Ion Solvation and Water Structure in an Aqueous Sodium Chloride Solution in the Gigapascal Pressure Range

Revealing the Molecular Origin of Anisotropy around Chloride Ions in Bulk Water

A clear picture of the local solvation structure around halide anions in liquid water remains elusive. This discussion has been stimulated by pioneering simulation results that proposed a "hydrophobic cavity" around anions in the bulk, which is analogous to air at the air-water interface. However, there is also sound experimental and theoretical evidence that halide ions are rather symmetrically solvated in the bulk, leading to a different viewpoint. Using extensive ab initio molecular dynamics simulations of an aqueous Cl- solution, we indeed find an anisotropic arrangement of H-bonded versus interstitial water molecules. The latter are not H-bonded to the anions and thus do not couple much electronically to Cl-. The resulting purely electronic anisotropy of the local solvation environment correlates with that structural anisotropy, which however should not be understood as an empty cavity─as it would be at the air-water interface─but rather contains interstitial water molecules.

The role of urea in formation of the sodium acetate trihydrate (SAT)-urea eutectic liquid: a neutron diffraction and isotopic substitution study

Neutron diffraction with isotopic substitution has been used to investigate the structure of the liquid sodium acetate trihydrate-urea eutectic (mole fraction (χurea) of 0.60) at 50 °C. Urea competes with acetate anions and water molecules in the solvation of sodium ions, displacing water and, simultaneously, stabilising the liberated 'excess' water through hydrogen bonding between water and urea molecules in the eutectic liquid. This provides a direct insight into the role of urea as both denaturant and hydrogen-bond network former in generating eutectic liquids.

The hydrogen-bond network in sodium chloride tridecahydrate: analogy with ice VI

The structure of a recently found hyperhydrated form of sodium chloride (NaCl·13H2O and NaCl·13D2O) has been determined by in situ single-crystal neutron diffraction at 1.7 GPa and 298 K. It has large hydrogen-bond networks and some water molecules have distorted bonding features such as bifurcated hydrogen bonds and five-coordinated water molecules. The hydrogen-bond network has similarities to ice VI in terms of network topology and disordered hydrogen bonds. Assuming the equivalence of network components connected by pseudo-symmetries, the overall network structure of this hydrate can be expressed by breaking it down into smaller structural units which correspond to the ice VI network structure. This hydrogen-bond network contains orientational disorder of water molecules in contrast to the known salt hydrates. An example is presented here for further insights into a hydrogen-bond network containing ionic species.

In situ Raman and X-ray scattering of a single supersaturated aqueous Mg(NO3)2 droplet ultrasonically levitated

A single liquid droplet in the air generated by ultrasonic levitation provides such analytical advantages as a small sample volume (~ μL) for expensive proteins, container-free condition for deeply supercooling and supersaturation, time-dependent observation, and homogeneous rapid mixing. The investigation of the properties and structure of a droplet at a molecular level is highly needed for understanding the physicochemical behaviors of a droplet and an underlying mechanism of processes in the droplet. We develop in situ Raman and synchrotron X-ray scattering methods of a single liquid droplet of ~ 1 mm size ultrasonically levitated. The composition of a supersaturated Mg(NO₃)₂ droplet and speciation in the droplet are determined by analyzing the nitrate N-O and the water O-H stretching vibrational Raman bands. The X-ray interference function of an supersaturated Mg(NO₃)₂ droplet is subjected to an empirical potential structure refinement modeling to reveal the ion solvation, association, and solvent water structure. Furthermore, crystallization of Mg(NO₃)₂⋅nH₂O from a saturated droplet is observed and identified.

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

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.

Ion association in hydrothermal aqueous NaCl solutions: implications for the microscopic structure of supercritical water

Knowledge of the microscopic structure of fluids and changes thereof with pressure and temperature is important for the understanding of chemistry and geochemical processes. In this work we investigate the influence of sodium chloride on the hydrogen-bond network in aqueous solution up to supercritical conditions. A combination of in situ X-ray Raman scattering and ab initio molecular dynamics simulations is used to probe the oxygen K-edge of the alkali halide aqueous solution in order to obtain unique information about the oxygen's local coordination around the ions, e.g. solvation-shell structure and the influence of ion pairing. The measured spectra exhibit systematic temperature dependent changes, which are entirely reproduced by calculations on the basis of structural snapshots obtained via ab initio molecular dynamics simulations. Analysis of the simulated trajectories allowed us to extract detailed structural information. This combined analysis reveals a net destabilizing effect of the dissolved ions which is reduced with rising temperature. The observed increased formation of contact ion pairs and occurrence of larger polyatomic clusters at higher temperatures can be identified as a driving force behind the increasing structural similarity between the salt solution and pure water at elevated temperatures and pressures with drawback on the role of hydrogen bonding in the hot fluid. We discuss our findings in view of recent results on hot NaOH and HCl aqueous fluids and emphasize the importance of ion pairing in the interpretation of the microscopic structure of water.

Solvation of simple ions in water at extreme conditions

The interaction of ions and water at high pressure and temperature plays a critical role in Earth and planetary science yet remains poorly understood. Aqueous fluids affect geochemical properties ranging from water phase stability to mineral solubility and reactivity. Here, we report first-principles molecular dynamics simulations of mono-valent ions (Li⁺, K⁺, Cl^-) as well as NaCl in liquid water at temperatures and pressures relevant to the Earth's upper mantle (11 GPa, 1000 K) and concentrations in the dilute limit (0.44-0.88 m), in the regime of ocean salinity. We find that, at extreme conditions, the average structural and vibrational properties of water are weakly affected by the presence of ions, beyond the first solvation shell, similar to what was observed at ambient conditions. We also find that the ionic conductivity of the liquid increases in the presence of ions by less than an order of magnitude and that the dielectric constant is moderately reduced by at most ∼10% at these conditions. Our findings may aid in the parameterization of deep earth water models developed to describe water-rock reactions.