CONDENSED SOLUTIONS OF THE RARE GASES

The condensed rare gases have a number of properties which make them suitable substances for an experimental test of solution theories. Their molecules have spherical symmetry and have no internal motions and their " radii " may differ by as little as 7 per cent. With the exception of helium, the rare gases crystallize in the same form-face-centred cubic. Measurements of the total vapour pressure of solid solutions of the two systems A-Kr and Kr-Xe have been reportedv-', These measurements have been compared with theory and in particular with the average potential model theory developed by Prigogine and his associates. This theory leads to an expression for the excess molar free energy (GE) of the form: (1) where Xl and X 2 are mole fractions of the components and a is a function of temperature (but not of composition) which may be calculated from a number of properties of one component. When a is positive, as it is fo