Donnan Contribution and Specific Ion Effects in Swelling of Cationic Hydrogels are Additive: Combined High-Resolution...

Area of Science:

• Materials Science
• Chemical Engineering
• Computational Modeling

Background:

• Hydrogel swelling is influenced by ionic content and environmental factors.
• Existing models often lack detailed consideration of specific ion effects and broad salt concentration ranges.
• Understanding these factors is crucial for predicting hydrogel behavior in diverse applications.

Purpose of the Study:

• To expand finite element modeling for hydrogel swelling analysis.
• To incorporate Donnan contributions and specific ion effects over an extended salt concentration range.
• To develop a predictive model for hydrogel response to aqueous salt solutions.

Main Methods:

• Experimental determination of hydrogel swelling dynamics and equilibrium using high-resolution interferometry.
• Step-wise increase in NaCl and NaBr concentrations up to 2 M for acrylamide and cationic acrylamide-based hydrogels.
• Development of a novel finite element model incorporating salt-specific, concentration-dependent Flory-Huggins parameters.

Main Results:

• Uncharged hydrogels showed increased swelling with salt concentration.
• Cationic hydrogels initially shrank up to 0.1 M NaCl/NaBr due to Donnan effects, then swelled at higher concentrations.
• Specific ion effects were additive to Donnan contributions and occurred in distinct salt concentration ranges.

Conclusions:

• The developed finite element model accurately accounts for hydrogel swelling behavior in aqueous salt solutions.
• This model uniquely integrates specific ionic effects, improving mechanistic insight into hydrogel swelling.
• The findings enable better prediction of hydrogel responses to environmental changes.