INFINITE DILUTION PARTIAL MOLAR PROPERTIES OF AQUEOUS SOLUTIONS OF NONELECTROLYTES. II. EQUATIONS FOR THE STANDARD THERMODYNAMIC FUNCTIONS OF HYDRATION OF VOLATILE NONELECTROLYTES OVER WIDE RANGES OF CONDITIONS INCLUDING SUBCRITICAL TEMPERATURES

Abstract

The volumetric equation proposed previously (Plyasunov et al., 2000), for estimating the infinite dilution Gibbs energy of hydration of volatile nonelectrolytes at temperatures exceeding the critical temperature of pure water, Tc, is extended to subcritical temperatures. The basis for the extension without inclusion of new fitting parameters besides the experimental values of the thermodynamic functions of hydration at 298.15 K, 0.1 MPa, is an auxiliary function, ΔhCp0(T, Pr), for the variation of the infinite dilution partial molar heat capacity of hydration of a solute in liquid-like water between temperatures T = 273.15 K and T = Ts = 658 K along the isobar Pr = 28 MPa. The analytical form of ΔhCp0(T, Pr) was found by globally fitting all available data for the seven best-studied solutes (CH4, CO2, H2S, NH3, Ar, Xe, and C2H4). Four constraints were used to determine the values of four terms of the ΔhCp0(T, Pr) function: the numerical values of the temperature increments between T = 298.15 K and T = Ts = 658 K for the Gibbs energy and the enthalpy of hydration, and numerical value of the heat capacity at Ts and at 298.15 K, all at the selected isobar Pr. This approach, in combination with the volumetric equation, may be used to describe and predict all the infinite dilution thermodynamic functions of hydration for nonelectrolytes over extremely wide ranges of temperature and pressure. The model allows calculation of the standard state partial molar properties, including the Gibbs energy of aqueous solutes in a single framework for conditions from high-temperature magmatic processes through hydrothermal phenomena to low-temperature conditions of hypergenesis.