DEPLETION FLOCCULATION OF POLYSTYRENE LATICES BY WATER-SOLUBLE POLYMERS

The stability of polystyrene latices has been studied in the presence of hydroxyethyl cellulose (HEC) sodium carboxymethyl cellulose (SCMC) and sodium polystyrene sulfonate (SPSS) at various ionic strengths. Only HEC was adsorbed onto the particles from water, whereas all three polymers were adsorbed at high ionic strength. Flocculation, monitored by turbidity measurements, was observed in the absence of electrolyte at critical polymer concentrations (C*) at 0.4%, 0.02% and 0.3% for HEC, SCMC and SPSS, respectively. The flocculation was reversible to dilution and weak shear forces and was attributed to polymer depletion. The results correlated well with predictions from potential-energy calculations incorporating van der Waals, electrostatic, steric and depletion terms. The potential-energy diagrams indicated that concentrations of ca. 0.3%, 0.006% and 0.45% HEC, SCMC and SPSS were required to give a sufficiently deep secondary minimum (ca. -5k(B)T) for aggregation to occur. C* values obtained experimentally for HEC decreased as the ionic strength of the system increased. This was expected since the only effect of the electrolyte was to compress the particle electrical double layer. In the case of the two polyelectrolytes increasing the ionic strength had a number of effects. As well as reducing the electrostatic potential V-E, the depletion potential V-D was also reduced, since the depletion-layer thickness and the osmotic pressure of the bulk polymer solution decreased while the steric potential V-S became significant owing to polymer adsorption. The state of aggregation was controlled by the subtle interplay of these forces.