Effect of counterion size on polyelectrolyte conformations and thermodynamics

Area of Science:

• Theoretical physics
• Polymer science
• Physical chemistry

Background:

• Polyelectrolyte (PE) chains exhibit complex behavior influenced by counterions.
• Understanding the role of counterion size is crucial for predicting PE properties.

Purpose of the Study:

• To develop a theoretical model investigating the effect of counterion size on PE behavior.
• To analyze energy and entropy contributions, including ion-pair interactions and excluded volume effects.

Main Methods:

• Utilized the Edwards-Muthukumar Hamiltonian for PE self-energy calculation.
• Modeled condensed ions within a cylindrical volume and interactions using virial coefficients.
• Incorporated dipolar attraction to study coil-globule transitions.

Main Results:

• Counterion size influences PE effective charge, size, and thermodynamic behavior.
• Identified competing effects of ion-pair energy, free ion entropy, and excluded volume interactions.
• Found that smaller counterions enhance condensation, reducing free ion entropy and affecting transition order.

Conclusions:

• The model provides insights into entropy- and energy-driven regimes of PE conformation.
• Counterion size is a critical parameter controlling PE condensation and thermodynamic properties.
• Model predictions align with simulation trends and extend theories with point-like counterions.