Surface Potential Driven Swelling of Polyelectrolyte Multilayers

Abstract

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 1H spin relaxation rates R2. 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 R2, a decrease of R2 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.