Liquid Mixtures Involving Cyclic Molecules: Xenon + Cyclopropane Jorge C. G. Calado,* Eduardo J. M. Filipe, Jose ´ N. C. Lopes, Jorge M. R. Lu ´ cio, Joa ˜ o F. Martins, and Luı ´s F. G. Martins Centro de Quı ´mica Estrutural, Instituto Superior Te ´ cnico, 1096 Lisboa, Portugal ReceiVed: January 13, 1997; In Final Form: June 17, 1997 X The total vapor pressure of liquid mixtures of xenon and cyclopropane has been measured at 161.39 K (the triple-point of xenon) and at 182.33 K (the triple-point of dinitrogen oxide), as a function of composition. At 182.33 K the liquid densities were also measured. The mixtures show positive deviations from Raoult’s law. Both the excess molar Gibbs energy (G m E ) and the excess molar volume (V m E ) were calculated from the experimental data. For the equimolar mixture, G m E ) 90.6 J mol -1 at 161.39 K, G m E ) 124.1 J mol -1 at 182.33 K, and V m E )-0.758 cm 3 mol -1 at 182.33 K. The excess molar enthalpy (H m E ) could be estimated from the temperature dependence of G m E and found to be -168 J mol -1 . The results were interpreted using the 1cLJ perturbation theory of Fisher et al. 1. Introduction Molecular shape cannot be ignored in any quantitative description of the thermodynamic properties of liquid mixtures of nonspherical molecules. With this in mind, we have recently started a systematic study on mixtures involving triangular molecules and reported results on the xenon + propane system. 1 Cyclopropane, the simplest three-membered ring molecule, provides a good model of a triangular molecule and the study of its mixtures with xenon, when compared to the xenon + propane mixture, should contribute to the elucidation of the role played by a cyclic structure in intermolecular forces. The differences and similarities between propane and cyclopropane can be used to understand the nature of the intermolecular interactions. For instance, in cyclopropane there is no rotation around the C-C bond, the CH 2 groups are all in an eclipsed configuration, and owing to the distortion of the bonding angles, there is an enormous strain pushing the electronic density out of the ring, probably interfering with the hydrogen atom interactions. As a result, it is likely that the electric field created by each CH 2 group in cyclopropane is different from the corresponding one in propane. A similar type of comparison, but for a polar group, can be made between dimethyl ether and ethylene oxide. The results for both systems, xenon + dimethyl ether and xenon + ethylene oxide, have been reported. 2,3 As far as we are aware no other work has been carried out on the xenon + cyclopropane mixture. Moreover, within thermody- namic studies of mixtures involving cyclic molecules, only cycloalkanes higher than cyclopentane have been studied. Lustig 4 modeled both propane and cyclopropane with a 3cLJ potential and calculated the thermodynamic properties of the pure substances using the perturbation theory of Fischer et al. The results were in excellent agreement with experiment, but the extension of the theory to mixtures is still to be developed. For this reason our experimental results were interpreted on the basis of a simple 1cLJ+1cLJ model. 2. Experimental Section The vapor pressure and density measurements were carried out in an apparatus described elsewhere, 5 using similar experi- mental procedures. As usual, a triple-point cryostat was used, the working temperatures being 161.39 K (the triple-point of xenon) and 182.33 K (the triple-point of dinitrogen oxide, N 2 O) The mixtures were prepared by condensing known amounts of each component into a calibrated pyknometer. Samples of xenon (99.995% purity from Air Liquide or 99.99% purity from Linde), dinitrogen oxide (99.99% purity from Air Liquide), and cyclopropane (99.0% purity from Merck) were further purified by fractional distillation in the laboratory low-temperature column. In the case of xenon and dinitrogen oxide, the final purity was checked by measuring the constancy of the triple-point pressure during melting. The values obtained were the following: for xenon, 81.669 ( 0.007 kPa, to be compared with the recommended value of 81.674 ( 0.011 kPa; 6 for dinitrogen oxide, 87.815 ( 0.010 kPa, to be compared with 87.865 ( 0.012 kPa. 6 In the case of cyclopropane, the triple- point vapor pressure is too low to be measured accurately by this method; however, experiments with other liquefied gases (including low hydrocarbons) show that the purity obtained using our distillation technique is never less than 99.99%. A further purity check was provided by the measured values of the vapor pressure and/or molar volume of the pure components at the triple-point temperature of dinitrogen oxide. In the case of xenon these values were p(Xe) ) 247.78 kPa and V m (Xe) ) 46.468 ( 0.006 cm 3 mol -1 , whereas in the case of cyclopropane p(C 3 H 6 ) ) 3.192 kPa. These results compare favorably with the values reported by Calado et al. 7 at the same temperature: p(Xe) ) 247.55 ( 0.21 kPa, V m (Xe) ) 46.485 ( 0.049 cm 3 mol -1 and p(C 3 H 6 ) ) 3.163 kPa. 8 The vapor pressures and densities of the Xe + C 3 H 6 mixtures were measured at the triple-point temperatures of N 2 O (182.33 K) and Xe (161.39 K). Most of the vapor pressure data were obtained with a fused quartz bourdon manometer (Texas Instrument model 145) with 130 kPa full range and 0.5 Pa resolution. In the case of the higher vapor pressures at 182.33 K, a similar manometer with 420 kPa full range and 1.4 Pa resolution was used instead. The density measurements were performed using a pyknometer (V pyk ) 2.2383 cm 3 ), calibrated with liquid ethane (Air Liquide, 99.995%) at 182.33 K on the basis of the experimental molar volumes of Haynes and Hiza. 9 The ancillary data necessary for the evaluation of G m E and V m E from vapor pressure and molar volume values have already been given as far as xenon is concerned. 7 For cyclopropane, the second virial coefficient at room temperature was taken from the compilation of Dymond and Smith 10 (B )-360 cm 3 mol -1 ). X Abstract published in AdVance ACS Abstracts, August 1, 1997. 7135 J. Phys. Chem. B 1997, 101, 7135-7138 S1089-5647(97)00195-8 CCC: $14.00 © 1997 American Chemical Society