Measuring changes in surface potential as two charged bodies approach in electrolyte solution.

Combining atomic force microscopy with an ion-sensitive field effect transistor we provide a novel potentiometer capable of measuring a surface potential change as small as 100 microV at 0.2 nm spatial resolution when two charged bodies approach in an electrolyte solution. The surface potential measured when two silica surfaces approach deviates significantly from values inferred indirectly by conventional analysis of force curves. The new data elucidate the way charge redistributes between two nonidentical surfaces and allow clear separation of electrostatics from other contributions to the interaction such as van der Waals and hydration forces.