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

An atomic force microscope has been used to measure forces interacting betw een a polymethyl methacrylate (PMMA) sphere and a flat plate, onto which ex pandable fluorine mica platelets have been coated. The surface thus behaves as a single macroscopic basal plane of expandable fluorine mica over conce ntrations 10(-5)-10(-2) M NaCl and a range in pH of 3.5-10. Most of the mea sured forces fit well to theoretically calculated forces by the DLVO theory at separations larger than 4 nm. A short-range repulsive hydration force c an be observed at separations below 4 MI which Varied in magnitude both wit h NaCl concentration and pH. The diffuse-layer potential was extracted from the best fit to the forces calculated from the DLVO theory and compared wi th 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 pote ntials were analyzed using the Gouy-Chapman-Stern-Graham model with dissoci ation-binding equilibria of surface groups. By assuming an extremely low va lue of the inner layer capacitance (similar to 5 mu F/cm(2)), the fit of th e outer Helmholtz layer potentials calculated to the diffuse-layer potentia l obtained experimentally were improved. (C) 1998 Elsevier Science B.V. All rights reserved.