Monomolecular adsorption layer

Extremely low interaction forces in monomolecular adsorption layers on liquids, Langmuir Blodgett films or in solid interfaces are the precondition for low energy surfaces. Perfluorocarbons or terminal fluorinated parts in amphiphilic molecules and fluoropolymers meet these demands. 

In the case of the formation of a monomolecular adsorption layer, the, fractional coverage 0 is often used instead of the adsorbed amount b to symbolize the extent of adsorption. It indicates the portion of the surface that is coated ... 

If a monomolecular adsorption layer is assumed, for the total surface area A we have... 

It is obvious that the revealed dependence (Figure 3.27) can be explained by the effect of the catalyst particle size decrease, when the polymer shell on their surface becomes close to the monomolecular adsorption layer. There is less probability of a cationic reaction between the internal double bonds of neighbouring macromolecules on a solid catalyst surface, which leads to the formation of crosslinked structures. This fact is confirmed by results in [45,46] the smaller the size of the catalyst particle, the bigger the polymer shell becomes (the thickness of the adsorption layer in a monomolecular layer is similar to the length of a macromolecule). Growing molecules on these particles are linked with the surface of the heterogeneous catalyst by one chain end. Revealed behaviour of the gel fraction, formed by linked macromolecules (a chemical gel ), will evidently be valid for the insoluble fraction of the chemically bound macrochains (physical gel ). 

In the case of large, compact particles, these force effects usually lead to the formation of monomolecular adsorption layers but they arc also able to form, in certain circumstances, adsorption films of several layers. In these films the binding strength of the individual solvent particle declines continuously from the surface of the adsorbent outwards, so that, in these cases, a variable adsorption or solvation potential has to be taken into account. 

If the gas A forms a monomolecular adsorption layer on the solid surface it can be treated as a dynamic process... 

The structure of the compact layer depends on whether specific adsorption occurs (ions are present in the compact layer) or not (ions are absent from the compact layer). In the absence of specific adsorption, the surface of the electrode is covered by a monomolecular solvent layer. The solvent molecules are oriented and their dipoles are distorted at higher field strengths. The permittivity of the solvent in this region is only an operational quantity, with a value of about 12 at the Epzc in water,... 

Only monomolecular chemisorbed layers are possible. Chemisorption is a specific process which may require an activation energy and may, therefore, be relatively slow and not readily reversible. The nature of physical adsorption and chemisorption is illustrated by the schematic potential energy curves shown in Figure 5.2 for the adsorption of a diatomic gas X2 on a metal M. 

Many adsorption isotherms are borderline cases between two or more of the above types. In addition, there are some isotherms which do not fit into Brunauer s classification at all, the most notable being the stepwise isotherms, an example of which is given in Figure 5.6. Stepwise isotherms are usually associated with adsorption on to uniform solid surfaces, each step corresponding to the formation of a complete monomolecular adsorbed layer (see page 133). 

Figure 5.6) shows at least two distinct steps, each corresponding to the formation of an adsorbed monomolecular layer. In most cases of multilayer physical gas adsorption, however, the adsorption energies are such that there is a greater or lesser tendency for the first monomolecular adsorbed layer to be completed prior to any adsorption into the second monolayer, but little tendency for the second monolayer to be completed prior to adsorption into the third and subsequent monolayers. 

Solute adsorption is usually restricted to a mono molecular layer, since the solid-solute interactions, although strong enough to compete successfully with the solid-solvent interactions in the first adsorbed monolayer, do not do so in subsequent monolayers. Multilayer adsorption has, however, been observed in a number of cases, being evident from the shape of the adsorption isotherms and from the impossibly small areas per adsorbed molecule calculated on the basis of monomolecular adsorption. 

Type I Correspond to monomolecular adsorption postulated by Langmuir. The volume of the gas adsorbed approaches a limiting value, just enough to complete a mono molecular layer even when the gas pressure is rather low. Further increase in pressure hardly produce any further rise in the amount of adsorption. The examples are furnished by adsorption of nitrogen or hydrogen on charcoal at temperatures close to -180 C. 

By introducing a number of simplifying assumptions, Brunauer, Emmett and Teller (1938) were able to extend the Langmuir mechanism to multilayer adsorption and obtain an isotherm equation (the BET equation), which has Type II character. The original BET treatment involved an extension of the Langmuir kinetic theory of monomolecular adsorption to the formation of an infinite number of adsorbed layers. 

