Depletion and structural interactions in charged colloidal systems

The presence of nonadsorbing (depleting) material in a variety of colloidal mixtures introduces subtle but important changes in the properties of these complex fluids, such as stability and rheology. Moreover, several potentially useful applications of the depletion and structural interactions are being actively pursued. Because of ubiquitous nature of charged surfaces in aqueous solutions, the depletion and structural forces between charged colloidal surfaces in the presence of charged macromolecules were investigated. The approach used was a series of fundamental model studies combined with direct measurement of these forces using the atomic force microscope (AFM).{09}Specifically, the effects of various solution conditions (e.g., macromolecule concentration, ionic strength, pH) on the depletion and structural forces between a large colloidal particle and a flat plate in systems containing either spherical depletant macromolecules or polyelectrolyte chains was studied.; In the case when spherical nanoparticles (Ludox silica) were used as depletant macromolecules, the theoretical and experimental force profiles exhibited the same trends with respect to changing macromolecule volume fraction, suspension ionic strength, pH, and composition ( i.e., when binary mixtures of different nanoparticles were employed). It was also found that the characteristic spacing between macromolecules in the gap region followed the space-filling behavior as expected in the bulk suspension. Furthermore, very good correspondence between experimentally-determined and predicted (based on the bulk number density) macromolecule-macromolecule spacing was determined.; With potassium polyacrylate chains employed as depletant macromolecules, the behavior of this polyelectrolyte in both dilute and semi-dilute solution regimes was examined and compared with the predictions of the scaling theory. Good agreement was found between the experimental and theoretical macromolecule-macromolecule spacing in both solution regimes, indicating that the polyelectrolyte chains in the gap region follow the bulk behavior. Specifically, the chains were space-filling in the dilute regime and formed a mesh in the semi-dilute regime. Moreover, the experimentally determined overlap concentration indicating the transition from the dilute to the semi-dilute polyelectrolyte solutions was found to match the predicted value.