ADSORPTION FROM DILUTE-SOLUTIONS AT UNCHARGED INTERFACES - A SEMI-THERMODYNAMIC TREATMENT

When an adsorbed film is formed at an uncharged interface, then an ele ctric field is established across this film created by the dipoles of the adsorbed species. Similarly, an electric field may be established parallel to the adsorbing surface from the in-plane dipoles of the ads orbed species. These electric fields affect the dipole-dipole interact ions and therefore they influence the surface activities of the adsorb ed species. Their effect on the surface activities was taken into acco unt by means of the electrostatic theory of dielectrics and the Gauss theorem which was used to correlate the potential drop across and para llel to the adsorbed layer with its composition. Then based exclusivel y on the properties of the various thermodynamic functions of the surf ace solution the chemical/electrochemical potentials of the constituen ts of this layer were derived without additional modelistic assumption s. In this approach the molecular information, which is fitted to ther modynamic arguments, is more general and therefore more reliable than that obtained from a certain model. The new adsorption isotherms, whic h arise from the chemical/electrochemical potentials, can take into ac count changes in the surface layer dielectric constant and the orienta tion of the adsorbed species during the adsorption process. The isothe rms were compared with a variety of experimental data and a good agree ment was found in all cases.