The nature of singular coexistence-curve diameters of liquid–liquid phase equilibria

We have examined experimental consequences of ‘complete scaling’, recently formulated by Fisher and co-workers [Phys. Rev. Lett. 85 (2000) 696; Phys. Rev. E 67 (2003) 061506], when extended to liquid–liquid coexistence in binary mixtures. A controversial issue of the proper definition of the order parameter in liquid mixtures is resolved. We show that a ‘singular’ diameter of liquid–liquid coexistence curves originates from two different sources: one is associated with a correlation between concentration and entropy, whereas another one is a correlation between concentration and molar density, generating a lower-order, and thus a more significant singular term.