Adsorption fundamental concepts

In 1997, Decher reported a fundamental concept regarding the multilayer nanoassemblies composed of polycations and polyanions [24]. The polymer assemblies were prepared by an alternate adsorption process, as shown in Figure 6.5. If a substrate has an anionic charge on its surface then polycation could adsorb onto the surface as a first layer. Polyanion could subsequently adsorb onto the surface by immersing in a... 

These and other experiments led Langmuir to develop some fundamental concepts on adsorption. One of these is that the forces involved in adsorption are just as large as those in chemical compounds. These forces are usually short-range forces. If the adatom is removed from the surface... 

The fundamental concept of chemical kinetics is that of reaction mechanism. In the broad sense, the word mechanism ("detailed , "intimate ) is the comprehensive interpretation of all experimental data accumulated on the complex reaction process. In this mechanism, one should discriminate individual stages and reaction steps, give characteristics for intermediates, describe transition states of individual steps, provide energy levels of substances, etc. As far as catalytic reactions are concerned, one should characterize surface properties, examine the adsorption character, etc. "I want to know everything about a complex chemical reaction this is the way one must understand chemists when they speak about their intention to investigate a detailed mechanism. Whether it is possible to realize such good intentions at a modern theoretical and experimental level will be another question. 

THE FUNDAMENTAL CONCEPTS of the physics and chemistry of surfaces are considered starting with Langmuir s work on thermionic emission and adsorption on metal surfaces and ending with the simple surface or phase boundary that occurs at a p-n junction in a semiconductor. Future progress depends on measuring both physical and chemical changes on the same surfaces at the same time. 

The initial broad effort established basic foundations and served for further expansion while the principal tenets have remained valid to date. When reported in the open literature, the new large pore materials immediately attracted attention of the scientific community, especially in catalysis and adsorption fields. Subsequent effort by many groups led to unforeseen expansion not only in terms of the overall effort and abundance of materials and phenomena but also new directions pursued. There is probably close to 10,000 publications on the subject [1], This presentation will focus primarily on fundamental concepts and ideas related to synthesis. For detailed and comprehensive treatments one is referred to the review publications that have been appearing regularly [13-20], For up-to-date overview of state of the art several recent reviews are especially recommended [17-20]. 

We begin in Chapter 2 with a discussion of the chromatographic process, developing the separate concepts of (1) equilibrium distribution of sample between adsorbent and solvent (or gas) and (2) bed efficiency or theoretical plate number. These two factors are then related in a general way to the problem of separation, and the various techniques of adsorption chromatography are introduced in terms of the different separation problems they are intended to solve. Chapter 3 provides a general discussion of adsorption, emphasizing those fundamental concepts which will be necessary in the discussions of later chapters. The effect of sample size on separation is treated in Chapter 4, particularly the factors which affect isotherm linearity. Chapter 5 provides a complete treatment of bed efficiency in liquid-solid systems. The distribution of sample... 

Another special feature of SEC is that the process of interaction between the solutes and the stationary phase is non-stoichiometric. For this reason, the fundamental concept on exchange capacity or adsorption capacity of the column is no longer applicable, and the concentration of the sample solution can be selected arbitrarily. Only the solubility of sample components in the mobile phase can set a threshold for their concentrations. Naturally there is a limit for the volume of the sample... 

The adsorption and ordering of ions at the electrode/electrolyte interface to form the electrochemical double layer is a fundamental concept of electrochemistry. In-situ SXS has been used to study both anion adsorption [19] and cation adsorption [20]. 

Chapter 6 introduces another powerful technique for determining the rate parameters involved in an adsorption column. The principle of chromatographic measurement implicitly contains many fundamental concepts concerning dynamic performance of a column reactor. The mathematical treatments introduced in this chapter can easily be extended to cover more complicated dynamic operations. 

