(12) THL-ABLRG, T . : IIundhitcA der hioloyincheTi :l,CpifsI,lelhotloi, Teil I, Heft i . Urban and Schwarzenbcrg, Berlin (1020). (131 ZSCHGILE,F. l'., AND COYAR, C. L.: Botan. Gaz. 102,463 (1941). (14) ZSCHPILE,F. P., A X I > HARRIS, D. G.: B o t a n Oaz. 104, 515 (1943).
EFFECTS OF HIGH-ESERGT RADIATIOS OK ORGANIC COVPOUSDP1 RI ILTOX BURTOW l l o n s a n t o Cheruical Company, Clinton Laboratories, Oak Ridge, Tennessee
Received November 8 , 1946 I. IKTRODUCTION
Organic compounds are hydrogenous materials. Thus, they are particularly effective as moderators for fast neutrons and at the same time become particularly susceptible to their influence. Quite apart from possible usefulness in the production of atomic energy, they definitely belong among those materials which might be introduced in or near a pile for the production of new and interesting chemical effects and chemical products. Furthermore, since piles and other portions of atomic energy plants are operated more or less remotely by human beings, discovery of the effects of all high-energy radiations on materials of byhich biological systems arc composed has become an increasingly important matter. The practical value of solution of the radiation-chemical problems of organic compounds is by no means the sole justification for their investigation. Just, as in reaction kinetics of thermal and photochemical systems, organic compounds nffer a very fruitful field for study because SO many different aspects of bond type and bond number and of molecular size, complexity, and relative stability (in the thermodynamic sense) can be independently and gradually varied in a manner advantageous for detailed study, for determination of general rules and correlations, and for discovery of underlying principles. The essential information required is knowledge of relative reactivity (under irradiation) of compounds and bonds of various types, of the relative effects of different kinds of radiation, and of the nature of the compounds produced. Much of this information is in the literature prior to t.he establishment of the Atomic Energy Projects, and some of it has been obtained outside of the projects, notably by the Massachusetts Institute of Technology group (7, 8, 14, 15),even during World War 11. 'Paper presented before the Symposium on Radiation Chemistry, which was held under the auspices of the Division of Physical and Inorganic Chemistry at the 110th hleeting of the American Chemical Society, Chicago, Illinois, September, 1946. 2Present address: Department of Chemistry, University of Xotre Dame, Notre Dame, Indiana. The work reviewed is in part taken from studies performed at the Metallurgical Laboratory, University of Chicago.
787
EFFECTS O F RADIATION O S ORGAKIC COJIPOUSDS
I t is not the purpose of this article to review all the information available on the radiation chemistry of organic compounds. Its subject matter is confined to decomposition processes exclusively. In this limited field, only general principles and hypotheses are offered and a few illustrative examples are selected from both public and project literature. 11. SPECIAL FE.4TURES
All the processes characteristic of photochemistry can occur in radiation chemistry for, apart from the other phenomena associated with high-energy radiation, there are ordinary excitations such as are produced by light of wave length > 1000 A . The processes include fluorescence, simple rupture at the locus of excitation, internal conversion of energy, transfer of energy to another molecule, and reaction with another molecule. Internal conversion is followed by collisional deactivation or by decomposition. In the latter case, the whole process (internal conversion plus decomposition) is called predissociation. Transfer of energy to another molecule may mean merely the conversion of the excitation energy to heat or it may cause reaction of the second molecule. In the latter case, the process is called photosensitization. In radiation chemistry. the excitation process may occur primarily as the result of action of the radiation on the molecule vithout ionization or it may occur secondarily after, or as a part of, ionization of the molecule. The distinctive primary process of radiation chemistry is the process of ionization. -4 numerically less frequent process is significant in certain special cases; it is an ion(or atom)-ejection process in which a particle is knocked out of lattice or molecular position either by gamma recoil effect (Szilard-Chalmers process) or by fast-neutron impact (Wigner effect). Such an ejection process never occurs alone; it always precedes or is accompanied by the far more extensively occurrent ionization processes. For overall knon-ledge of what happens to a system in a radiation field we therefore address our attention to the ionization process and its consequences. In the ionization process, given sufficient energy or appropriate conditions (as in ivater), the ion itself may dissociate. Usually, ho\vever, the sequence of events in organic chemistry may be represented by the three equations: , A+ f+e-
(la13
/
A
\ B++C+e-
\m+
-%+
+ e-
(1b)
--+&A*
A*-
(2)
x +Y hv
.
