Study of the Hydrogen Bond Network in sub-and supercritical Water by Molecular Dynamics Simulations

Abstract

Dedicated to Professor Manfred Zeidler on the occasion of his 65th birthday.For 12 points along the tangent to the saturation curve at the critical point the temperature dependen cies of the heights of the first maximum in the 0 -0 RDF, the average number of hydrogen bonds, and the self-diffusion coefficients have been calculated from MD simulations. The curves of these three properties show an inflection near the critical point. To improve the understanding of these changes in going from subcritical to supercritical water the librational spectra and the change in the fractions of wa ter molecules with a given number of hydrogen bonds as a function of temperature have been derived from the simulations, additionally.

Universality of hydrogen bond distributions in liquid and supercritical water

“…One can consider this as an indirect indication that the increasing orientational disorder due to the rising temperature remains essentially the sole mechanism of H-bond breaking under supercritical conditions as it is already well established for normal liquid water [41,42]. Another indirect indication of the reorientational mechanism of H-bond breaking under supercritical conditions can be found in the librational spectra of H2O molecules calculated from MD simulations [21,22,[31][32][33]. Even though these spectra are noticeably broadened under supercritical conditions, they still show the same characteristic librational frequencies in the range of 400-600 cm -1 as is observed in normal liquid water [43].…”

“…2a are characterized by 〈nHB〉 < 1.6, that is below the percolation threshold for the H-bonding network [44]. However, even though the continuous and infinite tetrahedral network of H-bonds is almost always broken in SCW, the fluid experiences significant local density inhomogeneities leading to the formation of H-bonded clusters of H2O molecules under these conditions [28][29][30][31][32][33][34].…”

“…The question of the ranges of temperature and density (or pressure) where H-bonding interactions can significantly affect the observable properties of water has long been considered very important for the construction of realistic structural models for this fluid, and the first X-ray diffraction experiments on dense supercritical water by Yu.E.Gorbaty already gave significant quantitative insight [7]. This conclusion was later confirmed by the early molecular computer simulations of supercritical water [18][19][20] Therefore, computational molecular modeling efforts are playing an especially important role in developing a detailed quantitative picture of hydrogen bonding in SCW as a locally heterogeneous system where the percolating network of H-bonds may be generally broken, but significant fractions of smaller patches (or clusters) of H-bonded molecules can still exist surrounded by a growing number of non-bonded H2O monomers with increasing temperature and decreasing density [28][29][30][31][32][33][34].…”

Supercritical water anomalies in the vicinity of the Widom line

“…The latter was also confirmed by the hydrogen bond distribution analysis. With the increase of temperature the height of the 1 st peak increases due to the low density cluster formation which start during the destruction of the hydrogen bond networks 32 .

Figure 1Supercritical isobars using different interatomic potentials (SPC/E, TIP4P/2005, BKE and SWM4-NDP) in comparison with experimental results. The filled symbols describe the temperature at which the maximum heat capacity line (point in the Widom line) is crossed (Supplementary Information).

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