Beyond Electrostatic Screening: Effect of Ion Pairing on Acid-Base Equilibria in Complex Electrolyte Solutions

Area of Science:

• Solution Chemistry
• Physical Chemistry
• Biochemistry

Background:

• Acid-base equilibria are fundamental but poorly understood in complex salt mixtures.
• Individual salts cause ion-specific shifts in weak acid pKa values.
• Molecular origins of these shifts in multicomponent electrolytes require further investigation.

Purpose of the Study:

• Investigate how ion pairing and specific ion effects govern acetic acid's pKa in single and mixed sodium salt solutions.
• Determine the role of cation-anion association versus electrostatic screening in pKa shifts.
• Develop a predictive model for pH control in multisalt environments.

Main Methods:

• Combined experimental techniques with constant pH molecular dynamics simulations.
• Studied acetic acid pKa in solutions with various sodium salts (sulfate, chloride, iodide).
• Analyzed ion-ion interactions and their impact on acid-base equilibria.

Main Results:

• Cation-anion association, not solely electrostatic screening, dictates pKa shifts across different anions.
• Mixed sodium chloride-sulfate solutions showed non-additive effects on acetic acid pKa due to complex ion interactions.
• An ion-pairing-augmented Debye-Hückel model was developed and validated.

Conclusions:

• Established a mechanistic basis for understanding pH-dependent phenomena in complex electrolyte systems.
• The developed model quantitatively captures pKa shifts, offering predictive power for multisalt environments.
• Findings are relevant to biochemistry, soft matter, and industrial applications requiring precise pH control.