APPROPRIATE AND INAPPROPRIATE STATISTICAL-MECHANICAL TREATMENTS FOR PHASE-TRANSITIONS AT ELECTRIFIED INTERFACES

The prerequisites for a statistical mechanical treatment of phase tran sitions of adsorbed layers on electrode surfaces to give results compa tible with both classical thermodynamics and experiment are studied. I t is shown that the use of the charge density as the independent elect rical variable, either in the canonical ensemble or the grand ensemble partition function of simple models, leads to artifacts which are at variance with thermodynamics. In contrast, the generalised ensemble De lta, presented in Nikitas et al. (J. Electroanal. Chem., 317 (1991) 43 ), which uses the potential drop across the layer which undergoes the transition as independent variable, gives results compatible with both experiment and thermodynamics under all circumstances. The results ob tained are compared with the predictions of Guidelli and Aloisi's thre e-dimensional lattice model of TIP4P water molecules. It is shown that in this case the choice of the charge density as independent variable leads to an incomplete description of the two-dimensional phase trans ition properties, since the vertical steps obtained in the adsorption isotherms at constant charge density may either indicate a new type of phase transition which has not been observed experimentally yet or th ey are artifacts which do not represent the properties of the phase tr ansition.