Ionisation and swelling behaviour of weak polyampholyte core-shell networks - a Monte Carlo study

Area of Science:

• Polymer Science
• Soft Matter Physics
• Computational Chemistry

Background:

• Polyampholyte microgels possess tunable network charge influenced by solution pH, enabling charge reversal.
• Traditional titration methods struggle to differentiate monomer ionization and charge distribution within microgels.
• Metropolis Monte Carlo simulations offer a powerful tool to determine individual monomer ionization degrees and spatial charge distribution.

Purpose of the Study:

• Investigate the pH-dependent ionization and swelling behavior of polyampholyte core-shell microgels.
• Compare the ionization and swelling characteristics of core-shell microgels with previously studied alternating polyampholyte microgels.
• Analyze the impact of spatial separation of acidic and basic monomers on microgel properties.

Main Methods:

• Utilized Metropolis Monte Carlo simulations with the constant-pH method.
• Employed a bead-spring model to represent the polyampholyte microgel network.
• Simulated microgel behavior under good solvent conditions across a range of pH values.

Main Results:

• Microgel volume exhibits a U-shaped dependence on pH, influenced by the dissociation constants of acidic and basic monomers.
• Microgel volume correlates linearly with the total network charge.
• Spatial separation in core-shell structures leads to less enhanced ionization compared to alternating structures, though inter-species ionization influence persists.
• The isoelectric point shifts to higher pH due to higher core charge density.
• Added salt narrows the U-shaped volume transition curve by altering Donnan equilibrium and providing electrostatic screening.

Conclusions:

• Metropolis Monte Carlo simulations accurately determine ionization and swelling in polyampholyte microgels.
• Core-shell architecture influences ionization and swelling differently than alternating structures.
• Salt concentration significantly impacts microgel volume transition behavior through electrostatic effects.