Ion pairing and/or ion clustering have recently been implicated as potential precursors for nucleation of crystalline materials (Demichelis et al1; Henzler et al2). However, predicting the stability of these species relies in part on interpretation by atomistic computational simulation, which should be validated against accurate experimental measurements. It is particularly imperative to determine the ion solvation structure to understand the underlying dynamics, since they are both intertwined and form a close functional relationship with one another. Here, classical molecular dynamics (CMD) results are refined by comparison to neutron diffraction with isotope substitution (NDIS) data of saturated salt solutions. NDIS method and the corresponding radial distribution functions (RDF) effectively measure the structure of solvation and ion pairing in a direct way and provide a valuable source for validation of MD simulations and force field parameterization. In this talk, we investigated CaCl2(aq)and ZnCl2(aq)solutions and used the Cl-isotopes to systematic explore the ion pair formation and changes in Cl-solvation structures. There has been much debate over the formation of ion-pairing between Ca-Cl, whereas Zn-Cl is known to form contact ion pairs. We find that there is an apparent inherent downside with formal, simple point charge-type force fields, because these force fields are not able to capture accurate descriptions of the solvation structures. Since the solvent structure of divalent ions are tremendously impacted by these effects, a previously developed method is implemented where charges on the dissolved species are scaled to reflect the electronic continuum correction (ECC). Modifications are made to these force fields to correct ion hydration structures by de-convoluting experimental NDIS/RDF and comparing them to MD simulation results. This leads to a clear picture of, and capacity to simulate, chloride ion-pair formation and solvation structures in the studied system. A comprehensive comparison between Formal charges and ECC helped us to determine that ECC is a better model to elucidate the solvation structure and ion pairing in aqueous zinc and calcium chloride solutions.

Chloride ion solvation, dynamics and ion-pairing interactions in concentrated aqueous solutions: Combined computational simulation and neutron diffraction with isotopic substitution

2019 Spring National Meeting

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