Adsorption on solid surfaces

Solving these three equations yields (before rounding for significant figures) 

Adsorbate equilibrium data on a specific adsorbent are often taken at a specific temperature and are referred to as adsorption isotherms. These functions or plots relate X , the maximum mass of adsorbate i that can be held by a unit mass of the adsorbent, to c, or pi. the concentration or partial pressure of adsorbate / in the fluid contacting the solid. 

Consider how an isotherm might be determined for the system carbon tetrachloride and activated carbon. 

Data resulting from a series of such experiments might appear as shown in Table 6.7-1. 

Chapter 16 of Perry s Chemical Engineers Handbook (see footnote i) gives physical properties of several important adsorbents and several different expressions for adsorption isotherms. Equilibrium data for specific adsorbent-adsorbate systems may be found in published articles, adsorbent manufacturers specification sheets, or company records. If no data can be found, isotherms must be obtained experimentally. 

9 ADSORPTION ON SOLID SURFACES 7.9a The Langmuir Equation Theory 

Throughout most of this chapter we have been concerned with adsorption at mobile surfaces. In these systems the surface excess may be determined directly from the experimentally accessible surface tension. At solid surfaces this experimental advantage is missing. All we can obtain from the Gibbs equation in reference to adsorption at solid surfaces is a thermodynamic explanation for the driving force underlying adsorption. Whatever information we require about the surface excess must be obtained from other sources. 

If a dilute solution of a surface-active substance is brought in contact with a large adsorbing surface, then extensive adsorption will occur with an attendant reduction in the concentration of the solution. To meet the requirement of a large surface available for adsorption, the solid —which is called the adsorbent — must be finely subdivided. From the analytical data 

TABLE 7.2 Some Familes of Commercial Surfactants and Specific Examples from Each 

Name of series General chemical nature Specific designation ( ) and chemical nature of example Example 

The power spectrum can be expressed in terms of the surface profile by substituting Eq. (2.87) into (2.90)  

So far we have considered the structure and properties of solid surfaces which are clean, i.e., they do not contain any foreign atoms or molecules. Such surfaces can be prepared under UHV conditions. However, in many practical cases, surfaces contain atoms and molecules from the surrounding gas phase which form more or less strong bonds with surface atoms. This phenomenon is called adsorption. Depending on the nature of the forces between the gas atoms or molecules (adsorbate) and the surface (adsorbent) one recognizes physical adsorption (or physisorption) and chemical adsorption (or chemisorption). 

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Bound moisture in a solia is that hquid which exerts a vapor pressure less than that of the pure hquid at the given temperature. Liquid may become bound by retention in small capillaries, by solution in cell or fiber walls, by homogeneous solution throughout the sohd, and by chemical or physical adsorption on solid surfaces. 

Adsorption is the preferential concentration of a species at the interface between two phases. Adsorption on solid surfaces is a very complex process and one that is not well understood. The surfaces of most heterogeneous catalysts are not uniform. Variations in energy, crystal structure, and chemical composition will occur as one moves about on the catalyst surface. In spite of this it is generally possible to divide all adsorption phenomena involving solid surfaces into two main classes physical adsorption and chemical adsorption (or chemisorption). Physical adsorption arises from intermolecular forces... 

If the data fit this model, a plot of 1/F0 versus 1/(S0) should be linear with a slope K/Vmax and intercept l/Vmax. It is analogous to that used in determining the constants in the Langmuir equation for adsorption on solid surfaces. Other forms that may be used to prepare linear plots are... 

In general, there is an array of equilibrium-based mathematical models which have been used to describe adsorption on solid surfaces. These include the widely used Freundlich equation, a purely empirical model, and the Langmuir equation as discussed in the following sections. More detailed modeling approaches of sorption mechanisms are discussed in more detail in Chap. 3 of this volume. 

Adsorption on solid surfaces requires the same information about the structure of the adsorbates and the adsorption site and configurations. 

King, D. A., and Woodruff, D. P. Adsorption on Solid Surfaces. Amsterdam Elsevier, 1983. 

In the Chapter 7, formation of monolayers in air-liquid interfaces and the resulting film pressure and phase transitions are discussed. This chapter also includes a brief discussion of adsorption on solid surfaces from solutions. 

The problem of the interaction of an atomic system with the surface of a metal at large distances is of significant interest for the theory of gas and vapor adsorption on solids surfaces. Just as in the interaction of two atomic systems, the universal attraction at large distances for neutral and non-polar systems may be obtained only in the second approximation of perturbation theory [1], Hitherto, only one attempt has been made in this direction, but the untenability of the assumptions, methods, and results of this attempt were obvious [2]. 

