ARGON-METHANE PHASE DIAGRAM

The argon-methane solid phase diagram has been determined between 15° and 65°K by x-ray diffraction techniques. The diagram shows a miscibility gap with a critical point for unmixing at 35% methane and 63°K, whereas published liquidus and solidus lines indicate a eutectic point at 39% methane and 71°K. Calculation of the interaction energy of the mixture from the properties of the constituents using the simplest forms of solution theory yields values which deviate by an order of magnitude from those derived from the experimental data. The discrepancy is in part due to an appreciable contribution to the excess free energy from strain energy and otherwise due to inadequacies of the methods of calculating both interaction and strain energies and due to imprecision in the experimental parameters required for these calculations.