Evaluation of Surface Ionization Parameters from AFM Data

A new application of atomic force microscopy (AFM) for evaluation of surface ionization parameters on the basis of a charge-regulation model is demonstrated. Interactions between symmetrical silicon nitride and silica surfaces have been measured in aqueous electrolytic solutions at different pHs and separations from a few Debye lengths down to zero. The obtained experimental data are reported and interpreted on a quantitative level in the framework of Derjaguin's approximation; the computational procedure used for this purpose is outlined. Surface charge density, surface potential, and species concentrations are calculated as functions of pH and separation, unveiling the limits of applicability of classical constant-charge and constant-potential approximations. The integrity of the results on the charge properties of interacting surfaces is discussed in reference to electrokinetic and titration data, potential problems caused by the existence of non-DLVO forces and surface irregularities being highlighted.