The equilibrium of three liquid phases upon approach to the tricritical point in water-acetonitrile-benzene-n-hexane mixtures

The region of three-liquid-phase coexistence has been determined in the system water-acetonitrile-benzene-n-hexane at 80, 85, 88, 89, 89.3 and 89.4°C. A method of direct analysis of the coexisting phases by gas chromatography was used. The tricritical point, inferred from these data by graphical and analytical methods is at a temperature of 89.65°C and has the composition (in mass fractions) Wwater = 0.156, Wacetronitrile = 0.540, Wbenzene = 0.165, Whexane = 0.139. Estimates of the tricritical exponents, β1, β2 and β3 (which characterize the vanishing of the three characteristic dimensions of the three-phase region upon approach to the tricritical point) gave β1 = 0.49, β2 = 0.95, β3 = 1.50 compared to the β1 = 12, β2 = 1, β3 = 32 predicted by the classical theory of Griffiths. There was no serious disagreement between estimates of the tricritical exponents derived from alternative measures of the characteristic dimensions of the three-phase region.