Only a few fundamental concepts will be summarized and discussed here, in that a detailed description of the thermodynamics of adsorption is out of the scopes of this Chapter. The interested reader is addressed to the exhaustive review on this subject published in 1992 by Cardona-Martinez and Dumesic [13]. 

The constant pattern concept has also been extended to circumstances with nonplug flows, with various degrees of rigor, including flow profiles in tubes [Sartory, Jnd. Eng. Chem. Fundam., 17, 97 (1978) Tereck et al., Jnd. Eng. Chem. Res., 26, 1222 (1987)], wall effects [Vortmeyer and Michael, Chem. Eng. ScL, 40, 2135 (1985)], channeling [LeVan and Vermeulen in Myers and Belfort (eds.). Fundamentals of Adsorption, Engineering Foundation, New York (1984), pp. 305-314, AJChE Symp. Ser No. 233, 80, 34 (1984)], networks [Aviles and LeVan, Chem. Eng. Sci., 46, 1935 (1991)], and general structures of constant cross section [RudisiU and LeVan, Jnd. Eng. Chem. Res., 29, 1054 (1991)]. 

In their description of metal ion adsorption, Benjamin and Leckie used an apparent adsorption reaction which included a generic relationship between the removal of a metal ion from solution and the release of protons. The macroscopic proton coefficient was given a constant value, suggesting that x was uniform for all site types and all intensities of metal ion/oxide surface site interaction. Because the numerical value of x is a fundamental part of the determination of K, discussions of surface site heterogeneity, which are formulated in terms similar to Equation 4, cannot be decoupled from observations of the response of x to pH and adsorption density. As will be discussed later, It is not the general concept of surface-site heterogeneity which is affected by what is known of x> instead, it is the specific details of the relationship between K, pH and T which is altered. 

Solubility and kinetics methods for distinguishing adsorption from surface precipitation suffer from the fundamental weakness of being macroscopic approaches that do not involve a direct examination of the solid phase. Information about the composition of an aqueous solution phase is not sufficient to permit a clear inference of a sorption mechanism because the aqueous solution phase does not determine uniquely the nature of its contiguous solid phases, even at equilibrium (49). Perhaps more important is the fact that adsorption and surface precipitation are essentially molecular concepts on which strictly macroscopic approaches can provide no unambiguous data (12, 21). Molecular concepts can be studied only by molecular methods. 

Chemisorption [9] is an adsorptive interaction between a molecule and a surface in which electron density is shared by the adsorbed molecule and the surface. Electrochemical investigations of molecules that are chemisorbed to electrode surfaces have been conducted for at least three decades. Why is it, then, that the papers that are credited with starting the chemically modified electrode field (in 1973) describe chemisorption of olefinic substances on platinum electrodes [10,11] What is it about these papers that is different from the earlier work The answer to this question lies in the quote by Lane and Hubbard at the start of this chapter. Lane and Hubbard raised the possibility of using carefully designed adsorbate molecules to probe the fundamentals of electron-transfer reactions at electrode surfaces. It is this concept of specifically tailoring an electrode surface to achieve a particularly desired goal that distinguishes this work from the prior literature on chemisorption, and it is this concept that launched the chemically modified electrode field. 

The concept of the surface diameter may be mostly used in the field of adsorption and reaction engineering, where the equivalent surface exposure area is important. The determination of the surface area depends on the method of measurements for example, permeametry can give a much lower area than does gas adsorption. The latter often includes the contribution of pore surface area, which is accessible to the gas molecules. The determination of particle surface area by gas adsorption is given in 1.2.2.4. The fundamentals of gas adsorption are further covered in 1.4.1. 

A fundamental advance in the theory of adsorption phenomena was made in 1916 when Langmuir suggested that adsorption was to be regarded as a chemical process occurring on an energetically uniform surface and that the adsorbed phase was a unimolecular layer of non-interactive molecules. However, it soon became impossible to describe some important phenomena of catalysis without denying the concept of a uniform surface. 

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