Effect of adsorbed polyethylene oxide on the rheology of colloidal silica suspensions

The viscosity behavior of aqueous suspensions of silica particles with phys ically adsorbed polyethylene oxide (PEO) molecules was investigated in rela tion to the adsorbed layer density and concentration of nonadsorbed polymer in the suspending fluid. Adsorption studies revealed the presence of two p lateaus in the adsorption isotherm, which was attributed to a change in con formation from a flat, "pancake"-type, to a more elongated "brush"-type con formation of the adsorbed polymer, where the terminal hydroxyl groups remai ned bound to the silica surface. Adsorption and rheological studies were pe rformed on suspensions of silica particles of three different sizes dispers ed in solutions of 0.01 M NaNO3 containing PEO of different concentrations and different molecular weights. The variation of shear viscosity with the adsorbed layer density, concentration of free polymer in the suspending med ia (depletion forces), and particle size are discussed. Results on the role of particle size on the viscosity of electrostatically and sterically stab ilized suspensions indicate that sterically stabilized systems may be treat ed as hard spheres at high shear rates depending upon the adsorbed layer de nsity and concentration of nonadsorbed polymer in the suspending media. Ele ctrostatically stabilized suspensions may show deviation from hard sphere b ehavior even at high shear rates depending on the range of electrostatic re pulsion between the suspended particles. Further investigation must be perf ormed to be able to explain the difference between the behavior of electros tatically and sterically stabilized systems in terms of the deformability o f the layers, i.e., "softness" of the electrical double layers around the p articles as compared with the "stiffness" of the adsorbed layers of the pol ymer, (C) 2000 Academic Press.