MOLECULAR MECHANISMS CONTROLLING THE SELF-ASSEMBLY PROCESS OF POLYELECTROLYTE MULTILAYERS

The distance dependent interaction between polyelectrolyte-covered mic a surfaces in aqueous solution was investigated with the surface force s apparatus. We find the following: (i) The surface charge changes sig n, when an oppositely charged polyelectrolyte from a concentrated poly electrolyte solution is adsorbed. (ii) Tails and loops of the adsorbed polyions dangle into the bulk phase, inducing a small steric force. I f polycations and poyanions are adsorbed on top of each other, a stron g short range attractive force is seen due to ion-pair formation after crossing a large repulsive electrostatic/steric barrier. (iii) Obviou sly, after polyelectrolyte adsorption, there are still nonoccupied bin ding places (point charges) on the substrate. We show that these adsor ption properties regulate the build-up of polyelectrolyte multilayers: Ion pairs between oppositely charged polyion segments and the substra te are formed, until the surface charge is inversed. The electrostatic barrier limits the adsorbed amount, guarantees the equal thickness of consecutively adsorbed layers, and furthermore causes a strong adsorp tion hysteresis, which leads to conveniently stable polyelectrolyte mu ltilayers in various environmental conditions.