Salting-out effects in colloidal dispersions of polymer-grafted spheres

J. Bergenholtz (University of Gothenburg, Sweden)

Model, waterborne systems of fluorinated spheres bearing surface grafts of poly(ethylene glycol) (PEG) have been studied, primarily by small-angle X-ray scattering and dynamic light scattering, as salting-out electrolyte is added. The ions are thought to be depleted in the vicinty of the PEG corona, leading to short-range attractions on overlap of such ion-depleted regions. The resulting attraction is shown to be of sufficient strength at high electrolyte concentrations to cause aggregation in spite of the near absence of the usual van der Waals attraction between particles. In other words, the study demonstrates unambiguously that the polymer graft plays a decisive role for the stability of these sterically stabilized particles. The dependence of the electrolyte-induced instability on PEG chain length has been evaluated, which shows that it is possible to fabricate systems that are impervious to added electrolyte. Moreover, at high electrolyte concentrations the particles are found to be expelled from the salt solution, which is likely caused by the PEG grafts only being partially wetted by the solvent under these conditions.

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