3054
NOTES
ampoules with weighed quantities of concentrated electrolyte solutions (4.500 m KC1 and 4.500 m LiCl and 2.500 m KBr and 2.000 m LiBr) or solid KCl were submerged and crushed in 450 g of 1 m HOAc or CHaN02, respectively. Enthalpies of dilution of the HOAc solutions due to transfer of small quantities of water from the ampoule were negligible. Variations in the quantity of electrolytes in the ampoule and, consequently, in the final concentration (over-all range 0.004-0).014m, the majority of runs being below 0.008 m) resulted in essentially constant enthalpies. A total of 27 runs, with a minimum of four per electrolyte, were carried out. Nitromethanelo and all other materialslb were described elsewhere.
Results and Discussion The enthalpies of transfer are shown in Table I. The over-all experimental error in all tabulated values of AH2 is f20 cal/mol, based on standard deviations of the means of measured enthalpies multiplied by factors necessary to give 90% confidence levels and estimated
Table I : Summary of Nonelectrolyte
HOAc
and A%a9
Z 8 ,
Electrolyto
cal/mol
osljmol
LiCl LiBr
110 30 190 -220 -150
110 60 150 190 150
-
KC1
CHdY"e
KBr KCI
-
errors for values from ref 6. The over-all experimental error for Z a on the same basis is kt10 cal/mol.*b It may be noted that in all but one case a 2 and AHa are equal, within the limits of experimental error. Thus eq 10 may be expressed in the form
Either set of enthalpies also allows prediction of the interaction-constant and activity-coefficient behavior as a function of temperature. Under limiting conditions the enthalpies are ionic properties;lb Le., they must consist of independent additive contributions from the cation and anion, respectively. Table I1 shows the average ion-contribution differences A(AH2)and A ( D 3 ) l bfor the HOAc systems. Table I1 : Ion-Contribution Differences A(aa),
Ion pnir
oal/mol
Li +-K + C1--Br -
270 50
The Journal of Phyaical Chemistru
A(aa), csl/mol
250 40
As a consequence of eq 11, values of A(B2) and A(AH3) must also be equal for a given ion-pair difference, and this additional requirement is met satisfactorily. Acknowledgment. The authors wish to thank the National Science Foundation for financial assistance.
The Thermal Decomposition
of Perfluoropropene by Richard A. Matula] Fluid Dynamics Laboratory, Department of Mechanical Engineering, The University of Michigan, Ann Arbor, Michigan (Received March 82, 1968)
The thermal decomposition of a few of the low molecular weight fluorocarbons have been studied. A number of investigatjors2-6 have studied the pyrolysis of tetrafluoroethylene in the temperature range 300-800°, It has been shown that this reaction system can be divided into three phases. At low temperatures (T < 550°), octafluorocyclobutane is the main product; a t medium temperatures (550 < T < 700"),perfluoropropene and a perffuorobutene are produced; and at high temperatures (T > 700") perfluoroethane and nonvolatiles are produced. The thermal decomposition of octafluorocyclobutane4~e-8 has been studied from 360 to 930". Atkinson and A t k i n ~ o nhave ~ ~ studied the pyrolysis of perfluoropropene and perfluoroisobutene in the temperature ranges 600-675 and 700750", respectively. The purpose of the present investigation is to study the thermal decomposition of perfluoropropene in the temperature range 550-675'.
Experimental Section The experiments were conducted in a cylindrical Vycor reactor vessel (250 mm long and a 60 mm i.d.), which was maintained a t a constant temperature by an electrically heated furnace. The reactor temperature, which was measured with the aid of four chromelalumel thermocouples, was controlled to within iO.5" (1) Department of Mechanical Engineering, Drexel Institute ob Technology, Philadelphia, Pa. (2) W. T.Miller, Jr., "Preparation, Properties, and Technology of Fluorine and Organic Fluoro Compounds," McGraw-Hill Book Co., Inc, New York, N . Y . , 1951,Chapter 32. (3) J. R. Lacher, G. W. Tompkin, and J. D. Park, J . Amer. Chem. Soc., 74, 1693 (1952). (4) (a) B. Atkinson and A. B. Trenwith, J. Chem. Soc., 2082 (1953); (b) B. Atkinson and V. A. Atkinson, ibid., 2086 (1957). (5) G. A. Drennan and R. A. Matula, Fluid Dynamics Laboratory Report No. 68-1,The University of Michigan, Ann Arbor, Mich., 1968. (6) B,F. Gray and H. 0. Prichard, J . Chem. SOC., 1002 (1956). (7) J. N. Butler, J. Amer. Chem. SOC.,84, 1393 (1962). (8) A. Lifshita, H. F. Carroll, and S. H. Bauer, J . Chem PhyS., 39* 1661 (1963).
