918
Vol. 67
NOTES
circles are for the violet solutions. The line to which they closely conform is the graph of the simple equation log UZ'
=
log xz
+ vzc$q2(14.1 - SJ2/1360
(2)
The best fit was obtained with = 0.256 and vz = 58.5 cc. These are only slightly different from the vaIues heretofore used : 0.258 and 59.0, respectively, obtained by extrapolation to 25' of the properties of liquid iodine. Among the aliphatic hydrocarbons, the compact molecule, cyclohexane, conforms well with the violet
solvents, but the three far from compact species, heptane, 2,2,4-trimethylpentane, and 2,2-dimethylbutane, fall far off the line. The complexing solvents, benzene, p-xylene, and mesitylene, deviate in order of increasing donor strength from the region on the line where they would fall if there were no complexing. The still stronger donor, ether, departs even more widely. Acknowledgment.--We express our thanks to Dr. C. L. Hobbs and the du Pont Company for the sample of CClB'CF3, and to the Atomic Energy Commission for its support of-the project.
NOTES THERMODYNARIICS OF LIQUID SURFACES : T H E SURFACE TENSION OF DIMETHYL SULFOXIDE A S D SOME DIMETHYL SULFOXIDE-ACETONE MIXTURES BY w.LAWRENCE CLEVER AND c. c. SNE.4D Department of Chemistry, Emory University, Atlanta 88, Georgza Received September IS', 1968
Theories of surface tension of binary mixtures succeed best when applied to symmetrical, non-polar, nonelectrolytes that differ in surface tension by only a few dynes/cm. This report gives data on a polar system whose pure components differ in surface tension by about 20 dynes/cm., but involves molecules of similar size, shape, and polarity so that they may fit into each others liquid structure with a minimum of interaction. Surface tension, density, and one heat of mixing were measured for dimethyl sulfoxide-acetone mixtures a t 30". The surface tension and density of dimethyl sulfoxide were measured a t several temperatures between 20 and G O O . Experimental Materials.-The dimethyl sulfoxide was a sample furnished and purified by the Crown Zellerbach Corporation Research Laboratory. They heated the dimethyl sulfoxide 1.5 hr. at 120" with 1 weight a/c KOH, then distilled it through a packed column a t reduced pressure. We fractionally crystallized the material twice just before use. Reagent grade acetone was dried over anhydrous K&O3 and distilled through a packed column just before use. Density.-Densities were determined in a 25-ml. pycnometer that had been calibrated a t each temperature with distilled water. Surface Tension.-Surface tensions were measured by the maximum bubble pressure technique on apparatus built and described by Quay1e.l The bubbler was calibrated with highly purified samples of benzene and n-heptane. The dimethyl sulfoxide-acetone mixtures were prepared by volume. The surface tension bubbler air was presaturated with vapor from the liquid mixtures to prevent evaporation losses during the measurement. Heat of Mixing.-One, relatively crude, heat of mixing was measured. The temperature increase on mixing 0.5 mole of acetone with 0.5 mole of dimethyl sulfoxide in a dewar flask was measured. The heat capacity of the resulting mixture was assumed to be the mean of the two pure liquid heat capacities. The dewar heat capacity was determined from a heat of neutralization experiment. (1) 0. R. Quayle, Chem. Eev., 53, 439 (1953).
Results and Discussion The surface tension, density, and parachor of dimethyl sulfoxide as a function of temperature are given in Table I. The least squares line y = 45.78 - 0.1145t1 where y is surface tension in dynes/cm. and t, centigrade temperature between 20 and 60°, reproduces the experimental surface tensions with an average deviation of 0.05 dyne/cm. Another sample of dimethyl sulfoxide fractionally distilled under reduced pressure and twice fractionally crystallized gave a surface tension 0.3 dyne/cm. higher than the results in Table I. Freezing point and other tests lead us to believe that sample might have been contaminated with as much as 1.5 mole water. The densities check well with some reported by Schlafer and Schaffernicht.* TABLE I SURFACE TENSIOS, DENSITY,AND PARACHOR OF DIMETHYL SULFOXIDE Temp., OC.
20 25 30 35 40 50 60
Density, gJcm.3
1.098
...
1,0913
...
1.0816 1.0721 1.0616
Surface tension, dynes/cm.
Paiachor
43 154 42.86 42 41 41.73 41.17 40.05 38.94
182.7 182.7 182 7 182 8 183 0 183 4 183 8
Table I1 gives the free energy, heat content, entropy, and latent heat for forming 1 cm.2 of new surface. The values were calculated from the surface tension and temperature depeiidence of surface tension3 for both acetone and dimethyl sulfoxide. The acetone surface tensions were taken from Prucino.4 The thermodynamic values can be converted to molar values by multiplying by the molar surface area. The molar rolumes and molar surface areas, calculated assuming spherical molecules, are, respectively, 71.6 and 146 X lo7 cme2for acetone; 74.5 cm.3 and 150 X lo7 cm.z for dimethyl sulfoxide. (2) H. L. Schlafer and W. cchaffernicht, Anpew. Chern., 72, (318 (1980). (3) W. D. Harkins and A. E. Alexander, Chapter XIV, "Physical Rfethods of Organic Chemistry," P a r t I, A. Weissberger, Ed., Intersrlence, New York, N. Y.. 1959. (4) L. J. Pruoino, P1i.D. Thesis, Emory University, 1951.
