Solvent adsorption affinity constants

For a given chromatographic system and a series of different substituted phenols, the parameters a and b are constant, since 0 and for each of the substituted phenols (in a vertical configuration) is constant. Equation (1 l-2a) is tested in Fig. 11-4 against the data for one of the solvent systems studied by Bark and Graham. A resonable correlation of R versus (7 is observed. Although the data of Fig. 11-4 show more scatter than those of Fig. 1 l-3(a), in all but two cases the adsorption affinity of the substituted... 

The D/R adsorption isotherm has a theoretical basis which is outside the scope of this book. Its use requires three types of information (1) a calculation of the thermodynamic amount of non-mechanical (i.e., chemical) work associated with a quantum of adsorption, without regard to the specific solvent or the specific adsorbent involved, but with regard to the relative volume of solvent available for adsorption (2) independently determined empirical constants — known as affinity constants, noted by the symbol P — which allow extrapolation from one solvent to another and (3) a characteristic energy of adsorption — noted by the symbol AEq — which is primarily based on the characteristic dimensions of the micropores. 

Adsorption isotherms are used to quantitatively describe adsorption at the solid/ liquid interface (Hinz, 2001). They represent the distribution of the solute species between the liquid solvent phase and solid sorbent phase at a constant temperature under equilibrium conditions. While adsorbed amounts as a function of equilibrium solute concentration quantify the process, the shape of the isotherm can provide qualitative information on the nature of solute-surface interactions. Giles et al. (1974) distinguished four types of isotherms high affinity (H), Langmuir (L), constant partition (C), and sigmoidal-shaped (S) they are represented schematically in Figure 3.3. 

Thus, deviations from the ideal Langmuir isotherm can be caused both by intermolecular interactions, which result in an enthalpy of mixing, and by area differences between molecules, which produce a non-ideal entropy of mixing [18]. For a simple case where the interactions are of the Frumkin type and the partial molar areas of solvent and surfactant are constant the entropic effect of area differences results in typical features of macromolecular adsorption, e.g., a steep initial increase of adsorption ( high affinity adsorption) and a very slow rise once the surface is approximately half filled [18]. 

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