Ion distribution in polyelectrolyte multilayers with standing-wave X-ray fluorescence.

Absolute ion concentration and its profile across polyelectrolyte multilayer films were studied. The films were prepared by alternating adsorption of polyanions and polycations from aqueous solution. Standing-wave X-ray fluorescence was used to map the ion profile. The well-studied multilayer system PSS/PAH was investigated, and bromide ions were used as probe entities. The results show that the sign of the charge of the outermost layer and the washing procedure after finishing the preparation have a decisive effect on the ion concentration and the ion profile. Multilayers with PSS as the outermost layer contain fewer bromide ions than the PAH-terminated multilayers. Exposure to water washes the ions out, but even after 6 h of washing, not all of the bromide ions had been removed.