Factors in Adsorption and the Formulation of Isotherms

The main interactions leading to adsorption at electrode interfaces are the following  

The electrode field will also tend to cause orientation of (a) solvent dipoles and (b) the reactant molecule if it is polar, or has a polar functional group within it. Also, in molecules where conformational fluctuations are possible, conformations having higher dipole moments will be preferred in the doublelayer field at the electrode interface. Examples are molecules such as dioxane and morpholine. 

Factor (b), the orientation of a functional group, may be important in the kinetics of reduction of such polar reactants as alkyl halides or ketones in the latter case, the orientation of the functional group will be the significant factor as discussed by Elving and Pullman.  

At finite temperatures, a distribution of orientations will be involved and the mean orientation will be determined by a function involving the ratio of the energy of the dipole group in the electrode field, to the thermal energy kT, For an angle of orientation 6 to the electrode surface, the energy We of interaction with the field will be cos 6 and the distribution function for orientation will be  

A more elaborate theory of orientation effects was developed by Watts-Tobin and Mott and applied by Bockris, Devanathan, and Muller and Bockris and Habibto the calculation of isotherms for adsorption of organic substances. Solvent molecules were regarded as being present in the double layer in two orientations, up or down (cf. theories of ferromagnetism) at respective surface coverages of N or N[. The sum iVf + is equal to the total coverage of the electrode by solvent molecules. 

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