Force Titrations and Ionization State Sensitive Imaging of Functional Groups in Aqueous Solutions by Chemical Force Microscopy

Chemical force microscopy (CFM) was used to probe interactions between ionizable and neutral functional groups in aqueous solutions. Force microscope probe tips and sample substrates have been covalently modified with self-assembled monolayers (SAMs) terminating in distinct functional groups. SAMs were prepared by treating Au-coated or uncoated tips and substrates with functionalized thiols or silanes, respectively. A force microscope has been used to characterize adhesive and frictional interactions between probe tips and substrates modified with SAMs that terminate in −NH2, −COOH, −OH, and −CH3 functional groups as a function of solution pH and ionic strength. In general, adhesion and friction forces were observed to depend on the chemical identity of the tip and sample surface and also were found to be highly sensitive to the changes in the ionization state of the terminal functionalities induced by varying the solution pH. Adhesion measurements made as a function of pH (force titrations) on amine-terminated surfaces exhibit a sharp decrease in the adhesion force at pH values below 4.5. The pKa estimated from this drop in adhesion, which is due to the protonation of amine groups on the sample and tip, 3.9, is similar to the value determined by conventional contact angle wetting studies of these same surfaces. The large decrease in the pK of the amine group relative to homogeneous solution was attributed to the relatively hydrophobic environment of the amine group in these SAMs. Adhesion measurements made as a function of pH on −COOH-terminated surfaces exhibited a large drop in adhesion force for pH values greater than 5. The pKa estimated from these data, 5.5, is similar to the free aqueous solution value. In addition, the adhesion force between nonionizable −OH and −CH3 groups was found to be independent of solution pH. The measured adhesive forces were interpreted using a contact mechanics model that incorporated the effects of the double layer free energy. Analyses of repulsive electrostatic forces and adhesion data recorded as a function of ionic strength were used to determine properties of the double layer. The pH dependence of the friction force between tips and samples modified with SAMs terminating in −COOH, −OH, and −CH3 groups was measured as a function of applied load. For a given pH, these data exhibit a linear dependence on load with the slope corresponding to the coefficient of friction. The coefficient of friction for −OH and −CH3 groups was independent of pH, while the friction coefficient for −COOH-terminated surfaces drops significantly at a pH corresponding to the pKa determined by adhesion measurements. The pH-dependent changes in friction forces for ionizable groups were exploited to map spatially changes in ionization state on surfaces terminating in −COOH and −OH functional groups.