THE JOURNAL OF
PHYSICAL, CHEMISTRY (Reuistered i n U. 8. Patent Ofice)
(8Copyn’oht, 1068, by the Amen’can
SEPTEMBER 16, 1963
VOLUME67, NUMBER9
_-
~
~~~~
Chemical Society)
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EVIDENCE FOR A TRACK EFFECT IN THE RADIOLYSIS OF LIQUID CYCLOHEXANE BY J. W. FALCONER AND MILTON BURTON Department of Chemistry and the Radiation Laboratory,‘ University of Notre Dame, Notre Dame, Indiana Received November 2, 1962 A detailed examination of the yields of liquid products in the Hg-photosensitized reactions and in the radiolytic reactions in liquid cyclohexane, induced, respectively, by 1.8-Mev. electrons, by Coao y-rays, and by Po210a-particles, shows that the ratio G(CeHlO)/G(C~~H,~) increases with LET.
(1960).
from impurities that may be present. Cyclohexane was purified. therefore, by three successive crystallizations in which one-half was recovered as solid in each crystallization, and by fractional distillation thereafter in a 45-plate spinning band column. This procedure reduced the total impurities detectable by gas chromatography from a range between 1 and 2% t o -0.1%. There are a t least four of these impurities which appear t o be saturated hydrocarbons. K and K Laboratories Research Grade cyclohexene was used without further purification. Gas chromatographic examination showed it to be ~ 9 9 pure, 7 ~ with cyclohexane as the main impurity. Aldrich Chemical Co. bicyclohexyl was used without further purification. Irradiations with Ultraviolet.-Mercury-photosensitized decomposition of liquid cyclohexane was conducted in Vycor cells, located on the axis of the helix of a Hanovia mercury resonance lamp SC 2537, operated a t 70 ma.; the output of the lamp was 5.3 X 10’8 quanta min.-l. After degassing by freeze-thaw technique, the sample containing a drop of mercury was heated to 70” for 30 sec. and then shaken as i t cooled to the operating temperature of the lamp (45 f lo). The gas phase was masked during exposures which varied from 60 t o 300 sec. Irradiations with the electron beam were conducted with a Van de Graaff generator (High Voltage Engineering Corp., Type 8,Model S) operated a t 1.8 Mv. and 1 fia. The same glass cell with a thin window ( ~ 1 mil) 0 was used for all the irradiations and it was placed almost touching the exit window of the generator to avoid energy loss and beam scattering in air. Under these conditions the dose rate in the volume being irradiated was -6 X loz4e.v. I.-’ min.-l. Ten-milliliter samples were stirred vigorously during irradiations with doses of lo2*to 1023e.v. 1. -1. It is important to note that total energy absorption wa8 not determined by the charge-input method but was, instead, established by assigning G(H2) = 5.6 to hydrogen production from cyclohexane , l o Irradiations with CosO?-radiation were carried out in a source similar to that described by Hochanadel and Ghormley.11 The dose rate was 3.77 X lozoe.v. 1.-l min.-’ based on G(Fe+S) = 15.6 for the Fricke dosimeter. Irradiations with a-particles were conducted with a 1.75-curie Po2l0source 5/8 in. in diameter supplied by Monsanto Chemical Company’s Mound Laboratories. The source sealed (as supplied) with a 1/S mil stainless steel window W‘ was fixed into the
(7) A. Kuppermann “Mffusion Kinetics in Radiation Chemistry,” C h a p ter I11 in Vol. 5 , ”Actions Chimiques et Biologiques des Radiations,” edited b y M. Eaissinsky, Masson e t Cie, Paris, 1961. (8) W. G. Burns, J . Phvs. Chem.. 65,2261 (1961). (9) J. L. Magee, Ann. Rev. Phys. Cham., 12,389 (1961).