3055
NOTES over a period of several hours. An Aerograph R/lodel 202-B dual-column, hot-wire, thermal-conductivity gas chromatograph and a Beckman IR-10 infrared spectrophotometer with a spectral range 300-4000 cm-1 were used to identify and quantitatively determine the gaseous products as a function of reaction time. A 5-ft column of 50-80 mesh Poropak Type N maintained a t 135" was used in conjunction with the gas chromatograph. The perfluoropropene used in these experiments was purchased from Air Products and Chemicals, Inc. Gas chromatographic analysis of the C3F6 indicated that it had a minimum purity of 99.8%.
Results and Discussion The pyrolysis of perfluoropropene (C3Fe)was studied over the temperature and initial pressure ranges 552676" and 50-410 torr, respectively. The order of the reaction with respect to perfluoropropene was determined by measuring the half-life of perfluoropropene as a function of initial perfluoropropene concentration a t 599". The half-lives for initial perfluoropropene pressures of 51, 102, and 204 torr were 70, 64, and 58 min, respectively. Based on these data, the reaction order was calculated to be 1-01. Therefore, the rate equation for the pyrolysis of perfluoropropene can be represented by the first-order expression
The average values of the rate constant (kd) calculated from the integrated form of eq 1 as a function of initial pressure and temperature are given in Table I. An Arrhenius plot of the first-order rate constant (k4) as determined from the data is given in Figure 1. A least-mean-squares fit to the experimental data yields
Gas chromatographic and infrared analysis of the reaction products indicated that perfluorobutene-2 and perfluoroisobutene were the major fluorocarbon reaction products. The perfluorobutene-2 was identified by its gas-chromatographic retention time. Since pure perfluoroisobutene was not available for calibration of the gas Chromatograph, the perfluoroisobutene in the reaction products was identified by its uniquely characteristic infrared absorption line at 10.05 p.4b I n addition to the above-mentioned fluorocarbons, traces of perfluorobutene-1 were detected under certain operating conditions. The perfluorobutene-1 was detected by both its characteristic retention time and its ir spectra. The concentration of perfluorobutene-2 as a function of reaction time for initial perfluoropropene pressures of 204 torr and various temperatures is shown in Figure 2. I n all cases, the rate of perfluorobutene-2 production increased as the initial perfluoropropene concentration was increased, and the order of the rate of
Figure 1. Arrhenius plot of kd
218.
1/T.
b
R-204 torr
t
,mln
Figure 2. Perfluorobutene-2 production in the perfluoropropene pyrolysis.
R * 204 torr
0
so
so
60 t
min
Figure 3. Perfluoroisobutene production in the perfluoropropene pyrolysis.
