The role of the potential of zero charge (PZC) and of the potential of maximum adsorption Em during the adsorption of neutral molecules at the electrode surface is not yet well understood. The phase transition of the adsorbed molecules to a condensed state adds further complications. But under certain circumstances this process can be utilized to obtain additional information about the adsorbate system and the electrochemical double layer. On these grounds a new method for the determination of the potential of maximum adsorption in condensed layers is proposed. This method is based purely on a qualitative analysis of the shape of current-time transients, which change their sign at the potential of maximum adsorption and become inverted. From this datum and the corresponding capacity-potential curves of the adsorbate system and of the pure electrolyte one can construct the true charge-potential characteristics of the system and obtain the value of the PZC. This method was applied to the system thymine/mercury/ 0.1 M NaClO4. One result is that in this system the potential of maximum adsorption is a function of the temperature, the pH-value, and the prepolarization potential. This latter result can only be explained in terms of a kinetic argumentation.

The condensation behaviour of adenine was investigated with respect to a possible reorientation during the phase transition. It was found that, in contrast to thymine, randomly adsorbed adenine yields a negative dipole contribution with regard to the displaced water molecules. During the condensation reorientation takes place in such a way that this negative dipole contribution is reinforced. Coadsorption of the complementary DNA bases adenine and thymine leads to destabilisation of both the condensed and the randomly adsorbed layers. In place of the attractive lateral forces between adenine or thymine in a pure condensed monolayer, another kind of interaction occurs between adenine and thymine in a mixed adsorbate. Hence, the phase transition disappears when a critical ratio of adenine to thymine is reached.

The influence of the cell resistance on the transient current signal after applying a potential step in condensation experiments has been investigated. The cell resistance was simulated by an external resistance in order to avoid chemical and double-layer effects. It could be shown that the shape of current transients depends sensitively on whether potentiostatic or nonpotentiostatic conditions exist on the surface during the phase transitions. For potentiostatic conditions four principally different current shapes could be detected in the system thymine/mercury, as was theoretically simulated with a model of coupled adsorption and condensation. The appearance of a single oscillation transient proved that the adsorption must be activation-controlled at least in a defined potential region, and commonly the supersaturation becomes time-dependent. Potentiostatic conditions are only fulfilled for small capacities and electrolyte concentrations higher than 0.1 mol/L on one hand and for relatively slow adsorption kinetics on the other. In the present case these conditions were fulfilled only for the mercury electrode. For higher capacities accompanied by faster adsorption kinetics, as is the case for gold electrodes with a diameter of 5 mm, the condensation takes place exclusively under nonpotentiostatic conditions. On gold electrodes in the physisorption region the simple interface model of a pure capacitor could not be applied. Probably additional reactions take place simultaneously with the adsorption process.

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