Free enthalpies of adsorption

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

Obviously AGad depends on the strength of the solvent-surfaee and the adsorbate-surfaee interaction in addition S-S-, A-A- and S-A-interactions contribute. Various simplifications are possible [74Tra]. As a further complication the electrical field being always present (except at at the electrode/solution-interface has to be considered. Consequently the free enthalpy of adsorption can be split into a chemical part and an electrical part ... 

Proper definition of the reference state is essential for the compatibility of values of the free enthalpy of adsorption results in values of obtained by various authors and with different methods [87Jas,... 

Table 4.1. Free enthalpies of adsorption of ions and organie moleeules at metal eleetrodes.

Closely eonneeted to this aspeet is the proeess of adsorption at eleetrode surfaees, whieh is a eompetetive proeess like at any other solid/liquid surfaee where a solvent moleeule is replaeed by an adsorbate moleeule. Compared to eommon adsorption proeesses at solid/liquid interfaees the strong eleetrie field at eleetroehemieal phase boundaries adds an influenee of utmost importanee. The strength of the interaetion between the eleetrode and the adsorbed speeies ean be expressed by the free enthalpy of adsorption. Employing a variety of methods many data have been reported, they are listed in this volume. 

Beside O P D it is well known that metal deposition can also take place at potentials positive of 0. For this reason called underpotential deposition (UPD) it is characterized by formation of just one or two layer(s) of metal. This happens when the free enthalpy of adsorption of a metal on a foreign substrate is larger than on a surface of the same metal [ 186]. This effect has been observed for a number of metals including Cu and Ag deposited on gold ]187]. Maintaining the formalism of the Nernst equation, deposition in the UPD range means an activity of the deposited metal monolayer smaller than one ]183]. 

The free enthalpy of adsorption is determined by the enthalpy as well as the entropy of adsorption. The... 

Frumkin was the pioneer of Russian electrochemistry [14], Frumkin assumed that there is some interaction between the adsorbates. The free enthalpy of adsorption is then AG = AGo + ys. If the adsorbates attract one another, then y > 0. If they repel, then y < 0. In the case that y = 0, we recover the Langmuir result. The Frumkin isotherm [15] is written as... 

Extending the approach of Dorris and Gray [6] to the PDMS oligomers, we may calculate an increment of free enthalpy of adsorption corresponding to a monomer unit, AGa, according to ... 

Finally, the adsorption will depend on the standard free enthalpy of adsorption ... 

Further information is obtained if the amount of liquid adsorbed on the surface of the particle is also determined, permitting the combination of the data on heat of immersion with those on the amount of adsorbed liquid. Thus, molar adsorption enthalpies can be given for the characterization of the stabilizing adsorption layer [12-16]. A further benefit of adsorption excess isotherms is that it is possible to calculate from them the free enthalpy of adsorption as a function of composition. When these data are combined with the results of calorimetric measurements, the entropy change associated with adsorption can also be calculated on the basis of the second law of thermodynamics. Thus, the combination of these two techniques makes possible the calculation of the thermodynamic potential functions describing adsorption [14,17-19]. 

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