THE EFFECTS OF DIFFERENT ELECTROLYTES ON THE FRACTAL AGGREGATION OF POLYSTYRENE LATEXES COATED BY POLYMERS

The effects of different electrolytes on the aggregation kinetics and fractal dimensions of aggregates of polystyrene latex particles coated by poly(N-isopropylacrylamide) (PNIPAM) and poly(ethylene oxide) (PEG ) have been studied using dynamic light scattering. Whilst PNIPAM is s oluble in water at low temperatures, on increasing the temperature and /or adding electrolyte it adopts a globular conformation and becomes h ydrophobic, in comparison to the more hydrophilic PEG. For the PNIPAM- coated latexes, the aggregation induced by various sodium salts was fo und to decrease in the following order: NaCl > NaBr > NaNO3 > NaI > Na SCN. This sequence is similar to that found previously for latexes coa ted by PEG. The concentrations of electrolyte required to reach the sa me fractal dimension at longer times corresponded to the reverse order . The longer time fractal dimensions were found to increase with incre asing electrolyte concentration so that the aggregate structures tende d to become more densely packed. All the results can be correlated wit h the sequential order of the viscosity coefficients B-eta of the elec trolytes studied. For the PEG-coated latex, the fractal dimension was found to be insensitive to electrolyte concentration. With respect to temperature, the fractal dimension changed only slightly between 40 an d 47.5 degrees C, but dropped dramatically up to 50 degrees C. The obs ervations have been interpreted in terms of the nature of the interpar ticle interactions.