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

The effects of CaCl₂, MgCl₂, and Na₂SO₄ on the swelling response and the thermal transition of poly­(diallyl­dimethyl­ammonium chloride)/poly­(styrene­sulfonate) (PDAC/PSS) layer-by-layer (LbL) assemblies are investigated. For MgCl₂ and CaCl₂, two swelling regimes are observed depending on the salt concentration of the overlying solution. Below 0.17 M MgCl₂ or CaCl₂, swelling appears to arise from electrostatic repulsion within the multilayer. Above 0.17 M MgCl₂ or CaCl₂, 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 Na₂SO₄. Unlike the swelling behavior, the thermal transition temperature observed in hydrated films shifts to higher temperatures with increasing concentration of Na₂SO₄. However, when hydrated with CaCl₂ or MgCl₂ solutions, the transition remains relatively constant within error. Results are discussed in terms of the salt ion’s hydration shell and Donnan exclusion