J . Phys. Chem. 1984,88, 69-72
69
Comparison of y-Radiolysis of Hydrocarbons in Glassy and Polycrystalline States. 2. Solid Alkane Mixtures B. Tilquin,* M. C. Pirsoul, and P. Claes Catholic University of Louvain, Department of Chemistry, CHAN B- 1348 Louuain- la- Neuue, Belgium (Received: March 7, 1983)
The dimer products formed by y-irradiation of frozen solutions of pentane-hexane (PH) or pentane-2,3-dimethylbutane (PD) systems have been measured by means of capillary gas chromatography (GCz). The G values of dimers are evaluated for certain compositionsof the solid mixtures. Energy transfer between the mixture constituents does not occur in solid alkane systems. Different dimer yields recorded can be readily accounted for by an effect of the physical properties of the solid.
Introduction The purpose of this work is to further clarify the subtle matrix effect on the chemical processes of solid-state radiolysis. It has been found that different nonradical and radical processes can occur between the glassy and polycrystalline states of the same compound.’ Moreover, different radicals are formed by y-irradiation of different solid-state phases of some compounds.2 When frozen solutions of alkane mixtures are exposed to ionizing radiation at 77 K, several experimental findings support the view that no transfer of chemical activation O C C U ~ S . ~ This is evidenced by the yields of trapped radicals and of dimer compounds as studied by means of ESR and capillary gas chromatography (GC2) measurements, respectively. In this paper, gas chromatography measurements have been carried out for several compositions of the n-pentane-n-hexane (PH) and n-pentane-2,3-dimethylbutane(PD) systems. When PD mixtures are rapidly cooled to 77 K in liquid nitrogen, transparent (glassy?) solids are formed. For PH systems, natural (poly) crystallization phenomena occur. Experimental Section The methods used for sample preparation, y-irradiation, ESR, and GC2 measurements, have been d e ~ c r i b e d . ~ 2,3-Dimethylbutane (2,3-DMB) was purified by fractional distillation, passage through a 20-cm column packed with freshly activated silica gel, storage over CaO and CaH, and trap-to-trap vacuum distillations. Residual impurities are less than 0.1% as tested by GC2. Mixtures of different compositions are made on a vacuum line and the samples were prepared by quench freezing the roomtemperature liquid to 77 K in liquid nitrogen. The pentane-hexane (PH) mixture forms a polycrystalline system; the pentane-2,3dimethylbutane mixture was obtained as either a glass or a polycrystal depending on the mixture composition. This composition was measured by G C and expressed as the electron fraction of pentane (ep). Some stratified systems are made by successive quench freezing of each component. After y-irradiation (dose = 4 Mrd), the samples were warmed up and the heavy products analyzed by GC2 with a Varian 3700 chromatograph. A column of soft glass was coated with SE 30 (50 m, 0.25-mm i.d.). The published retention i n d i ~ e s were ~ ~ J used for the identification of the compounds. The response of the flame ionization detector was found to be proportional to the number of carbon atoms. In order to compare the dimer distribution in a pure phase with that in a mixed phase, an internal product distribution is used; the reference is the yield of isomeric dimers. Absolute G values are determined by using n-nonane and n-dodecane as internal standards. The thermal decay of trapped radicals was recorded on a JEOL-ME- 1X spectrometer. Measurements at temperatures > (1) B. Tilquin et al., Radial. Eff.,32, 37 (1977). (2) (a) M. Kat0 et al., Bull. Chem. SOC.Jpn., 46, 1036 (1973); (b) B. Tilquin, C. Bombaert, and P. Claes, Radial. Res., 85, 262 (1981). (3) M. C. Pirsoul, R. van Elmbt, and B. Tilquin, Radial. Phys. Chem., in
press.
0022-3654/84/2088-0069$01.50/0
77 K were performed on the sample in the Dewar sleeve of a variable-temperature device which passed through the microwave cavity; the sample temperature was maintained at a constant value within f 2 OC. Relative yields of radicals for different compositions of the mixtures were determined by comparing the double integral of the ESR signal; an internal reference (Mn) in the cavity served to monitor the sensitivity of ESR response.
