Bi-layer adsorption

Let the concentration of solvent (B) in equilibrium with the silica gel surface be (c) g/ml. Let a fraction (a) of the surface be covered with a mono-layer of the polar solvent (B) and, of that fraction (a), let a fraction ( 3) be covered by a second layer of the polar solvent (B). The number of molecules striking and adhering to the surface covered with a mono-layer of polar solvent (A) and that covered with a mono-layer of solvent (B) per unit time will be (n ) and be (n ) respectively. Furthermore, let the number of molecules of solvent (A) leaving the mono-layer surface and the bi-layer surface per unit time be ni and 2 respectively. Now, under conditions of equilibrium, 

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This equation is the mono-layer adsorption isotherm of solvent (B) and is exactly the same as the previously derived Langmuir adsorption equation but somewhat differently expressed. 

The total area covered (S), expressed as fraction of the total area, will be 

Equation (7) is the Langmuir function for bi-layer adsorption. The expression gives a value for the total amount of moderator on the surface (Cg/g), which is measured 

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Consider the bi-layer adsorption of strongly polar solvent (B) (e.g., ethyl acetate) from a solution in a dispersive solvent (A) (e.g., n-heptane) onto a silica gel surface, as depicted in Figure 6. 

A theoretical curve for bi-layer adsorption was calculated from experimental data [3] and is given in Figure 7. The actual values obtained are superimposed on the... 

Figure 7. The Bi-layer Adsorption Isotherm of Ethyl Acetate on Silica Gel...

The identification of bi-layer adsorption of polar solvents on the surface of silica gel arose from some work by Scott and Kucera (5) who measured the adsorption isotherms of the some polar solvents, ethyl acetate, isopropanol and tetrahydrofuran from n-heptane solutions onto silica gel. The authors found that the experimental results for the more polar solvents did not fit the simple mono-layer adsorption equation and, as a consequence, the possibility of bi-layer adsorption on the silica gel surface was examined. 

Bi-layer adsorption is not uncommon and the development of the bi-layer adsorption isotherm equation is a simple extension of that used for the mono-layer equation. The Langmuir equation for bi-layer adsorption is as follows ... 

In contrast, the mono-layer of methanol is built up much more slowly and is not complete until the concentration of methanol in the aqueous mixture is about 35%w/v. The behavior of methanol on the reverse phase is reminiscent of the adsorption of chloroform on the strongly polar silica gel surface. The complementary nature of the silica gel surface and that of the reverse phase is clearly apparent. It is also clear that strongly dispersive solvents might form bi-layers on the reverse phase surface just as polar solutes form bi-layers on the highly polar surface of silica gel. In fact, to date there has been no experimental evidence furnished that would support the formation of bi-layers on the surface of reverse phases, although their formation is likely and such evidence may well be forthcoming in the future. 

Although monolayers at the gas-water interface are useful to study adsorption phenomena of e.g. proteins at membranes they are not a very good model, since they represent only one half of a biological membrane. Attempts have therefore been made to extend this concept of polymer monolayers to bilayers and particularly to liposomes. It was to prove, whether the monomers (Table I.) could form bilayers and whether a polyreaction within these bi-layers was possible under retention of the structure and the orientation of the molecules. 

Surface aggregates formed by ionic surfactant adsorption on oppositely charge surfaces have been shown to be bi layered structures (1.) and are called admicelles<2) in this paper, though they are sometimes referred to as hemimicelles. The concentration at which admicelles first form on the most energetic surface patch is called the Critical Admicellar Concentration (CAC) in analogy to the Critical Micelle Concentration (CMC), where micelles are first formed. Again, in much of the literature, the CAC is referred to as the Hemimicellar Concentration (HMC). 

Strictly thermodynamically, the electrosorption valency only shows that no other co-adsorption of other ions occurs. However, there may be a compact adsorption layer. STM gives images for the two surfaces (Figure 4.32). For Pb on Ag(l 11) the image was interpreted to be a so-called filled honeycomb 3(2X2) structure and a compressed hep layer. For Pb on Ag(lOO) a c(2X2) structure was observed. Otherwise the adsorbed mass was similar to the Ag(l 11) substrate which lead to the model of a bi-layer. 

Adsorption of cationic surfactants (cationic soaps) on silica has been studied by Ter-Minassian-Sarage (88) and by Bijsterbosch (89), who have shown that either a monolayer or a bi-layer may be formed. Similar complexities may exist with some micelle-forming dyes, and special conditions would have to be worked out for each adsorbate to eliminate confusion. Also, the adsorption of these ionic species is affected by pH and by aluminosilicate ion impurities on the silica surface. Use of cationic dyes and surfactants is generally restricted to rapid comparison of surface areas of a series of silica powders of the same type. 

In addition to homopolymers a large variety of other architectures can be used to achieve colloidal stabilization. We investigate here the interfacial behavior of diblock and random copolymers. Diblock copolymers adsorb[7,8] in a bi-layer structure which depends on several physico-chemical parameters like the surface alBnity, the solvent quality or the mutual incompatibility of the blocks. We discuss next the adsorption from a selective and a non-selective solvent. 

While the pzc of Hg in F solution has not changed by more than 1 mV for over 70 years, marginal variations are visible for Ga, Tl, In, Cd, Bi, Sn, and Sb that are related to electrolyte effects (weak specific adsorption or disturbance of the adsorbed water layer, as for Ga).847 Important variations can be seen, on the other hand, for polycrystalline Ag, Zn, Ni, Fe, and Cu. For all these metals a drop of the pzc to much more negative values has been recorded this is evidently related to an improvement in the preparation of the surface with more effective elimination of surface oxides. All these metals, with the exception of Ag, are naturally sensitive to atmospheric oxygen. Values of pzc for single-crystal faces first appeared in a 1974 compilation,23 in particular for the three main faces of Ag and for Au (110). Values for a number of other metals were reported in 1986.25 However, for sd-metals, an exhaustive, specific compilation of available experimental data was given by Hamelin etal. in 1983.24... 

The control experiment in pure supporting electrolyte (dotted lines in Fig. 13.2) shows a sharp faradaic current spike, which is mainly due to pseudocapacitive contributions (adsorption of (bi)sulfate and rearrangement of the double layer) plus oxidation of adsorbed Hupd (dotted lines in Fig. 13.2a), but no measurable increase in the CO2 partial pressure (m/z = 44 current) above the background level (dotted lines in Fig. 13.2b). Therefore, a measurable adsorption of trace impurities from the base electrolyte can be ruled out on the time scale of our experiments. Moreover, this experiment also demonstrates the advantage of mass spectrometric transient measurements compared with faradaic current measurements, since the initial reaction signal is not obscured by pseudocapacitive effects and the related faradaic current spike. 

Fabrication processing of these materials is highly complex, particularly for materials created to have interfaces in morphology or a microstructure [4—5], for example in co-fired multi-layer ceramics. In addition, there is both a scientific and a practical interest in studying the influence of a particular pore microstructure on the motional behavior of fluids imbibed into these materials [6-9]. This is due to the fact that the actual use of functionalized ceramics in industrial and biomedical applications often involves the movement of one or more fluids through the material. Research in this area is therefore bi-directional one must characterize both how the spatial microstructure (e.g., pore size, surface chemistry, surface area, connectivity) of the material evolves during processing, and how this microstructure affects the motional properties (e.g., molecular diffusion, adsorption coefficients, thermodynamic constants) of fluids contained within it. 

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