(10) This value is somewhat lower than the value G(HJ = 5.8 previously bsed in communications from this Laboratory b u t is consistent with the results of other worlP$ra. (11) C. J. Hoohansdel a n d 6 . A, Ghormley, Rev. Sci.Inetr., 21, 473 ( ~ 9 6 1 ) ~
Introduction Linear energy transfer (ie., -dE/dx or LET) has a pronounced effect on distribution of products in the radiolysis of liquid water2 but, according to data in the literature,3 4 apparently none in the radiolysis of liquid cyclohexane. The former result is consistent with the spur-diffusion model of Samuel and Magees and with the refined computations of Dynee and of Kuppermann.’ Burns* suggests that the cause of this difference may be found in an initial radius of the spur about 10 times as great in cyclohexane as in water; however, there appears to be no obvious explanation for such a pronounced difference in spur radius. This paper reports yields of liquid products from liquid cyclohexane in mercury-photosensitized decomposition and in radiolysis by Coe0yradiation, by 1.8MeV. electrons, and by Pozloa-particles. By contrast, the first case represents a condition of relatively homogeneous kinetics. The results show an effect of LET on the liquid products of radiolysis which would tend to be obscured at the high conversions used in previous studies and which may be obscured in studies of hydrogen yield alone. Experimental Reagents.-Both Eastman Reagent Grade and Fisher Spectralanalyzed cyclohexane were used. Because, in this work, Bome significance is given to products with low G-value, it is important to establish that such products come from cyclohexane and not (1) The Radiation La.boratory is opelateti under contract with t h e Atomic Energy Commission. (2) A . 0. Allen, “The ]Radiation Chemistry of Water a n d Aqueous Solutions,” 13. Van Nostrand Co., Inc., New York, N. Y., 1962. (3) R. H. Schuler a n d A. 0. Allen, J . A m . Chem. SOC.,77, 507 (1955). (4) H. A. Dewhurst a n d R. H. Schuler, i b i d . , 81, 3210 (1959). (5) A. H. Samuel a n d J. L. Magee, J. Chem. Phys., 21, 1080 (1953); A. K . Ganguly and J. L. Magee, ibid., 26, 129 (1956). Kennedy, Can. J. CSem., 36, 1518 (1958); 38, 6 1 (6) P. J. Dyne a n d J.
1743
1i 4 4 The liquid products were analyzrd by gns chromatography using :til F and 11 )lode1 001) flmie ionimtion chromatograph. e in. and tlicrc W:LY no sample I h c cdrirnns ernplo? et1 ~ c r 0.25 splitting cithcr berole or after the column; 5-x samples of irriidiated sollition n e w used aithout prior concentration. Several stilnd:lrds of appropri:ite concentration 11 ere run with each series u1 :tn:dyscs. ( ” 6 products were resolved on a 16-ft column, of ,9,,9’-oxydi111q)ionitrilc on firebrick, operated :it 43”; the C l g products were resolved on a 12-ft. column, of ~ ) o l y c t llene i ~ glycol succinate, oiierated :it 160”. The latter coliinin gives good resolution between bicyclohe~yll~ and an unidentified product U (which, retention time data suggests, may be c)cloticsylc).clohcxen~) and also separates these produc t8 froin c i clohc~anone and cyclohexanol. The lattcr produc’ts can be forrrwd if solutions arc not completely degaascd. No standards w r e avniliLblc for products other than bicyclohrryl and cyclohcxcne. For such products the sensitivities were cstimatcd from thobe of bic hexene on the assuniption that concentration is proportional to peak nren for msteri:tls with the same numtwr of c a bon ~ xtoms. r 7
He
-
Fig. I .--l’o”l0
-He
a-source holder arid irradiation vessel.
