Feb., 1950
HEATCAPACITY OF
THE
SYSTEMCCL~-PERFLEOROMETHYLCYCLOHEX.4NE
297
It may be that such a decrease would occur a t Bi components tend to show positive deviations from compositions greater than those studied. The Henry’s law constants obtained above for Bi were greater thaii unity. That is, they indicate positive deviations from Raoult’s law for the Bi in dilute solution. Thus, in this syetem both
Raoult’s law. Acknowledgments.-The authors are indebted to Mr. William E. Robbins, who performed the experimental work, and to Dr. C. M. Kelley for fruitful discussions.
THE HEAT CAPACITY OF THE SYSTERiI CARBON TETRACHLORIDEPERFLUOROMETIPYLCYCLOHEXANE THROUGH THE CR,ITICAL REGION BY HARTLASD SCHMIDT, GEORGE JURA AND JOEL H. HILDEBRAND Contribution from the Departme/& of Chemistry, University of Calijornia, Berkeley and Riverside, California Received September 8 4 , 1068
The heat content of a mixture of citrbon tetrachloride and perfluoromethylcyclohexane having the critical composition has been determined through a range of temperature spanning the critical by means of an ice calorimeter. This s l l o w ~ t8imefor the system to come to equilibrium a t the initial temperature. Close to the critical point, several days were reH o vs. t give the heat capacity of the system. The heat c;Lpucity of the mixture quired. Tangents to the curve for H t continues to fall through some 7’ above the critical point before reaching the approximate constancy of a completely homogeneous mixture. This confirms earlier, exploratory results.
-
The senior author,’ in 1052, discussed the fluctuations in density occurring in two-component liquid systems just above the temperature where the meniscus disappears and suggested: (a) that the meniscus would reappear in a centrifugal field, and (b) that the volume and entropy of the system would reflect the rapid change occurring in the structure of the liquid. The former suggestion (:L) was verified by Hildebrand, Alder, Beams and I)ixon,2 who found that in a centrifugal field with an acceleration of lo8 em. sec.-I the meniscus temperature of the system perfluoroheptane-2,2,4trimethylpentane was raised 10” of which 1.9’ was attributable to sedimentation of the denser component, the remainder to hydrostatic pressure. The second expectation (b) was verified by Jura, Fraga, Maki and Hildebrand,a who found that a liquid-liquid system of the critical composition has, for many degrees above the two-phase region, a volume-temperature curve that is convex up ward and a heat capacity considerably in excess of its value a t still higher temperature. Because this last investigation was exploratory only, the present one mas undertaken in order to have more precise figures for this important thermodynamic quantity. We found that equilibria in the immediate supra-critical region is attained very slowly, and that stirring could affect the structure of the liquid so that it was necessary to discard the usual cdorimetric method of measuring the rise in temperature following the addition of electrical energy in favor of ice calorimetry, which permitted holding the system for as long a ? necessary a t the initial, higher temperature, with no stirring during equilibration. The differences between the heat content of the system a t a series of upper temperatures and its heat content a t 0” were measured. These differences were plotted (1) J . H. Hildebrand. J . Coll. Sei., 7 . 551 (1952). (2) J. H. Hildebrand, B. J. Alder, J. W. Beams and H. M. Dixon, THIB JOURNAL,58, 577 (1954). (3) G. Jura, D. Fraga, G. Maki and J. H. Hildebrand, Proc. Nut. A c a d . Sei., 39, 19 (1953).
