A closer look at supercritical water.

Water, the fluid of life at ambient pressure (P) and temperature (T), is mostly present under supercritical conditions in the Earth’s crust and mantle (1): that is, above the vaporliquid critical point (647 K and 221 MPa). As a free fluid or dissolved in silicate minerals, supercritical water greatly influences the structure and dynamics of our planet. From a technological standpoint, there are several, new emerging applications for supercritical water as a green solvent (2), including the catalytic conversion of biomass into fuels and the oxidation of hazardous materials. In PNAS, the article by Sahle et al. (3) points at two intertwined issues: the yet unsolved structural properties of supercritical water: in particular, whether the system is homogeneous and hydrogen bonds (HBs) are still present, and the need to combine simulations and experiments to understand the complex changes the fluid undergoes under supercritical conditions. The spectroscopic measurements presented in the report provide benchmarks for atomic and electronic structural models and represent a step forward in gathering the data required to unravel the complexity of supercritical water. As pointed out in numerous studies (3–5), water’s properties exhibit dramatic changes under supercritical conditions: the fraction of HB molecules greatly decreases with respect to ambient P and T, and there appears to be a consensus on the persistence of some HBs up to at least 600 °C and 134 MPa (3). The HB network, where present, is substantially distorted. Whether supercritical water is homogeneous [or composed of patches of clearly defined HB regions and non-HB ones (5)], and over which length-scale possible density heterogeneities might appear, are still matters of debate. The presence of inhomogeneous patterns in the density has long been a contentious topic for the liquid at ambient conditions as well (6–10). Again, a tenuous consensus is forming in the scientific community around the idea that possible heterogeneities represent transient rather than equilibrium states of the liquid or supercritical fluid (8–10). Changes in structure and bonding as a function of T and P are accompanied by variations of the dielectric constant (11), and hence the reactivity of water, which can then itself work as an acid or base catalyst. The variation detected in the dielectric constant ɛ, from ɛ ∼78 at ambient conditions to ∼6 at the vapor-liquid critical point, affects the solubility of minerals in the Earth (1) with important implications [e.g., for the transport of oxidized carbon (11)]. In general, the solubility and nucleation rates of various inorganic and organic compounds are modified (2), making it possible to use water in hydrothermal processes to produce a variety of functional materials without the use of polluting organic solvents. For example, supercritical water is used in the chemical recycling of waste polymers, including resins widely present in electronic devices, and in oxide material recovery (2). Measurements in supercritical conditions are challenging, and experiments are difficult to interpret without the help of detailed microscopic models that account for the changes in chemical bonding as a function of T and P. The need for suchmodels calls for quantummechanical, first principles descriptions of water, as provided for example, by ab initio molecular dynamics (MD) (12), where interatomic forces are evaluated using density functional theory. This is themethodology of choice in the report of Sahle et al. (3). Numerous ab initio MD simulations of water have been carried out in the last 20 y and have contributed significantly to understanding basic structural and electronic properties of aqueous solutions and hydrated surfaces. However, the description of liquid water from first principles is an ongoing challenge and intense research is underway in many groups to establish the accuracy of different density functional descriptions, with focus on the performance of hybrid (13) and van der Waals functionals (14), and on the role of proton quantum effects (ref. 15 and references therein). In addition, special care must be exercised in obtaining robust numerical convergence of ab initio MD (16). Hybrid and some van der Waals functionals overcome (13, 14) some of the deficiencies of semilocal ones, such as slow diffusion of the liquid at ambient conditions and the overestimate of tetrahedral order and underestimate of entropy. Under supercritical conditions, where the proportion of HBs is greatly decreased, simple semilocal functionals seem to perform better than at ambient T and P: for example, for the equation of state of the fluid (11), the melting line (17), and the dielectric properties (11, 17). However, the larger error bars of high-pressure measurements may conceal some of the inaccuracies of the theory in the supercritical regime. A crucial advantage of the description of interatomic forces provided by ab initio MD, compared with simulations using empirical or ab initio fitted potentials, is the simultaneous access to the atomistic structure and the electronic properties of the system. Therefore, one can simulate not only diffraction patterns but also spectroscopic data (3, 9, 10), and understand the interplay between specific structural motifs, and the electronic states of the system. This is particularly important in the case of water, where a detailed and wellaccepted structural model is not yet available from experiments. Neutron (18) and X-ray data (19) have yet to come to a full consensus Fig. 1. Three-dimensional surface representing the square modulus of the lowest unoccupied single-particle state of liquid water, as obtained in an ab initio MD simulation with a cell of 64 water molecules (16). White and red spheres represent hydrogen and oxygen atoms, respectively. (Inset) A simplified, pictorial representation of excitation processes occurring in X-ray Raman experiments of the type described in ref. 3.