(78a) Polyelectrolytes in Multivalent Ionic Media: New Physics and New Materials

Multi-valent interactions in systems of polyelectrolytes can exhibit dramatic, non-monotonic effects, for example, switching forces from repulsive to attractive, and back to repulsive again, in some cases. We have been studying these patterns of behavior with the surface forces apparatus (SFA) and with electrochemical methods, such as cyclic voltametry, which enables the quantitative determination of the number of multi-valent ions residing in thin layers of charged polymers. We have looked at di-valent ions such as Ca²⁺, tri-valent metal ions such as Al³⁺ and La³⁺, surfactants such as CTAB, as well as the complex ions useful for electrochemistry. At fixed ionic strength, all cause strong shrinkage and condensation of poly(styrene sulfonate) brushes over a narrow range of ratio multi-valent to mono-valent ions. When the multi-valen ion is an oppositely charged polymer, new fluid phases can form. Charged blocks in copolymers leads to materials with new types of ordered phases. Effects of these multi-valent interactions on supermolecular and biomolecular assembly will be discussed. There are many possibilities for the creation of new materials based on electrostatic assembly involving mutli-valent interactions.