Structure of Supercritical Water

The most important information about the structure of a molecular liquid is calculated in molecular computer simulations in the form of atom-atom radial distribution functions, gij(r). These functions give the probability of finding a pair of atoms i and j a distance r apart, relative to the probability expected for a completely random distribution of atoms at the same density. 

The atom-atom radial distribution functions, goo r), gonir), and gHn( ) for two 

The sharp first peak and following deep minimum of gon(A ) for normal liquid water (at 1.8 A and 2.4 A, respectively) become much less pronounced under supercritical conditions (Fig. 7b). These two characteristic features of gon( ) observed in computer simulations, as well as experimentally (Soper et. al. 1997), are the basis of a simple geometric definition of a hydrogen bond, whereby the bond is assumed to exist between any pair of H2O molecules whose respective O and H atoms are separated by less than i HB = 2.4 A. Integration under gon(A ) up to the chosen threshold distance provides a convenient way to quantitatively estimate the average number of H-bonds in which an individual molecule participates under various thermodynamic conditions (e.g., 

Guissani and Guillot 1993). At ambient conditions, this geometric criterion gives the average number of H-bonds per a water molecule (whb)g 3.2. 

The most striking feature of the neutron diffraction data at the supercritical temperature of 673 K (Fig. 9) is the disappearance of the first maximum of goH(T) at 1.8 A. However, a significant shoulder still remains at its place. This feature is best reproduced by the ab initio MD simulations of Fois et al.(1994) where no assumptions on 

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R249 M.-C. Bellissent-Funel, Structure of Supercritical Water , J. Mol Liq., 2001,90,313... 

T. Tassiang, M.-C.Bellisent-Funel, D. Beysens, B. Guillot and Y. Guisani, Neutron diffraction and MD simulation studies of the structure of supercritical water... 

Figure 4 Schematic diagram for the pulse radiolysis of supercritical water and cell structure.

In this chapter, we have reviewed some of our own work on solvation properties in supercritical fluids using molecular dynamics computer simulations. We have presented the main aspects associated with the solvation structures of purine alkaloids in CO2 under different supercritical conditions and in the presence of ethanol as co-solvent, highlighting the phenomena of solvent density augmentation in the immediate neighborhood of the solute and the effects from the strong preferential solvation by the polar co-solvent. We have also presented a summary of our results for the structure and dynamics of supercritical water and ammonia, focusing on the dielectric behavior of supercritical water as functions of density and temperature and the behavior of excess solvated electrons in aqueous and non-aqueous associative environments. 

S. T. Cui and J. G. Harris, Ion association and liquid structure in supercritical water solutions of sodium chloride a microscopic view from molecular dynamics simulations, Chem. Eng. Sci. 49,2749-2763 (1994). 

H2O is a major detonation product. A simple exp-6 potential model, however, does not naturally represent the hydrogen-bonded tetrahedral structure of water. We find that an effective two-species model is effective in representing the equation of state of supercritical water over a wide range of conditions. In the two-species model, we represent water by two species in chemical equilibrium non-associated water (H2O) and associated water H20(a). The non-associated water has standard state parameters given by gaseous water. In associated water, however, standard state parameters are chosen closer to that of liquid water the standard enthalpy and entropy are both less than that of gaseous water. 

R 724 N. Matsubayashi and M. Nakahara, NMR Study of the Structure and Dynamics of Supercritical Water , Koatsuryoku no Kagaku to Gijutsu, 2003,13, 320... 

Bellissent-Funel, M.-C., Tassaing, T., Zhao, H., Beysens, D., Guillot, B. and Guissani, Y. (1997) The structure of supercritical heavy water as studied by neutron diffraction, J. Chem. Phys. 107,2942-49. 

Matubayasi, N., Wakai, C. and Nakahara, M. (1997) Structural study of supercritical water. 

While NDIS has been used earlier to study the structure of water at non-ambient conditions [149], the first complete study of supercritical water, and comparison with new results for ambient water results, was reported by Soper and collaborators in late 1993 [121], followed shortly by two other contributions [150,151]. For convenience, we will refer to the NDIS results published in Refs. [121,150,151] as NDIS-93 and the earlier Soper-Phillips results [96] as NDIS-86. Because the NDIS-93 results were somewhat controversial, they generated a very enthusiastic and healthy scientific discussion which still goes on [103]. We present a brief account of the significant progress that followed the publication of NDIS-93 in elucidating of water microstructure and the fruitful interplay between experiment and simulation (see [152]). We first review the experimental results, including those from neutron and X-ray diffraction, NMR, and Raman. 

Cui, S. T. Harris, J. G. (1994) Ion Association and Liquid Structure in Supercritical Water Solutions of Sodium-Chloride A Microscopic View from Molecular Dynamics Simulations, Chemical Engineering Science 49, 2749-2763... 

This model has the 4-point geometry (Fig. lb), but a much more complicated functional form with parameters derived from ab initio quantum chemical calculations. The flexible version for this model (MCYL) has also been developed (Lie and dementi 1986). The MCY model was used by Impey et al. (1981) in their MD studies of the structure of water at elevated temperatures and high density, and by O Shea and Tremaine (1980) in the MC simulations of thermodynamic properties of supercritical water. It is well known, however, that this potential reproduces poorly the pressure at a given density (or the density at a given pressure). Even the... 

Matubayasi N, Wakai C, Nakahara M (1997b) Structural study of supercritical water. I. NMR spectroscopy. J ChemPhys 107 9133-9140... 

Supercritical fluids can be applied to remove polluting materials from the environment. Theory and practice of this technology is of increasing interest at the present time. In Supercritical fluids and reductions in environmental pollution by Koji Yamanaka and Hitoshi Ohtaki focus their attention to start with the thermodynamics and structure of supercritical fluids and then describe the supercritical water oxidation process, the extraction of pollutant from soils with supercritical carbon dioxide and other supercritical fluids, and recycle of used plastic bottles with supercritical methanol. Andrew I. Cooper et al. report on Supercritical carbon dioxide as a green solvent for... 

Three reviews have been published entitled Structure and dynamics of water from ambient to supercritical / Computer simulation of supercritical water and aqueous solutions /and Recent developments in the ligand chemistry of tellurium , which contains Se NMR data. ... 

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