The Adsorption Phenomenon

The strict definition for osmotic pressure is in terms of molarity, tt = MRT. Osmotic pressure, tt, has the units of pressure (atmospheres) M (mol/L) and T (kelvins). 

When we use the approximation that M = we might think that the units do not balance. For veiy dilute aqueous solutions, we can think ofM as being numerically equal to m, but stUl having the units mol/L, as shown above. 

Sports drinks were developed to counteract dehydration while maintaining electrolyte balance. Such solutions also help to prevent cell dam e due to the osmotic pressure that would he caused hy drinking laige amounts of water. 

The scattering of light from automobile headlights by fogs and mists is an example of the Tyndall effect, as is the scattering of a light beam in a laser show by dust particles in the air in a darkened room. If the air didn t have any suspended solid particles you would not be able to see the laser beam. 

Much of the chemistry of everyday life is the chemistry of colloids, as one can tell from the examples in Table 14-4. Because colloidal particles are so finely divided, they have a tremendously high total surface area in relation to their volume. It is not surprising, therefore, that an understanding of colloidal behavior requires an understanding of surface phenomena. 

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Not pursuing the objective to describe the adsorption phenomenon in detail, we would like, however, to dwell briefly on its principle ideas as well as on main theoretical and experimental results necessary for further understanding of the concept of the method of semiconductor sensors. 

When this happens the adsorption phenomenon follows the so-called Langmuir isotherm and the current intensity at the maximum of the forward peak is correlated with the parameter To (that represents the quantity of Ox, in mol m-2, initially adsorbed on the electrode surface) and with the scan rate according to the relationship ... 

The active sites of CBH II (or Core II), on the other hand, participated in the adsorption phenomenon both onto (semi)crystalline (Avicel) and amorphous cellulose. 

Table 6.10 gives a list of several isotherms, including the ones already discussed, and some of their main characteristics. The isotherms we have presented here are focused on particular aspects of the adsorption phenomenon. For example, the Langmuir isotherm focuses on the basic process of transferring a molecule from the bulk to the electrode the Temkin isotherm focuses on interpreting the adsorption process in terms of the heterogeneity of the adsorbing surface etc. 

Many calculated figures of adsorption energies given in the literature which, because of their close approximation to experimental values, are said to prove that a supposed force is responsible for the adsorption phenomenon, are in fact fallacious, because of the improbably low figures for the distances used in obtaining these calculated results (13). 

The adsorption phenomenon described so far deals with the adsorption of a molecule at an adsorbent surface site. It is possible for a molecule to be adsorbed at two or more sites. For example, in a molecule with two or more functional groups, each functional group may be adsorbed on one site. Suppose that a molecule is adsorbed at two different surface sites. The rate of adsorption is written as ... 

Although in a number of cases chromatography is used in packed-bed mode, there are an increasing number of examples of the use of solid phases in fluidized-bed mode. This does not change the adsorption phenomenon based on the complementarity of the solute for the solid phase. Fluidized-bed columns are essentially used to resolve specific problems related to the feedstock, as detailed later. 

The Gibbs representation provides a simple, clear-cut mode of accounting for the transfer of adsorptive associated with the adsorption phenomenon. The same representation is used to define surface excess quantities assumed to be associated with the GDS for any other thermodynamic quantity related with adsorption. In this way, surface excess energy (U°), entropy (Sa) and Helmholtz energy (Fa) are easily defined (Everett, 1972) as ... 

Whatever quantity is used to determine the amount adsorbed (pressure, mass or gas flow), one has to choose between the two basic procedures, discontinuous and continuous. In the past there has been some misunderstanding about the significance of these terms. We prefer to focus attention on the adsorption phenomenon itself, as will be made clear in the following sections. 

The results summarized in this review represent the early stages of an ongoing research effort at 3M, which in effect is a three-dimensional approach to surface chemistry , i.e. an area of science between surface chemistry and solution chemistry. The sorption sites on the polymer are not representable as an immobile surface between solid and liquid domains, but rather are string-like polymer molecules undulating in a sea of liquid, so that the adsorption phenomenon must be represented three-dimensionally on a molecular scale rather than two dimensionally. 