Results E S R Measurements. Pentane-Hexane Systems. Thermal Decay of Radicals. No measurable decay of the trapped radicals takes place after a few hours at 77 K. As the temperature of the irradiated sample is raised, a “stepwise” decay begins. As previously observed for trapped radicals in y-irradiated polycrystalline organic compounds, at each new temperature there is an initial rapid decay followed by a very low rate of decrease. This stepwise decay is the result of varied softening points of crystallites and surface irregularities of different sizes4 Concurrent with the loss in radical concentration resulting from the raising of the temperature, there is no change in the ESR line structure. The temperatures of radical decay are not related to the mixture composition. Radical Yields. Within the experimental uncertainty, the yields of trapped radicals are equivalent for all the mixtures. Pentane-2,3-Dimethylbutane System. Thermal Decay of Radicals. The kinetics of the decay of radicals in PD rigid matrices are of two types. One type is shown by radicals produced from mixture with a high pentane content (ep = 0.8); the radical decay is stepwise as for cracked polycrystals. For tp values ranging from 0.27 to 0.64, the second type of radical decay occurs; this decay proceeds rapidly on heating. Such an effect is observed for radicals trapped in glassy matrices. Radical Yields. Other composition effects on ESR measurements include a decrease (1:3) in G for trapped radicals from mixtures with tp equal to 0.8 to glassy mixtures. During the irradiation of the mixtures, some radical may recombine. This decrease suggests the possibility that the trapped-radical yields are lower for glassy states than for polycrystalline ones. It has been ~ h o w nthat ~ , ~the glassy state favors the lower yield. Chromatographic Measurements. Pentane-Hexane Polycrystalline Mixtures. Figure 1 shows a typical capillary gas chromatogram (GC2) in the range of the heavy (dimeric) compounds. The retention indexes ( I , Table I) were calculated for isothermal GC2which leads to long analysis time for these complex mixtures of dimer isomers. Formally, the products of Table I can be built up by the combination of alkyl radicals; e.g., combination of primary pentyl radical (PI) and secondary hexyl radical center at the 3-position (H3) gives the PlH3 dimer. In Figure 1, solid peaks correspond to compounds which are only formed in the irradiated mixture (peaks 7-14, Figure 1, Table I); these PxHy compounds are called mixed compounds. (4) D. Wilkey, H. Fenrick, and J. Willard, J . Phys. Chem., 81,220 (1977). ( 5 ) B. Tilquin, R. van Elmbt, C. Bombaert, and P. Claes, Radial. Res., 86, 419 (1981).
0 1984 American Chemical Society
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The Journal of Physical Chemistry, Vol. 88, No. 1, 1984
Tilquin et al.
x
2 4
-.
Figure 1. Gas-chromatographicprofile of the dimeric compounds from the radiolysis of pentane-hexane mixture (ep = 0.48) obtained by using a 50 m X 0.25 mm glass capillary column coated with a 0.2-fimfilm of SE 30 employing a N2 flow rate of 0.9 cm'min-', a split ratio of 25:1, and appropriate temperature programming. Numbered chromatographic peaks are identified in Table I.
TABLE I: Retention Indexes and Identification of Chromatographic Peaks of a r-Irradiated Pentane-Hexane Mixture peak retention indexa identification 1 2 3 4 5 6
943.1 946.9 949.5 962.6 968.9 1000 1023.7 1027.9 1039.6 1041.2 1043.5 1052.9 1058.2 1061.3 1066.8 1100
7 8 9 10 10' 11 12 13 14 15 16 16' 17 17'
Figure 2. Dimer yields vs. electron fraction; plots of dimer yields in pentane-hexane polycrystalline mixtures; distribution in percent: ( X )
decanes: (0) dodecanes; ( e )mixed compounds.
+ I
*
.+
+
b L'*+
1
1103.2 0
1120.7
?
05
EP
'
Y
01
0
05
EP
'
Figure 3. Internal distribution of the decane isomers (100% is for the
decane yield) from the irradiated pentane-hexane solid mixture; see Table I for the identificationof decanes: (+) P,P,; (A)P2P,; ( 0 )P,P,; PIPI; (x) PIP?,. 20 21 a
1156.5
HIH, HIH,
1200
SE 30. 85 'C.
