Radiolysis of Liquid Carbon Tetrachloride
167
Free Radical cavenging at High Dose Rates in the f Liquid Carbon Tetrachloride' Ned E. Bibler Savafinah River Laboratory, E. 1. du Pont de Nemours & Company, Aiken, South Carolina 29801 (Received Juiy 17, 1972) Publication costs assisted by the U. S. Atomic Energy Commission
The kinetics of radical scavenging at dose rates of -1025 eV/(g sec) were investigated by pulse radiolysis of samples of liquid carbon tetrachloride containing varying concentrations of Br2. At Br2 concentrations greater than 0.045 M , the 100-eV yields of radiolytic products were equal to published values determined at dose rates -lo9 times lower. As the Br2 concentration decreased below 0.04 M in the pulse radiolysis experiments, G(C2C16) increased, G(CC12Br2) and G(CC13Br) decreased, and G(C2C14) remained unchanged. These results are consistent with the free radical mechanism developed a t law dose rates after the inclusion of recombination reactions involving CCl3 and C1 radicals. Quantitative agreement for the effect of scavenger concentration on G(CC13Br) and G(C2C16) was attained between observed results and those calculated by numerical integration of rate equations derived from the free radical mechanism assuming a homogeneous distribution of intermediates.
Introduction In radiation chemistry, the addition of solutes to liquids being irradiated has been extensively applied as a technique for scavenging free radical intermediates.2 This technique serves not only to identify the intermediates but also, within certain solute concentration limits, to estimate their 100-eV yields. The concentration dependence of the scavenging process has been examined in two previous studies.3 , 4 In both investigations, the scavenging rate a t dose rates lower than 1015 eV/(g sec) was proportional to the half power of the dose rate and to the first power of the solute Concentration. These observations are consistent with a reaction mechanism involving competition between radical-solute reactions and radical recombination reactions. In one study,4 the concentration dependence at high dose rates wa5 also investigated with a Van de Graaff and a linear accelerator. Actual radiation intensities were not measured; thus, the results were interpreted only qualitatively. The relative frequency of radicalsolute reactions compared to radical recombination at any specific concentration of solute was lower a t higher dose rates. However, the overall dependence of the yield of scavenger product upon solute concentration was similar at all the observed dose rates. This article presents the results of a detailed investigation of radical ecavenging by bromine at high dose rates in the pulse radiolysis of carbon tetrachloride. The observed dependence of scavenging efficiency on solute concentration is comparcbd with that calculated by numerical integration of rate equations based on a free radical mechanism.
Experimental Section Carbon tetrachloride (Matheson Co.) was purified as previously d e s ~ r i b e d .Samples ~ of carbon tetrachloride and bromine were dried by passage through a 5-A molecular sieve column attached directly to a vacuum line, degassed by several freeze-pump-thaw cycles, and then distilled into the irradiation cells (1.7 cm square by 2 cm high). The cells3 were sealed from the vacuum line while the samples were fro.zen with liquid nitrogen. After irradiation, free halogens were determined spectrophotometri-
cally, and organic products were determined by gas-solid chromatography with procedures described p r e v i ~ u s l y . ~ Irradiations were performed at ambient temperatures (-25") with pulsed X-rays produced by a Febetron6 Model 705 pulsed electron accelerator. An electron pulse (maximum energy = 2.0 MeV) was impinged on a 20-mil tungsten target creating an intense X-ray pulse, which passed through a 5-mil tantalum foil and then the samples. Maximum duration of the pulse was 40 nsec with -80% of the total dose delivered within 20 nsec.7 The effective pulse duration (the ratio of the time integral of the dose rate to the peak dose rate) was 19 nsec.? Samples were irradiated with up to 63 pulses; the time between pulses was at least 2 min. The dose/pulse absorbed by the samples was determined with manganese-activated LiF as a secondary standards by procedures previously described.9 The estimated10 energy spectrum of the X-rays had a peak of 200 KeV, with >90% of the photons having energies greater than this. X-Rays having energies less than '70 KeV were essentially removed from the beam by the tantalum foil. The LiF response was calibrated with the Fricke dosimeter and 6OCo radiation. A mass-energy absorption coefficient for CC14 was determined by graphical integration of monoenergetic absorption coefficientsT3 over the energy The information contained in this was developed during the course