NOTES
April, 1963
TABLE I1 THERMODYNAMICS OF Nmw SURFACE FORMATION : COMPARISON OF ACETONEAND DIMETHYL SULFOXIDE AT 30”
Acetone Dimethyl sulfoxide
Surface free energy, ergsjom.2
Entropy, ergs/deg./ cm.2
IIeat content, ergs/cm.P
Latent heat, ergs/cm.2
22.42 42.41
0.169 0.115
73.6 77.1
51.2 34.7
919
SURFACE ACETONE
45
TENSION
-
DIMETHYL-
40
The surface tension and density of five acetoiie-dimethyl sulfoxide mixtures are given in Table 111. The surface tensions of the mixtures indicate considerable enrichment of the surface in acetone. Excess volumes of mixing, calculatemdfrom the densities, are large, positive, and misymmetrical, with a maximum between 0.75 and 0.50 mole fraction of acetone.
0.0
.lo
.25 .50 .75 .90 1.00
Density, g./cm.a
Surface tension, dynes/cm.
Excess volume, om.a/mole
Excess heat, cal./mole
0.7793 .SO03 .8356 .9160 ,9968 1.056 1.091
22.42 23.02 24.10 26.36 32.64 36.74 42.41
0.0 0.8 1.7 1.3
... ... ...
1.o 0.2 0.0
-80 f 10
... ... .
I
.
Guggenheim5p6suggests that the surface tension of an ideal mixture obeys the single parameter symmetrical equation where Y1, YZ
Y 21,
k
T a
2%
are the pure component surfare tensions the surface tension of the mixture the bulk mole fraction the Boltzmann constant absolute temperature average surface area per molecule, w l k h for a spherical molecule is ( V / S ) 2 / 3
Figure 1 shows the results of several efforts to fit this equation to the data. Values of a tried were 24.5 X 10-l6 cm.2/molecule, the average area calculated from the molar volumes; 43.5 X 1O-l6, calculated from the parachor as a corresponding state molar volume al; unit surface tension; 55.0 X 10-16; and 69.6 X cm.2/molecule. The last value forced a fit at 0.5 mole fraction. No one value fits the data well over the entire composition raiige. The last value fits the data within experimental error between 0 and 0.5 mole fraction of dimethyl sulfoxide, but gives too lorn a value of surface tension a t high concentrations of dimethyl sulfoxide. Acetone and dimethyl sulfoxide are near enough alike in size, shape, and polarity that they meet the require-. ments of a regular sollution. The interaction energy between an acetone-dimethyl sulfoxide molecular pair ( 5 ) E. A. Guggenheim, Trans. Faradug Soc., 41, 150 (1945). (6) E. A. Guggenheim, “Mixtures,” Clarendon Press, Oxford, 1952. Chapter IX.
1 -ch
...2
9
3 0 2z
s
v1
TABLEI11 SURFACETENSIOKS, DEXSITIES,EXCESS ‘VOLUMESOF MIXIKG, AND EXCESS HEATOF RIIXINQOF SOMEACETOXE-DIMETHYL SULFOXIDE MIXTURES AT 30’ Dimethyl sulfoxide, mole fraction
8
35
25
20 0.0
0.2
0.4
0.6
0.8
1.0
Dimethyl sulfoxide, male fraction.
Fig. 1.-Surface tension us. mole fraction dimethyl sulfoxide. The curves show the fit of the ideal solution equation with various values of surface area: curve a, 24.5 X curve b, 43.5 m X 10-16; curve c, 55.0 X and curve d, 69.6 x 10-’6 cm.2 per molecule.
can be calculated from the heat of mixing at 0.5 mole fraction and is 55 X 10-l6 ergs. Curves aa’ of Fig. 2 show the fit when the Guggenheim5z6regular solution equations
and y = YZ
+ --kal ’ In-x2 + (xlf2 lCzf
lw
x12) -
a
W
- mxI2a
are solved simultaneously for, y, the mixture surface tension, and x’, the surface layer mole fractions, assuming a close packed lattice with 1 = l/z and rn = 1/4 and an a of 24.5 X cm.2/molecule and w of 55 X ergs/molecular pair. Curve a’ gives the surface composition and curve a the surface tension. Obviously the calculated surface tensions are too high. The fit is improved only several tenths of a dyne over the fit of the ideal equation with the same surface area. The fit is no better if the simple cube model is used. The equation can be forced to fit the experimental surface tension a t 0.5 mole fraction if w is increased about 13-fold to 730 X 10-16 ergs/molecular pair (1050 cal./mole). Curves bb’ of Fig. 2 show calculated values of surface tension and surface composition with the larger interaction energy. The fit to the experimental points is reasonably good over the entire composition range.
