Direct experimental verification of the isomorphism hypothesis for critical phenomena

The heat capacity at constant volume of a mixture of methane and 0.0345 mole fractions of heptane is experimentally studied over a wide range of densities and temperatures. In the case when the transition from a three-phase state takes place in the presence of a noncritical liquid phase, it is found that the behavior of the heat capacity in the vicinity of the upper end critical point is fully isomorphic with the behavior of the heat capacity in the vicinity of the liquid-vapor critical point of one-component fluid. It is shown that the measured quantity in this experiment is the heat capacity at constant volume and constant chemical potential μ of the heavy impurity component C _{v, μ}. Thus, it has been confirmed by direct measurements that the anomaly of this heat capacity completely coincides in character with the anomaly of the heat capacity at constant volume in the vicinity of the liquid-vapor critical point of one-component fluids.