The Influence of Polyelectrolytes on the Stability of Colloids

Abstract: The interaction between two colloids with grafted polyelectrolytes in aqueous solution is studied within the primitive model by Monte Carlo simulations. For neutral colloids with grafted polyelectrolytes and in absence of salt the interaction is strongly repulsive in absence of salt. The interaction depends on the extension of the polyelectrolytes from the surfaces and the interaction is derived and the interaction can be estimated for different chain length. The interaction between two charged colloids depends on the amount of surface charge neutralized by polyelectrolytes. The interaction has a minimum when the surface charge is perfectly neutralized by polyelectrolytes. When the energy term dominates in a double layer almost all ions are found in the vicinity of the charged surfaces forming alternating layers of counterions and coions. Coions have the same charge as the surface and counterions the opposite charge. The energy term becomes dominant at high surface charge density, low dielectric constant, low temperature or by reducing the entropy by a increasing the counterions valency. Another way to reduce the importance of the entropy is replace the counterions by polyelectrolytes. In the latter case the effect increases with the chain length. If the polyelectrolytes are only grafted to one of two equally but oppositely charged surfaces a more long-range attraction occurs than if both surfaces are neutralized by either ions or polymers. The origin of this extra atrraction is the small ions, which are spread over the whole volume and causes a charge imbalance of the system. The MC results are shown to be in qualitatively agreement with results from surface force measurements.