Ion distribution in multilayers of weak polyelectrolytes: A neutron reflectometry study.

Neutron reflectometry was used to determine the distribution of salt ions and water in thin poly(acrylic acid) and poly(allylamine hydrochloride) polyelectrolyte multilayers assembled with and without salt. Increasing salt concentration reverses the exclusion of water from the substrate region, eventually leading to an asymmetric segregation of water near the substrate at high salt concentration. The counterions were found to localize near the substrate in films that were either assembled with salt or were exposed to salt solutions. In addition, the capping layer of the film was found to greatly influence the counterion distribution in the multilayer.