Structure and Dynamics of Low-Density and High-Density Liquid Water at High Pressure

Abstract

Liquid water has a primary role in ruling life on Earth in a wide temperature and pressure range as well as a plethora of chemical, physical, geological, and environmental processes. Nevertheless, a full understanding of its dynamical and structural properties is still lacking. Water molecules are associated through hydrogen bonds, with the resulting extended network characterized by a local tetrahedral arrangement. Two different local structures of the liquid, called low-density (LDW) and high-density (HDW) water, have been identified to potentially affect many different chemical, biological, and physical processes. By combining diamond anvil cell technology, ultrafast pump-probe infrared spectroscopy, and classical molecular dynamics simulations, we show that the liquid structure and orientational dynamics are intimately connected, identifying the P-T range of the LDW and HDW regimes. The latter are defined in terms of the speeding up of the orientational dynamics, caused by the increasing probability of breaking and reforming the hydrogen bonds.

Pressure-induced ion pairing in MgSO4 solutions: Implications for the oceans of icy worlds

“…The experiments indicate a transition from an open structure in low-density water (LDW) to a structure with a collapsed second coordination shell in high-density water (HDW) (Soper and Ricci, 2000). Based on femtosecond infrared pump-probe spectroscopy on a D 2 O-H 2 O mixture, the LDW-HDW transition is at ~0.25 GPa at 273 K and ~0.2 GPa at 298 K, but not observed at 363 K (Fanetti et al, 2014).…”

“…This was not observed at 50-150 °C, where the percentage of CIP in 0.75 and 2.25 molal MgSO 4 solutions generally decreased with pressure along all Figure 1 Raman spectroscopic results at 21 ± 1 °C as a function of pressure. The shadowed band is the pressure range of the LDW-HDW transition at about 20 °C (Fanetti et al, 2014). (Helgeson et al, 1981).…”

“…This assumption yields a minimum difference in predicted and observed CIP. The shadowed band is the pressure range of the LDW-HDW transition at about 20 °C (Fanetti et al, 2014).…”

“…Based on Coulomb's law, the observed increased contact ion pairing must therefore be caused by a decrease in the distance between cation and anion. An increasing tendency to form ion pairs with pressure can thus be related to increased hydrogen-bond breaking with pressure resulting in a collapsed second coordination shell in HDW (Soper and Ricci, 2000;Krisch et al, 2002;Fanetti et al, 2014). Therefore, the inferred relationship between water structure and ion pairing implies that significant interaction between ions and water exists at distances larger than the first shell, which has long been a matter of debate (c.f.…”

“…About 75 % of the salts produced by low-temperature aqueous alteration of chondrite under oxidising conditions are magnesium sulphate hydrates (Hogenboom et al, 1995 et al, 2012), Europa (Spohn and Schubert, 2003;Hand and Chyba, 2007;Valenti et al, 2012), Ganymede (Vance et al, 2014;Saur et al, 2015), and Callisto (Kuskov and Kronrod, 2005) with approximate depths of the subsurface ocean boundaries in km. The shadowed band is the pressure range of the LDW-HDW transition at about 20 °C (Fanetti et al, 2014).…”