REORIENTATION PROCESSES AND SURFACE PHASE-TRANSITIONS .1. A STATISTICAL-MECHANICAL TREATMENT

A statistical mechanical treatment of the critical and transition properties of monolayers absorbed on electrode surfaces is presented. The monolayer has a lattice structure and consists of solvent molecules occupying one adsorption site and adsorbate molecules present in two distinct states occupying r1 and r2 adsorption sites, respectively. Therefore, the adsorbate molecules can reorientate during the adsorption. General expressions for the adsorption isotherms and equation of state are obtained by means of the Bragg-Williams approximation. It is shown that when nearest-neighbour interactions prevail among similar species a phase separation process takes place. This process is favoured by the asymmetry of the adsorbate molecules and thus it occurs at higher temperatures with increasing deviations of the ratio r1/r2 from unity. At a certain electrode potential the coexisting equilibrium phases are characterized by different surface coverages of the adsorbate and its adsorbed states. The thermodynamic properties of the monolayer expressed via the plots of surface coverage vs. electrode potential and adsorbate chemical potential, charge density vs. electrode potential, surface pressure vs. adsorbate chemical potential and charge density vs. surface coverage and adsorbate chemical potential are also presented and discussed.