of work under Contract No. AT(07-2)-1 with the U. S. Atomic Energy Cornmission. For application to organic iiquids see R . A. Hoiroyd, "Fundamental Process in Radiation Chemistry," P. Ausioos, Ed., Interscience. New York, N. Y . 1968, p 456. For aqueous systems see I. G. Draganic' and Z. D. Draganic', "The Radiation Chemistry of Water," Academic Press, New York, N. Y., 1971, p 123. R. A. Holroyd, J. Phys, Chem., 66, 730 (1962). A. Charlesby, W. H. T. Davison, and P. G . Lloyd, J. Phys. Chem., 63,970 (1959). N. E. Bibier, J. Phys. Chem., 75, 24 (1971). Field Emission Corp., McMinnville, Ore. Field Emission Corp., Technical Bulletin, Vol. 4, No. 1 (1965). E. M. Fielden and E. J. Hart, Advan. Chem. Ser., No. 8 f , 585 (1968). N. E. Bibler, J. Phys. Chem., 75, 2436 (1971). F. M. Charbonnier, Field Emission Corp., McMinnville, Ore., private communication. (11) E. Strom and H. I . Israei, "Photon Cross Sections from 0.001 to 100 Mev for Elements 1 through 100," USAEC Report No. LA-3753 (1967). The Journal of Physical Chemistry, Voi. 77, No. 2, 1973
Ned E. Bibler
168
spectrum of the X-rays. This coefficient was only -10% higher than that determined for energy absorption predominantly by the Compton process. Reproducibility of the dose/pulse was 8% or less at all positions where samples were irradiated. Overall relative uncertainty in determining the dose received by each sample is less than 15% and results primarily €rom slight differences in the position of samples and dosimeters in the X-ray beam. Results and ~ i ~ c u ~ ~ ; ~ ~ n The 100-eV ytelds for all the radiolysis products are presented in Table I. Relative errors in the results for BrCl may be as high as 30% because of analytical errors associated with measuring spectrophotometrically small amounts of I3rCI in the presence of a nominal tenfold excess of Brz. All other analytical results are accurate within 5% or less. Fractional conversion of the initial Brz to BrCl and CCl3Br was less than -IO%, except for 60% conversion at the lowest Br2 concentration. As will be shown later, this largri conversion apparently did not affect the kinetics significantly. From 0.045 to 0.0’1 M Bra, the product 100-eV yields were almost iclenlical with those determined from the steady radiolysis of CC14-Br2 solutions with dose rates - ~ 1 0times ~ lower..3 This agreement implies that the same reaction mechanism is applicable at both intensities. At 0.33 M Bra, G(CC13Br) increased and G(CzC16) decreased in the pulse radiolysis experiments due to scavenging within the spurs. Tetrachloroethylene is the only product in Table I whose 100-eV yield is independent of Brz concentration (G(C2Cl4) = 0 074 f 0.004). This value agrees with that determined at a dose rate -109 times lower5 and confirms the absence of a dose rate effect,. Specific reactions have not been propc)sed for C2Cl4 formation, but these results imply that the mechanism may involve an efficient radical-solvent reaction similar to that proposed for CzC14 formation in the radiolysis of liquid chloroform.12 In that system, G(C2C14) is also independent of the presence of a wavenger*2 and of dose rate variati0ns.~-~3 A mechanism involving a 13ichloromethyleneprecursor has been suggested12 on the basis of data from the pyrolysis of CHC13 vapor.14 Such a ~nechaniemis not unreasonable in CCl4 because CClz species have been detected in Cc14 vapor excited by electric discharge15 or ultraviolet light.le The effect of EErz concentration on CC12Brz formation from pulse radiolysis IS compared in Figure 1 to that obTABLE !: 100-OV Yields GCX4-.ESr2 Solutionsa
in the Pulse Radiolysis of
____
Molecules/lOO eV
--
jBrr]~,~M
CSlgBr
BrCl
C2Cl6
CpCll
3.7
4,O
5.1 5.5 5.3
5.9 2.0 5.5
0.95 0.83
