Specific ionic effects on weak polyelectrolyte multilayer formation

The layer-by-layer deposition method to prepare multilayers of polyelectrolytes of alternating charge has been followed in situ by means of optical reflectometry. It has been shown previously that in solutions containing both weak polyelectrolytes and appropriate salt the buildup of multilayers is modified and becomes an adsorption/dissolution process. The influence of different salts (phosphates, chlorides, and nitrates) and polyelectrolyte molecular weight on formation and erosion of multilayers on silica surfaces was investigated. In all experiments, the anionic polyelectrolyte was poly(acrylic acid). As the cationic polyelectrolyte, poly(dimethylaminoethyl methacrylate), poly(allylamine hydrochloride), and poly(2-vinyl-N-methylpyridinium iodide) were used. It has been shown that at very low ionic strength (1 mM) regular buildup of multilayers is observed independent of the salt used. However, at higher ionic strength, dissolution also takes place, and the critical "glass-transition ionic strength" needed for the multilayer to be dissolved depends on the salt used, as well as on the polycation/polyanion pair studied.