Adsorption theories

This theory relies on the orientation of surface forces resulting in van der Waals interactiOTi. There are three general types of van der Waals forces (a) Keesom, (b) Debye, and (c) London. 

The two main theories of catalysis are (i) intermediate compound formation theory and (ii) adsorption theory. 

According to this theory, the catalyst reacts with one of the reactants to give an intermediate, which reacts with another reactant to yield products and the catalyst as follows  

The heterogeneous catalysis e.g. gaseous reaction on a solid surface, is explained by this theory as follows  

Generally, following four steps are involved in the heterogeneous catalysis  

the surface provides the chances for reactants to combine and form activated complex and accelerate the rate of reaction. An increase in the surface area (increasing peaks, cracks, corners) of the catalyst increases the rate of reaction. 

Donor-acceptor bonds Bronsted acid-base interactions Up to 1000 

Secondary bonds Hydrogen bonds Hydrogen bonds involving fluorine Hydrogen bonds excluding fluorine van der Waals bonds Up to 40 10-25 

Permanent dipole-dipole interactions Dipole-induced dipole interactions Dispersion (London) forces 4-20 Less than 2 0.08  

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Isotherms of the type now characterized as Type IV have played an essential part in the development of adsorption theory and practice, as being the first kind of isotherm to be studied in detail. Already in 1888, half a century before the BDDT classification had appeared, van Bemmelen had... 

The mechanisms of adhesion are explained by four main theories mechanical theory, adsorption theory, diffusion theory, and electrostatic theory. 

There are various theories on how passive films are formed however, there are two commonly accepted theories. One theory is called the oxide film theory and states that the passive film is a diffusion-barrier layer of reaction products (i.e., metal oxides or other compounds). The barriers separate the metal from the hostile environment and thereby slow the rate of reaction. Another theory is the adsorption theory of passivity. This states that the film is simply adsorbed gas that forms a barrier to diffusion of metal ions from the substrata. 

In view of the fact that there are two opposing views on the mechanism of passivity it is not surprising that a similar situation prevails concerning the mechanism of breakdown of passivity. The solid film theory of passivity and breakdown of passivity is dealt with in some detail in Section 1.5, so that it is appropriate here to discuss briefly the views based on the adsorption theory. 

This relationship is in accordance with the hydroxyl adsorption theory where polarisation is decreased with increasing hydroxyl concentration... 

Use the adsorption theory (of Section 2-1-3) to explain why adsorptive stripping voltammetry results in nonlinear calibration plots. 

To evaluate the time-dependent function, X(t), a simple model of diffusion is proposed. Starting from Langmuir adsorption theory, we consider that liquid molecules having diffused into the elastomer are localized on discrete sites (which might be free volume domains). In these conditions, we can deduce the rate of occupation of these sites by TCP with time. Only the filhng of the first layer of the sites situated below the liquid/solid interface at a distance of the order of the length of intermolecular interaction, i.e., a few nanometers, needs to be considered to estimate X(t). 

Adsorption Theory, Modeling, and Analysis, edited by Jozsef Toth... 

We investigate theoretically how the adsorption of the polymer varies with the displacer concentration. A simple analytical expression for the critical displacer concentration is derived, which is found to agree very well with numerical results from recent polymer adsorption theory. One of the applications of this expression is the determination of segmental adsorption energies from experimental desorption conditions and the adsorption energy of the displacer. Illustrative experiments and other applications are briefly discussed. 

An illustrative example is the work of Clark et al, on the conformation of poly(vinyl pyrrolidone) (PVP) adsorbed on silica 0). These authors determined bound fractions from magnetic resonance experiments. In one instance they added acetone to an aqueous solution of PVP in order to achieve theta conditions for this polymer. They expected to observe an increase in the bound fraction on the basis of solvency effects as predicted by all modern polymer adsorption theory (2-6), but found exactly the opposite effect. Their explanation was plausible, namely that acetone, with ability to adsorb strongly on silica due to its carbonyl group, would be able to partially displace the polymer by competing for the available surface sites. 

