Vapour-liquid and gas-gas equilibria in simple systems

The shape of the coexistence surface of the system neon-xenon has been determined in great detail, in the neighbourhood of the critical line, by means of isobaric measurements. The critical line has a temperature-minimum, the so-called critical double point, at about 1040 bar, above which gas-gas equilibrium occurs. A mathematical analysis of the measurements shows that the isobars can be represented by a power law with the critical exponent βP, to which a single value, 0.348, can be assigned for all the isobars. This is in accordance with the idea of universality of the critical exponents. By the use of a geometrical transformation, the power law behaviour of isotherms has also been investigated. The corresponding critical exponent βT has, in general, the same value as βP but, in the immediate vicinity of the critical double point, a deviation from this behaviour has been found in accordance with theoretical arguments.