Local pH and Effective pK of a Polyelectrolyte Chain: Two Names for One Quantity?

Area of Science:

• Physical Chemistry
• Polymer Science
• Supramolecular Chemistry

Background:

• Local pH near polyelectrolytes is often measured indirectly using fluorescent pH indicators.
• This method relies on shifts in the indicator's effective acidity constant.
• The precise meaning and accurate determination of

Purpose of the Study:

• To investigate the accuracy of indirect local pH measurements near polyelectrolyte chains.
• To elucidate the factors contributing to discrepancies between measured and actual local pH.
• To define the physical meaning of "local pH" in the context of polymer-bound indicators.

Main Methods:

• Combined experimental techniques with molecular simulations.
• Utilized fluorescent pH indicators attached to polyelectrolyte chains.
• Analyzed correlations between polymer conformation, small-ion distribution, and indicator ionization state.

Main Results:

• The observed shift in the ionization constant is slightly lower than the theoretical difference between bulk pH and local pH.
• An offset, caused by coupled fluctuations, was quantified.
• These fluctuations involve chain conformation, small-ion distribution, and fluorophore ionization.

Conclusions:

• Indirect local pH measurements using fluorescent indicators require correction for correlated fluctuations.
• The term "local pH" and its experimental determination are nuanced due to these dynamic effects.
• Accurate characterization of polyelectrolyte microenvironments necessitates considering these complex interplay.