Free energy of adsorption

Thermodynamics information of the adsorption process at infinite dilution can be obtained from the retention volume. At infinite dilution, the standard free energy to transfer 1 mol of adsorbate from the gas phase to the surface at standard state, defined as the variation in the standard free energy of adsorption, (J/mol), can be expressed as  

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Some representative plots of entropies of adsorption are shown in Fig. XVII-23, in general, T AS2 is comparable to Ah2, so that the entropy contribution to the free energy of adsorption is important. Notice in Figs. XVII-23 i and b how nearly the entropy plot is a mirror image of the enthalpy plot. As a consequence, the maxima and minima in the separate plots tend to cancel to give a smoothly varying free energy plot, that is, adsorption isotherm. 

Fig. XVII-23. (a) Entropy enthalpy, and free energy of adsorption relative to the liquid state of N2 on Graphon at 78.3 K (From Ref. 89.) b) Differential entropies of adsorption of n-hexane on (1) 1700°C heat-treated Spheron 6, (2) 2800°C heat-treated, (3) 3000°C heat-treated, and (4) Sterling MT-1, 3100°C heat-treated. (From Ref 18.)...

FIG. 9 Solvent contribution to the free energy of adsorption of I on Pt(lOO) at different temperatures as indicated. (From Ref. 164.)... 

Finally, two sets of physical properties have been correlated by the Hammett equation. Sharpe and Walker have shown that changes in dipole moment are approximately linearly correlated with ct-values, and Snyder has recently correlated the free energies of adsorption of a series of substituted pyridines with u-values. All the reaction constants for the series discussed are summarized in Table V. 

In the absence of specific interactions of the receptor - ligand type the change in the Helmholtz free energy (AFadj due to the process of adsorption is AFads = yps - ypi - Ysi, where Yps, YPi and ys, are the protein-solid, protein-liquid and solid-liquid interfacial tensions, respectively [5], It is apparent from this equation that the free energy of adsorption of a protein onto a surface should depend not only of the surface tension of the adhering protein molecules and the substrate material but also on the surface tension of the suspending liquid. Two different situations are possible. 

The free energy of adsorption, /16r Ds is potential-dependent and, in the case of an adsorbate which is an uncharged molecule,... 

The term free enthalpy of adsorption is synonymonsly used for free energy of adsorption and Gibbs energy of adsorption. 

In the ease of the reactive chemisorption the electrode redox potentials assigned to the chemisorption step represent the thermodynamic free energy of adsorption according to AGad - n F Em- This can be visualized by eonsidering the example of the reactive adsorption of an n-aUcanethiolate on a silver electrode surfaee. The reaction is... 

In Fig. 3.15, the calculated shift in free energy of adsorption due to the electric field is shown for various intermediates of interest for the ORR on Pt(l 11). The effect of the electric field was taken into account explicitly by adding an external electric field to the... 

The solvent strength peuT2uieter (c ) is defined as the free energy of adsorption of the solvent per unit of surface area. Its magnitude, therefore, is dependent on the particular adsorbent... 

Figure 40 (a) Plot of free energy of adsorption of n-alkane probes vs carbon number 110°C,... 

In addition to the specific adsorption of ions, it is also possible to adsorb neutral organic molecules at the interface and both Butler and Frumkin independently arrived at equivalent expressions for the free energy of adsorption of a neutral dipolar species, A, replacing the solvent, S. The free energy change takes the form ... 

The idea that the surface will adsorb A with essentially the same enthalpy for all values of 0 is clearly very simplified and attempts have been made to establish more realistic isotherms. Perhaps the best known of these is the Temkin isotherm which is based on the notion that the magnitude of the enthalpy of adsorption would decrease as the coverage increases. If the free energy of adsorption at 0 = 0 is AG , then the simplest assumption is that... 

If the surface is heterogeneous, consisting of minute patches whose free energy of adsorption differs incrementally. 

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 adsorption of fatty acids on the non-polar hydrophobic surface (Hg) is dominated by their hydrophobic properties. The extent of adsorption increases with increasing chain length. The following relationship of the free energy of adsorption, AGacis. and the number of C atoms, nc, of the fatty acids can be established ... 

The mechanisms of adsorption of organic solutes—including hydro-phobic, polar, and ionic species—onto surfaces have been summarized previously (2 5). Assuming that the various adsorptive mechanisms act independently, the free energy of adsorption (AGa[Pg.192]

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