For acetonitrUe/water systems it was found that acetonitrile forms thick adsorbed layer on the surface of hydrophobic bonded phase, while methanol adsorption from water formed a classical monomolecular adsorbed layer [166]. The thick adsorbed layer of acetonitrile provides a suitable media for the adsorption of liophilic ions on the stationary phase adding an electrostatic component to the retention mechanism, while monomolecular adsorption of methanol should not significantly affect adsorption of ions. 

According to Stem, the charge in the dense part of the EDL can be determined from Langmuir s theory of monomolecular adsorption, discussed in Chapter II. The work required to transfer the z -th ion from the solution bulk to the dense part of the EDL, Wi (x = d) should contain the term - = -(p0 - poS) )/NA, describing the purely adsorption interaction of the ion with the surface (see Chapter 11,2), and the term due to work against electrostatic forces of the interaction of the ion with a charged surface, z,eipd, where (p(/ is the dense layer potential. Thus, the term (p0 - PqS) )/NA in eq. (11.21) should be replaced with O, - z,ebulk concentration, n,(), can be expressed as... 

In the beginning of 1933 we published a paper entitled "The Heterogeneous Catalysis of the Para-Hydrogen Conversion" in which we summarized all our results obtained on metals, charcoals and salts (21). Our conclusion was that there are two mechanisms for the conversion. At low temperatures the conversion is monomolecular, occurs in the van der Waals adsorption layer and is probably caused by magnetic effects. At higher temperatures the conversion involves atomic adsorption. 

When A = 1, this expression reduces to Langmuir s equation for monomolecular adsorption. If 0 < A < 1, there is a finite maximum hygroscopic moisture content. While many textile fibers approach such a moisture content asymptotically at high relative humidities, some man-made fibers such as nylon and viscose appear to have well-defined maximum hygroscopic moisture contents [9]. In many cases, the coefficient k is greater than 1. Jaafar and Michalowski [8] interpret this behavior as the thermal effect of adsorption being equal to the heat of condensation only after a multimolecular layer has been formed. 

NOj(N02/N204) gas and the effect of gas adsorption was monitored using visible and infrared spectroscopy the high sensitivity surface-enhanced resonance-Raman spectroscopy (SERRS) allowed to observe the reversible chemical absorption of NO2 on a monomolecular LB layer of PrPc2 and PrPc. ... 

Diamond and sapphire differ in that they have lower than normal ju.f values in the area of 0.1, and its value depends on the load, as might be expected for materials that deform elastically rather than plastically. Such materials also begin to show surface damage beyond a certain load. Under very clean conditions, m for diamond has been found to rise to 0.6, suggesting that some mechanism such as the adsorption of a monomolecular water layer or slight surface oxide formation may act to lubricate the diamond surface naturally. 

Figure 11. Double-layer model for monomolecular oxide layer with chemisorbed oxygen ions, a, Potential distribution b, charge distribution c, equivalent circuit diagram with regard to adsorption equilibrium of oxygen ions. Symbols defined in text. ...

The term adsorption was proposed by E. du Bois-Reymond but introduced into literature by Kayser [78,79] and during the next few years the terms isotherm and isothermal curve were used for describing the results of adsorption measurements at a constant temperature. Kayser also developed some theoretical concepts which became a basis for the monomolecular adsorption theory. He assumed that the molecules on the solid surface possess free unsaturated attractive forces. Due to that they are able to attract and accumulate molecules of gas or liquid which form a monomolecular layer on the solid surface. According to Kayser, a pressure increase is favourable for adsorption but a temperature increase causes thermal vibrations which disturb and decrease adsorption. 

The BET theory, despite many restrictions, was the first attempt to create an universal theory of physical adsorption. It describes the whole course of the isotherm including the areas of monomolecular adsorption, polymolecular adsorption and capillary condensation. Unfortunately, the last area is described in an unsatisfactory way, particularly, when the solid is of a heterogeneous porous structure including pores of various capillary widths. This theory cannot be applied at temperatures higher than the critical temperature because it is not possible to assume equality of heats of adsorption and condensation as well as stability of the adsorption coefficients ratio with regard to individual adsorption layers. 

Equation (12) already incorporates the assumption that there exists a concentration range where the volume of the adsorption layer—on a constant surface area—is independent of the composition of the bulk phase. However, no statement can yet be made as to whether this layer is monomolecular or multilayered. 

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