6.3 7.4 7.1 7.0
6.9 5.3
0.072 0.081 0.075 0.069 0.068 0.072 0.080 0.075
CCIpBr2
-lll_
0.00088 0.0039 0.0047
0.0057 0.019 0.045 0.070 0.330
d d
0.61 0.52 0.46 0.49 0.38 0.26
C C C
0.04 0.08
0.12 0.14
0.15
a Dose/pulse = 0 64 x 1Ol7-l 9 x 10’’ eV/g, maximum instantaneous dose rate =: 0 34 X IOz5-l 0 X l o z 5 eV/(g sec), temperature Gz [CCh[Br& i- 0 5[BrCI], i25’ initial Br2 cmcentrahons, [Br2]0 [CCI2Br2], after dose d Not detected Not determined Sr],
+
The Journai of I’hysicai Chemistry, Vol. 77, No. 2, 1973
tained from steady radiolysk5 At each Brz concentration, 100-eV yields at both intensities agree within experimental error, indicating the absence of a dose rate effect. This implies that the reactions forming CClzBrz and the reactions competing for its precursor are first order with respect to the precursor. A mechanism involving a radicalsolvent reaction in competition with a radical-solute reaction is consistent with this observation. The data in Table I show that CClzBr2 and C2C14 do not have the same precursors. As the Brz concentration increases above 0.0057 M , G(CCl2Brz) increases (indicating that more precursors are being scavenged) with no corresponding decrease in G(CzC14). The following mechanism involving CC13 and C1 radicals formed by ionic or neutral dissociation processes and charge neutralization reactions has been proposed for major product formation in the radiolysis of CC14-Br2 solutions .5
C1+ Br2 CC13 f Brz
+ Br CC13Br + Br
BrCl
---+
2Br
Br2
(2) (3)
As stated earlier, the same mechanism appears applicable for the pulse radiolysis results at Brz concentrations above 0.045 M . At this Brz concentration, C2Cls formation occurs only in the spurs,5 and the identity of its precursor has not been established. The effects of Brz concentration on G(CC13Br) and G(c2Cl6) are shown in Figures 2 and 3. Results obtained from the radiolysis of CC14-Br2 solutions with a COCOsource5 are also included in the figures. Before the solute concentration dependence of CC13 scavenging are discussed, the effect of BrCl and CC13Br formation in the system must be considered. If the concentration of BrCl increased sufficiently to compete’with Br2 and CC13 radicals, G(CCl3Br) would be lowered. Similarly, if CClaBr reacted with C1 atoms, G(CC13Br) would be decreased. A t initial Bra concentrations above 8.8 X M , the final ratio of CC13Br or BrCl to Brz was less than 0.12. Experiments in this laboratory have shown that below this relative concentration BrCl and C1 reactions do not affect G(CC1Br).17 Consequently the values of G(CCl3Br) lower than 7 at Brz concentrations greater than 8.8 X 10-4 M must result from a dose rate effect. M will be discussed The value obtained a t 8.8 X later. The data in Figure 2 clearly show that at concentrations less than 0.01 M , Br2 scavenges a lower fraction of Ccl3 radicals at the higher dose rate. This decrease with an increase in dose rate is in agreement with the results of Charlesby and coworkers.4:Also, the overall curvature of the line in Figure 2 obtained at the high dose rate is very similar to their result^.^ However, at their highest solute concentration, they did not obtain product yields that were equal at all dose rates, presumably because of in(12) H. R. Werner and R. F. Firestone, J. Phys. Chem., 69,840 (1965). (13) F. P. Abramson and R. F. Firestone, 2. Phys. Chem., 70, 3596 (1966). (14) A. I. Shilov and R. D. Sabirova, Russ. d. Phys. Chem., 34, 408 (1960). (15) P.Venkateswarlu, Phys. Rev., 77,79 (1950). (16) T. E. Khalfawi and A. Johannin-Giilis, C. R. Acad. Sci., 242, 1716 ( 1956). (1 7) These experiments were performed with previously described proc e d ~ r e s A. ~solution of 0.015 M B,r. in CCL was irradiated with various doses until the ratio of (CC138r) or (BrCI) to (Brp) was 0 13 The yield of CC13Br was linear with dose, and G(CCI3Br) equalled 6 9 Thus, within these limits there is no effect of BrCl or CI on G (CC13Br)
Radiolysis of Liquid Carbon Tetrachloride
169 8.0, 8 .0
,
I 1 1 1 1 1 1
I
I
I I I Ill/
I
I
I I I I I I
6.0
0.0
b
0.001
-
-
-
0.0 I
d
rii
d0.I
2
V
4.0
[5r*Io3 M
1
Fiqure 1. variation af (2 (CCI2Br2) with initial Br2 concentration
at 25':
0 ,pulse radiolysis (this work), maximum dose rate a,6oCoradiolysis (ref 5 ) , dose rate = 1.8
-loz5 eV/(g sec); x 10'6 eV/3(g S d C ) .