against the initial temperatures and the slope of the curve a t any point gave the heat capacity. The fluorocarbon thus purified gives a. critical temperature of 26.2’ with carbon tetrachloride.2 Higher values that have been reported for this system may have been due to traces of carbon disulfide in the carbon tetrachloride or of air in the fluorocarbon, either of which would raise the observed critical temperature. Experimental Crude erfluoromethylcyclohexane supplied by Du Pont Organic 8hemicals and Minnesota Mining Fluorochemiciils Departments was passed through a 2 meter silica column of 1 em. i.d. This material was then refluxed for 6 hours with saturated KMn04 solution, washed several times with H?SO,, water arid 10% NaOH, and dried over “Drierite.” The liquid wiw aistilled on a packed helix column operating a t about 30 theoretical plates efficiency. The boiling point range of the middle cut used was 76.38-76.45’ (760 mm.)l The resulting sample gave greater than 77% transmission throughout the range 208 to 290 millimicrons through 1 em. of the C1F14. Th,e dcnsity of the material was 1.7891 f 0.0002 g./cc. a t 25 The CCl, used was Eastman “Spectra-grade.” This was dried over CaS04 and distilled on a Podbielniak “HyDerCnl” column under conaitions giving more than 100 theoretical plates. The boiling range of the fraction used was 0.07’ and the boiling point assumed (76.75’ a t 760 mm.)4 was used to calibrate the thermocouple. The density of the CC14 was 1.5858 i 0.0002 g./cc. a t 25’ (lit.5 1.58431.5858), and n D 1.4580 0.0003 (lit 1.4576). The Bunsen isothermal ice calorimeter was similar i n design t o one described and calibrated by Ginnings and Curruccini.6 Use of an isothermal method to measure the heat content difference HT - H278.~a made it possible to determine the heat capacity over arbitrarily long equilibration periods. The “static” heat capacity could thus be determined without the difficulties due to heat leak, thermal equilibration and mechanical heating during stirring which have plagued previous attempts to measure heat crtpacit,ies in this region by conventional adiabatic methods. The calorimeter was calibrated electrically from time t o time as described by the Bureau of Standards workers6 and their calibration factor of 270.33 abs. joules per g . of Hg disphced was roughly (0.06% av. dev.) verified. Closer calibration
.
*
(4) J. Timmermans, “Physico-Chemical Constants of Pure Organic Compounds,” Elsevier Publishing Co., Inc., New York, N. Y.,1950. (5) D. D. GinniDgs and R. J. Curruccini, J . Research Nall. Bur. Standards, 38, 553 (1947).
298
HARTLAND SCHhfIDT,
GEORGEJURA A K D JOEL H. HILDEBRAND
G
18
20
22
24
26
28
30
32
34
t, "C. Fig. 1.
8 d
0
20
40 60 Hours a t 25.91'. Fig. 2.
SO
100
120
was not attempted in this series of measurements since the reproducibility of the experimental runs gave sufficient indication of the precision of the method, and greater absolute accuracy was unnecessary. A controlled flow of 0.14 cc./sec. of air dried by flow through saturated KOH solution and CaS04, and chilled to the ice-point in the ice-bath surrounding the calorimeter, served to sweep moist air out of the central sample chamber and prevent drift due to water condensation during runs. The ice mantle surrounding the sample well was formed and shaped by suspending a small copper bucket of solid COZa t various heights in the sample chamber, using care to see that the ice mant,le extended all the way t o the top of the ice-water chamber. Great care was taken to exclude dissolved air in preparing the sample. A carefully cleaned copper sample bulb of about 60-cc. capacity, tapered to slide easily down the calorimeter well, was equipped with a metal-to-glass s e d entrance tube connected to a degassing chamber. The sample bulb contained three fine mesh copper screen disks soldered horizontally for thermal conduction. The assembly was isolated under vacuum and found to leak less than 10-2 mm. in 24 hours. The purified components of the solution were then weighed into the degassing bulb; the assembly was linked t o a vacuum line with a short piece of Tygon tubing; the solution was frozen with solid COZ, and the bulb was evacuated briefly. The solution was isolated, melted, refrozen and degassed several times before it was distilled over into the sample bulb. After