Thermophysical properties of supercritical water and bond flexibility

Molecular dynamics results are reported for the thermodynamic properties of supercritical water using examples of both rigid $\left(\mathrm{TIP}4\mathrm{P}\text{/}2005\right)$ and flexible $\left(\mathrm{TIP}4\mathrm{P}\text{/}2005\mathrm{f}\right)$ transferable interaction potentials. Data are reported for pressure, isochoric and isobaric heat capacities, the thermal expansion coefficient, isothermal and adiabatic compressibilities, Joule-Thomson coefficient, speed of sound, self-diffusion coefficient, viscosities, and thermal conductivity. Many of these properties have unusual behavior in the supercritical phase such as maximum and minimum values. The effectiveness of bond flexibility on predicting these properties is determined by comparing the results to experimental data. The influence of the intermolecular potential on these properties is both variable and state point dependent. In the vicinity of the critical density, the rigid and flexible potentials yield very different values for the compressibilities, heat capacities, and thermal expansion coefficient, whereas the self-diffusion coefficient, viscosities, and thermal conductivities are much less potential dependent. Although the introduction of bond flexibility is a computationally expedient way to improve the accuracy of an intermolecular potential, it can be counterproductive in some cases and it is not an adequate replacement for incorporating the effects of polarization.

Figure 1

Pressure as a function of density predicted by the $\mathrm{TIP}4\mathrm{P}\text{/}2005$ (blue  ) and $\mathrm{TIP}4\mathrm{P}\text{/}2005\mathrm{f}$ (red  ) intermolecular potentials and compared to iapws-95 reference data [38] (black ■). Lines through the data points are given only for guidance.

Figure 2

Isothermal (a) and adiabatic (b) compressibilities as functions of density predicted by the $\mathrm{TIP}4\mathrm{P}\text{/}2005$ (blue  ) and $\mathrm{TIP}4\mathrm{P}\text{/}2005\mathrm{f}$ (red  ) intermolecular potentials and compared to iapws-95 reference data [38] (black ■). Lines through the data points are given only for guidance.

Figure 3

Comparison of the oxygen-oxygen radial distribution functions at $673\mathrm{K}$ obtained from (a) experiment [40], and calculations with the (b) $\mathrm{TIP}4\mathrm{P}\text{/}2005$ and (c) $\mathrm{TIP}4\mathrm{P}\text{/}2005\mathrm{f}$ potentials at densities of $870\mathrm{kg}/{\mathrm{m}}^3$ (blue), $730\mathrm{kg}/{\mathrm{m}}^3$ (green), $660\mathrm{kg}/{\mathrm{m}}^3$ (red), and $580\mathrm{kg}/{\mathrm{m}}^3$ (black).

Figure 4

Distributions of $\mathrm{TIP}4\mathrm{P}\text{/}2005\mathrm{f}$ H-O-H angles and O-H bonds at densities of (a) $1000\mathrm{kg}/{\mathrm{m}}^3$, (b) $600\mathrm{kg}/{\mathrm{m}}^3$, (c) $400\mathrm{kg}/{\mathrm{m}}^3$, and (d) $200\mathrm{kg}/{\mathrm{m}}^3$.

Figure 5

Isobaric (a) and isochoric (b) heat capacities as functions of density predicted by the $\mathrm{TIP}4\mathrm{P}\text{/}2005$ (blue  ) and $\mathrm{TIP}4\mathrm{P}\text{/}2005\mathrm{f}$ (red  ) intermolecular potentials and compared to iapws-95 reference data [38] (black ■). Lines through the data points are given only for guidance.

Figure 6

Thermal expansion coefficient as a function of density predicted by the $\mathrm{TIP}4\mathrm{P}\text{/}2005$ (blue  ) and $\mathrm{TIP}4\mathrm{P}\text{/}2005\mathrm{f}$ (red  ) intermolecular potentials and compared to iapws-95 reference data [38] (black ■). Lines through the data points are given only for guidance.

Figure 7

Joule-Thomson coefficient as a function of density predicted by the $\mathrm{TIP}4\mathrm{P}\text{/}2005$ (blue  ) and $\mathrm{TIP}4\mathrm{P}\text{/}2005\mathrm{f}$ (red  ) intermolecular potentials and compared to iapws-95 reference data [38] (black ■). Lines through the data points are given only for guidance.

Figure 8

Speed of sound as a function of density predicted by the $\mathrm{TIP}4\mathrm{P}\text{/}2005$ (blue  ) and $\mathrm{TIP}4\mathrm{P}\text{/}2005\mathrm{f}$ (red  ) intermolecular potentials and compared to iapws-95 reference data [38] (black ■). Lines through the data points are given only for guidance.

Figure 9

Self-diffusion coefficient as a function of density predicted by the $\mathrm{TIP}4\mathrm{P}\text{/}2005$ (blue  ) and $\mathrm{TIP}4\mathrm{P}\text{/}2005\mathrm{f}$ (red  ) intermolecular potentials and compared to experimental data [39] (black ■). Lines through the data points are given only for guidance.

Figure 10

Dynamic (a) and kinematic (b) viscosity as functions of density predicted by the $\mathrm{TIP}4\mathrm{P}\text{/}2005$ (blue  ) and $\mathrm{TIP}4\mathrm{P}\text{/}2005\mathrm{f}$ (red  ) intermolecular potentials and compared to iapws-95 reference data [38] (black ■). Lines through the data points are given only for guidance.

Figure 11

Thermal conductivity as a function of density predicted by the $\mathrm{TIP}4\mathrm{P}\text{/}2005$ (blue  ) and $\mathrm{TIP}4\mathrm{P}\text{/}2005\mathrm{f}$ (red  ) intermolecular potentials and compared to iapws-95 reference data [38] (black ■). Lines through the data points are given only for guidance.