Colloid stability and electrokinetic characterization of polymer colloids prepared by different methods

The electrokinetic characterization and colloidal stability of functionalized latexes, two sulfonated polystyrene model colloids prepared by different methods, are described. JM1 latex was synthesized by an emulsion copolymerization process from styrene and sodium styrene sulfonate (NaSS) in the absence of an emulsifier. SN9 latex was prepared by a two-stage ''shot-growth'' emulsion polymerization process without emulsifier and using NaSS as an ionic comonomer. The two latexes had nearly the same particle size but very different surface charge densities. When the electrophoretic mobility of these functionalized model colloids was studied as a function of pH and ionic strength, the latex with the lower surface charge yielded higher mobility values. The experimental results for the electrokinetic characterization suggest that the surface of the particles prepared with a second injection is covered by a layer of oligomers or polymer chains which shift the shear plane towards the bulk solution, increasing the anomalous surface conductance of the particle-electrolyte solution interface. The stability factor and the critical coagulation concentration were studied for the two samples. Better colloidal stability was observed for the latex prepared with the second injection, which has a higher surface charge density. There is an important electrostatic contribution to the stability, but there also seems to be a steric contribution due to the structure of the electric double layer which surrounds the particles. (C) 1997 Elsevier Science B.V.