Results Mercury-Photosensitized Reactions.-In ihis n o r k the rate of dcconiposition was rcproduc*ihIconly within a factor of -2, presumably because of tlifTewncrs in concentrat ion of mercury dissoll-ed in the liquid cyclohcxanc. The dose, thcreforc, is reprcwiitcd most conveniently in ai4)itrary units rclatcd t o the fixrtion of CGH12 converted to products. l‘hc Liter nab cstimatedLfronithe relation -AC6TI,2/CGTI,,‘v (C61T,,
0
5
10
13
Approximate conversion in units of 10-4.
Fig. 2 --Yields of major liquid products from Hg-photosensitized decomposition of cyclohesnne. aluminum source holder as illustrated in Fig. 1. A thrcaded p h g
P positioned the source above, and almost in contact with, a second 1 /8 mil stainless steel window \V” attached by epoxy resin to the a1:lminum source holder. Thc intciior of the so~ir(’e holder was continuously purged with helium to increase the range of aparticles and also t o reduce corrosion of the window by radiolysis products from air. For each run, the glass irradiation cell was first purged with helium, the stopcoc’ks w r e closed, and the degassed solution (2-4 ml.) was introdured through the septum with a Hamilton gnu-tight syringe. h magnetic stirrer bi stirred the solutlon S at 1000 r.p.m. during irradiation; the volume of solutmn employed was nlways adcquittc for iininersiori of the window W”, The total energy flux wxs -2 x 101’ e.v. niin.-’ incident on an area -1.3 cni.2. The maximum energy of the a-pthrticlcs, after correction for self-ithsorption by the polomuin and for absorption by 0.3 cm. helium and the two st:iinlcss stcc,l uindows, 19 -3.7 hIev. Product Analysis.-Products volatile at - 196” were collected by a reflux tectiriique12; n u s s spectral annlyses confirmed thfLt the g:is was -XI$;, h> drogen i n all cases.
+ 2R2 + %)/C6H12
where, as clscwhcrc in this paper, 12 is nriitcii for CaIIll and, for purposes of stoichiometry, u c assunic U RC6H9. The yield of T i is so small that tliib assumption introduces no significant crror into t hc calculntion. I’igurc 2 shows yields of cyclohexenc, of hicyclohcxyl, aiid of U as a fuiiction of tlic estimated aniouiit of conversion of CrfT,,. Yields of CGHlo and of 112 are lincar with dose at low conversions; we describe such products as “initial products.” We avoid tlic usage “primary products” as employed in a similar sriise hy Dyne and Stonei4 twcause the term “pyimary” has special meaning in regard to reaction mcclianism in reaction kinetics generally. The term “initial products,” as here employed, d e r s to an experimental ohscrvation, not to any notions of niccf::iiii~m. Ori the other harid, as nearly as can hr drt(mnincd, 1; appears only after -2 x conr.ersion aiicl its yield is not a linear function of dose. Radiolysis with 1.8-Mev. Electrons.-Figurc 3 shows yields of liquid produrts as a function of dose for J‘an de Graaff cilectroii l~ombaidmcntat an intensity of -4 x loz4e.\.. 1.-’ niiii.-l in thr actually irradiated volume. Cyclohexcne, i)icyc~lohcsyI,and thc iinidcntificd product U are all prcsrnt as initial products. ‘I’hc3 100-e.v. yields given by tlic straight-line slopes arc G(C61110)= 2.86, G(It2) = 1.55, and G(U) 0.068. It should be noted that thew valiics, takcn toget>hcrwith our knowledge of G(IJ2), do riot give a material balance and that, in contrast to the mcrcury-photosrnsitized reaction, U is present as ai1 initial product. Radiolysis with CoF0 y-Radiation.-Figure 4 shows yields of products as a function of dose in irradiation with Co60gammas at a (lose rate of 3.77 X lozoP.V. 1.-’
-
(12) I,. 31. Theard and 11.Burton, J. I’hvs. Chem.. 67, 89 (1983). (13) The yield of bicyclohexyl I S ~ r r scnwtivc y to tr&ccamounts of oxhgrn Iltacauso ot mpid dcl)letion of oxggen, the effert is n o t important a t rnorlvrntr conversions (%.e..>lo-’). (1.1) 1’. J 1)prrt. . ~ n dJ A. Stone, Can. J. Chem., 39,2381 (1961).
