Measurements of Electrostatic Double-Layer Forces Due to Charged Functional Groups on Langmuir-Blodgett Films with an Atomic Force Microscope

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Abstract
Using an atomic force microscope, we measured the forces between a Si3N4 tip and monolayers with different polar functional groups ( –NH2, –COOH, –CONH2, and –OH) prepared by the Langmuir-Blodgett method while varying the pH value of aqueous solutions. The obtained force vs distance curves were related to the surface charges of the tip and the dissociation of the functional groups from the pH dependency, and the charged state of functional groups could thus be discerned. In addition, the electrostatic origin of the force has been confirmed assuming constant potentials on both surfaces.