The interface between a metal and an electrolyte solution is the most important electrochemical system, and we begin by looking at the simplest case, in which no electrochemical reactions take place. The system we have in mind consists of a metal electrode in contact with a solution containing inert, nonreacting cations and anions. A typical example would be the interface between a silver electrode and an aqueous solution of KF. We further suppose that the electrode potential is kept in a range in which no or only negligible decomposition of the solvent takes place - in the case of an aqueous solution, this means that the electrode potential must be below the oxygen evolution and above the hydrogen evolution region. Such an interface is said to be ideally polarizable, a terminology based on thermodynamic thinking. The potential range over which the system is ideally polarizable is known as the potential window, since in this range electrochemical processes can be studied without interference by solvent decomposition. As we pointed out in the introduction, a double layer of equal and opposite charges exists at the interface. In the solution this excess charge is concentrated in a space-charge region, whose extension is the greater the lower the ionic concentration. The presence of this spacecharge region entails an excess (positive or negative) of ions in the interfacial region. In this chapter we consider the case in which this excess is solely due to electrostatic interactions; in other words, we assume that there is no specific adsorption. This case is often difficult to realize in practice, but is of principal importance for understanding more complicated situations. A simple but surprisingly good model for the metal-solution interface was developed by Gouy and Chapman as early as 1910. The basic ideas are the following: The solution is modeled as point ions embedded in a dielectric continuum representing the solvent; the metal electrode is considered as a perfect conductor. The distribution of the ions near the interface is calculated from electrostatics and statistical mechanics.
Multi-configurational Ehrenfest simulations of ultrafast nonadiabatic dynamics in a charge-transfer complex