In recent years, new insights have been gained by employing advanced analytical techniques (solid-state NMR, FTIR, XPS) for the study of silane adsorption on solid surfaces and for the characterization of the substrate/coupling agent interphase, particularly for metal substrates. 

Cartalade D, Vemhet A. 2006. Polar interactions in flavan-3-ol adsorption on solid surfaces. J Agric Food Chem 54 3086-3094. 

In developing his later views on the social practice of science, Polanyi explicitly drew upon his earlier career experiences in the 1920s and 1930s. Two series of investigations at the boundaries of chemistry and physics were prominent among his examples of scientific practice and the distribution of merit within the scientific community. I turn now to these two cases the potential theory of adsorption on solid surfaces and X-ray studies of the solid state. 

The increase in the hydrophilic head group size reduces the amount of adsorbed surfactant at surface saturation. On the other hand, increasing the hydrophobic tail length may increase, decrease or maintain the surfactant adsorption. If the surfactant molecules are not closely packed, the increase in the chain length of the tail increases surfactant adsorption on solid surfaces. If the adsorption of surfactant on the solid surface is due to polarisation of tc electrons, the amount of surfactant adsorbed on the surface reduces at surface saturation. If the adsorbed surfactants are closely packed on the solid surface, increasing the chain length of the surfactant tail will have no effect on the surfactant adsorption. 

Soviet physicists and chemists have been reporting research on the peculiarities of adsorption on solid surfaces and the apparent deviations from the derived laws expressing adsorption phenomena, e.g., deviations from the Langmuir laws. S. Z. Roginskil has treated the nonuniformity of the surface and adsorption under conditions of limited extent of coverage of the surface in a number of articles. 

The UV/Vis absorption of solute molecules can be influenced not only by surrounding solvent molecules but also by other environments, e.g. embedment in solids, polymers, glasses, micelles, or proteins, as well as adsorption on solid surfaces. Therefore, the more general term peri-chromism (from the Greek peri = around and chroma = colour) has been recommended (Prof. E. M. Kosower, Tel Aviv/Israel, private communication to the author). Solvatochromic shifts caused by dye inclusion into protein interiors have been called enzymichromism [438]. 

It may be good to note here that various molecular cross-sections have now been considered. In the treatment of adsorption on solid surfaces was introduced. Interpreting this area in terms of lattice models is not a property of the adsorptive molecule but of the adsorbent. It is possible to imagine a situation where greatly exceeds the real molecular cross-section. On the other hand, for mobile monolayers on homogeneous surfaces is the real molecular cross-section or, for that matter, it is the excluded area per molecule. To avoid an undue abundance of symbols we have used the same symbol for both situations, for instance in table 3.3 in sec. 3.4e. It is to be expected that a and a, obtained by compression of monolayers, are more similar to the a s for adsorbed mobile monolayers on homogeneous substrates than to those for localized monolayers. 

The chemical behaviour of a given species strongly depends on the nature of the other molecules involved in the interaction. For a specific type of reaction, an appropriate model is needed to simulate the chemical environment of the species of interest. In the present work, the interest is focused on the initial response of the molecule to a particular type of chemical situation, independent of the value of those parameters that characterize one specific reaction. In other words, the intrinsic capabilities of the chemical species are studied and modelled as derivatives of the electronic properties with respect to an appropriate independent variable. For example, in those processes where charge transfer is involved (such as Lewis acidity and basicity, electrophile-nucleophile interactions and coordination compounds), the number of electrons must be an independent variable when a small molecule interacts with a very large counterpart (such as enzyme-substrate interaction and adsorption on solid surfaces), the chemical potential of the large partner will be imposed on the small molecule, and its number of electrons will not be independent. 

Scheutjens-Fleer (SF) Theory. A conceptual model for the effects of NOM on colloidal stability can be developed by using existing theoretical and experimental investigations of polymer and polyelectrolyte adsorption on solid surfaces and of the effects of macromolecules on colloidal stability. The modeling approach begins with the work of Scheutjens and Fleer for uncharged macromolecules, termed here the SF theory (3-5). This approach has been extended to the adsorption of linear flexible strong polyelectrolytes by van der Schee and Lyldema (6), adapted to weak polyelectrolytes (7-9), and applied to particle-particle interactions (8, 10). 

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