Energy of adsorption, critical

The theory of adsorption at porous adsorbents predicts the existence of a finite critical energy of adsorption e, where the macromolecule starts to adsorb at the stationary phase. Thus, at > the macromolecule is adsorbed, whereas at e < e the macromolecule remains unadsorbed. At e = Ec the transition from the unadsorbed to the adsorbed state takes place, corresponding to a transition from one to another separation mechanism. This transition is termed critical point of adsorption and relates to a situation, where the adsorption forces are exactly compensated by the entropy losses TAS = AH [2, 7]. Accordingly, at the critical point of adsorption the Gibbs free energy is constant (AG = 0) and the distribution coefficient is Kj = 1, irrespective of the molar mass of the macromolecules. The critical point of adsorption relates to a very narrow range between the size exclusion and adsorption modes of liquid chromatography. It is, therefore, very sensitive towards temperature and mobile phase composition. 

Chemisorption may be rapid or slow and may occur above or below the critical temperature of the adsorbate. It is distinguishable, qualitatively, from physical adsorption in that chemical specihcity is higher and that the energy of adsorption is large enough to suggest that full chemical bonding has occurred. Gas that is chemisorbed may be difficult to remove, and desorption may be... 

The theory of polymer adsorption is complicated for most situations, because in general the free energy of adsorption is determined by contributions from each layer i where the segment density is different from that in the bulk solution. However, at the critical point the situation is much simpler since the segment density profile is essentially flat. Only the layer immedia-... 

The application of liquid chromatography at the critical point of adsorption to block copolymers is based on the consideration that Gibbs free energy AGab of a block copolymer AnBm is the sum of the contributions of block A and block B, AGa and AGB respectively. 

An optimum catalysis of the oxygen eleetrode reaction depends on a critical compromise of adsorption energies of the intermediates. It is, therefore, not surprising that no electrocatalyst has been found where this reaction occurs reversibly at ambient temperatures. In both directions rather large overvoltages are required to reduce O2 to H2O or to oxidize H2O to O2. 

This equation delivers minimum Heniy coefficients. Dealing with polar adsorptive molecules and adsorbents with electrical charges such as NH3 (yU, = 5x10 ° Cm) and CHF3 (p = 5.34 x 10" Cm), the Henry coefficient can be by a factor up to 2,000 higher than the minimum value based only on the Hamaker energy and critical data of adsorptive molectrles. 

In work by Bockris and his co-workers [1, 10, 119-123, 137, 138] the dependence of reversible adsorption of organic substances on the potential at platinum metals was interpreted on the basis of the assumption that there is competition between the organic molecules and the water molecules for sites on the surface and that the standard free energy of adsorption of water depends on the electric field at the electrode — solution interface. This approach has been criticized [9,139] from the point of view of the analysis made... 

There is no reason why the distortion parameter should not contain an entropy as well as an energy component, and one may therefore write 0 = 0q-sT. The entropy of adsorption, relative to bulk liquid, becomes A5fi = sexp(-ca). A critical temperature is now implied, Tc = 0o/s, at which the contact angle goes to zero [151]. For example, Tc was calculated to be 174°C by fitting adsorption and contact angle data for the -octane-PTFE system. 

The quantity zoi will depend very much on whether adsorption sites are close enough for neighboring adsorbate molecules to develop their normal van der Waals attraction if, for example, zu is taken to be about one-fourth of the energy of vaporization [16], would be 2.5 for a liquid obeying Trouton s rule and at its normal boiling point. The critical pressure P, that is, the pressure corresponding to 0 = 0.5 with 0 = 4, will depend on both Q and T. A way of expressing this follows, with the use of the definitions of Eqs. XVII-42 and XVII-43 [17] ... 

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