Abstract: The tricritical phenomena of quasi-ternary solutions of H2O＋formamide(FA)＋cyclohexane＋diethylene glycol monobutyl ether have been studied by measurements of the volume ratios of the coexisting phases for a series of solutions with various overall compositions at various temperatures. Taking (H2O＋FA) as a quasi “pure” component, the mass fraction FA in the quasi “pure” component controlled the approach to the tricritical point. As the mass fraction increased from 0 to 0.206, the ΔT of the three-phase region reduced from 1.500 ℃ to 0.228 ℃. By extrapolation of the experimental data, the tricritical mass fraction of FA in the quasi “pure” component and the tricritical temperature were obtained to be 0.286±0.003 and (54.8±0.69) ℃, respectively. It was found that the water-oil sides of three-phase triangles at the middle temperatures of the three-phase regions were almost parallel to the water-oil sides of the triangle coordinates. The dependences of lengths of the water-oil side of the three-phase triangle and the corresponding altitude to the water-oil side on ΔT were consistent with the classical theory.

Key words: Phase volume, Quasi-ternary system, Tricritical phenomena, Diethylene glycol monobutyl ether