NUMERICAL-SIMULATION OF SURFACE-IONIZATION AND SPECIFIC ADSORPTION ONA 2-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 io ns, is analysed. The effect of various parameters, such as site concen tration and strength, ion-adsorption equilibrium constants and solutio n composition, are considered and it is shown how these parameters inf luence the extent of surface ionisation and adsorption, as well as the surface charge. Special consideration is given to the point of zero c harge (pzc) and the factors which determine its value. Of particular n ote is the finding that when specific adsorption occurs the value of t he 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 th e consequences of this dependence is that, although cation adsorption tends to decrease the pzc and anion adsorption tends to increase it, e qual adsorptions will not necessarily cancel out the opposing changes. Furthermore, under appropriate circumstances the pzc can decrease eve n though the anion is adsorbed more strongly and vice verse. A graphic al proof that the mass-titration method for the determination of the p zc is valid, even when specific adsorption of ions occurs, is also pre sented.