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

Abstract

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 423K at 0.1 kbar and then increasing the pressure from 0.1 to 33.8 kbar at 423K, 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.