Molecular Dynamics and Self‐Diffusion in Supercritical Water

Abstract

The dynamics of the hindered molecular translations and librations and their dependence on temperature and density has been investigated for supercritical water in the range 630<T<780 K and 0.166<ϱ< 1.284 g/cm3 in terms of velocity autocorrelation functions obtained from molecular dynamics simulations. Calculated self‐diffusion coefficients of supercritical water agree well with available experimental data and results of other simulations. The validity of the Stokes‐Einstein relation has been tested using the experimental data on viscosity of supercritical water.

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].…”

“…The densities were chosen to correspond to the pressure range of 25<P<3000 MPa. The flexible BJH water model [36], employed in the simulations, has already been proven to provide quite accurate thermodynamic, structural, diffusional, and vibrational properties of SCW [21,22]. The total potential is represented by separate intermolecular and intramolecular contributions and an isolated H2O molecule has the same geometry as the TIP4P model.…”

“…Nevertheless, a significant clarification of this picture has gradually emerged over the last 15-20 years due to the concerted efforts of many experimental [7][8][9][10][11][12][13][14][15][16][17] and computational molecular modeling [18][19][20][21][22][23][24][25][26][27] research groups.…”

Molecular Simulations of Liquid and Supercritical Water: Thermodynamics, Structure, and Hydrogen Bonding

“…The pronounced effects of temperature and density are also reflected in the spectral densities of the hindered translational motions of water molecules, which can be calculated by Fourier transformation of the velocity autocorrelation functions (e.g., Allen and Tildesley 1987). Such spectra for two high-density supercritical MD simulations (Kalinichev and Heinzinger 1992;Kalinichev 1993) are shown in Figure 18 as dotted and dash-dotted lines, while similar spectra for near-ambient liquid water are shown as solid and dashed lines for the BJH and SPC models, respectively. In normal liquid water, the peaks at ~60 cm -1 and ~190 cm -1 are usually assigned to the hydrogen bond O••O••O bending motion and O••O stretching motions, respectively (e.g., Eisenberg and Kautzmann 1969).…”

“…The dynamics of different modes of molecular librations (hindered rotations) and intramolecular vibrations in supercritical water can now be analyzed in terms of velocity autocorrelation functions for the corresponding projections Heinzinger 1992, 1995;Kalinichev 1993). The velocity autocorrelation functions calculated for the quantities Q i (Eqns.…”