3417
THERMODYNAMICS OF BINARY SOLUTIONS OF NONELECTROLYTES
Thermodynamics of Binary Solutions of Nonelectrolytes with Z,Z,P=Trimethylpentane. 11. Phase EQuilibrium Study with Cyclohexane and a New Cooling Curve Apparatus
by Rubin Battino' Department of Chemistry, Illinoia Institute of Techhlogy, Chicago, Illinois 60616
and George W. Allison Oak Park High School, Oak Park, Illinois
(Received February 17, 1966)
The phase equilibrium of the cyclohexane2,2,4-trimethylpentane system was established over the entire range of composition by cooling curve analysis. There is a eutectic a t -113.95' and a mole fraction of cyclohexane of 0.25. The phase transition for cyclohexane (solid I to 11) is at -87.06' and the break in the solubility curve comes at a mole fraction of cyclohexane of 0.65. The calculated ideal solubility of cyclohexane is close to the experimental solubility, confirming the nearly ideal behavior that this system exhibited in other studies. A new cooling curve apparatus is described.
The phase equilibrium study of mixtures of cyclohexane and 2,2,4-trimethylpentane (isooctane) was undertaken to supply supplemental information for other studies,2to establish the phase diagram, and to test a new cooling curve apparatus.
venience. The apparatus takes 50-60-cc samples. Reciprocating stirring action is achieved v i a a long Kel-f stirring rod, G. Both the amplitude and the rate of stirring are adjustable. The lower end of the stirring rod has two Kel-f disks across which l-mm platinum wire is strung to assist the stirring action and increase the rate of attainment of thermal equilibrium. The cooling chamber can be purged with dry air through stopcocks F and tube D. Tube C is an initiation well for diminishing supercooling by the addition of a few drops of liquid nitrogen or a few specks of Dry Ice. The standard-taper (55/50) joint E permits easy disassembly and also the maintenance of a dry and sealed atmos-
Experimental Section Materials. The cyclohexane used in these studies was Phillips pure grade which was subjected to several fractional distillations and crystallizations. The final material had a density of 0.77381 g/cc at 25" and a melting point of 6.43" which indicates a purity of 99.95 mole %. These values compare well with a freezing point of 6.555" and a density of 0.77390 g/cc of the highly purified cyclohexane of Everett and S ~ i n t o n . ~ (1) Correspondence should be addressed to this author a t the DepartThe isooctane used was Phillips pure grade and was ment of Chemistry, Wright State College, Dayton, Ohio 45431. purified by methods described earlier.435 The density (2) Part I: R. Battino, J. Phys. Chem., 70, 3408 (1966). of the isooctane was 0.68776 g/cc at 25", which com(3) D.H.Everett and F. L. Swinton, Trans. Faraday SOC.,59, 2476 (1963). pares well with 0.68774 g/cc reported in ref 3 and 4. (4) 9. Weissman and S. E. Wood, J. Chem. Phys., 3 2 , 1153 (1960). For the freezing point of isooctane, see Table I. (5) S. E. Wood and 0. Sandus, J . Phys. Chem., 60, 801 (1956). Apparatus. The cooling curve apparatus is shown (6) A. R. Glasgow, Jr., A. J. Streiff, and F. D. Rossini, J. Res. Natl. in Figure 1. It, is basically a modification of the design Bur. Std., 35, 355 (1945); A. R. Glasgow, Jr., N. C. Krouskop, J. Beadle, G. D. Axilrod, and F. D. Rossini, Anal. Chem., 20, 410 used by Rossini and co-workers,6 but has several im(1948); A. R. Glasgow, Jr., N. C. Krouskop, and F. D. Rossini, provements for control of the cooling rates and conibid., 2 2 , 1521 (1950). Volume 70, Number 11
November 1966
3418
~~
RUBINBATTINO AND GEORGE W. ALLISON
Cyclohexane-Isooctane Solutionsapb X(CoH1d
0.0000 0.0983 0.2032 0.2311 0.2374 0.3054 0.4697 0.5319 0.6513 0.6605 0,7048 0.7554 0.7736 0.8614 0.8835 0.9225 0.9545 0.9634 0.9803 1.0000 a
1, o c
- 107.365
.--
-109.68
- 112.49
- 113.42 - 113.60
I
-G
-109.93 -97.70 -92.65 -87.04 -83.33 -71.55 -56.61 -47.67 -24.98 -21.25 -12.41 -4.43 -2.33 2.07 6.43
Phase transition for cyclohexane a t -87.06'.
