Adsorption isotherms for neutral organic compounds—A hierarchy in modelling: Part V. Models for the inner layer potential difference in the presence of dipolar adsorption

Abstract

The relations for the inner layer potential difference ( E) in the presence of adsorbed organic molecules are derived for three hierarchical models, in terms of molecular constants like permanent dipole moments, polarizabilities, etc. It is shown how the experimentally observed patterns of the E vs. θ plots (linear in all ranges of σ M, non-linear in one or both regions of σ M, etc.) can be understood in a semi-quantitative manner from the simplest model in our hierarchy, viz. the two-state site parity version. Two-state multi-site and three-state (site parity) models are also analysed and the slope (∂ E/∂θ) σ M tabulated for these also. The results for the Esin-Markov effect are derived for all the models and compared with the earlier result of Parsons. A comparison with the GSL phenomenological equation is presented and its molecular basis, as well as the limitations, is analysed. In particular, two-state multi-site and three-state (site parity) models yield E−σ M relations that are more general than the “unified” GSL equation. The possibility of viewing the compact layer as a “composite medium” with an “effective dielectric constant” and obtaining novel phenomenological descriptions is also indicated.