Specific Ion Effects of Divalent Cations on the Formation and Properties of Polyelectrolyte Multilayers

Abstract

Multilayer films made of strong polyelectrolytes poly(diallyldimethylammonium chloride) and poly(sodium 4-styrenesulfonate) were built-up on silicon wafer using the layer-by-layer method. The films were built-up in the presence of various divalent cations (Mg²⁺, Ca²⁺, Sr²⁺, Ni²⁺, Zn²⁺, and Cu²⁺) to examine how the nature of cations affects the properties of the film. The results have shown that the thickness, morphology, and roughness of films prepared in the presence of transition metal cations are not significantly different. In contrast, these properties varied for multilayers prepared in the presence of alkaline earth metal cations. The difference in the ion-specific behavior of these two classes of cations was explained by the difference in the hydration of these ions and by the bridging of polyelectrolyte chains with ions. While transition metal cations have similar hydration parameters, alkaline earth cations have different degrees of hydration and a better ability to form bridging bonds with polyelectrolyte monomers.