Surface Roughness and Surface Force Measurement:  A Comparison of Electrostatic Potentials Derived from Atomic Force Microscopy and Electrophoretic Mobility Measurements

The force of interaction between pairs of silica colloids in aqueous electrolyte has been measured using an atomic force microscope . The measured force of interaction has been compared to predictions based on the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, and only partial agreement has been obtained. Specifically, the force−separation data is entirely repulsive, failing to manifest the attraction predicted from the van der Waals interaction. The experimental decay length of the repulsive force was found to be in good agreement with the predicted Debye length, implicating an electrical double layer interaction. At low electrolyte concentration (<5 mM KNO 3 ), good agreement between the fitted diffuse layer potential ( ψ fit ) and the ζ -potential, derived from electrophoretic mobilities, was obtained. At higher ionic strengths, a systematic divergence of ψ fit and ζ was observed with increasing ionic strength. The divergence at high ionic strength is consistent with the enhanced importance of the surface roughness as the Debye length shortens. The shift in origin of the DLVO calculation required for agreement of the ψ fit and ζ has been shown to be well within the peak-to-valley range of the surface. Therefore, while the force of interaction cannot wholly be explained in terms of DLVO theory, the nonagreement can be quantitatively rationalized in terms of surface roughness.