THE QUESTION OF NEGATIVE CAPACITANCE AND ITS RELATION TO INSTABILITIES AND PHASE-TRANSITIONS AT ELECTRIFIED INTERFACES

The question of negative differential capacitance (C) at electrified i nterfaces is critically reviewed. The importance of the thermodynamic constraints and the physical differences between systems under charge and potential control is emphasized. For a system under potential cont rol, C can never be negative, while no such general constraint is mand ated for systems under charge control. However, the appearance of a ne gative C domain in isolated systems can be limited by a phase transiti on to a non-uniform state. The occurrence of such transitions depends on the 'stiffness' of the interface. Theoretical calculations that pre dict C < 0 are analysed. We investigate a series of electromechanical analogues of the existing models of double layer, illustrating the app earance of C < 0 in 'rigid' systems under equilibrium conditions, and the transition to the non-uniform state in systems with moderate 'surf ace tension', narrowing the C < 0 domain. The effect of intrinsic non- homogeneity is also addressed. The appearance of domains where C is ne gative under charge control implies the occurrence of surface phase tr ansitions under potential control. The characteristics of phase transi tions in isolated and open systems, and the relationship between them, are considered. Some biophysical implications are discussed.