Strong stretching theory for pH-responsive polyelectrolyte brushes in large salt concentrations

Area of Science:

• Polymer physics and physical chemistry
• Soft matter science
• Electrochemistry

Background:

• Polyelectrolyte (PE) brushes are crucial in various applications, but their behavior at high salt concentrations is poorly understood.
• Existing theories often fail to capture the complexities of PE brushes in regimes with several molar salt concentrations.
• Molecular simulations encounter these high salt conditions, necessitating advanced theoretical models.

Purpose of the Study:

• To develop a theoretical framework for describing the thermodynamics, configuration, and electrostatics of strongly-stretched, pH-responsive polyelectrolyte brushes at high salt concentrations.
• To investigate the unexplored regime of several molar salt concentrations for PE brushes.
• To incorporate non-Poisson-Boltzmann effects into the theoretical model.

Main Methods:

• Utilized the augmented Strong Stretching Theory (SST) to model PE brushes.
• Incorporated three non-Poisson-Boltzmann (non-PB) effects: ion-ion correlations, solvent polarization, and finite ion size.
• Analyzed the individual and combined impacts of these non-PB effects on brush properties and electric double layer (EDL) electrostatics.
• Validated theoretical predictions against all-atom molecular dynamics (MD) simulations.

Main Results:

• Ion-ion correlations and solvent polarization reduce brush height, increase monomer density, and strengthen the near-wall EDL potential.
• Finite ion size increases brush height, decreases monomer density, and weakens the near-wall EDL potential.
• Ion-ion correlations and solvent polarization effects dominate over finite size effects, dictating overall brush behavior.
• Theoretical predictions show excellent agreement with MD simulation results.

Conclusions:

• The developed theory accurately describes PE brush behavior in high salt concentrations by including non-PB effects.
• Ion-ion correlations and solvent polarization are key factors influencing PE brush configuration and electrostatics at high salt.
• The findings provide a theoretical basis for understanding and designing PE brush systems in demanding ionic environments.