The use of AFM for direct force measurements between expandable fluorine mica

Abstract

An atomic force microscope has been used to measure forces interacting between a polymethyl methacrylate (PMMA) sphere and a flat plate, onto which expandable fluorine mica platelets have been coated. The surface thus behaves as a single macroscopic basal plane of expandable fluorine mica over concentrations 10 −5–10 −2 M NaCl and a range in pH of 3.5–10. Most of the measured forces fit well to theoretically calculated forces by the DLVO theory at separations larger than 4 nm. A short-range repulsive hydration force can be observed at separations below 4 nm which varied in magnitude both with NaCl concentration and pH. The diffuse-layer potential was extracted from the best fit to the forces calculated from the DLVO theory and compared with electrokinetic potential. There is good agreement between the two values in acidic pH range and at concentrations less than 10 −3 M NaCl, although slight discrepancies between the potential were recognized in the alkaline pH range and in the high NaCl concentration region. The diffuse-layer potentials were analyzed using the Gouy–Chapman–Stern–Graham model with dissociation-binding equilibria of surface groups. By assuming an extremely low value of the inner layer capacitance (∼5 μF/cm 2), the fit of the outer Helmholtz layer potentials calculated to the diffuse-layer potential obtained experimentally were improved.