production of perfluorobutene-2 with respect to the perfluoropropene concentration was between 1 and 2. The concentration of perfluoroisobutene as a function of reaction time for initial perfluoropropene pressures of Volume 72,Number 8 August 1968
3056
NOTES
Table I : Average Values of the First-Order Rate Constant ka T, O C
676 676 650 650 650 599 599 599 552 552
Initial pressure, torr
No. of data points
204 102 204 102
7 9 11 6 6 6 7 10 10 9
51 204 102 51 408 204
106ka, seo-1
141 149 73.5 71.2 73.2 20.5 18.7
18.5 3.73 2.67
204 torr and various temperatures is shown in Figure 3. Since pure perfluoroisobutene was not available for calibration of the gas chromatograph, the quantitative results for this compound are based on the assumption that the thermal conductivity detector had the same sensitivity to perfluoroisobutene as the average of the perfluorobutene-2 and octafluorocyclobutane sensitivities. The rate of production of perfluoroisobutene with respect to perfluoropropene also had an order between 1 and 2. I n addition to the fluorocarbons mentioned above, the gas-phase reaction products contained carbon monoxide, carbon dioxide, and silicon tetrafluoride. Butler' also noticed side reactions with the wall when he studied the pyrolysis of octafluorocyclobutane in a Pyrex vessel in the temperature range 360460". In all cases, these three compounds were significant products. A mass balance on the carbon, including all of the gaseous carbon containing compounds, a t 650" indicated that a small carbon mass loss of 10-25% occurred. This mass loss was attributed to the small flakes of white dust that condensed in the cooler parts of the system. The present experimental results indicate that the pyrolysis of perfluoropropene, in the temperature and initial pressure ranges 550-675' and 50-410 torr, respectively, can be represented by a first-order reaction. When the pyrolysis of perfiuoropropene was carried out in a nickel vessel in the same temperature range, Atkinson and A t k i n ~ o nreported ~~ that their data could be represented by a reaction of order 1.5. In their paper Atkinson and Atkinson4bdo not specify the range of initial CaFe concentrations which were considered. However, these authors reported that the perfluoropropene half-life was approximately 60 min when the reaction temperature and initial CaFs pressure were approximately 600' and 400 torr, respectively. As discussed earlier, the half-life of the peduoropropene pyrolysis a t 599" determined in the present investigation varied between 58 and 70 min when the initial perfluoropropene pressure was varied from approximately 50 to 200 torr. A reaction of order 1.5 The Journal of Physical Chemistry
requires that the half-life be approximately doubled when the initial reactant concentration is decreased by a factor of 4. Considering these comments and the fact that the half-life as measured by Atkinson and Atkinson4b is approximately the same as determined in this investigation, it is reasonable to suggest that the pyrolysis of perfluoropropene is a first-order reaction.
Acknowledgment. The author acknowledges the help of Mr. W. C. Kelly and M r . M.J. Siemion in the experimental and data-analysis phases of the program. This research was sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research, United States Air Force, under Grant No. AF-AFOSR1144-67.
Nuclear Magnetic Resonance Study of Micelle Formation in Sodium Perfluorocaprylate and -propionate by Rizwanul Haque Oregon State University, Corvallis, Oregon 97831 (Received March 26, 1968)
The various phenomena occurring in or due to surfactant materials as studied by nuclear magnetic resonance (nmr) technique include phase transition,l behavior in liquid crystalline ~ t a t e , membra ~ ' ~ ne^,^ interaction in s o l u t i ~ n ,and ~ solubilization s t u d i e ~ . ~ ~ ~ The evidence of micelle formation in-soap solutions has been obtained from the concentration dependence of proton chemical shift*and spin relaxation time of waterg and solute protonlOJ1 resonance. Changes in the proton chemical shift of quaternary ammonium soaps are small at different concentrations.12 The shielding of the fluorine nucleus is more susceptible to the environment than the shielding of the proton, and hence (1) T. J. Flautt and K. D. Lawson, Advances in Chemistry Series, No. 63,American Chemical Society, Washington, D. C., 1967, p 26. (2) A. Saupe, B.Englart, and A. Povh, ref 1,p 51. (3) M. P. McDonald, Arch. Sei. (Geneva), 12,141 (1959). (4) D. Chapman, V. B. Kanat, J. DeGier, and 5.A. Penkett, Nature, 213, 74 (1967). (5) R. M. Roseberg, H. L. Crespi and J. J. Kats, Abstracts, 155th National Meeting of the American Chemical Society, San Francisco, Calif., April 1968. (6) J. C. Eriksson, Acta Chem. Scand., 17, 1478 (1963). (7) T.Nakagawa and K. Tori, Kolloid Z.,194 (1964). (8) J. Clifford and B. A. Pethica, Trans. Faraday Soc., 60, 1483 (1964). (9) J. CWord and B. A. Pethica, ibid., 61, 182 (1965). (10) J. Clifford, ibid., 61, 1276 (1965). (11) K. D. Lawson and T. J. Flautt, J . Phys. Chem., 69, 3204 (1905). (12) H.Inoue and T. Nakagawa, ibid., 70,1108 (1966).
