Effects of Ammonium Chloride on the Rheological Properties and Sedimentation Behavior of Aqueous Silica Suspensions

The influence of ammonium chloride (NH 4 Cl) on the rheological properties and sedimentation behavior of aqueous silica (SiO 2 ) suspensions of varying solids volume fraction (φ s ) was studied. SiO 2 suspensions with low NH 4 Cl concentration (≤0.05 M , pH 5.2) exhibited Newtonian behavior and a constant settling velocity ( U ). The volume fraction dependence was well described by the Richardson–Zaki form, U = U 0 (1 −φ s ) n , where n = 4.63 and U 0 = 1.0419 × 10 −5 cm/s. At higher NH 4 Cl concentrations (0.07–2.0 M , pH 5.2), suspensions exhibited shear thinning and more complicated sedimentation behavior due to their aggregated nature. For all suspensions studied, however, the apparent suspension viscosity, characteristic cluster size, and initial settling velocity were greatest at ∼0.5 M NH 4 Cl and exhibited a similar dependence on salt concentration. Above 0.5 M NH 4 Cl, considerable restabilization was observed. This behavior cannot be explained by traditional DLVO theory.