Polyelectrolyte complex coacervation by electrostatic dipolar interactions

Area of Science:

• Physical Chemistry
• Polymer Science
• Materials Science

Background:

• Complex coacervation involves liquid-liquid phase separation in oppositely charged polyelectrolyte solutions.
• It is driven by the spontaneous formation of polycation-polyanion complexes, forming a coacervate phase and a dilute phase.

Purpose of the Study:

• To model complex coacervation using a mean-field theory.
• To compute coacervate phase diagrams considering polymer composition, salt concentration, and temperature.
• To investigate the interplay of hydrophobicity and electrostatics in driving phase separation.

Main Methods:

• Utilized a mean-field theory accounting for ion entropy, electrostatic interactions, and polymer-solvent hydrophobicity.
• Treated polyelectrolyte complexes as flexible chains with dipolar and uniformly charged segments.
• Computed phase diagrams based on varying system parameters.

Main Results:

• Demonstrated that combined hydrophobicity and electrostatics, not either alone, drive phase separation for moderately hydrophobic polyelectrolytes.
• Observed that increased salt concentration, temperature, and composition asymmetry suppress coacervation.
• Noted that increased chain length promotes coacervation and preferential salt partitioning into the dilute phase.

Conclusions:

• The study provides a theoretical framework explaining key experimental observations in complex coacervation.
• Predicts the emergence of instability loops with two critical points, offering new insights into phase behavior.
• Highlights the crucial role of enhanced effective hydrophobicity due to dipolar attractions in driving phase separation.