NOTES
920
1
SURFACE
TENSION
procedures previously employed are quite unsatisfactory; however, methods depending 011 the radiochemical nature of the tritium molecule may be used. This article describes the results of separation factor determinations using th.e transpiration apparatus previously described and radiochemical analyses of the equilibrium phases.
45
I
40
35 --. 5 8
4 fi
.3 1
30
$ m2
25
0.0
0.2
0.4
0.6
0.8
Vol. 67
’0 1.0
Dimethyl sulfoxide, mole fraction.
Fig. 2.-Surface tension vs. mole fraction dimethyl sulfoxide. The curves show the fit of the regular solution equations: curves sa’ are for an a of 24.5 X 10-16 and a w of 55 X lo+; curves bb’ are for an a of 24.5 X 10-le and a w of 730 X curves a and b refer to bulk composition; curves a’ and b’ refer to surface composition.
Acknowledgment.-We thank NIr. William Chase for checking several of the dimethyl sulfoxide surface tensions. We thank Mr. E. M. Seidel and the Crown Zellerbach Corporation for a purified sample of dimethyl sulfoxide. This work was supported in part by Yational Science Foundation Grant 7357. DETERMINATION OF THE SEPARATION FACTOR, FOR THE VAPORIZATIOX OF MIXTURES OF PROTIUM AND TRITIUM OXIDES BY HILTONA. SMITHAND KARLR. FITCH Dspartment of Chemistry, Unmersity of Tennessee, Knoxville, Tennessee
Experimental The general method and apparatus used for saturating the nitrogen carrier gas passing over a mixture of protium and tritium oxides was identical with that already described.’ This equipment involves four efficient equilibration units in series, with virtually no pressure drop across the saturator. As in earlier work, most of the water picked up by the nitrogen carrier gas passing through the saturator at flow rates of 15 t o 25 l./hr. was condensed in a trap at 0 ” , with the remaining moisture removed by a chemical drying agent. The time required for collection of a liquid sample varying in volume between 0.5 and 1ml. was from 0.5 t o 3 hr., depending on the temperature of the run. The minimum quantity required for accurate analysis was 0.5 ml. It was found that best results were obtained when runs were made one after another with minimum interruption of the nitrogen flow. In fact, the first run of a series always gave a value of the separation factor which was high, and was excluded from consideration. The apparatus used in analyzing the tritium samples was patterned after that in common use in the determination of the tritium content of urine.3 The water sample to be analyzed was decomposed by reaction with calcium metal in an all-glass system and the tritium gas collected in a 250-ml. capacity Borkowski type ionization chamber. The chamber then was placed on a Gary Model 31 vibrating reed electrometer, and the tritium activity was measured by the rate of charge counting procedure. The analytical apparatus was calibrated with a tritium oxideprotium oxide sample, portions of which were placed in the analytical apparatus. Various pressures of gas evolved by reaction with the calcium metal were allowed to enter the ionization chamber, and the rate of charge of the electrometer determined. A plot of the reciprocal of the time for reaching a given charge os. the pressure of gas in the chamber was linear over the fourfold change in pressure studied (13 to 52 mm.). Since the separation factors to be calculated involved ratios of activities, no absolute determination of the tritium activity in a sample was necessary. The actual procedure involved the passage of dry nitrogen through the four-stage separator and condensation of the moisture in the drying trap. Bttenipts to remove the moisture content of the chemical absorbers as well as the cold trap and combine the two led to erratic results, while use of the water samples collected in the cold traps only gave no difficulty as long as 0.5 ml. or more of liquid was obtained. A liquid sample corresponding t o each condensed vapor sample was removed from the final stage of the saturator after each run, and was analyzed. Because of the very efficient operation of each cell of the saturator, the composition of the fourth stage was constant within experimental error throughout the run.
Experimental Calculations and Results The separation factor, a, to be determined is given by the equation
Received September BO, 196%
An apparatus for the determination of the separation factor accompanying the vaporization of mixtures of protium oxide and deuterium oxide has been described previous1y.l Analytical procedures by the falling drop method allowed precise determinations when appreciable fractions of the heavier hydrogen isotope were presenL2 While this transpiration apparatus is quite satisfactory for use in determining the separation factor for protium oxide-tritium oxide with tracer quantities of tritium oxide added to ordinary water, the analytical (1) H. A. Smith, R . L. Combs, and J. & Googin, ‘I. J. Phys. Chem., 68, 997 (1954).
(2) R. 1,. Combs, J. M. Googin. and H. A. Smith, ibid., 6 8 , 1000 (1954).
where H / T represents the atomic ratio of protium and tritium in a sample, g refers to the gaseous phase, and 1 to the liquid phase. Since th.e tritium was present in tracer quantities oiily, the fraction of the hydrogen in the form of protium was essentially unity in both phases, and the expression for the separation factor may be written as (3) J. MoClelland, M. F. Milligan, B. P. Bayhurst, B. C. Eutsler, W. W. Foreman, B. M. Head, R. D. Hiebert, R. J. Watts, and W . E. Wilson. Report No. LA-164.5 (Decl.), Los Alamos Scientific Laboratories, LOS Alamos, New Mexico, March. 1954.
