Poly(ethylene oxide) Is Positively Charged in Aqueous Solutions

Area of Science:

• Polymer Chemistry
• Physical Chemistry
• Solution Behavior of Polymers

Background:

• Controversies exist regarding cation binding to poly(ethylene oxide) (PEO) chains in aqueous solutions.
• Understanding PEO chain interactions with ions is crucial for its applications in various fields.

Purpose of the Study:

• To provide single-molecular evidence for the charging of PEO chains by cation binding in aqueous solutions.
• To investigate the influence of salt concentration and ion identity on PEO chain behavior.

Main Methods:

• Photon-counting histogram method to determine local pH near PEO chains.
• Single-molecular electrophoresis experiments.
• Fluorescence correlation spectroscopy to measure charging extent and hydrodynamic radii.

Main Results:

• PEO chains exhibit a higher local pH near the chain, indicating positive charging due to cation binding (hydronium, sodium ions).
• PEO chains are weakly charged (~5% extent), with charging increasing in moderate NaCl concentrations.
• Increased salt concentration leads to PEO chain expansion followed by shrinkage, demonstrating polyelectrolyte-like behavior.
• The charging capacity of alkali cations follows the order: Li+ < Na+ < Cs+ < K+.

Conclusions:

• Single-molecule techniques confirm cation binding leads to positively charged PEO chains in aqueous solutions.
• PEO chains display polyelectrolyte behavior influenced by salt concentration and ion identity.
• The findings resolve controversies and provide insights into PEO-ion interactions for material science and nanotechnology.