For each pentane-hexane mixture studied, Gdlmer,the yield of dimer isomer per 100 eV absorbed during y-irradiation, is very low. The composition effects on these G values are shown in Figure 2; within the experimental uncertainty, the total G value (decane undecane dodecane) is independent of the composition of the mixture; i.e., G = 0.67 (a = 0.05). The approximately equal yields for mixed compounds at all compositions are noteworthy. By contrast, there is a dramatic change in the internal product distribution as a function of the electron fraction. The distribution of the decanes changes with the composition of the mixtures (Figure 3); 100% in the figure corresponds to the yield of the decanes. Distribution of the dodecanes shows similar, though less dramatic, changes. Pentane-2,3-DimethylbutaneMixtures. Previous works from this laboratory have described the exact nature of the radiolytic products from solid 2,3-dimethylbutane. Unsaturated heavy products are observed as main products. Formally, these compounds could be built up by a combination of alkyl (Rp, primary; R,, tertiary) and allyl (B, C) radicals with
+
a" e-
+
B
C
Figure 4. Gas-chromatographicprofile of the heavy products from the irradiated pentane-2,3-dimethylbutane solid mixtures. Conditions were the same as those given in Figure 1. PxPycorresponds to the decanes (see Table I); A-I indices are for mixed compounds; for the identification of the heavy products from the 2,3-dimethylbutane,see the text.
Figure 4 is a typical chromatogram from an irradiated PD mixture, for which tp = 0.51. The mixed compounds (solid peaks in Figure 4, A-I) are unidentified. However, peaks A, E, and H correspond to unsatured compounds; hydrogenation of the radiolysis products allows the identification of unsaturated compounds. The yields for several compositions of the mixture are shown in Figure 5. This figure also shows the distribution of the products. The total G values are dependent on the mixture composition. For tp values ranging from 0.1 to 0.7, the average yield is 0.39 (CT = 0.07). In contrast to this low value for glassy
The Journal of Physical Chemistry, Vol. 88, No. 1 , 1984 71
y-Radiolysis of Hydrocarbons 4
i
I 0
0.5
Ep
A
t
0
1 0
1
,
0
’ 05
0
0 Ep
0
1
os
Ep
1
Figure 7. Internal distribution of the CI2heavy compounds from the irradiated pentane-2,3-dimethylbutane solid mixture; see text for the
0.5
identification: (A)BRpBR,;( 0 )CR,; (X) CR,; (A)R,R,; (a)R,R,; (0) RtRt.
50
1
Rf
0 0
05
Ep
1
Figure 5. Dimer yields as a function of electron fraction in pentane cp; plots of dimer yields in irradiated pentane-2,3-dimethylbutane solid
mixtures: (X) decanes; (0)heavy products from 2,3-DMB; ( 0 )mixed compounds. 05
0 0
IO,
0
.;’
Ep
1
Figure 8. Relative yields of fragments pentyl (X) and 2,3-dimethylbutane (0)from PD mixtures. Values are calculated from the total dimer yields. ;
0
05
05
1 Ep
1
0 0
05
EP
’
Figure 6. Internal distribution of the decane isomers (100% is for the decane yield) from the irradiated pentane-2,3-dimethylbutane solid mixture; see Table I for the identification of decanes: (+) P2P2;(A)P2P,; ( 0 )P3P3; (0) P,P2; PIP,; ( X ) P,P,.
mixtures is the value of 0.69 ( 0 = 0.006) for the polycrystalline system (eP = 0.8). The internal distributions of the decanes and of the heavy products (dodecanes and dodecenes) from the 2,3-DMB are shown in Figures 6 and 7, respectively.
Discussion Glassy Systems. PD Mixtures with tp Valuesfrom 0.1 to 0.72. A previous paper3 on the radiolysis of alkane glassy mixtures (pentane-2-methylpentane) suggests the absence of a chemical activation transfer between the precursors of the dimer isomers and of the trapped radicals. For these glassy mixtures, the relative yields of mixed compounds are important, >40% at ep = 0.5. In the present work, values up to 45% are measured in the PD glassy mixtures. Mixtures of pentane and 2,3-dimethylbutane thus form a homogeneous system if the processes leading to the dimer formation are considered. Trapped radicals diffuse randomly in the matrix and combine with each other statistically. The identity of the trapped radicals is not known in this case because of the broad overlapping ESR lines observed in solid protiated alkanes.
