Structure and dynamics of aqueous NaCl solutions at high temperatures and pressures

Autor: Annalisa Polidori, Mathieu Salanne, Anita Zeidler, Burkhard Annighöfer, Stefan Klotz, Ruth F. Rowlands, Henry E. Fischer, Philip S. Salmon
Rok vydání: 2021
Předmět:
Zdroj: Polidori, A, Rowlands, R, Zeidler, A, Salanne, M, Fischer, H, Annighoefer, B, Klotz, S & Salmon, P 2021, ' Structure and dynamics of aqueous NaCl solutions at high temperatures and pressures ', Journal of Chemical Physics, vol. 155, no. 19, 194506 . https://doi.org/10.1063/5.0067166
ISSN: 1089-7690
0021-9606
DOI: 10.1063/5.0067166
Popis: The structure of a concentrated solution of NaCl in D2O was investigated by in situ high-pressure neutron diffraction with chlorine isotope substitution to give site-specific information on the coordination environment of the chloride ion. A broad range of densities was explored by first increasing the temperature from 323 to 423 K at 0.1 kbar and then increasing the pressure from 0.1 to 33.8 kbar at 423 K, thus mapping a cyclic variation in the static dielectric constant of the pure solvent. The experimental work was complemented by molecular dynamics simulations using the TIP4P/2005 model for water, which were validated against the measured equation of state and diffraction results. Pressure-induced anion ordering is observed, which is accompanied by a dramatic increase in the Cl–O and O–O coordination numbers. With the aid of bond-distance resolved bond-angle maps, it is found that the increased coordination numbers do not originate from a sizable alteration to the number of either Cl⋯D–O or O⋯D–O hydrogen bonds but from the appearance of non-hydrogen-bonded configurations. Increased pressure leads to a marked decrease in the self-diffusion coefficients but has only a moderate effect on the ion–water residence times. Contact ion pairs are observed under all conditions, mostly in the form of charge-neutral NaCl0 units, and coexist with solvent-separated Na+–Na+ and Cl−–Cl− ion pairs. The exchange of water molecules with Na+ adopts a concerted mechanism under ambient conditions but becomes non-concerted as the state conditions are changed. Our findings are important for understanding the role of extreme conditions in geochemical processes.
Databáze: OpenAIRE
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