CRITICAL PHENOMENA IN HYDROTHERMAL SYSTEMS - STATE, THERMODYNAMIC, ELECTROSTATIC, AND TRANSPORT-PROPERTIES OF H2O IN THE CRITICAL REGION

The critical point defines the vertex of the near-parabolic H2O vaporization boundary; hence, geometric considerations ensure that specific partial derivatives of any representative H2O equation of state will diverge to +/- infinity at criticality. This inherent divergency of isothermal compressibility, isobaric expansivity, and higher-order derivatives effectively controls not only thermodynamic, electrostatic, and transport properties of H2O, but also dependent transport and chemical processes in hydrothermal systems. The Gibbs-Duhem equation provides several thermodynamically-equivalent functional expressions for P-V(m)-T and mu-rho-T equations of state. Specific statements of these expressions, however, are distinguished by their mathematical representation of the critical point as either analytic or an isolated singularity, a distinction that leads to, respectively, so-called classical or nonclassical asymptotic behavior of equation-of-state derivatives. In the critical region, nonclassical mu-rho-T formulations are preferred because they closely approximate experimental observations in the context of statistical mechanics descriptions of critical-point phase transition. The nonclassical equation of state for H2O developed by Levelt Sengers and others (1983) from modern theories of revised and extended scaling affords accurate prediction of state and thermodynamic properties in the critical region. This formulation has been used together with the classical, empirically-augmented virial equation proposed by Haar, Gallagher, and Kell (1984) and predictive equations for the dielectric constant (this study), thermal conductivity (Sengers and others, 1984), and dynamic viscosity (Sengers and Kamgar-Parsi, 1984; Watson, Basu, and Sengers, 1980b) to present a comprehensive summary of fluid H2O properties within and near the critical region. Specifically, predictive formulations and computed values for twenty-three properties are presented as a series of equations, three-dimensional P-T surfaces, isothermal and isobaric cross sections, and skeleton tables from 350-degrees to 475-degrees-C and 200 to 450 bars. The properties considered are density, isothermal compressibility, isobaric expansivity and its isobaric temperature derivative, Helmholtz and Gibbs free energies, internal energy, enthalpy, entropy, isochoric and isobaric heat capacities, the dielectric constant, Z, Q, Y, and X Born functions, dynamic and kinematic viscosity, thermal conductivity, thermal diffusivity, the Prandtl number, the isochoric expansivity-compressibility coefficient, and sound velocity. The equations and surfaces are analyzed with particular emphasis on functional form in the near-critical region and resultant extrema that persist well beyond the critical region. Such extrema in isobaric expansivity, isobaric heat capacity, and kinematic viscosity delineate state conditions that define local maxima in fluid and convective heat fluxes in hydrothermal systems; in permeable media, these fluxes diverge to infinity at the critical point. Extrema in the Q, Y, and X Born functions delineate state conditions that define local maxima or minima in the standard partial molal volumes, enthalpies, entropies, and heat capacities of aqueous ions, complexes, and electrolytes; these properties diverge to +/- infinity at criticality. Because these fluxes and thermodynamic properties diverge to +/- infinity at the critical point, seemingly trivial variations in near-critical state conditions cause large variations in fluid flow velocity, thermal energy transfer rates, and the state of chemical equilibrium.