The "good," the "bad," and the "hidden" in neutron scattering and molecular dynamics of ionic aqueous solutions

Denys Biriukov, Hsiu Wen Wang, Nikhil Rampal, Carmelo Tempra, Patrik Kula, Joerg C. Neuefeind, Andrew G. Stack, Milan Předota

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18 Scopus citations

Abstract

We characterize a concentrated 7.3 m CaCl2 solution, combining neutron diffraction with chloride isotopic substitution (Cl-NDIS) in null water and molecular dynamics (MD) simulations. We elucidate the solution structure, thermodynamic properties, and extent of ion pairing previously suggested as concentration-dependent and often not observed at lower concentrations. Our Cl-NDIS measurements designate the solvent-shared ion pairing as dominant and the contact ion pairing (CIP) as insignificant even under conditions close to the solubility limit. The MD models parameterized against neutron diffraction with calcium isotopic substitution (Ca-NDIS) overestimate CIP despite successfully reproducing most of the Cl-NDIS signal. This drawback originates from the fact that Ca2+-Cl- interactions were primarily "hidden"in the Ca-NDIS signal due to overlapping with Ca2+-Ow and Ca2+-Hw contributions to the total scattering. Contrary, MD models with moderate CIP and possessing generally good performance at high concentrations fail to reproduce the NDIS measurements accurately. Therefore, the electronic polarization, introduced in most of the recent MD models via scaling ionic charges, resolves some but not all parameterization drawbacks. We conclude that despite improving the quality of MD models "on average,"the question "which model is the best"has not been answered but replaced by the question "which model is better for a given research."An overall "good"model can still be inappropriate or, in some instances, "bad"and, unfortunately, produce erroneous results. The accurate interpretation of several NDIS datasets, complemented by MD simulations, can prevent such mistakes and help identify the strengths, weaknesses, and convenient applications for corresponding computational models.

Original languageEnglish
Article number194505
JournalJournal of Chemical Physics
Volume156
Issue number19
DOIs
StatePublished - May 21 2022

Funding

D.B. and M.P. were supported by the Ministry of Education, Youth, and Sports of the Czech Republic, Project No. LTAUSA17163. C.T. acknowledges the International Max Planck Research School for Many-Particle Systems in Structured Environments hosted by the Max Planck Institute for the Physics of Complex Systems, Dresden, Germany. H.-W.W., N.R., J.C.N., and A.G.S. were supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. The research at the Spallation Neutron Source at Oak Ridge National Laboratory was supported by the Scientific User Facilitates Division, BES, DOE. The authors also thank Pavel Jungwirth and Hector Martinez-Seara for fruitful discussions related to this research. The sharing of ab initio molecular dynamics data by Tom\u00E1\u02C7s Martinek is also appreciated.

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