. For a gas-phase species, the normalized concentration is equal to pipa where p is the partial pressure of the species and po is its saturation vapor pressure. The endpoint for physisorption occurs when the concentration of the adsorbing species reaches its saturation value. For a gaseous contacting phase, condensation occurs at this point, (i.e., when p = po). 

FIGURE 9.3. In multilayer adsorption on a solid surface the first adsorbed layer may be physically adsorbed or chemisorbed. Subsequent layers will be physically adsorbed. 

Irving Langmuir developed an equation for adsorption on a solid surface under low pressure, where the number of adsorbate molecules present is not sufficient to cover the solid surface. At equilibrium. 

The adsorption on a solid surface, the types of adsorption, the energetics of adsorption, the theories of adsorption, and the adsorption isotherm equations (e.g., the Langmuir equation, BET equation, Dubinin equation, Temkin equation, and the Freundlich equation) are the subject matter of Chapter 2. The validity of each adsorption isotherm equation to the adsorption data has been examined. The theory of capillary condensation, the adsorption-desorption hysteresis, and the Dubinin theory of volume fllhng of micropores (TVFM) for microporous activated carbons are also discussed in this chapter. 

Figure 4.26 shows the effect of the interaction parameter BpQ on the isotherm shape. Positive values of BpQ, corresponding to repulsive interactions (see Equation 4.65), cause the coverage, for fixed activity, to decrease as Bp increases. On the contrary, negative (attractive) BpQ cause faster adsorption as the activity increases notice that for BpQ < -4, a sharp rise in is observed, which can be interpreted as a bidimen-sional phase transition (Hill 1986) even when for localized adsorption on a solid surface, this concept is questionable, it means here that the adsorbate molecules are binding preferentially next to others already adsorbed. Of course, one can question for these cases the validity of the assumption of random adsorbate distribution, but nevertheless the prediction is important, as this is the simplest isotherm equation predicting such phenomena. 

Gas-solid chromatography actually preceded gas-liquid chromatography by several years. The two techniques are more or less similar to each other, the primary differenee being that the partition within the column is caused by adsorption on a solid surface rather than solubility in a liquid phase. Since the techniques are similar in most aspects, the instrumentation for GSC is identical to GLC with very minor differences. 

Exponentially decaying orbital are required for accurate representation of the atom-atom interactions involved in molecular adsorption on a solid surface. The present application involves CO adsorption on copper. This modifies the carbon partial charge so that it becomes the seat of nucleophilic attack. 

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