The extent of ion association in aqueous salt solutions is critically important to understand both equilibrium (e.g., speciation, activity-concentration relationships) and dynamic (e.g., viscosity, diffusivity, and conductivity) processes. However, the molecular structure of the associated ions, and their propensity to form, is unclear even for ‘simple’ salt solutions. In addition, existing estimates of the equilibrium constants for ion association found in thermodynamic speciation databases are often fit parameters from activity-concentration relationship models which can be incoherent with literature data. As a model system to explore ion association, we have investigated ZnCl<sub>2</sub>(aq) and CaCl<sub>2</sub>(aq) solutions because of their propensity to form ion pairs and uncertain ion pair formation, respectively. These are explored using Neutron Diffraction with Isotope Substitution (NDIS) and Classical Molecular Dynamics (CMD) simulations. A force field for the CMD is corrected for ion hydration structures by deconvoluting the experimental pair distribution function of the solutions. This leads to a clear picture of, and capacity to simulate, chloride ion-pair formation and solvation structures in the studied system. The tuned force field is used to study the concentration-dependent reaction dynamics. We find that both water solvent and counter-cations control local specific bonding interactions and the free energy barriers for the interconversions from one stable coordination state to another increase at higher concentration. This suggests hindered reaction dynamics with ions assuming locally-favored structures, which in turn control the solution structure and alter the reaction dynamics in highly concentrated solutions.

Computational insights into the origin of concentration-dependent reaction mechanisms in aqueous salt solutions

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.