Measuring local surface charge densities in electrolyte solutions with a scanning force microscope

To show that local surface charge densities can be measured with a scanning force microscope purple membranes adsorbed to alumina were imaged in electrolyte solutions. Force versus distance curves were measured on purple membranes and on the bare alumina with standard silicon nitride tips. By comparing the electrostatic force measured on both substances, the surface charge density of purple membranes could be calculated from the known charge density of alumina. The charge density of purple membranes was estimated to be -0.05 C/m².