Structural characterization of eutectic aqueous NaCl solutions under variable temperature and pressure conditions

Abstract

Structural characterization of eutectic aqueous NaCl solutions under variable temperature and pressure conditions †The structure of amorphous NaCl solutions produced by fast quenching is studied as a function of pressure, up to 4 GPa, by combined neutron diffraction experiments and classical molecular dynamics simulations. Similarly to LiCl solutions the system amorphizes at ambient pressure in a dense phase structurally similar to the e-HDA phase in pure water. The measurement of the static structure factor as a function of pressure allowed us to validate a new polarizable force field developed by Tazi et al., 2012, never tested under non-ambient conditions.We infer from simulations that the hydration shells of Na + cations form well defined octahedra composed of both H 2 O molecules and Cl À anions at low pressure. These octahedra are gradually broken by the seventh neighbour moving into the shell of first neighbours yielding an irregular geometry. In contrast to LiCl solutions and pure water, the system does not show a polyamorphic transition under pressure. This confirms that the existence of polyamorphism relies on the tetrahedral structure of water molecules, which is broken here.

Effect of salt on the H-bond symmetrization in ice

“…Due to its high vitrification ability, LiCl-water also represents the ideal system to produce the salty amorphous ice phases serving as precursors for salty ice VII crystallization under pressure (37-39) (details are given in Sample Preparation). Despite a different ability to vitrify (47)(48)(49) and to undergo polyamorphic transitions at ambient and moderate pressures (39), LiCl and more naturally abundant NaCl produce very similar modifications on water structure and equations of state at high pressure (39)(40)(41)(42).…”

“…by R = 6 aqueous solutions loaded into highpressure chambers of 30-40 μm in diameter in a rhenium gasket between two type IIa synthetic diamonds mounted on a symmetric Mao-Bell-type cell. A salt-doped amorphous sample was produced by hyperquenching R = 12 aqueous solution (39), loaded under liquid nitrogen, compressed to 4 GPa, and then annealed at this pressure, to promote salt inclusion in the ice lattice (37). Finally, R = 30 and R = 50 LiCl aqueous solutions were loaded in the cell at ambient temperature and compressed to 5 GPa, warmed at 400°C, and then quenched to 77 K by immersing the cell in liquid nitrogen.…”

“…Salty ice VII samples were produced following the procedure described in refs. [37][38][39]. by R = 6 aqueous solutions loaded into highpressure chambers of 30-40 μm in diameter in a rhenium gasket between two type IIa synthetic diamonds mounted on a symmetric Mao-Bell-type cell.…”

“…Recently, unexpected evidence has emerged that high-pressure water polymorphs can contain a substantial amount of small ions in salty ice structures (37)(38)(39)(40)(41)(42), and, similarly to what is observed in ice clathrates (43,44), they transform under pressure into bcc O lattices with ions either occupying interstices (37) or being substitutional to water molecules (37,39). These "filled" ice structures can be stable over several gigapascals and hundreds of Kelvins, conditions encountered in large ice bodies in the universe.…”

“…These "filled" ice structures can be stable over several gigapascals and hundreds of Kelvins, conditions encountered in large ice bodies in the universe. Combined neutron scattering experiments and molecular dynamics simulations have allowed sketching of the pressure and Temperature (p-T) phase diagram of LiCl-water and NaCl-water mixtures up to a few gigapascals, and have shown that the presence of salt suppresses the ordered ice VIII phase and modifies the boundary of the proton-disordered ice VII phase (37)(38)(39)(40)(41)(42). Orientational disorder of water molecules is also enhanced by the presence of salt, as demonstrated by the Significance Ice-VII and ice-X phases are the most stable forms of ice at high temperature and extreme pressures, typical of the interiors of satellites and planets.…”