Adsorption isitherms for neutral organic compounds-a hierarchy in modelling: Part IV. Charge of maximum adsorption-Statistical mechanical basis

Abstract

The charge at which adsorption of orgamc compounds attains a maximum ( σ_max^M) at an electrochenucal interface is analysed using several multi-state models in a hierarchical manner The analysis is based on statistical mechamcal results for the following models (A) two-state site parity, (B) two-state muhl-slte, and (C) three-state site parity The coulombic interactions due to permanent and reduced dipole effects (using mean field approximation), electrostatic field effects and specific substrate interactions have been taken into account. The simplest model in the hierarchy (two-state site parity) yields the exphcit dependence of ( σ_max^M) on the permanent dipole moment, polarizability of the solvent and the adsorbate, lattice spacing, effective coordination number, etc Other models in the baerarchy bring to hght the influence of the solvent structure and the role of substrate interactions, etc As a result of this approach, the "composition" of σ_max^M terms of the fundamental molecular constants becomes clear. With a view to use these molecular results to maxamum advantage, the derived results for (σ_max^M) have been converted into those involving experimentally observable parameters lake Co, C 1, E N, etc Wherever possible, some of the earlier phenomenologlcal relations reported for (σ_max^M), notably by Parsons, Damaskm and Frumkln, and Trasattl, are shown to have a certain molecular basis, vlz a simple two-state sate panty model.As a corollary to the hxerarcbacal modelling, σ_max^M and the potential corresponding to at (Emax) are shown to be constants independent of θ _max or C_org for all models The lmplication of our analysis for σ_max^M with respect to that predicted by the generalized surface layer equation (which postulates σ_max and E_max variation with θ) is discussed in detail Finally we discuss an passing σ_min^M and the electrosorptlon valency an this context.