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 electr olyte has been measured using an atomic force microscope. The measured forc e of interaction has been compared to predictions based on the Derjaguin, L andau, Verwey, and Overbeek (DLVO) theory, and only partial agreement has b een obtained. Specifically, the force-separation data is entirely repulsive , failing to manifest the attraction predicted from the van der Waals inter action. The experimental decay length of the repulsive force was found to b e in good agreement with the predicted Debye length, implicating an electri cal double layer interaction. At low electrolyte concentration (< 5mM KNO3) , good agreement between the fitted diffuse layer potential (psi (fit)) and the zeta -potential, derived from electrophoretic mobilities, was obtained . At higher ionic strengths, a systematic divergence of psi (fit) and zeta was observed with increasing ionic strength. The divergence at high ionic s trength 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 psi (fit) and has been shown to be well withi n the peak-to-valley range of the surface. Therefore, while the force of in teraction cannot wholly be explained in terms of DLVO theory, the nonagreem ent can be quantitatively rationalized in terms of surface roughness.