Thermodynamic behavior of polar and nonpolar fluids within and beyond the anomalous region: Similarities, deviations, and universal compressibility points

A remarkable discovery of two thermodynamic states has been made, where the loci of the minima of modified isothermal compressibility (κ* = ∂v/∂P) along the isotherms and isobars cross for the 19 fluids considered here—from helium to water, including the van der Waals fluid and air. These unique Universal Compressibility Points (UC1 and UC2), in terms of the reduced pressure (P/Pc) and temperature (T/Tc) of (1.001, 0.875) and (1.065, 0.802), exist in the subcritical vapor region. They do not involve any phase transitions like triple point, critical point, and dwij point. The existence of these states reinforces the need for an integrated analysis of the anomalous region from subcritical to supercritical states. Intricate, complex similarities and deviations in fluids' behavior are demonstrated when the thermodynamic/transport properties and the fluids' characteristics, such as the acentric factor, dipole moment, degree of freedom, and spherical symmetry, are considered together. Although the property behavior and the shape and size of the anomalous regions may remain similar for many polar, nonpolar, and special fluids, the anomalies in certain fluids, e.g., helium, ethanol, and methanol, may be far more complex. These findings raise new questions about the mechanisms that produce and strengthen/weaken the anomalies. Also presented are the correlations for the boundaries of the anomalous regions of 15 fluids that are extremely accurate against their real diagrams. The similarity among these fluids has led to one set of correlations, which can serve as the first estimate of the anomalous regions of regular fluids.