THE ORDER OF ADDING POLYELECTROLYTE AND SALT AFFECTS SURFACE FORCES AND LAYER STRUCTURES

The order in which salt-and polyelectrolyte are added to a solution influences the structure of adsorbed layers on surfaces in contact with the solution and the interactions between surfaces across such solutions. By performing surface force measurements, it is shown that the strong attraction observed between polyelectrolyte-coated surfaces in low ionic strength solutions is changed into a long-range repulsion with a solution of high ionic strength. This repulsion has a longer range and larger magnitude when the salt is added before the polyelectrolyte than when additions are done in the reverse order. When increasing the ionic strength in steps, the layer thickness and short-range repulsion increases. We suggest that this is due to an increased adsorption of the polyelectrolyte. Mean field calculations (Scheutjens-Fleer model) were used to rationalize experimental findings. The results show that the adsorbed layer is much more extended in solutions of high ionic strength. Furthermore, we show that the adsorption of polyelectrolyte is increased with ionic strength when the nonelectrostatic interaction between polymer segments and the solvent is unfavorable.