Molecular mechanisms controlling the self-assembly process of polyelectrolyte multilayers

The distance dependent interaction between polyelectrolyte-covered mica surfaces in aqueous solution was investigated with the surface forces apparatus. We find the following: (i) The surface charge changes sign, when an oppositely charged polyelectrolyte from a concentrated polyelectrolyte solution is adsorbed. (ii) Tails and loops of the adsorbed polyions dangle into the bulk phase, inducing a small steric force. If polycations and poyanions are adsorbed on top of each other, a strong short range attractive force is seen due to ion-pair formation after crossing a large repulsive electrostatic/steric barrier. (iii) Obviously, after polyelectrolyte adsorption, there are still nonoccupied binding places (point charges) on the substrate. We show that these adsorption properties regulate the build-up of polyelectrolyte multilayers: Ion pairs between oppositely charged polyion segments and the substrate 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 adsorption hysteresis, which leads to conveniently stable polyelectrolyte multilayers in various environmental conditions.