Dresden 2026 – scientific programme
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CPP: Fachverband Chemische Physik und Polymerphysik
CPP 13: Charged Soft Matter, Polyelectrolytes and Ionic Liquids
CPP 13.2: Talk
Monday, March 9, 2026, 15:30–15:45, ZEU/0260
Beyond electrostatic screening: Effect of ion pairing on acid-base equilibria in complex electrolyte solutions — •Varun Mandalaparthy1, Johannes Hunger2, Mischa Bonn2, and Nico van der Vegt1 — 1Technical University, Darmstadt — 2Max Planck Institute for Polymer Research, Mainz
The modulation of acid-base equilibria by salts is central to solution chemistry, yet the molecular origins of these effects in multi-component electrolyte mixtures remain incompletely understood. Building on evidence that individual salts induce ion-specific shifts in the pKa values of weak acids, we combine experiments with constant-pH molecular dynamics simulations to examine how ion pairing and specific ion interactions regulate the pKa of acetic acid in single and mixed sodium salt solutions. Across anions of differing charge density (sulfate, chloride, iodide), we show that cation-anion association-rather than electrostatic screening alone-governs the observed pKa changes. In solutions containing mixtures of sodium chloride and sodium sulfate, the impact on acetic acid pKa deviates from simple additivity because of complex ion-ion interactions. We present an ion-pairing-enhanced Debye-Hückel model that quantitatively reproduces these behaviors, offering a predictive framework for pH regulation in multi-salt environments. This work provides a mechanistic foundation for interpreting and forecasting pH-dependent behavior in complex systems relevant to biochemistry and soft matter.
REF: Mandalaparthy et al. ChemRxiv. 2025; doi:10.26434/chemrxiv-2025-p2chl (under review at J. Phys. Chem. Lett.)
Keywords: Constant pH MD simulations; Hofmeister chemistry; Ion specific effects; Debye-Hückel theory; Solvation thermodynamics