Non-swellability of polyelectrolyte gel in divalent salt solution due to aggregation formation

The swelling behavior of polyelectrolyte gels depends on their ambient conditions owing to the ionic pressure of the Gibbs-Donnan equilibrium. In a polyelectrolyte gel with a high fixed ion density, the counterion conden-sation effect influences the Gibbs-Donnan equilibrium. In particular, the mechanism of the divalent salt effect remains unclear due to its complexity. This study investigated the swelling properties of polyelectrolyte gels in a divalent CaCl2 salt solution using model polyelectrolyte gels called tetra-polyacrylic acid-polyethylene glycol gels (Tetra-PAA-PEG gels). In [Ca2+] < 0.1 mM, the swelling ratios increased with increasing pH. With increasing Ca2+ ions concentration, the gel showed the non-swellability independent of pH, which cannot be described by the simple mean-field theories. The viscoelastic and small-angle X-ray scattering measurements indicate that an aggregation formation due to the divalent salt induces an anomaly even in the model system. These findings will help understand the swelling behavior of polyelectrolyte gels in divalent salt systems.

Automated summary

The swelling behavior of polyelectrolyte gels is influenced by the ambient conditions, including the ionic pressure of the Gibbs-Donnan equilibrium and the conden-sation effect of counterions. This study investigated the swelling properties of polyelectrolyte gels in a divalent CaCl2 salt solution using model polyelectrolyte gels. The results showed that the swelling ratio increased with increasing pH, but the gel showed non-swellability independent of pH with increasing Ca2+ ion concentration.