(3)
radiation chemistry the symbol --+ has the same significance as has + In photochemistry. It is intended t o show that the consequences of a process (i.e,, exposure t o high-energy radiationj, not its details, are being indicated.
788
MILTON BURTON
The reactions are successively ionization, ionic discharge, and decomposition (4). It has been known for a long time (cf. Hustrulid, Kusch, and Tate (10)) that the initial ionization step may be by a number of paths. Reaction l a represents the simplest such step. Reaction l b represents a group of reactions in which different ionic species and molecules, free radicals, or atoms are formed either in the primary act or stepmise in subsequent acts. For example, Hipple, Fox, and Condon (9) have shown definitely in studies on the ionization of hydrocarbons in the mass spectrograph that ions such as A+ or B+ may be metastable and dissociate with production of a variety of products; e.g.,
4+ or B+ -+ E+
+F
where F maybe a free radical but is usually a stable molecule. Innormal butane, for example, C4Hlof may be produced (by reaction la) in the primary act. Thereafter, it can (but does not necessarily) decompose; two reactions are reported for the decomposition of CaHlo+.
+ CHB -+ C3Ha+ + CHa
CaHio+ -+ CsH: CaHio’
Reaction 1 does not stipulate which electron of the molecule is removed nor how much excess energy is conferred on the molecule in the ionization process. In general, because of the nature of the ionization act, the chances are apparently equally good that any one of a large number of electrons in the molecule may be ionized. Thereafter, conceivably, the charge of the molecule may shift to a preferred position to the accompaniment of further excitation of various orbitals. In any event, the Franck-Condon principle applies. The atoms of the molecule do not have an opportunity to shift during the ionization process, but they do shift immediately afterwards to positions of greater stability for the ion. Such a shift is accompanied by processes in which great vibrational excitation of a part of the molecule is dissipated either in collisions with other molecules or to other degrees of freedom in the same molecule. Usually, the final ionic configuration is distended in comparison to that of the parent molecule I t is not apparent whether this distension is over the whole molecule or is localized in the region of ionization. There are certain cases where the molecule is quite symmetrical, as in benzene, in which it is not unreasonable to assume that all the atoms shift and that the missing orbital of the ionic state is, so to speak, “smeared” over the whole ion. This assumption is discussed in further considerations below (Section 111, D,(2) and (3)). Reactions 2 and 3 are prototypical. Actually, B+ or E+ may be written for A+ and B* or E* for A*; the excited particles can be either excited molecules or excited atoms. Thorough understanding consequently requires detailed consideration, in the particular case under study, of reactions additional or alternative to 2 and 3. Whatever the details of the situation are, the fact is that the ion which enters
EFFECTS O F RADIATIOK ON ORGAKIC COMPOUTDS
789
into reaction 2 is distended in comparison with the unexcited molecule A.' The Franck-Condon principle holds also during the process of ionic discharge. The molecule A* formed in reaction 2 has the same configuration as the ion A+ from which it was produced. Thus, it is an excited molecule whose excitation may be fairly well localized or may be spread over a number of bonds, depending on the degree of localization of distension of the parent ion. I t is sufficient here to say that something in the mechanism makes possible the process of ionic discharge (without immediate subsequent ionization) and that the excess energy consequently liberated in the molecule is far more than sufficient to break any one or two of the bonds in that molecule. Whether reaction 3 occurs a t all depends on whether the excess energy in .4* becomes available for a decomposition process before collisional deactivation occurs.~ The products X and Y may be either radicals or stable molecules. For occurrence of decomposition into radicals, greater localization of excess energy is required. Perhaps such localization is the more ordinary phenomenon in ionic discharge. In such circumstances a (rearrangement) mechanism involving formation of ultimate molecules in the primary act (Le., in reaction 3) is improbable. On the other hand, if the excitation energy of A* is distributed over a large number of bonds, the chance of a free-radical split in reaction 3 is reduced and, if collisional deactivation does not occur first, the predominant process in reaction 3 may be an ultimate-molecule split.6 111. DECOMPOSITIOX PRODUCTS
A . Relationship to groups in molecule In radiation chemistry, as in photochemistry, many decomposition processes involve simple rupture and formation of free atoms or radicals as a first step in the chain. Usually, in photochemistry, at a particular wave length only a special part of the molecule is activated. In radiation chemistry, on the other hand, no part of the molecule should be preferentially ionized' and usually no special part of the molecule is preferentially excited. Thus, in photochemistry 'In the ensuing discussion attention will be concentrated on A and its derivatives.
