(162w) Direct Force Measurements for Analyzing Nanoparticle Stabilized Suspensions

The use of nanoparticles to stabilize colloidal suspensions has been known for a number of years, but the exact stabilization mechanisms have not been fully elucidated. While the role of van der Waals and electrostatic interactions are fairly well defined, the exact role of depletion forces within the system is still obscure. This is due, in part, to the use of secondary techniques, such as settling behavior, in examining the mechanisms involved. In this work, the Colloidal Probe technique has been utilized to directly measure the interaction forces between two silica surfaces at pH 1.5 over a 4 order of magnitude range of zirconia nanoparticle concentration. The spring constant of the Atomic Force Microscope cantilever is calibrated in a pH 7 solution just prior to use in the nanoparticle solutions. Based on these direct measurements of the interaction forces, the role of the van der Waals, electrostatic and depletion forces will be explained utilizing DLVO theory and several equations for the depletion force, starting with the classic model of Asakura and Oosawa. Difficulties in both the experimental technique and the mathematical modeling will also be discussed.