pH Variations of Surface Forces as Probed by Chemically Modified Tips

Chemical information about systems studied by atomic force microscopy can be obtained with probe tips covalently modified with self-assembled monolayers (SAMs) that terminate in well-defined functional groups. This new chemical force microscopy technique (CFM) has been used to probe adhesion and friction forces between distinct chemical groups (COOH, OH, NH 2 and CH 3 ) in organic and aqueous solvents. These forces varied with the chemical identity of the tip and sample surface, solvent composition and changes in the ionization state of the terminal functionalities and followed trends expected from the strengths of the intermolecular interactions. Contact mechanics provides a framework to model adhesion between molecular groups. Adhesion and friction measurements made as a function of pH (force titrations) on ionizable surfaces were used to assess the pK a of surface groups. The basic knowledge of adhesive and frictional forces provides a basis for rationally-interpretable mapping of a wide range of chemical functionality.