Surface potential driven swelling of polyelectrolyte multilayers

The adsorption of polyelectrolytes to charged surfaces can be applied to build up multilayers of polyelectrolytes of alternating charge by layer-by-layer assembly. The hydration of such multilayers adsorbed to colloidal particles in dilute aqueous dispersions is investigated by NMR, namely, water H-1 spin relaxation rates R-2. For the system of alternating layers of poly(diallyl dimethylammonium chloride) (PDADMAC) and poly(sodium 4-styrenesulfate) (PSS) adsorbed to silica particles (PDADMAC/PSS), a linear increase of multilayer thickness and R2 with the number of layers is found, reflecting an increasing amount of hydration water. For the system (PAH)/PSS, involving the weak polyelectrolyte poly(allylamine hydrochloride) (PAH), it is found that while adsorption of a positive layer leads to an increase of R-2, a decrease of R-2 is observed after the adsorption of a negative layer. This reveals an increase of the net water mobility involving dehydration. Evidence is given that the reversible swelling behavior is a property of the internal layers, not the outer layer. Furthermore, it is driven by changes of the electric potential of the outer layer: A positive surface charge leads to a swelling of the multilayers, while due to a negative surface charge deswelling occurs. These effects can be due to uncompensated charges within the multilayer assembly, possibly involving changes of the dissociation equilibrium.