Co-adsorption of two components on an electrode as described by the Alekseev-Popov-Kolatyrkin model coupled with a set of Frumkin isotherms

The Alekseev-Popov-Kolotyrkin (APK) model complemented by two mixed Frumkin isotherms is used to derive equations and develop a procedure for calculat ing co-adsorption of two solution components (ions or neutral molecules) an d relevant potential dependences of the charge and differential capacitance of the electrode and the interfacial tension. Under the conditions of a st rong attractive interaction between specifically adsorbed cations and anion s, the model predicts two-dimensional phase transitions leading to the form ation or destruction of condensed films. It is shown that allowing for a li miting monolayer coverage of the surface by the adsorbate removes restricti ons inherent in the APK model, which emerge when the model is coupled with a set of virial isotherms. On the other hand, such an allowance affects the relationship between the surface excess of cations and the electrode charg e, thus additionally complicating the interpretation of the Esin-Markov eff ect on the basis of this relationship.