=-J----o-o.
complete scaivenging a t the high dose rates. Their highest solute Concentration was -100 times smaller than that used in this study. The increase in G(C<ij coupled with the decrease in G(CC13Br) at Brz concentrations less than 0.01 M is consistent ,with a competition for CC13 radicals beLween reactions 2 and 4. As shown in Table I, for any CCls + CC13
-
C2Cl6
+ CC13--, CC14
(5)
Rate equations derived from this high dose rate mechanism were used to cwlculate product yields as a function of Brz concentration. For completeness, reaction 6 was in-
c1 + 61
-
Cl2
0.I
M
[%lot
Figure 2. Variation of G (CCI3Br2) with initial Brz concentration at -25": points experimental, line calculated as described in text; 0, pulse radiolysis (this work), maximum doss rate -IOz5 eV/(g sec); 0 ,6oCo radiolysis (ref 5), dose rate = 1.8 X 1016 ev/ (g sec).
(4)
specific decrease in B P concentration, ~ two times the increase in G((>&16) does not account for the total decrease in G(CC13Br). Gon!sequently, reaction 4 cannot be the only process co:mpet.ing with reaction 2 for CC13 radicals. Recombination of CC18 and C1 radicals would be a suitable process that may compete since its occurrence would lower G(CC13Br:r. CI
0.01
0.001
(6)
cluded. The rate equations were integrated by a fourthorder Runge-Kntta techniquels with a digital computer. The program also contained a subroutine that simulated the time dependence of the intensity of the radiation pulse.? Rate equations1 were derived from reactions 1-6 for CCl3, C1, andl Er radical production and depletion as well as for product production, assuming a homogeneous distribution of eadicals. The 100-eV yields of CC13 and C1 radicals were set at '7.0.5 When 99.9% of the total amount of radicals produced by the radiation dose had reacted, the calculation was rterminated, and G values were calculated for single pulses. Of the rate constants for eq 1-6, only kq has been determined in the liquid phase. In carbon tetrachloride at 24", 2k4 = 7 X IO7 M - l S ~ C - Values ~ . ~ ~ for k 3 , ha, and kg can be estimated from the modified Debye equation20 if the reaction rates are assumed to be diffusion controlled. The estimated values were k~ and hg = 7 . 2 X lo9 M - l sec-1 and h5 = 8.1 :X 109 619-1 sec-1. As a starting point, k l was set equal to the value determined in the vapor phase, 2.3 X 108 M-f S I ? C - ~ , ~ : I . and k 2 was varied in an attempt to calculate the observed results. When k z was 2.3 x lo6 M-1 586-1, good agreement with the observed results was obtained as indicated !by the calculated curve in Figure 2. Values for the total 100-eV yield far CzC16 were calculated by adding the spur :yield of CzCle (0.44 molecules/100 eV) to the yield calculated for reaction 4 with the computer. Best agreement was obtained when 2124 equalled 6 X lo7 M--1 sec-1, and these results are plotted as the line in Figure 3. The observed increase in G(CzC16) at Bra con-
0.2
0.001
0.01 [B'2Io9
0. I
M
Figure 3. Variation of G(C2C16) with initial Br2 concentration at -25": points experimental, line calculated as described in text; 0 , pulse radiolysis (this work), maximum dose rate -loz5 eV/(g sec); 0 ,6oCo radiolysis (ref 5), dose rate = 1.8 X 1016 eV/(g sec). centrations below 0.005 M is apparently due to reaction 4. Decreasing 2k4 from 7 x 107 to 6 x 107 M - 1 sec-1 had a negligible effect on the other calculated 100-eV yields. At 0.00088 M Brz, the calculated values for G(CC14) (from reaction 5 ) and G(C12) (from reaction 6) were 2.7 and 1.2, respectively. The agreement between the observed and calculated results for G(CC13Br) at 0.00088 M Brz deserves brief comment. This sample was irradiated with 31 pulses at 1.7 X 10x7 eV/g per pulse. During irradiation, the CCl3Br and BrCl concentrations increased to -1.3 times the final Br2 concentration. Statistically, at these product concentrations, -36% of the C1 atoms could react with CC13Br and -60% of the CC13 radicals could react with BrC1; in either case, G(CC13Br) could be lowered. Agreement with calculations based on a mechanism that does not contain reactions of CC13Br or BrCl is either fortuitous resulting from the choice of rate constants or suggests that GC13Br and BrC1, even at this high relative concentration, do not significantly affect G(CC13Br). There are some data to support the latter. Prolonged radiolysis of CCl4-Br2 solutions yields CC13Er as the only brominated product and as- the only inorganic product.22 This implies that so S. Gill, Proc. Cambridge Phil. SOC.,47.96 (1951). D. J. Carlsson, J. A. Howard, and K. U. Ingold, J . Amer. Chem. Soc., 88, 472 (1966). H. L. J. Backstrorn and K. Sandros. Acta Coem. Scand., 14, 48 (1960). M. J. Christie, R. S. Roy. and B. A, Thrush, Trans. Faraday Soc.. 55, 1139 (1959). F. P. Abramson, B. M. Buckhold, and R. F. Firestone. J. Arner. Chern. Soc., 84, 2285 (1962). The Journal of Physical Chemistry, Voi. 77, No. 2, 1973
