Water Distribution in Multilayers of Weak Polyelectrolytes
Tanchak, O.M.; Yager, K.G.; Fritzsche, H.; Harroun, T.A.; Katsaras, J.; Barrett, C.J. "Water Distribution in Multilayers of Weak Polyelectrolytes" Langmuir 2006
Neutron reflectometry measurements were applied to polyelectrolyte multilayer thin films. These experiments allowed us to demonstrate unambiguously that water is distributed asymmetrically within the layer stack. Specifically, water localizes preferentially to the free surface, forming a sharp boundary. This is true both in the case of swelling from humid atmosphere, and when films are exposed to bulk water.
The water localization in thin polyelectrolyte multilayers assembled from poly(acrylic acid) and poly(allylamine hydrochloride) was investigated with neutron reflectivity in an atmosphere of controlled humidity and with bulk water. Water was found to be distributed asymmetrically within the multilayer and to localize preferentially at the polymer surface. The diffusion of water into the multilayer did not completely penetrate to the substrate, but instead there appeared to be an exclusion zone near the Si substrate. These results help to explain previous observations of anomalous water transport kinetics in weak polyelectrolyte systems.