Adsorption Isotherms and Site Distribution

The bi-Langmuir model (equation (4)) or tri-Langmuir model, the sum of two or three Langmuir isotherms, correspond to models that assume the adsorbent surface to be heterogeneous and composed of two or three different site classes. Finally the Freundlich isotherm model (equation (5)) assumes no saturation capacity, but instead, a continuous distribution of sites of different binding energies. 

Depending on the template-functional monomer system, the type of polymer, the conditions for its preparation and the concentration interval covered in the experiment, the adsorption isotherms of MIPs have been well fitted with all the isotherm models [24,25,41,30], 

To determine the adsorption isotherm, the equilibrium concentrations of bound and free template have to be reliably measured within a large concentration interval. Since the binding sites are part of a solid this experiment is relatively simple. Thus, it can be done in a batch equilibrium rebinding experiment or by frontal analysis. 

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The application of the composite isotherms enables us to model the ion exchange on heterogeneous surfaces, such as rocks and soils. When the structure and composition of the sorbent is well known, we can choose the most probable site affinity distribution function. If not, it is desirable to fit the composite isotherm by different models. The just-described four isotherms provide an opportunity for this. In addition, when adsorption and ion exchange can take place simultaneously, adsorption and ion-exchange isotherms and site distribution functions can be combined (Cernik et al. 1996). 

The Freundlich isotherm and the distribution coefficient K,i) adsorption models assume an infinite number of sorption sites are available, whereas the Langmuir i.sotherm and ion-exchange models assume a limited or maximum number ol sorption sites. Write sorption reactions that correspond to each of these models and explain the above statements in terms of those reactions. 

More specific information on the nature of active sites can often be deduced from adsorption isotherms and heat of adsorption distribution measurements (by calorimetry or GSC) with individual reactant gases or vapours. These can be backed up with many of the spectroscopic techniques referred to above, to characterize the nature of the adsorbed layer and chemisorbed species, and their effects on the structure and oxidation state of the surface itself Acidity is frequently an important parameter in the chemisorption process or general catalytic behaviour. The extent (and heat) of adsorption of various... 

Most calculations of f(Q) for a heterogeneous surface, using an adsorption isotherm assume a patchwise distribution of sites. Explain for what kind of local isotherm functions,/((2,P, T) this assumption is not necessary, and for which it is necessary. Give examples. 

Results of adsorption experiments for butylate, alachlor, and metolachlor in Keeton soil at 10, 19, and 30°C were plotted using the Freundlich equation. A summary of the coefficients obtained from the Freundlich equation for these experiments is presented in TABLE IV. Excellent correlation using the Freundlich equation over the concentration ranges studied (four orders of magnitude) is indicated by the r values of 0.99. The n exponent from the Freundlich equation indicates the extent of linearity of the adsorption isotherm in the concentration range studied. If n = 1 then adsorption is constant at all concentrations studied (the adsorption isotherm is linear) and K is equivalent to the distribution coefficient between the soil and water (Kd), which is the ratio of the soil concentration (mole/kg) to the solution concentration (mole/L). A value of n > 1 indicates that as the solution concentration increases the sorption sites become saturated, resulting in a disproportionate amount of chemical being dissolved. Since n is nearly equal to 1 in these studies, the adsorption isotherms are nearly linear and the values for Kd (shown in TABLE IV) correspond closely to K. These Kd values were used to calculate heats of adsorption (AH). 

As has been depicted in Fig. 1, various conformations are possible for adsorbed polymers, depending on polymer-polymer, polymer-solvent, and polymer-interface interactions and the flexibility of polymers. To determine experimentally the conformation of adsorbed polymers only adsorption isotherm data are insufficient. The average thickness of the adsorbed polymer layer, the segment density distribution in this layer, the fraction of adsorbed segments, and the fraction of surface sites occupied by adsorbed segments must be measured. Recently, several unique techniques have become available to measure these quantities. 

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