Two-phase asymmetry in the phase diagram of critical binary liquied systems: carbon disulphide + nitromethane and cyclohexane + acetic anhydride

Abstract

The coexistence curves of two binary liquid systems, carbon disulphide + nitromethane and cyclohexane + acetic anhydride, are examined over a temperature range 10‒6< —∊= — (Tc—T)/Tc< 10‒1. The critical indexβfor the two systems are 0.316 and 0.362, which agree with other experiments. The diameters of the coexistence curves for both systems show significant deviations nearTcfrom the rectilinear diameter law. Fort=Tc— T > 10 K, rectilinear behaviour is shown. Fort< 10 K, the deviation ∆xfrom the straight line is described by ∆x= Ftƒexp (—Gth), with ƒ ≈ 0.9, showing that the derivative ∂(∆x)/∂tdiverges weakly like the heat capacity ast→ 0. This is in striking agreement with recent theoretical models. A three-variable scaling theory is formulated to describe the critical behaviour of binary liquid systems. In addition to giving the usual scaling-law equalities, the theory shows that the heat capacity, the com­pressibility and the thermal expansion of binary liquid systems have the sameα-divergence atTc. The theory is also capable of accommodating the diameter singularity, in which form it shows that, close toTc, ∆xbehaves ast‒αand that the difference between the fluctuations, in com­position, in two phases diverges astβ‒1.