REORIENTATION PROCESSES AND SURFACE PHASE-TRANSITIONS .2. A THERMODYNAMIC INTERPRETATION OF THE ELECTROCAPILLARY DATA OF METHYLPYRIDINES ADSORPTION ON A MERCURY-ELECTRODE

The thermodynamic method proposed in J. electroanal. Chem. 309, 1 (199 1) for analysis of adsorption data of organic compounds at electrified interfaces is extended to adsorption layers of two-state adsorbates. It is shown that the information which can be gained in such systems i s considerably low in comparison with adsorption layers of constant-or iented adsorbates. Thus, the excess free energy of mixing and excess e ntropy cannot be calculated and only under optimum circumstances the G ibbs energies of adsorption and the particle-particle interaction para meter can be estimated. The thermodynamic method is applied to the stu dy of the adsorption properties of 2- 3- and 4-methylpyridines on a me rcury electrode from aqueous solutions of 0.1 N Na2SO4. It is shown th at methylpyridines reorientate on the electrode surface from the flat state to the normal one with the N atom facing the solution. At potent ials more negative than -0.7 V (sce) the reorientation of 2- and 3-met hylpyridines is accompanied with a phase separation process. The Gibbs energies of adsorption and the particle-particle interaction paramete r are estimated by means of the thermodynamic method and a correlation with interfacial interactions is attempted. Finally, the values of th e interaction parameter are compared with theoretical calculations and discussed.