Adsorption approach

Fig. 4.18 shows the result of Cd2+ adsorption on illite in presence of Ca2+ (Comans, 1987). The data are fitted by Freundlich isotherms after an equilibration time of 54 days. It was shown in the experiments leading to these isotherms that adsorption approaches equilibrium faster than desorption. Comans has also used 109Cd to assess the isotope exchange he showed that at equilibrium (7-8 weeks equilibration time) the isotopic exchangeabilities are approximately 100 % i.e., all adsorbed Cd2+ is apparently in kinetic equilibrium with the solution. The available data do not allow a definite conclusion on the specific sorption mechanism. 

It is difficult to make water-soluble peptides form complexes with titanium alkoxide, because they are not soluble in organic solvents. Therefore, such templates cannot be imprinted by the complexation approach. Instead, they could be imprinted in Ti02-gel films by the alternate adsorption approach with Ti(0 Bu)4. Figure 6.24a shows a plot of alternate layer-by-layer assembly of 100 mM titanium butoxide (3 min adsorption in toluene/ethanol) and 10 mM glycyl-L-tyrosine (Gly-L-Tyr, 10 min adsorption in water). The template molecule was removed by treatment with 10 mM aqueous sodium hydroxide, as... 

More activated carbon than this would be put into the container, for several reasons. First, since the rate of adsorption approaches zero as the adsorbent approaches saturation, it would take an infinite amount of time for the mole traction of CCU in the gas phase to reach 0.001. If more carbon is present, the target mole fraction would be reached in a finite amount of time (before the carbon becomes saturated). Second, the Langmuir isotherm is an approximate correlation with parameters obtained by fitting scattered experimental data, and so the estimated adsorption capacity of the adsorbent (X ) could be too high. Third, we have assumed that nothing but CCU is adsorbed on the carbon. If any oxygen, nitrogen, or other species that mav be present in the gas is adsorbed, it could lower the amount of carbon tetrachloride adsorbed. 

The primary differences between limiting conditions of solubility and Belenkii s critical condition of adsorption approach 4, 5) are the use of a thermodynamically poor (bad) eluent, or even an eluent that is a nonsolvent for the polymer, whereas the polymer is dissolved and injected in a thermodynamically good solvent. The limiting condition... 

The surface adsorption approach has been used to make a DPI device. First, a fatty acid or fatty alcohol derivative or a poloxamer is dissolved or dispersed in a solvent in which drugs and carriers are insoluble. The preferred solvent is n-hexane or... 

Figure 13.2. Evidence for reversibility for sorption on illite. Adsorption-desorption equilibrium for Cd(II) on illite after 54 days of equilibration. The solution contains HCO, 2 X 10 M Ca, and has a pH = 7.8. Freundlich isotherms are based on separate adsorption ( ) and desorption (o). Adsorption approaches equilibrium faster than desorption. The data do not allow a conclusion on the specific absorption mechanism. (Data are from Comans, 1987.)...

Kaneko et al. [25-28] have studied He adsorption of activated carbon at 4.2 K in order to develop a new characterization method for ultramicropores in stead of N2 adsorption. He adsorption almost finishes within 5 min in activated carbon samples having micropores of w > 0.7 nm regardless of the low temperature conditions. The He adsorption isotherm has a sharper uptake below P/Pq = 0.02, but the amount of He adsorption approaches to that of N2 adsorption with the increase of P/Pq- The He adsorption isotherm has no hysteresis. Fig. 4 shows the wide pressure range adsorption isotherms of He at 4.2 K... 

When the Gibbs adsorption approach is applied to any monolayer in which the Gibbs dividing plane is located, where n = 0 for the solvent excess, then we have, from Equation (225)... 

In this context the isosteric heat of adsorption approaching zero coverage Qo = Qiso 9 0) is another important quantity. Its experimental determination is more delicate and may be obscured by the fact that the initial adsorption may preferentially occur at surface defects (steps) or impurities due to a stronger binding of the noble gas atoms to these defects. In fact, the increase of Qi at very low coverage observed, e.g., on stepped or high index snrfaces allows to extract the adsorption energy of the defect sites and can even be used as a quantitative measure to determine the defeet concentration. 

Two principle methods of surface modification are well known they can be summarized as the physical adsorption method and the chemical immobilization technique of the organic modifier.Each method is experienced with certain advantages and disadvantages over the other. For example, the physical adsorption approach is commonly accomplished in a single-step reaction this requires less time for obtaining the final modified solid extractor, but the active donor centers or atoms in the physically modified phases may be consumed in the adsorption process. In addition, these modified phases were found to suffer from leaching or desorption processes under the influence of... 

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