Polyelectrolytes interacting with interfaces: Surface forces and theoretical results

The order in which salt and polyelectrolyte is added to a solution influences the structure of the adsorbed polyelectrolyte on surfaces in contact with the solution, and the interactions between surfaces across such solutions, These effects were investigated using the interferometric surface force technique. The results show that in low ionic strength (0.1 mM KBr), there is a strong attraction between two mica surfaces covered with oppositely charged polyelectrolytes. When the ionic strength is increased (up to 100 mM KBr), a long-ranged repulsion is developing. This repulsion is more long ranged and larger in magnitude when the salt is added before the polyelectrolyte than when salt is added to a system which already contains polyelectrolyte. This indicates a more extended adsorbed layer when the adsorption is carried out directly from the higher ionic strength solution, This more extended adsorbed layer is irreversibly compressed upon the first approach of the surfaces. Mean field calculations (Scheutjens-Fleer model) were used to rationalize experimental findings. The results from these calculations show that the adsorbed layer is much more extended in high ionic strength solutions, which is in good agreement with the experimental results.