Polystyrene (PS) latex dispersions were prepared by surfactant-free emulsion polymerization. Poly(methyl methacrylate) (PMMA) latex dispersions were prepared using the same procedure except by adding INUTEC SP1 as emulsifier. The two latex dispersions were characterized by photon correlation spectroscopy (PCS) to obtain the average particle size and the polydispersity index. The stability of these latex dispersions was measured by determination of the critical coagulation concentration (CCC) of three electrolytes, namely, NaCl, CaCl2, and Al2(SO4)3. The CCC was 0.375 mol·dm-3 for NaCl, 0.007 mol·dm-3 for CaCl2, and 0.0004 mol·dm-3 for Al2(SO4)3. A polymeric surfactant, namely, a graft copolymer of polyfructose on which alkyl groups were grafted to the backbone, were added to latex dispersions and their stability was investigated. On addition of this polymeric surfactant, the stability of the latex dispersions was significantly increased and the CCC became very high above a critical polymer concentration. For the PS latex, the CCC of CaCl2 was higher than 4.3 mol·dm-3 when the polymeric surfactant concentration was 0.25 wt%. The results could be rationalized in terms of the enhanced steric repulsion resulting from the adsorption of the graft copolymer. It was assumed that the molecule produces large "loops" of polyfructose between the adsorbed alkyl groups, forming a hydrated layer thickness of approximately 4 nm. In addition, these polyfructose chains were still hydrated even in the presence of high electrolyte concentrations.

The authors gratefully acknowledge financial support from ORAFTI Non-Food and Generalitat de Catalunya DURSI (grant 2001 SGR-00357).

Peer reviewed