Swelling and Thermal Transitions of Polyelectrolyte Multilayers in the Presence of Divalent Ions

The effects of CaCl 2, MgCl 2, and Na 2 SO 4 on the swelling response and the thermal transition of poly(diallyldimethylammonium chloride)/poly(styrenesulfonate) (PDAC/PSS) layer-by-layer (LbL) assemblies are investigated. For MgCl 2 and CaCl 2, two swelling regimes are observed depending on the salt concentration of the overlying solution. Below 0.17 M MgCl 2 or CaCl 2, swelling appears to arise from electrostatic repulsion within the multilayer. Above 0.17 M MgCl 2 or CaCl 2, the multilayer swells linearly with the concentration of the overlying solution due to doping; the degree of swelling in this linear regime depends on the hydration shell of the doping ions. Alternatively, swelling with no concentration dependence was observed for Na 2 SO 4 . Unlike the swelling behavior, the thermal transition temperature observed in hydrated films shifts to higher temperatures with increasing concentration of Na 2 SO 4 . However, when hydrated with CaCl 2 or MgCl 2 solutions, the transition remains relatively constant within error. Results are discussed in terms of the salt ion’s hydration shell and Donnan exclusion.