期刊
JOURNAL OF CHEMICAL PHYSICS
卷 155, 期 19, 页码 -出版社
AIP Publishing
DOI: 10.1063/5.0067166
关键词
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资金
- EPSRC [EP/J009741/1]
- Royal Society-EPSRC Dorothy Hodgkin Research Fellowship
- Institut Laue Langevin (ILL)
- University of Bath [ILL-1353.1]
- EPSRC [EP/J009741/1] Funding Source: UKRI
The structure of a concentrated solution of NaCl in D2O and the coordination environment of the chloride ion were investigated through experimental and simulation studies. It was found that increased pressure resulted in an increase in coordination numbers and a decrease in self-diffusion coefficients. Contact ion pairs were observed under all conditions, with the exchange mechanism between water molecules and Na+ changing as state conditions varied.
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 ClMIDLINE HORIZONTAL ELLIPSISD-O or OMIDLINE HORIZONTAL ELLIPSISD-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.
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