The structuring of nonadsorbed nanoparticles and polyelectrolyte chains in the gap between a colloidal particle and plate

The force profiles between negatively charged silica and titania surfaces in solutions containing additives of either rigid spherical nanoparticles or polyelectrolyte chains, also negatively charged, were measured using an atomic force microscope (AFM). The effects of various solution conditions (e.g., additive concentration, solution ionic strength, pH) on the nature of the measured force profiles were investigated. The primary focus of the work, however, was the long-range oscillations in the force profile and the specific dependence of the wavelength of these oscillations on both solution chemistry and the bulk additive concentration. In the case when spherical nanoparticles (Ludox silica) were used as depletants, the characteristic spacing between macromolecules in the gap region followed the space-filling behavior expected of the bulk suspension (i.e., the wavelength of the oscillations in the force profile scaled with the bulk nanoparticle concentration, c, as c(-1/3)). In addition, the actual magnitude of this spacing was approximately equal to n(-1/3), where n is the bulk number density. For a system in which the additive was potassium polyacrylate, measurements of the force profile was made in both the dilute and semidilute regimes. Good agreement was found between the experimental and theoretical chain-chain spacing in both solution regimes, indicating that the spacing between the polyelectrolyte coils in the gap region is controlled by the bulk behavior. Specifically, the chains were space-filling in the dilute regime (i.e., the wavelength of the oscillations scaled as c(-1/3)) and formed a mesh in the semidilute regime, with the wavelength of the oscillations scaling as c(-1/2). In addition, the concentration at which this change in scaling behavior occurred agreed with the expected overlap concentration.