Recent polymer adsorption theories, such as those of Roe (3) and of Scheutjens and Fleer (h) allow the calculation of displacement isotherms, so that we could study the dependency of these isotherms on various parameters by numerical methods. However, all the essential features of displacement can also be demonstrated by means of a simple analytical expression for the critical point, which can be derived in a straightforward way. 

A major advance in adsorption theory generalized the treatment of monolayer adsorption and incorporated the concept of multilayer adsorption. This is known as the BET theory after Brunauer, Emmett and Teller [32], The adsorption of a gas on a solid surface can be described by... 

Physical adsorption, theory of, 4 211 PhysisorpUon, 26 356-358 Piezo-QEXAFS, 42 330-344 Pillared clays, 33 345-346... 

The adsorption theory states that the bioadhesive bond formed between an adhesive substrate and tissue or mucosae is due to van der Waals interactions, hydrogen bonds, and related forces. Alternatively, when mucus or saliva are interacting with a solid dosage form, the molecules of the liquid are adsorbed on the solid surface. This is an exothermic process. The free energy of adsorption is given by Eq. (1). 

Although the problem of the liquid membrane potential was solved in principle by Nemst, a discussion developed in the ensuing two decades between Bauer [6], who developed the adsorption theory of membrane potentials, and Beutner [10,11,12], who based his theories on Nernst s work. This problem was finaly solved by Bonhoeffer, Kahlweit and Strehlow [13], and by Karpfen and Randles [49]. The latter authors also introduced the concept of the distribution potential. 

The Gibbs adsorption theory (Birdi, 1989,1999, 2002, 2008 Defay et al., 1966 Chattoraj and Birdi, 1984) considers the surface of liquids to be monolayer. The surface tension of water decreases appreciably on the addition of very small quantities of soaps and detergents. The Gibbs adsorption theory relates the change in surface tension to the change in soap concentration. The experiments that analyze the spread monolayers are also based on one molecular layer. The latter data indeed conclusively verifies the Gibbs assumption (as described later). Detergents (soaps, etc.) and other similar kind of molecules are found to exhibit self-assembly characteristics. The subject related to self-assembly monolayer (SAM) structures will be treated extensively (Birdi, 1999). However, no procedure exists that can provide information by direct measurement. The composition of the surface of a solution with two components or more would require additional comments. 

If one adds an inorganic salt, such as NaCl, instead of detergent, then no foam is formed. Foam formation indicates that the surface-active agent adsorbs at the surface, and forms a TLF (consisting of two layers of amphiphile molecules and some water). This has led to many theoretical analyses of surfactant concentration (in the bulk phase) and surface tension (consequent on the presence of surfactant molecules at the surface). The thermodynamics of surface adsorption has been extensively described by the Gibbs adsorption theory (Chattoraj and Birdi, 1984). 

Further, the Gibbs adsorption theory has also been used for systems other than solutions (such as solid-liquid or liquidj-liquid2, adsorption of solute on polymers, etc.). In fact, the Gibbs adsorption theory will be applicable to any system in which adsorption takes place at an interface. 

H.H. Dubinin, V.V. Serpinsky and K.O. Murdmaa (Eds.), Adsorption and Adsorbents (Russ.) (Proc. 6th Allunion Conf. Adsorption Theory, Moscow, November 18-21, 1985), Nauka, Moscow, 1987. 

The deviations from the Szyszkowski-Langmuir adsorption theory have led to the proposal of a munber of models for the equihbrium adsorption of surfactants at the gas-Uquid interface. The aim of this paper is to critically analyze the theories and assess their applicabihty to the adsorption of both ionic and nonionic surfactants at the gas-hquid interface. The thermodynamic approach of Butler [14] and the Lucassen-Reynders dividing surface [15] will be used to describe the adsorption layer state and adsorption isotherm as a function of partial molecular area for adsorbed nonionic surfactants. The traditional approach with the Gibbs dividing surface and Gibbs adsorption isotherm, and the Gouy-Chapman electrical double layer electrostatics will be used to describe the adsorption of ionic surfactants and ionic-nonionic surfactant mixtures. The fimdamental modeling of the adsorption processes and the molecular interactions in the adsorption layers will be developed to predict the parameters of the proposed models and improve the adsorption models for ionic surfactants. Finally, experimental data for surface tension will be used to validate the proposed adsorption models. 

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