170
Ned E. Bibler
long as other scavengers such as C1z (or BrCl and BIZ) are present, CCl3Br is protected from attack by C1 atoms. Also, the fact that BrC1 formation eventually decreased to zero on extended ridiolysis22 suggests that CCl3 radicals may react with BrCl by Br abstraction even though this reaction is b s s favored thermodynamically then formation of CClc. If Br abstraction does occur, BrCl would react similarly to 13rz and not affect the results. As an extension of the mechanism, the possibility of combination reactions of Br atoms and C1 or CCl3 radicals were included. Rate constants for r.eactions 7 and 8 were estimated with the modified Debye equationlg (k7 = 7.9 x 109 M - 1 sec-I). When k l was equal to 2.3 X lo2 M - l CCl3
+ Br
--
CC13Br
(7)
C1 I- Br BrCl (8) see-I, hz had to be lowered to 3.8 X 105 to obtain agreement with the experimental results for G(CC13Br). When this was done, the calculated values for G(C2Cls) were -10% higher thair the observed results up to 0.019 M Brz concentration. In tlwse calculations, -70% of the CCl3Br resulted from reaction 7 . Unfortuntitely, the above calculations cannot be used to establish unique values for any rate constants or even for ratios of rate constants. The calculated results were dependent on the choice of k l (a constant that has not been determined in liquid CC14). When k l was lowered by one half, the calculations reproduced the data for G(CC13Br) when k~ wars 6 X IO6 M - 1 sec-1 and reactions 7 and 8 were ignored, or when k.2 was 9.2 x 105 M-1 sec-1 and reactions 7 and 8 were included. In both cases, the calculated values for G(C2Cla) were -20% lower than the observed values up LO 0.019 M BrZ but still indicated the observed curvature. Above 0.019 M Br2, good agreement was attained. Also the calcuiated results were dependent on estimates of diffusion-controlled rate constants. These calculations do indicate though that the result8 can be
The Journa! oil Physical Chemistry, Vol. 77, No. 2, 1973
quantitatively interpreted by a homogeneous mechanism containing reasonable values for rate constants. However, even though there is no necessity to postulate an increased significance of intraspur events affecting the yields of C1 and CC13 radicals, there are too many parameters in the calculations to unequivocally rule out such a possibility. Homogeneous kinetics have been successful in describing reactions of transients produced by pulse radiolysis of many organic and aqueous systems.23 Also, calculations based on a homogeneous mechanism have been successful in quantitatively predicting the effect of dose rates of -1028 eV/(g sec) on the yield of Hz from pulse radiolysis of water a t pH >3.23 However, these calculations were not successful in predicting the effect of dose rate on scavenging of aqueous electrons by nitrous oxide ([NzO] = 0.047 M).24Agreement between calculated and observed results was attained only when k(N2O I- eaq-) was three to five times lower than the presently accepted value.25 This difference was attributed to scavenging within the spurs in an inhomogeneous mechanism. TO determine if such an effect is actually occurring in the carbon tetrachloride system, independently determined values of the appropriate rate constants are necessary. However, analysis of the data of both systems indicates that the processes competing with scavenging reactions are primarily recombination reactions to form the parent solvent (cc14 or H2O).
Acknowledgment. The author wishes to thank Robert C. Kuckuck and his staff at Lawrence Radiation Laboratory, Livermore, Calif., for performing the irradiations. (23) For a review. see M. S. Matheson and L. M. Dorfman, “Pulse Radiolysis,” The MiT Press, Cambridge, Mass., 1969, pp 61 and 148. (24) C. Wiliis, A. W. Boyd, A. E. Rothwel!, and 0. A . Miiler. Iflf. J . Radiat. Pbys. Chern., 1, 373 (1969). (25) M. Anbar and P. Neta, Int. J. Appl. Radiat. Isotopes, 18, 493 (1967).
