Bi layers

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. 

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,... 

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... 

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...

It is clear that such a surface offers a wide range of sorption and displacement processes that can take place between the solute and the stationary phase surface. Due to the bi-layer formation there are three different surfaces on which a molecule can interact by sorption and three different surfaces from which molecules of solvent can be displaced and allow the solute molecule to penetrate to the next layer. During a chromatographic separation under these circumstances, all the alternatives are possible. Nevertheless, depending on the magnitude of the forces between the solute molecule and the molecules in each layer, it is likely that one particular type of interaction will dominate. The various types of interaction are included in Figure 11. 

Figure 11. The Different Types of Solute Interaction that can Occur on a Silica Surface Containing a Solvent Bi-layer...

Bi-layer angular movement during current flow, 348, 349, 350 Biological processes, mimicked, 425 Bipolaronic bands as a function of oxidation depth, 342 Bipolar iron-selective film, 226 Bismuth... 

This will be true for all solvent mixtures where there is no association between solvents and is useful to remember when adjusting solvent composition to obtain the desired selectivity and retention. Unfortunately, when one solvent is polar, a bi-layer of solvent can be formed on the surface and solute/solvent interactions with the silica gel are even more complex. 

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 ... 

Scott and Kucera fitted their data to equations (2) and (3), solved for the constants and reconstructed the isotherms for each layer which are included in figure 3. The formation of a solvent bi-layer is diagramatically represented in figure 4. 

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. 

Presently, several clinical trials with liposome-encapsulated agents are under way and more are planned (Zonneveld and Crommelin, 1988 Klausner, 1988). During the last 5 years, key issues related to the pharmaceutical manufacturing of liposomes such as stability, sterilization, upscaling, and reproducibility have been successfully addressed. Although it is generally believed that a proper selection of the bi-layer components can minimize the occurrence of toxic effects due to the use of natural body constituents, the issue of liposome-related toxicity is not a trivial one and should be carefully studied,... 

Reactor 27 [R 27] [Bi-layer Contactor High-aspect-ratio Heat Exchanger] -Reaction System... 

Figure 4.27 Schematic of the bi-layer contacting reaction platelet and photographs of details of the transfer region micro channel-collecting channel (left top) and of the array of the collecting channels (left bottom) [56].

Reactor type [Bi-layer contactor high-aspect-ratio heat exchangerj-reaction system Residence times at 750 ml h 1-10 s 1 ml h for different 14 min - 2 h combinations of plates... 

In conclusion, both catheters appear similar in terms of construction and polymeric materials, although the infrared spectra suggest that a different nylon may have been used for each, and that the inner polyethylene-based bi-layers have been co-extruded from different polyethylenes. Another variation noted was the slight difference in thickness of layers between the two samples. 

As it follows from Table 5, many catalysts contain metallic platinum. We have developed bi-layer porous hydrophobic air electrodes, which do not contain platinum metals, are active and can be cycled [24, 25] (Figures 4-6). These bifunctional catalysts are pyrolized Co - macrocyclic compounds. Said catalyst has high catalytic activity for the oxygen reduction and also features acceptable stability, however its activity for the oxygen evolution is not high enough. 

If transition temperatures other than those allowed by superconducting pure metals are required, two metallic layers can be deposited to form a bi-layer TES. In most cases, only one of the two metals is a superconductor. In this case, the Cooper pairs diffuse from... 

Huang W and Gopalan S. Bi-layer structures as solid oxide fuel cell interconnections. J. Power Sources 2006 154 180-183. 

Zhang X, Robertson M, Deces-Petit C, Xie Y, Hui R, Qu W et al. Solid oxide fuel cells with bi-layered electrolyte structure. J. Power Sources 2008 175 800-805. 

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