a t very l o w conversions
--
RADIOLYSIS OF LI(JXTICTCLOHICXANE
Scpt., 1963
Approximate conversion in units of 10-4.
min.-’. G-values, calculated from initial slopes, are G(C6HlO) = 3.02, G(Rp) = 1.67, and G(G) -0.07; they arc similar to yields found for 1.8-Nev. electron irradiation, in which the dose rate nas h i g h by a factor of 104. Two very minor peaks of similar size h a w also been found in the chromatograms of the Clz products. I n each case the G-values are 5 0 . 1 and show no marked dose drpcndence. Chromatograms of thc Co products show three unidentified peaks which, from thrir sppearance times, may indicate products which contain one or two double bonds or be of molcrular w i g h t h t i lecn those of Coand C12 products; rach such G is -0.1. The observed value of G ( H p )for radiolysis of cyclohexane may be compared with that ca!culaied from yields of liquid products; i.e.
+
+
+ 1.67 + 2
X 0.OGG
1745
4
0 “[’
8
12
+
G ( 1 1 4 ~ ~ ~ ~=1 cG(CsTI,o) ~1 G(R2) 2G(U) .T 8 C(otIiCr C~Z’S) ?/ 2 G(0thcr C6’s) =
+
3.03
+ -0.2 + -0.3
N
5.3
Vnlike Dyne and Stone, we are unable to report a bal0 5 10 1: ance between hydrogen and hydrogrn-deficirnt, prodDose, units of 102%e.\-. l.?. ucts. A reasonable iiifcrcrice from our work is that Fig. 3.-Yields of major liquid produck from radiolysis of either some additional, thus far unidentified unsaturatrs cyclohexane by 2-hlev. Van de Graaff electrons. are produced or, in the equation above, x or g or both are greater than unity (e.g., that w-e have C6 products ccnApproxiniate conversion in units of 10-4. 0 4 a taining two doublr bonds). Radiolysis with Po2loa-Particles.-The steel window in the irradiation cell precluded dosimetry with Fricke solution. Calculations of approximate dose rate from geometry of the source, thickness of the windows, and iriteiisity of the source indicated that the yields observed were of the order which might be expected. Figure 5 shows yields in molecules of liquid products plotted as a function of time of exposure (corrected for Po21o decay) and of rstimated fraction of cyclohexane con/ lOxU/ verted. The sloprs of the lines indicate that the ratio G(C6H1O)/G(R2) = 3.0 cornpared with ratios 1.9 and 1.8 for 1.8-ILIcv. electrons and Co60 y’s, rrspectivcly, and nith the corresponding average ratio of quantum yields in €IF;-photosensitization of 1.47. The product T i is again prcsent as an initial product with G(U)/G(CoIIlo) 0.04 compared with the value 0.023 obtained nith 1.RMev. electrons and Coo0 yradiations. The minor Clz product yields are aboutJthe same as for radiolysis at low LET while the minor Cs products are about 2-3 times as large. It is emphasized that the data reported for the aradiolyses relate to low fractional conversion ( S 1.3 X 0 2 4 F 8 and are thus of more immediate significance than Dose, units of 1 0 2 2 e.v. 1.-‘. values for higher fractioiial conversion ( 2 3 X lo-*) Fig. 4.-Yields of major liquid products from Coco y-radiolysis reportrd clscwhrre. of cyclohexane.
Discussion Hg-Photosensitization.-Figure 2 suggests that U is thr product of reactions rssentiallj diffwent from those which yield C,,ZI,,l and (CJIIJ2. A rrnionable mwlianisni is
+
2R +Col-I,o (’6T€,2
(2)
2R + 122 0) followed, after C6Hlo has had an opportunity to accumulate, by reactions which irivolvc that, coinpound ; e.g.
