Investigation of the nonideal behavior of mixtures of simple molecules is an important step toward improved understanding of the theory of solutions. This work describes an equilibrium apparatus suitable for the measurement of vapor-liquid and vapor-liquid-liquid equilibria of simple fluid mixtures at cryogenic temperatures. Vapor-liquid data are reported for the argonethane and nitrogen-carbon tetrafluoride systems. Contrary to expectations, limited liquid-phase miscibility was not observed for either of these mixtures. These results and those for other nonideal mixtures of simple molecules are compared with predictions of upper critical solution temperatures by the theory of regular solutions. Usefulness of this theory is impaired by its neglect of the order-disorder phenomenon and, more seriously, by the geometric-mean assumption for the intermolecular forces between dissimilar molecules.
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