the sample bulb was sealed off, the assemhly was again weighed to see if there was an appreciable loss i n weight due to the evacuation of the frozen sample. The loss indicated was 0.1% of the sample weight, and, since the two coinponents have practically identical vapor pressu w, it was assumed that the composition was not changed adreciably by this procedure. The total amount of air in the sample, including residual dissolved air and air from leakage during the six months of the measurements, should not have been more than that represented by a partial air pressure of a few mm. over the solution. The sample was suspended by a Nylon string in a copper pipe surrounded by a 12 kg. copper block for heat ballast. This pipe was mounted to extend through a well-insulated water thermostat of about 100-liters capacity. The temperature fluctuations were recorded continuously on a 1 mv. full scale Brown recorder connected to a quick responding thermistor bridge in the water-bath. The mean tempera-
Vol. 63
ture of the bath remained constant to 0.003' over periods of several days, and the total amplitude of the fluctuation over the thermostating cycle of about 2 minutes was of the order of 0.01". The absolute temperature of the bath was determined on a Brooklyn Thermometer Co. certified mercury thermometer graduated to 0.01' in conjunction with two other thermometers which were used to get an average relative temperature over the interval studied. Thus, the over-all accuracy of the temperature of the snmple itself was about 10.01' while the relative temperature scale of the runs was accurate to about 0.005'. I t \.vas found by trial that the heat content became constant n'fter six hours of thermostating at t.emperntures far from the observed heat capacity discontinuity, but that equilibration times were very much longer close to that temperature. The snmple was always heated to about 50' and mixed. tho!oughly before it was suspended in the thermost,at, since it was found that equilibration of the unstirred sample was attained more rapidly if the samplc temperature W:IS approached from above. To initiate the measurement the sample was allowed to fall free into the calorimeter to within a fraction of an inch of tjhe bottom of the sample well by release of a loop in the Nylon string. The string was then completely released to allow the sample to rest on the bottom of the well. Calculations indicated that heat losses of the sample during the fgll were negligible and that possible variations in the mode of dissipation of the gravitational energy would have no detectable heat effect. Corrections for drift and changes of Hg height in the reading capillary, were less than 1 % and usually less than 0.3y0 of the total heat effect. The heat capacity of the bulb was estimated to be 6.5 ca1. deg.-l. It contained0.427 mole of mixture in which the mole fractions were: CC14, 0.7216; C?Fla,0.2784.
Results The results expressed as calories per mole between t aiid 0" are shown in Table I. Each value represents the mean of a t least two runs with an average deviation from the respective means of less than 0.05%. A total of 43 runs were completed of which 5 were rejected on the basis of large deviatioils from the mean of the other runs a t that temperature. It should be noted that the temperature and heat content values given have the indicated precision only relative to other values in the set since only a crude estimate of the heat capacity of the sample bulb, assumed constant over this range of temperature, was used to obtain the net molar hea,t content of the sample. Figure 1 gives values of the heat capacity in calories per mole per degree obtained from a large scale plot of the data. The points in the neighborhood of 26" are somewhat uncertain on account of extreme slowness in attzk~ingequilibrium. Figure 2 plots AH us. time for repented runs a t t = 25.00 f 0.01", showing that several days seemed to be necessary for reaching a not very esactly reproducible equilibrium. This effect, observed only in this narrow region, makes it impossible to fix the shape of the peak of the A-type curve of Fig. 1. t, OC.
19.385 24.734 '25.163 25.439 25.857
Ht
- Ho,
TABLL I Ht
- Ho,
cal. mole-1
OC.
oal. mole-1
970 1250 1275 1293 1307
2G.014 26.107 26.323 26.940 28.165
1316 1322 1342 1379 1448
t.
HL t,
"C.
