Principles of Adsorption Chromatography by Lloyd R. Snyder

By Lloyd R. Snyder

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If, for a given configuration of an adsorbed molecule, there is appreciable overlap of the van der Waals radii of any pair of nonbonded atoms (in the adsorbate or surface), significant steric hindrance exists and the net adsorption energy is decreased as a result. Relative steric hindrance (and decrease in adsorption energy) can be assumed to be greater the greater the overlap of van der Waal's radii of nonbonded atoms. 6 A in the planar configura­ tion (preferred for adsorption). Since the steric hindrance in this configura­ tion (planar) is quite large, the adsorption energy of 2-methyl biphenyl is correspondingly reduced (compared to that of 3- or 4-methyl biphenyl).

3-5оДпеп gives Kth — Px(I - 0) which rearranges to give Eq. (3-5). For adsorption of X from the liquid phase, the derivation of the Langmuir isotherm proceeds similarly. f X11, + SA^XA + S (1) (3-5c) where a molecule of X in the liquid phase "reacts" with a site-solvent adsorption complex SA to give adsorbed X (XA) and a molecule of solvent S in the liquid phase. Again the thermodynamic equilibrium constant for the above "reaction" can be defined in terms of the activities of "reactants" and "products": Kth = ^ (3-5d) Oxd,a Standard states are again defined in terms of concentrations at small sample concentrations: The mole fraction of X in the liquid phase Nx equals ax, the mole fraction of S in the liquid phase 1 — Nx equals as, and the mole fractions of adsorbed sample and solvent в and 1 — 0 equal the activities axa and am, respectively.

It has been suggested (35) that as pore diameter is reduced, and the curvature of the surface within the (cylindrical) pore is increased, a point will eventually be reached where the adsorption of a planar molecule becomes inhibited because the planes of adsorbate and adsorbent are no longer matched. This effect should be more pronounced for wider adsorbate molecules. Thus the average distance separating a curved surface and a planar adsorbate molecule will be greater for wide molecules, and the resulting interaction energy must therefore be less.

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