Not only delayed radical processes but also prompt nonradical processes contribute to dimer formation in the solid state.6 There is evidence that, at 90-100 K, the radical decay in glassy solids, by combination of alkyl parent radicals, yields a maximum of 50% of the dimer isomers. Therefore, the prompt combination of reactive unknown “fragments” during radiolysis at 77 K must be taken into account. The radiolytical yield (Gfi) of all the fragments i can be estimated from the radiolysis: Gfi
= 2Gdimer ii -k
lGmixed dimer ix
The previous work3 implies that the relative yield of pentyl fragments (RP) varies with the composition of the mixture (ep, electron fraction in pentane) according to the following rule: I
with Gfi = O G f i t i where O G f i is the fragment yield in a pure phase with identical physical properties as the mixture. Figure 8 shows the variation of the relative yield of pentyl fragments with ep, In the same figure, results for all the fragments from 2,3-DMB are also reported in relative yields as a function of the mixture composition expressed by the electron fraction of pentane. On the right side of Figure 8 (ep 3 0.7) relative values from polycrystalline samples are also indicated. In the vitreous zone, the fact that the experimentally estimated fragment yields agree with relationship 2 suggests that the previously described mechanism3 of solid-state radiolysis has validity. (6) B. Tilquin, P. Tilman, and P. Claes, Radiat. Phys. Chem., 16, 321 (1980).
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The Journal of Physical Chemistry, Vol. 88, No. 1, 1984
Figure 9. Relative fragment yields vs. electron fraction of pentane in the
pentane-hexane solid mixture. With regard to glassy alkane states, earlier work implies (1) that all reactions which may conceivably be responsible for the production of the fragments during y-irradiation are monomolecular and, ( 2 ) that the deposition of radiation energy in vitreous solids initially released in the molecule follows the mixture law, Le. is proportional to the electron fraction and does not cause radiation damage in a selected alkane molecule. It is then necessary to discard mechanisms of energy transfer such as exciton, migration of positive holes, selective abstraction of hydrogen atom from a solvent molecule, etc. Polycrystalline Systems. Figure 9 shows the relative yields of fragments from the radiolysis of pentanehexane polycrystalline mixtures. Experimental data are in agreement with relationship 2 demonstrated in vitreous mixtures. However, in the PH mixtures the very low yield of mixed compounds is independent of the composition of the mixture (Figure 2). This indicates inhomogeneity of this mixture'as far as intracomponent radical-radical or fragment-fragment reactions are concerned. It is plausible that the smaller fraction of mixed compounds (20%) in polycrystalline systems is due to the crystallization out of the components, which tends to eliminate the mixture formation in this solid system at low temperatures. However, there is some reaction at the interzones and it is surprising that this is rather independent of the electron fraction. This independence may indicate a large zone for each component and considerable energy migration to the area connections. To
Tilquin et al. study this hypothesis, we have measured the relative yields of mixed compounds from y-irradiated stratified PH systems. The strata are ca. 103-105 A thick and the yield of mixed compounds can be up to 12%. Comparison with the results from the PH mixture is not possible because of the difficulty in obtaining the estimated thickness of the strata. However, the 12% relative yield for mixed compounds suggests that considerable energy migration occurs between connection areas. Internal Dimer Distribution. From individual decane yields, it is possible to estimate the yields of primary fragments (Pl) or of secondary fragments (P2 and P3). These yields are independent of the mixture composition in all the mixtures studied. However, the internal dimer distribution varies with the electron fraction (Figures 3 and 6). Moreover, for glassy and polycrystalline samples, the composition dependence is similar. This dimer distribution is a function of the composition of the mixture but is invariant with the matrix effect. The changes in the internal dimer distribution for the dodecenes (Figure 7, left side) and the dodecanes (Figure 7, right side) on increasing the electron fraction of pentane are different for the glassy and polycrystalline states. All the previous investigations have shown major differences between distribution of dimer yields from the glassy and the polycrystalline states. 1,2b To explain this problem, additional experiments are needed. This is important to improve understanding of radiation chemistry and solid-state chemistry.
Conclusion In organic polycrystalline matrices, the characteristics of dimer formation indicate the preferential localization of reactive species near defect sites in the matrix. This model was first developed by Voevcdsky in 1967 in an effort to explain the kinetics of growth of radicals during the y-irradiation of ethylene glycol at 77 Ke7 However, a simple model with homogeneous distribution of spurs as in the liquid phase was used. This model can explain neither the selectivity of radical formation from solute* nor the effect of the physical properties of the solid on radiation-induced mechanisms.' Acknowledgment. We gratefully acknowledge financial assistance from the F.D.S. (U.C.L.). Registry No. 2,3-Dimethylbutane,79-29-8;pentane, 109-66-0;hexane, 110-54-3. (7) V. K. Ermolaev and V. V. Voevodsky, "Proceedings of the 2nd Tihany Symposium on Radiation Chemistry, Akademiai Kiado, Budapest, 1967". (8) B. Tilquin et al., T. Miyazaki et al., Radiat. EfJ,66, 9 (1982).
