Adsorption shapes

Fig. 10. The ESR signal produced at various points on the resonant line in a magnetic field modulated spectrometer. The vertical magnetic field modulation interacts with the bell-shaped adsorption curve [F(H)1 to produce the horizontal ESR signal. Here AH is the half amplitude line width and Hu is the center of resonance (S3).

Finally, one must be aware of possible modifications of the sample during the adsorption process itself. This is what was shown to happen when adsorbing a nonionic surfactant (nonyphenoloxyethylene, with 9-10 ethoxy groups) on kaolin in the presence of 1% NaCl at 40°C. In this case one step only was visible in the normal L-shaped adsorption isotherm whereas two steps were seen in the microcalorimetric recording. The first was attributed to the displacement of water by the surfactant and the second to a partial opening and hydration of the sheet-like structure of kaolin under the action of surfactant and salt (Rouquerol and Partyka, 1981). 

The rotary bed adsorber (also called adsorption wheel) provides a truly continuous TSA system. It uses a shallow wheel-shaped adsorption bed that continuously turns about an axis inside a fixed supporting frame. A section of the wheel is continuously used for adsorbing impurities from a gas while the other section is continuously regenerated by heating it with an impurity free gas. The adsorbent is made from a honeycomb-shaped alumina substrate that can be coated with layers of silica gels, activated carbons, or zeolites [14], It has been used for gas dehumidification, solvent vapor recovery, VOC removal, and deodorization of a gas stream. 

The integral exchange enthalpy isotherm calculated from the flow microcalorimetric measurements is shown in Fig. 12. The course of the isotherm - in case of a U-shaped adsorption excess isotherm - is determined by the composition of the interfacial phase ( ) according to the following equation[45-47] ... 

A two-step adsorption mechanism has been proposed (Gu and Zhu, 1990 Gu, 1992) for the various types of S-shaped adsorption isotherms (non-Langmuir) that are sometimes obtained. In the first step, the surfactant molecules are adsorbed as individual molecules or ions. In the second step, the adsorption increases dramatically as surface aggregates form through interaction of the hydrophobic chains of the surfactant molecules with each other. 

FIGURE 2-11 S-shaped adsorption isotherm for an ionic surfactant on an oppositely charged substrate. 

Adsorption of the cationic compounds on organic soil colloids resulted in isotherms which were L-shaped (Figure 6, bottom). Giles et al. 71) suggest that L-shaped adsorption isotherms are the most common type and occur when the adsorbent has a moderately high aflBnity for the solute. They are characterized by a curvilinear response at all concentrations used. The isotherms often seem to level oflF at a certain adsorption maximum. Adsorption of the cations was accompanied by a... 

For a pore of slit shape, adsorption will occur on two surfaces of the pore when pressure increases. As the pressure approaches a certain pressure, the adsorbed layers of the two opposing walls meet and this pore will be no longer available for... 

In summary, just the acceptance of the fact that commercially available surfactants are mixtures of various molecular weight surfactants may explain the unusually shaped adsorption isotherms that have been observed. 

FIG. 4 Schematic representation of the enthalpy of displacement isotherm in binary mixtures in the case of U-shaped adsorption excess isotherms. 

On a more detailed level, it is observed that, in agreement to previous studies, the models perform worse for the total amount adsorbed than for the adsorption isotherms (with the error almost doubled in the latter case). As expected, none of the models can predict well those cases with S-shape adsorption curves, e.g. acetone/hexane and methanol/hexane in MS-Carbon-5A. 

The varied mass action constants in Fig. 27 represent the relative strength of the nonionic-surface site eomplexation bond. The varied combinations of mass action constants for a single nonionic solute adsorbing on all three types of surface site produces many different shaped adsorption curves. As noted previously for different solutes, greater adsorption occurs when the mass action constant for a particular type of solute-surface site complex is largest. 

Figure 11.2 Schematic representation of particle deposition on heterogeneous surfaces bearing disk-shaped adsorption sites (a) and spherically-shaped sites (b).

Figure 11.4 (a) Dependence of the jamming coverage of particles 0 on the coverage of the disk-shaped adsorption sites the points denote the results of numerical simulations, performed for V = ( ) 10, (2) 5, (3) 2 and (4) 1 dashed lines show the results derived from the equation =X s- (From Ref. [51].) (b) Dependence of jamming coverage of particles... 

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