The influence of pH on friction between oxide surfaces in electrolytes, studied with lateral force microscopy: Application as a nanochemical imaging technique

We have investigated the lateral force (frictional) signal between the SI₃N₄ tip of an AFM and SiO₂ and AlO_x surfaces in a 1 mM NaCl solution, as a function of pH. It was found that the frictional force depends strongly on both the pH and the isoelectric points (IEP) of the materials under investigation. A simple linear model describing the dependence of the lateral force on the total normal force has been used to account qualitatively for the observed lateral force signal from the single-asperity contact between tip and surface on a nanometer scale. The observed pH-dependence of the lateral force signal on different oxide surfaces has been applied to reverse the chemical contrast on an appropriate sample composed of two different oxides, thus demonstrating the potential of this method for "chemical imaging".