THERMODYNAMIC PROPERTIES OF cis-DECALIN-EY~~S-DECALIN
2883
The Thermodynamic Properties of the Binary System cis-Decalin-trans-Decalin
by M. La1 Unilever Research Laboratory, Port Suntight, Birkenhead, Cheshire, England
and F. L. Swinton .Department of Pure and Applied Chemistry, University of Strathclyde, Olasoow, (7.1., Scotland
(Received January 17, 106'0)
The excess enthalpy of mixing at 30 and 70" and the excess volume of mixing at 30" have been measured for the binary system cis-decalin-trans-decalin. Both excess functions are found to be negative for this system. The experimental results are used to test various statistical theories of liquid mixtures, and no current theory is found to be wholly satisfactory in accounting for the experimental excess functions. Recent advances in the experimental techniques of calorimetry and dilatometry have made possible the experimental investigation of the excess thermodynamic properties of binary liquid mixtures previously thought to be ideal. Mixtures of isotopes112and of isotopically substituted molecules, for example, have been shown to possess small but nonzero excess free energies, enthalpies, and volumes of mixing. Isomeric mixtures should also deviate from ideality to a rather small extent. Assuming that the deviations can be measured with sufficient accuracy, the thermodynamic properties of such systems can be used to test existing statistical theories of liquid mixtures. The isomeric decalins are pseudospherical molecules which exist in the liquid state over a wide temperature range a t atmospheric pressure. Some early density measurem e n t ~indicated ~ that the excess volume of mixing, vE, was small and negative. This is a rather unexpected result so it was decided to investigate this system more fully.
Experimental Section The calorimeter used to measure the enthalpy of mixing was a differential forma of the successful design of Larkin and McGlashan.6 The excess volumes of mixing were measured directly using a style of dilatometer which has been described previously.e The thermal pressure coefficients of the two pure components were measured using an apparatus similar to that described by Allen, et al.' Materials. The samples of cis- and trans-decalin were obtained from K & K Laboratories Inc., Plainview, N. Y. A vpc analysis of the trans isomer showed that its purity was 99.5%, the 0.5% impurity being cis-decalin. The freezing point was found to be in reasonable agreement with accepted values.8 The sample of cis-decalin was found to contain 5% of the trans isomer. Twenty successive recrystallizations
from the melt reduced the trans content to