The type II isotherm is associated with solids with no apparent porosity or macropores (pore size > 50 nm). The adsorption phenomenon involved is interpreted in terms of single-layer adsorption up to an inversion point B, followed by a multi-layer type adsorption. The type IV isotherm is characteristic of solids with mesopores (2 nm < pore size < 50 nm). It has a hysteresis loop reflecting a capillary condensation type phenomenon. A phase transition occurs during which, under the eflcct of interactions with the surface of the solid, the gas phase abruptly condenses in the pore, accompanied by the formation of a meniscus at the liquid-gas interface. Modelling of this phenomenon, in the form of semi-empirical equations (BJH, Kelvin), can be used to ascertain the pore size distribution (cf. Paragr. 1.1.3.2). 

For both the continuous and non continuous methods, there are three possible types of experimental techniques that can be used, differing by the parameter sensitive to the adsorption phenomenon which is measured ... 

The BET model is used to determine the specific surface area of solids with the properties mentioned above. Thus, solids for which specific surface areas can be determined using this model are limited to meso- and macroporous solids. In the case of microporous solids, the adsorption phenomenon cannot be described using the above hypotheses. However, in the absence of a universally accepted model, the BET equation is usually used to calculate the specific surface of a microporous solid. 

Figure 1.12 Energy diagram showing the adsorption phenomenon.

In the study of the adsorption phenomenon by means of adsorption isotlierms it must also be realized that the initial concentration of the adsorbate, the carbon dosage, and particle size all influence the results obtained [3 ]. Some of the observed effects are to be attributed to the complex nature of humic substances so that their adsorption is a multicomponent process. 

The Tyndall Effect 14-17 The Adsorption Phenomenon 14-18 Llydrophilic and Hydrophobic Colloids... 

Recognize and describe colloids the Tyndall effect, the adsorption phenomenon, hydrophilic and hydrophobic colloids... 

One of the factors causing fouling in ultrafiltration membranes is the adsorption of solutes in the membrane pores. Since fouling, in general, has been discussed in the previous chapter, the discussion presented here will be restricted to the adsorption phenomenon. Clark et al. [37] studied the relationship between membrane fouling and protein adsorption on alumina ultrafiltration membranes. Equilibrium adsorption of bovine serum albumin (BSA) was measured by the standard static method at 7°C. Their study covered the concentration range between 1 and 10 g/1, pH values between 2 and 10 and NaCl... 

Another feature of adsorption from solution is the variety and complexity of molecules that may be involved in the processes. Indeed one can be interested either by a simple organic molecule, like benzene and its derivatives, or by much larger molecules like proteins, surfactants, or polymers, which bear many different chemical functions and may adopt a large number of conformations at the interface. For such molecules, a good knowledge of both the surface chemistry and the accessibility of porous materials are crucial to understand the adsorption phenomenon. 

The adsorption phenomenon of the stationary phase is substantially reduced once a large part of the adsorption sites are occupied. This creates an effect of elongation of the spots. As a result, the Rf (retardation factor) of a compound in the pure state is slightly different from the Rf of the same compound present in a mixture. 

The surface electron spectroscopies are powerful e >erimental methods for investigating the sorption process. This section deals with some of the important applications of APS to the study of the adsorption phenomenon of gases on transition metals. 

Surfactants are the compounds at an interface that reduce the interfacial tension. It follows from thermodynamics, that this property is due to the ability of these compounds to undergo the transfer from within an adjacent fluid (liquid or gas) phase to the interface. In fact, this ability is just the adsorption phenomenon. Clearly, an accumulation at the interface is possible for substances consisting of two parts, each of them separately exhibiting the affinity to one of the contacting phases. Such a property is characteristic of amphiphilic molecules possessing polar (hydrophilic) and nonpolar (lipophilic) parts. 

Thermodynamically, we do explain the adsorption phenomenon of a fluid at the surface from the Gibbs free energy. It should be spontaneous and, thus, A Gads < 0-However, the final entropy of the system diminishes too, since the disorder is lower when molecules are adsorbed, or AS < 0. 

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