Abstract: The equilibria of the three-liquid-phase for ternary system water-cyclohexane-diethylene glycol monobutyl ether were determined by the improved volume method, at the same time, the coexisting curves of the temperatures versus the volume fractions and the mass densities of the coexisting three liquids phases are reported. It is found that the coexisting curves of the temperatures versus the volume fractions of water, cyclohexane and diethylene glycol monobutyl ether are respectively inverse “S” type curve, positive “S” type curve and “knot-shape” curve. Simultaneously, when the volume fractions of coexisting three-liquid-phases are converted into mass densities, it is also found that the coexisting curve of temperatures versus mass densities of phases is also standard inverse “S” type curve. Furthermore, the three “S” type curves are all asymmetric, and the mass density may be used to describe the phase-equilibrium, moreover, the mass density may really be a good order parameter to describe the critical scaling behaviour.

Key words: Cexisting curve, Density, Oder parameter, Water-cyclohexane-diethylene glycol monobutyl ether