Scaling fields and the nature of liquid-gas asymmetry in fluids

Fisher and coworkers [Phys. Rev. Lett. \textbf{85}, 696 (2000); Phys. Rev. E \textbf{67}, 061506 (2003).] recently suggested that in fluids the two theoretical scaling fields, commonly known as ``ordering'' and ``thermal'', are mixtures of three physical fields, namely, chemical potential, temperature, and pressure. We have examined experimental consequences of this formulation (``complete scaling'') with regard to the asymmetry of vapor-liquid coexistence in real fluids. By analyzing the coexisting curves of various fluids, we have shown that the vapor-liquid asymmetry originates from two different sources: one from mixing of chemical potential and pressure into the thermal field and another one from mixing of pressure into the ordering field. The first source is attributed to a correlation between entropy and density, whereas the second source is associated with the excluded volume. Real fluids can be mapped into the symmetric lattice-gas (Ising-like) model by a redefinition of the order parameter that can be now expressed through a combination of density, entropy, and molar volume. We have also demonstrated which molecular parameters of fluids control these two sources of vapor-liquid asymmetry.