Thermodynamics of Fluid Phase Phenomena at High Pressures

Abstract

AbstractThe experimental investigation of fluid mixtures covers three main areas: liquid‐liquid equilibrium, gas‐liquid equilibrium and the so‐called gas‐gas equilibrium.In recent years this last type of phenomenon has been much investigated, especially as the associated critical phenomena can now be related to the general theory by Griffiths and Wheeler. The phenomenon of gas‐gas equilibrium was already predicted by J. D. van der Waals in 1894 and the theory was further worked out by Kamerlingh Onnes and Keesom in 1907. In the gas‐gas equilibrium of the “first type” the critical curve, starting from the critical point of the least volatile component, goes to higher temperature with increasing pressure. In the gas‐gas equilibrium of the second type the critical curve, with increasing pressure, first goes to lower temperatures and then shows an inversion at the so‐called double critical point.Two examples are presented of recent work from the Van der Waals‐laboratory which show a double critical point in the system Ne — Kr at 1240 atm and — 108.43 C and for the system Ne — Xe at 1025 atm and — 9.39 C. A detailed analysis has shown that the associated critical exponent β has the value 0.348 which is very close to the generally accepted value for pure substances. This would seem to confirm the principle of universality as applied to critical exponents in binary mixtures at high pressure.A simple theoretical model has been developed for the region of the critical double point. One striking result is that from this the geometrical structure of the isotherms in the vicinity of the CDP can be predicted. In particular, the crossing over of isotherms can be excluded. A discussion is also given of the critical exponents in terms of the Griffiths and Wheeler theory.As regards the liquid‐liquid equilibrium, some interesting results have been obtained by the group in Bochum for systems containing fluorated hydrocarbons.A good example of a recent investigation of gas‐liquid equilibrium is the system Ar — Kr which was studied in the Van der Waals‐laboratory and shown to be very near to an ideal solution. Its critical curve can be predicted fairly well by using perturbation theory.The conclusion from the present trend in the study of mixtures is undoubtedly that in the coming years the phenomenon of gas‐gas equilibrium will still be much in the foreground of scientific interest.