20.785 30.869 31.05 33,87
- Ho,
oal. mole-'
1533 1583 1593 1730
Discussion The A-shape of the C , curve is similar to that found in the earlier study. Its most significant
NOTES
Feb., 1959 feature, as in the other case, is the fact that some 7 degrees beyond the crit,ical temperature is iiecessary for C, t o descend to the approximately constant, value of a homogeneous liquid. Below the critical point, the system would gain entropy with rising temperature more rapidly than if the components were separate, because of the increasiiig disorder attending mutual penetration. ,.ibove that point, the randomness is still far from complete, as shown also by its milky appearance. The right-hand braiich eventually sinks lower than the left as the randomness becomes maximum. Tlie curve s h o w clearly the temperature rn,iige beyond which raiidomiiess can be regarded as virtually complete. In studying the heat capacity of a system such as this, one would expect, even an.iiy from the critical composition, a cusp rather than iz first order transition in the C, us. T curve when the phase bouiiclary is crossed. The cusp rather than the typical line of tl first-order transition would be observed simply because oiily ail infinitesimal amount of one of the phases is present at the transition. We would expect the cusp to become a typical A-curve at the critical concentration if the true C, of the two phases below the critical temperature were known. I n the data presented, the Cp's in t#lie two phase region represent oiily the gross
29 9
heat capacity of the entire contents of the calorimeter. Tlie necessary heats of solution and dilution hare not been measured. Consequently, the shape of the curve below the critical deviates from that expected from a typical A-curve, while above t,his teiiiperiiture the Cp cunre behaves as expected. Our dat'a suggest that the gross C, has a maximum just lielow the critical : however, the data are too meager to establish this point. We helieve that if we had available all of the thermodynamic data iiecessary to obtain the true heat capacity, this maximum would disa,ppenr :md we would obtain a typical A-curve. Two lion-polar liquid coniponents that are sufficiently different in interiial forces as to be able to form two liquid phases dissolve i n one another with expansion and absorption of heat. As the temperature rises, the rate of mutual solubility increases, making nn increasing coiit,ribution to the heat cnpncit,y of the syst'em. This accounts for the low temperature braiich of the C,, curve. This was t'o be expected. Our interest, however, has been primarily in the upper branch, above the meniscus tempernt ure. Acknowledgment.-We express our appreciation t'o t'he Itomic Energy Conimissioii and to the Research Corporation for support of this work.
NOTES EFFECT O F MICELLAR BEH.\T'IOR ON ADSORPTION C H ~ ~ R ~ ~ C T E R I S TOF ICS TWO SURFACTANTS BY \V. E. BELL' U n i o n Research Center, U n i o n 011 Cornpanu o j CaTijornza, Bren, Received J u n e 19, 1858
I n the process of determining degrees of adsorption of vnrious surfactants on mirieral surfaces, i t appeared that micellnr behavior affected the adsorption characteristics of the surfactants. This cnn be illustrated by compariiig the adsorption characteristics of Igepal CA-630 (an alkyl aryl polyether nlcohol) and Pluronics L-44 (a condelisate of ethylene oxide with propylene glycol and oxide) 011 silica srirfnces (45-60 mesh sand) from aqueous solution. Both surfactants were commerchl 100% active products coiitaining molecular sizes followiiig a theoretical distribution. Figure 1 compares adsorpt'ioii isotherms for the two surfactants. The Igepal solutions were an-
adsorbed on the sand surface reaches a maximum a t a concentration of about 300 p.p.m. and remains coiibtaiit as the conceiitratioii increases. On the other hand, the amount of Pluroiiics adsorbed continues to iiicrease as the conceiitratioii increases. Based 011 the sand surface area and the amounts of t-
-125
2
/ /O
2 100-
/
1
5
PLURONICS L 4 4
1 0
0
z"
/
//a
3 75
-
Q// /
/ / /
/
5 50 -
-2z 0
/
25 -
//O
/
/
/
/O
IGEPAL C A - 6 3 0 0
0
3
0 (1) J o h n J a y Hopkins Laboratory for Pure and Applied Science, General Atomic Division of General Dynamics Corporation, San Diego, California. (2) L. Hsiao and H. N. Dunning, THISJOURNAL, 69, 362 (1955).
EQUILIBRIUM CONCENTRATION(PPM 1. Fig. 1.-Adsorption of Pluronics L44 and Igepal CA-G30 from aqueous solution by sand.
