Numerical simulation of surface ionisation and specific adsorption on a two-site model of a carbon surface

A two-site (one acidic, one basic) model of the ionisation of a carbon-type surface, in the presence or absence of specifically adsorbing ions, is analysed. The effect of various parameters, such as site concentration and strength, ion-adsorption equilibrium constants and solution composition, are considered and it is shown how these parameters influence the extent of surface ionisation and adsorption, as well as the surface charge. Special consideration is given to the point of zero charge (pzc) and the factors which determine its value. Of particular note is the finding that when specific adsorption occurs the value of the pzc depends not only on the strength of the adsorption itself, but also on the concentration and ionisation constant of the acidic site (for cation adsorption) or basic site (for anion adsorption). One of the consequences of this dependence is that, although cation adsorption tends to decrease the pzc and anion adsorption tends to increase it, equal adsorptions will not necessarily cancel out the opposing changes. Furthermore, under appropriate circumstances the pzc can decrease even though the anion is adsorbed more strongly and vice versa. A graphical proof that the mass-titration method for the determination of the pzc is valid, even when specific adsorption of ions occurs, is also presented.