Extreme dependence of dynamics on concentration in highly crowded polyelectrolyte solutions

Area of Science:

• Polymer Physics
• Soft Matter Science
• Biophysical Chemistry

Background:

• Polyelectrolytes are crucial in biological and synthetic systems.
• Understanding their dynamic behavior is key to controlling system properties.

Purpose of the Study:

• To investigate the concentration dependence of polyelectrolyte dynamics.
• To compare polyelectrolyte behavior with neutral polymer systems.
• To explore the role of environmental conditions (pH, counterions) on dynamics.

Main Methods:

• Single-particle tracking to measure individual polyelectrolyte chain diffusivity.
• Viscosity measurements of concentrated polylysine solutions.
• Application of the Vrentas-Duda free-volume theory.

Main Results:

• Observed strong scaling laws: diffusivity D ~ c^-6.1 and viscosity η ~ c^7.2.
• These scaling behaviors deviate significantly from theoretical predictions.
• Dynamics were found to be universal across different pH and counterion conditions.
• Excluding counterion mass unified dynamics across all tested conditions when analyzed with free-volume theory.

Conclusions:

• Concentrated polyelectrolyte solutions exhibit hindered dynamics, likely due to large effective excluded volumes leading to glassy behavior.
• The Vrentas-Duda free-volume theory provides a framework to understand these dynamics, especially when counterion mass is considered.
• Findings are relevant for understanding mobility in crowded biological environments like the cell interior.