Sensitivity of Friciionll Forces to pH on a Nanometer Scale: A Lateral Force Microscopy Study

An atomic force microscope (AFM) operating in the lateral (frictional) force mode (LFM) was used to investigate the pH dependence of kinetic friction between a silicon nitride tip and an oxidized silicon surface under aqueous electrolyte solutions. It was found that, besides the known pH dependence of the normal force curves, the frictional force is highly sensitive to the pH value of the solution. A linear relationship between frictional force and adhesion hysteresis (i.e., the dissipated energy), as recently shown to be valid for some systems by Israelachvili et al., is supported. The implication is that many chemical species on surfaces can be readily differentiated using AFM-LFM by varying the pH value of the surrounding medium and that a possible nanochemical imaging method could be developed, based upon this principle.