Use of atomic force microscopy for the study of surface acid-base properties of carboxylic acid-terminated self-assembled monolayers

Atomic force microscopy was used to measure the forces between a silica probe and a carboxylic acid-terminated self-assembled monolayer (SAM) on a gold substrate in the presence of KCl electrolyte solutions of different pH. Silica-silica interaction force measurements were conducted at different pH values to determine the silica probe surface electrostatic potentials under these conditions. The interaction between two silica surfaces is repulsive and can be accurately predicted (except at short distances) by the Dejaguin-Landau-Verwey-Overbeek theory. The interaction between silica and clean gold surfaces,exhibits an attractive interaction at neutral pH. The interaction between a silica probe and carboxylic acid-terminated SAM-covered gold substrate was a strong function of the pH value of the electrolyte. The surface electrostatic potentials of the surface-confined monolayers of carboxylic acid were obtained by theoretical fits of the force data to solutions of the complete nonlinear Poisson-Boltzmann equation, with the knowledge of silica probe surface potentials, at different solution pH values. The surface titration curve was obtained by correlating the surface potentials to the different electrolyte pH values. A theoretical fit to the titration curve provides the surface pK(a), and an explanation for the broadening of the titration curve.