Flocculation of silica particles by adsorbing and non-adsorbing polymers

Abstract

The stability of silica sols has been studied in the presence of adsorbing (hydroxyethyl cellulose) and non-adsorbing (sodium carboxymethyl cellulose and sodium polystyrene sulphonate) polymers. The state of aggregation has been monitored by turbidity measurements and for the adsorbing polymer, flocculation due to polymer bridging was observed at low polymer additions while at higher polymer concentrations flocculation induced by a depletion mechanism was observed for all samples studied. The critical polymer concentrations required to induce depletion flocculation were 0.2, 0.1 and 0.05% for hydroxyethyl cellulose (HEC), sodium carboxymethyl cellulose (CMC) and sodium polystyrene sulphonate (PSS), respectively. Varying the charge density of the CMC had no apparent effect on the critical polymer concentration required to induce flocculation.

The pair potentials of the particles for the various systems have been calculated incorporating contributions from van der Waals, electrostatic, steric (for HEC) and depletion forces and the depth of the secondary minima in the energy curves have been used as an index for the onset of flocculation. A critical value for this minima of 5 k_BT for the onset of flocculation was established from stability measurements determined using simple electrolyte. Good correlation was obtained between theory and experiment for HEC and PSS providing that values of osmotic pressure and radius of gyration of the latter were measured at the same solution ionic strength as in the silica dispersion. In the presence of CMC, however, flocculation was predicted to occur at 0.01% polymer which is an order of magnitude lower than the value found experimentally. These discrepancies may be due to the polydisperse nature of the CMC samples.