Underscreening in concentrated electrolytes: re-entrant swelling in polyelectrolyte brushes

Area of Science:

• Polymer Science
• Physical Chemistry
• Materials Science

Background:

• Hypersaline environments and technological processes involve high salt concentrations.
• Empirical studies show a discrepancy between predicted and observed screening lengths at high salt, termed underscreening.
• Classical electrolyte theory struggles to explain phenomena at extreme salt concentrations.

Purpose of the Study:

• To investigate the phenomenon of underscreening using a cationic polyelectrolyte brush.
• To analyze the structural changes and swelling response of polymer brushes in varying salt concentrations.
• To compare experimental findings with theoretical predictions.

Main Methods:

• Synthesis of cationic polyelectrolyte brushes: Poly(2-(methacryloyloxy)ethyl)trimethylammonium (PMETAC).
• Neutron reflectometry and spectroscopic ellipsometry to monitor internal structure and swelling.
• Experimental measurements across a range of monovalent and multivalent electrolyte concentrations.

Main Results:

• Monotonic collapse of PMETAC brushes with increasing monovalent electrolyte concentration.
• Non-monotonic re-entrant swelling of PMETAC brushes in multivalent electrolytes at high concentrations, indicating underscreening.
• Numerical self-consistent field theory predictions align with experiments at low-to-moderate salt concentrations.

Conclusions:

• Classical electrolyte theory is insufficient for describing screening lengths at high salt concentrations.
• Re-entrant swelling in polyelectrolyte brushes is consistent with the underscreening phenomenon.
• The study highlights the need for refined theories to explain polymer behavior in hypersaline conditions.