The pH inside a swollen polyelectrolyte gel: poly(N-vinylimidazole)

Area of Science:

• Polymer Chemistry
• Physical Chemistry
• Electrochemistry

Background:

• Internal pH of polyelectrolyte coils and ionic polymer networks is not directly measurable.
• Accurate internal pH is crucial for understanding polymer behavior, especially for ionizable polymers.
• Traditional pKa determination methods rely on external pH measurements, which can be inaccurate.

Purpose of the Study:

• To develop a simple model for calculating the internal pH of swollen ionic polymer networks.
• To investigate the influence of counterion condensation on internal pH and pKa.
• To provide a method for accurate pKa determination of polyelectrolytes.

Main Methods:

• A model based on Donnan equilibrium, counterion condensation, and mobile ion balance was developed.
• The model requires polymer concentration, initial solution pH, and equilibrium bath pH as inputs.
• Experimental validation was performed using chemically cross-linked poly(N-vinylimidazole) in acidic solutions.

Main Results:

• The calculated internal pH of the polymer gel was several units higher than the external bath pH.
• Counterion condensation slightly decreased the polymer's basicity.
• Apparent pKa values determined using external pH were significantly lower than actual internal pKa values.

Conclusions:

• The developed model accurately predicts internal pH in polyelectrolyte networks.
• External pH measurements lead to underestimation of polyelectrolyte pKa.
• Accurate internal pH determination is essential for precise characterization of ionizable polymers.