Molecular-Scale Remnants of the Liquid-Gas Transition in Supercritical Polar Fluids

V. P. Sokhan, A. Jones, F. S. Cipcigan, J. Crain, and G. J. Martyna

Phys. Rev. Lett. 115, 117801 – Published 11 September 2015

Abstract

An electronically coarse-grained model for water reveals a persistent vestige of the liquid-gas transition deep into the supercritical region. A crossover in the density dependence of the molecular dipole arises from the onset of nonpercolating hydrogen bonds. The crossover points coincide with the Widom line in the scaling region but extend farther, tracking the heat capacity maxima, offering evidence for liquidlike and gaslike state points in a “one-phase” fluid. The effect is present even in dipole-limit models, suggesting that it is common for all molecular liquids exhibiting dipole enhancement in the liquid phase.

Received 14 June 2015

DOI:https://doi.org/10.1103/PhysRevLett.115.117801

Authors & Affiliations

V. P. Sokhan¹, A. Jones², F. S. Cipcigan^1,2, J. Crain^1,2, and G. J. Martyna³

¹National Physical Laboratory, Hampton Road, Teddington, Middlesex TW11 0LW, United Kingdom
²School of Physics and Astronomy, The University of Edinburgh, Mayfield Road, Edinburgh EH9 3JZ, United Kingdom
³IBM T. J. Watson Research Center, Yorktown Heights, New York 10598, USA

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Vol. 115, Iss. 11 — 11 September 2015

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Images

Figure 1
Average molecular dipole moment as a function of density at $T = 673 K$ (blue diamonds), and at $T = 873 K$ (orange circles). The triangle denotes the isolated monomer value (parameter of the model). Green squares and right scale, average number of hydrogen bonds per molecule as a function of density at $T = 673 K$ . The lines are linear fits to the data. Insets illustrate the variation in electron density with pressure, where the pink and blue isosurfaces correspond to gain and loss of electron density.
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Figure 2
( $T, p$ ) phase diagram of water in the proximity of the critical point in units of critical temperature $T_{c}$ and critical pressure $p_{c}$ . The liquid-vapor coexistence curve (pink), the loci of maxima in heat capacity (green), thermal expansivity (orange), and isothermal compressibility (blue), obtained using the IAPWS-95 reference equation of state [20]. The blue diamonds represent our results derived from the analysis of Fig. 1. The black dot marks the critical point. The inset, the corresponding ( $ρ, T$ ) phase diagram in reduced units, shows the gas and liquid branches of the coexistence curve, the locus of the heat capacity maxima, and superimposed are linear fits to the dipole moment (the blue dashed lines) in gaslike and liquidlike regions with vertical offsets to map the corresponding temperatures at the crossover. The crossover densities are denoted by the diamonds.
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Figure 3
Density of water along the $T = 673$ , 773, and 873 K isotherms (from top to bottom) as obtained in APIMD QDO simulation (symbols) versus experimental data (full lines). All simulated states, apart from the first four in each isotherm, are in the supercritical region [2]. The dashed line indicates the crossover densities. 3D plots (insets) showing first oxygen (red) and hydrogen (white) coordination shells, illustrate remarkable persistence of tetrahedral structure at $T = 673 K$ .
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Figure 4
The partial radial distribution functions obtained in the QDO water model simulation under supercritical conditions at $T = 673 K$ and $p = 340 MPa$ (red lines). Also shown for comparison are distribution functions obtained from neutron scattering [30] at the same thermodynamic conditions (blue lines).
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Figure 5
Relative (to the gas-phase value) enhancement of the molecular dipole moment as a function of the reduced density. Red diamonds, quantum Drude model; green circles, classical Drude oscillator model (dipole limit); blue triangles, polarizable Stockmayer model (dipole limit). All calculations were performed at a reduced temperature $T^{*} = 1.036 T_{c}$ in terms of corresponding critical temperature $T_{c}$ . Straight lines are to guide the eye.
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