Force titration of a carboxylic acid terminated self-assembled monolayer using chemical force microscopy

omega-Mercaptoundecano acid self-assembled monolayer (SAM) on gold was titrated with buffered water and chemical force microscopy (CFM) was used to follow the titration process by measuring the adhesion force between the SAM-modified substrate and probe tip. The adhesion force vs pH curve thus obtained, which can be named as force titration curve, was found to show a big hump around pH 5. Taking the contact-angle titration result together, an interpretation on this unique force titration behavior is made in terms of the rearrangement of the surface carboxylic acid groups when the probe tip and the substrate come into contact.