H H
+ C6HIO +I12 +
+ CeHio
+R
c61I9
(4)
(5)
J. W. FALCONER ASD MILTON BURTON
1746
Approximate conversion in units of 104. 10
5
15
80
sC
Vol. 67
lead to the conclusion that the difference in the G(CBHIO)/G(RZ) ratios is a consequence of differences in the spatial distribution of precursors associated with a change in LET; Le., we must consider secondary reactions taking place inside the spur. Possible radical-radical reactions inside the spur are
60
H+H---tHz
1
+R H +R
.c.
II)
H
$
m
2 40
2
3
I
I
200 400 600 Exposure, min. (cor. for decay).
0
800
of major liquid products from radiolysis of cyclohexane by Poz10a-particles.
Fig. 5.-Yields
R
+ C6H9 -+- RCsHg
E
U (?)
+ H2 C6H11 + H
CBHIZ -+CsHio Ci",
--
(9
(ii) The value of Gi(C6H10), the yield in reaction i, is given by Gi(ceH10) = G(C~HIO) - kZG(RZ)/k3 Thus, for Coco y and electron radiation, if we employ the value k z / k s = 1.47 given by the mercuryphotosensitized reaction studies, G, (COHIO)is -0.6. I n the case of Po210 radiolysis the product ratio G(C~H~O)/G(RZ) is 3.0, i e . , some 50% greater than that observed in the lorn LET radiolyses. It is reasonable to assume that quality of high-energy radiation does not affect the relative probability of primary modes of decomposition, i.e., of reaction i and ii. Thus we are
+ C6HlO
C6HY2
(8) (9)
The failure of Schuler and Allen3 to detect a significant effect of LET on hydrogen production may indicate that reactions 7 and 9 are not of major importance. Although dosimetry was not possible in our a-radiolysis work we can assign approximate values to the yields. From the work of Schuler and Allen G(H2) is -5.5. Increased amounts of minor products suggest a product imbalance of 1.5; Le., G(CsHlo) 3.0 and G(R2) 1.0. Because G(U) is very low, we may write
-
+ +
G(ctiH10)N (2% Gi
-
Gg
But
Gz = kz/k3 G(Rz)
(6)
The ratio of initial slopes of the CsH1o and Rz yieldcurves gives the ratio of disproportionation to combination, kZ/k3 = 1.47. Radiolysis.-Because radiolysis may involve modes of product formation other than those given by reactions 1 to 6, we no longer can obtain kz/k8simply from the ratio G(C6Hlo)/G(R2). The observation that this product ratio is greater in radiolysis than in photolysis is consistent with a frequent method of describing the decomposition of cyclohexane, namely
-
--+HZ
(7)
so that Gs E G(CsH10) - kz/& G(R2) - Gi
Assuniing G, is 0.6 in a-radiolysis as in Co60 y and electron radiolysis, it follows that Gg 'u 1; i e . , approximately 20YGof the radicals are reacting in the spur a t high LET. Possible explanations for predominance of reaction 8 over reaction 9 may involve steric considerations as well as considerations of the effect of the energy of the hydrogen atom. It may be significant that the yield of the minor product C has approximately doubled in the a-radiolysis compared to the lorn LET radiolysis; i e . , that some reaction (e.g., 4) contributes more greatly as a spur reaction to the production of U in the high LET process than in the low LET processes. Regarding the rather large material balance discrepancy in the cases of electron and a-particle irradiation (where local dose rates are high) as compared with the y-irradiation (where Zoea7 dose rates are low), we merely suggest that this effect may be liiiked with the reported variations of G(H2) with dose and with the susceptibility of that variation to dose rate. Acknowledgment.-The authors are greatly indebted to Dr. D. B. Peterson and Dr. F. W.Mellows for criticisms and advice on the preparation of this paper.
