Structure and phase behavior of poly(acrylic acid)-ferric ion complex aqueous solutions

Area of Science:

• Polymer Science
• Solution Chemistry
• Materials Science

Background:

• Poly(acrylic acid) (PAA) is a weak polyelectrolyte in aqueous solutions.
• Ferric ions (Fe3+) interact with PAA's carboxylic acid groups, influencing solution properties.
• Understanding these interactions is crucial for materials design and applications.

Purpose of the Study:

• To investigate the conformational evolution and phase behavior of PAA solutions with added ferric ions.
• To identify and characterize the transitions occurring upon increasing ferric ion concentration.
• To elucidate the molecular mechanisms governing these transitions through detailed analysis.

Main Methods:

• Small-angle X-ray scattering (SAXS) to probe polymer conformation.
• Turbidity measurements to detect phase separation.
• Zeta-potential and pH measurements to understand charge dynamics and ion coordination.

Main Results:

• Two key transitions were observed: a polyelectrolyte to apparent good solution transition and a phase separation.
• SAXS showed the disappearance of the polyelectrolyte peak, indicating conformational changes.
• Turbidity increased sharply during phase separation, correlating with charge inversion and dense Fe3+ coordination.

Conclusions:

• The study identifies three distinct regions: polyelectrolyte, good solution, and phase separation, based on pH and Fe3+ concentrations.
• Ferric ion coordination with PAA dictates the solution's conformational evolution and phase behavior.
• Phase separation occurs when PAA chains become densely coordinated with Fe3+ ions, leading to charge inversion.