SPACER
Eutectic a t
- 113.95'. phere. The rate of cooling can be partially controlled by the degree of vacuum in the outer vessel via stopcock H. This outer vessel is silvered except for a narrow viewing strip. The insulating arrangement for the apparatus is shown in cross section in Figure 1. The wool felt which was used provides very good insulation, and the aluminum foil was used to provide zones of reasonably uniform temperature. Two thermocouples were used in conjunction with the resistance thermometer to determine temperature differentials. For most of the runs, the temperature difference between the platinum thermometer and surface L was maintained at 8-10' by the addition of small amounts of liquid nitrogen (into the space marked "Air"). Thermometer Calibration. The temperature in the thermometer well was measured using a four-lead miniature platinum resistance thermometer identical with the ones described in part I. Silicone oil in the well aided thermal contact. The thermometer was calibrated against the triple point of water, a benzoic acid cell (see part I), and the melting point of pure isooctane (-107.365"'). The cooling curve of the isooctane used showed no detectable freezing point lowering with a flat plateau which lasted for 52 min and with no supercooling. Temperatures were calculated using the Callendar-Van Dusen equation, and repeated calibrations showed the thermometer to be stable to *0.02' over The Journal of Physical Chemistry
*-
'lh 4 % .
~
Table I: Freezing Points of
17cm I
29cm
I
SIDE VlEW
I
PURGING TUBE D
r
- - LIQUID -
LEVEL
INlTlATDN
I
P t WIRE
I
dL Figure 1. Cooling curve apparatus.
the period of measurement. It is believed that the temperatures reported are accurate to f0.05"and that the melting points of the mixtures are reproducible to f0.1' as determined by repeat measurements.
Results and Discussion The results for the phase equilibrium of cyclohexaneisooctane mixtures are shown in Figure 2 and tabulated in Table I. The heayy lines are for the smoothed data and the circles are for the experimental points. The eutectic temperature is -113.95' and the eutectic composition is a t a mole fraction of cyclohexane of 0.25. The phase transition for cyclohexane (solid I to 11) is a t -87.06' and the break in the solubility curve (7) "Selected Values of Properties of Hydrocarbons," National Bureau of Standards Circular No. C401, U. S . Government Printing Office, Washington, D. C., 1947, p 40.
THERMODYNAMICS OF BINARY SOLUTIONS OF NONELECTROLYTES
O
A
I-
-40
3419
-
("c
t - * O ~
-.20 I 3.5
4.5 1/T
-601r 0 iyi
I
4 .O X
5.0
5.5
1000
Figure 3. Experimental and ideal solubility of cyclohexane for cyclohexane mole fractions of 0.65 to 1.0.
.2
.4 xcyclo
.6
.8
1 cycio
Figure 2. Phase equilibrium of the cyclohexane-isooctane system.
comes at a mole fraction of cyclohexane of 0.65. The transition point for cyclohexane is in excellent agreement with the value of -87.07' reported by Aston, Szasz, and Fink8 and in agreement with the value of -87.3' reported by Parks, et d Q The compositions of the mixtures were determined by density measurements after each run using the volume of mixing resuXts for this system at 25" as presented in part I. The error in the composition is *0.0003 in the mole fraction. Figure 3 shows the experimental and ideal solubility of the system from a 0.65 mole fraction of cyclohexane up to 1. The ideal solubility was calculated by stand-
ard methods using the heat capacity and heats of transition data in ref 8 and 9. The excess chemical potential for cyclohexane is about 17 cal/mole at a mole fraction of 0.65. This very small positive value is additional verification for the almost ideal behavior of this system which was demonstrated in part I from vapor-liquid equilibrium and volume of mixing measurements, The forma.tion of a simple eutectic (without any indication of compound formation) is also in accord with the ideality of this system. Acknowledgment. The authors gratefully acknowledge the support of the Petroleum Research Fund (PRF Grant No. 975-A3) for this work and the National Science Foundation for the support of G. A. as a summer research participant for three summers. We thank Professor Scott E. Wood for many helpful discussions and Dr. E. L. Washington for making some of the density measurements. (8) J. G. Aston, G. J. Szasz, and H. L. Fink, J . Am. Chem. SOC.,65, 1136 (1943). (9) G.9. Parks, H. M. HutTman, and S. B. Thomas, ibid., 52, 1032 (1930).
Volume 70,Number 11 Nmember 1066
