Adsorption on a surface

In this assumption, the singlet oxygen adsorption on a surface may be considered a quasistationary process and described by the Langmuir Adsorption Isotherm. Then the equilibrium constant is... 

Fig. 27a-c. Conformations of cylindrical brushes upon adsorption on a surface a extended coil b two-dimensional helix c globule... 

However, it is also the case that slopes different from mr 1 can occur even under equilibrium conditions, due to the nature of the molecular interactions involved in adsorption on a surface. For example, Goss and Schwarzenbach (1998) propose a modified formulation of Eq. (VV) that more explicitly takes into account the interactions between the SOC and the surface on a molecular level. Thus, based on work by Goss (1997), they express the relationship between a gas-particle partition coefficient K fds, defined as the adsorbed con-... 

Figures la and lb are typical curves found for the heat of immersion of polar solids in water (and are also foimd for polar and nonpolar solids in organic liquids). An example of Fig. la is found in the immersion of chrysotile asbestos having known and increasing amounts of physically adsorbed water on its surface (S ). The linear relationship between the heat of wetting and the volume adsorbed up to about a monolayer is significant and indicates surface homogeneity since the heat evolved is proportional to the amount of bare surface present. In accord with this finding, the isosteric heat values calculated from adsorption isotherms increased with coverage to a maximum near the monolayer as expected for adsorption on a surface possessing nearly uniform sites.

Turner G., Sachara J., McKinley J., and Smith S. C. (1996) Surface-charge properties and UO adsorption on a surface smectite. Geochim. Cosmochim. Acta 60, 3399-3414. 

Figure 10.4. A diagram of multilayer adsorption on a surface (A) and the resulting BET adsorption isotherm (B). P is the saturation vapor pressure of the adsorbing gas.

STM and AFM have both proved to be powerful tools for obtaining information on the packing order of molecular adsorption on a surface. ... 

In a practical sense the effects described in the foregoing discussion place a severe limitation on the applicability of spectral studies of adsorbed molecules to the detailed elucidation of the adsorption process and of the stereochemistry involved in surface catalysis. Since the absorption intensity may be either enhanced or decreased as a result of adsorption on a surface, and may either increase or decrease with variation in surface coverage, it becomes very difficult indeed to use spectral data as a measure of the surface concentration of adsorbed species. This is of particular importance when more than one species occupies the surface e.g., physisorbed and chemisorbed species. In this case the absolute concentration of either species on the surface cannot be measured directly nor can it be reliably inferred from a comparison of the intensity of the bands corresponding to these two species. Moreover, in the identification of an adsorbed species the relative intensities of two or more bands characteristic of that species e.g., the CH stretching and the CH deformation frequencies for adsorbed hydrocarbons, cannot be used as evidence for the structure of the adsorbed species since the absorption coefficients of the individual bands may change in opposite directions as a function of surface coverage. Thus the relative intensities of such bands cannot be compared to the relative intensities of the same bands observed in solution or in the gas phase. A similar difficulty arises when attempts are made to use the electronic spectra of adsorbed molecules to complement the infrared spectra for identification purposes. 

An important step in the consideration of surface reactions is the equilibrium level of adsorption on a surface. In terms of the example A -f S A S, the rate is shown to be reversible and equilibrated, so the rate of adsorption equals the rate of desorption. It is convenient to think of this as a process of dynamic equilibrium, where the net rate of change is zero. From equations (3-3) and (3-4) we have... 

Hydrogels are highly hydrated 3D physically or chemically crosslinked polymer networks. Smart hydrogels can respond to environmental stimuli such as light, temperature, electric or magnetic fields, pH, ions, and chemical or biochemical molecules. Here, the responses may include gelation, reversible adsorption on a surface, and alteration between hydrophilic and hydrophobic states [114, 115],... 

Equations (6.13) and (6.16) were developed to describe adsorption on a surface of topological dimension Aop = 2, in three-dimensional space d = 3>. Equation (6.17) can also be generalized to the case of an arbitrary topological and space dimension [3]. This generalization may be important because, in the case of the adsorption on fractal soil materials including clusters of aggregated colloids, polymeric substances (e.g. humic substances), etc., Aop = 1 rather than Aop = 2. Finally, it should be noted... 

Equating eqs. (2.5-8) and (2.5-9) yields the following adsorption isotherm for the case of mobile adsorption on a surface... 

Most of the above problems can be overcome by using immobilized enzymes in other words, enzymes which are confined in a well-defined region of space by means of a selective membrane, or immobilized by, for example, absorption or entrapment within the polymeric matrix of a membrane. Enzymes are prohibited, due to their molecular size, from diffusing out or permeating through the membrane, while substrates and products can readily permeate the membrane. The enzymes retain their catalytic properties and can be repeatedly and continuously used. Traditional immobilization techniques are summarized in Rg. 1.1. They include adsorption on a surface, covalent binding to an insoluble support, co-polymerization with a proteic carrier, encapsulation in a membrane shell and confinement in a gel. A comparison between some different techniques is also reported in Table 1.2. Hollow fibre membranes are commonly used for membrane reactors... 

The concept of the spreading pressure as the appropriate intensive variable for the discussion of surface adsorption was first introduced by Gibbs. However, the Gibbs formulation implies a two-dimensional adsorbed phase which is unnecessarily restrictive. Essentially the same thermodynamic relations may be developed in terms of the surface energy, and this approach has the advantage that it is equally applicable to two-dimensional adsorption on a surface or three-dimensional adsorption in a microporous solid. 

The energy of adsorption on a surface atom increases with increasing coordinative unsaturation of the surface metal atom(s). This agrees with ideas proposed by the Bond Order Conservation Principle, which would indicate that the strength of the chemical bond increases when the number of atoms which share bonds to different adorbates decreases. 

The simplest model of adsorption on a surface is that in which localized adsorption takes place on an energetically uniform surface without any interaction between adsorbed molecules. When surface coverage or fractional filling of the micropore is 0 (=gfgo) and the partial pressure in the gas phase, p, which is to be replaced by C = pIRT) when the concentration in the fluid phase is used, the adsorption rate is expressed as Kp 1 — 0) assuming first order kinetics with desorption rate given as keff. Then equilibration of adsorption rate and desorption rate gives the equilibrium relation as... 

Figure 7.1 Schematic models of protein adsorption on a surface - Secondary adorption grafted with polymer brushes.

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