A more general discussion requires repeated reference t o B+ and the possibility that B represents a free radical rather than a stable molecule. I n order to avoid unnecessary complication, i t is left t o the reader mentally to insert the necessary additional analogous statements or t o add statements entailed by the reactivity of B in any medium under specific consideration. $The reader will note an analogy here t o ordinary thermal and photochemical processes. SA contrast t o photochemistry is here notable. I n a photochemically excited molecule, the excitation energy is very localized. Kevertbeless, under certain conditions (i,e,, when the excitation energy is not sufficiently high) decomposition cannot occur a t the locus of absorption but must occur in another way. I n such circumstances, the ultimatemolecule mechanismmay occur (3). I n radiation chemistry, on the other hand, the excitation energy is always sufficiently high for bond rupture. 'This presumable absence of initially preferential ionization does not exclude the possibility of a n ultimate preferential ionization or excitation of a certain part of the molecule in special cases.
790
MILTOlV BURTON
it may occur that in a particular class of compounds (e.g., the aldehydes) one single bond is preferentially broken (e.g., the C-C bond adjacent to the carbonyl group (6)). In radiation chemistry such a phenomenon should be exceptional, and we expect that the probability of rupture of a particular bond is more or less closely related to its fractional “concentration” in the molecule. Thus, in a hydrocarbon containing a number of methyl groups, the number of free methyl radical splits should be proportional to their number, just as the number of free hydrogen atoms initially formed should be roughly related to the number of C-H bonds in the molecule.
FIG. 1. Relationship between product yield and parent groups in a series of hydro carbons, according t o Schoepfle and Fellows. The curve is through the normal hydro carbons. Point B is for 2,Z-dimethylhexane; point .\ is for 2,2,5-trimethylpentane
Schoepfle and Fellows (8) offered an interesting example of such a relationship many years ago. Figure 1, taken from their data, shows that the relationship between the ratios (Hzyield)/(CH1 yield) and (number of C-H bonds)/(number of C-CHa bonds) is nearly linear for a series of normal aliphatic hydrocarbons. However, the points for two isomeric octanes lie distinctly off the curve, with the fraction of hydrogen in the yield much exceeding the expected value. Table 1 is taken from the same set of data as is figure 1. Comparison of 2.5-dimethylhexane with n-hexane shows that the yield of methane is nearly what would be expected on the basis of number of C-CHI bonds but it is then difficult, on the same basis, t o account for the large decrease in hydrogen yield in the former
791
EFFECTS O F RADIATION ON ORGAXIC CO?dPOUKDS
case. In order to account for the very high Hn/CI14 ratio in all the cases, Schoepfle and Fellows suggested that a significant portion of the hydrogen yield comes from the decomposition of large radicals. I t is sufficient for our purposes here to say that it must come from a process which does not necessarily involve a primary C-H split. As a matter of fact, it is important to note that the type of data summarized by figure 1 cannot be considered evidence for any freeTABLE 1 Effert of 170-kv. cathode rays (0.8 m a , SO min.) on some hydrocarbons (Schoepfle and Fellone: Ind. Eng. Chem. 23, 1396 (1931))
I i
HYDROCARBON ~~
TOTAL GAS
CC.
%Hexane . . . . . . . . . . . . . . . . . . . . . . . n-Heptane.. . . . . . . . . . . . . . . . . . . . . . . %-Octane.. . . . . . . . . . . . . . . . . . . . ! n-Decane. . . . . . . . . . . . . . . n-Tetradecane . . . . . . . . . . . .
57.6 51.4 48.3
2 5-Dimethylhexane 2 2,4-Trimethylpentane
a
~
12
l
~
~1 cc. 1 cc
I
11.6
,
31.9
I
38.3 39.5 38.0 32.5 31.8
1
210 176
8 503
OTHER CAS (CALCCLATED)
H?
’
! I
5 8 7 6
CC.
16.2 9.8 8.0 7.11 2.5
3 1 2 1 1.4 0 9 0.6
1
1
1
230 251
-
8 -
.4
-
I
I
I
radical or free-atom mechanism at all. Decomposition via rearrangement could and should give precisely similar data. More recently, Honig ( 8 ) noted a similar “group to product” relationship between the ratio (Hz yield)/(CO CO, yield) and number of hydrogen atoms in three fatty acids exposed in the solid state to alpha-particle bombardment. Figure 2, given by Honig, s h o w a nearly linear relationship. The data are insufficient to indicate whether the departure from linearity is real.
+
792
MILTON BURTON
B. Gas production in relation to unsaturation One of the unfortunate features of many of the data of radiation chemistry is that so little basic quantitative information is given. Very frequently, only gas analyses appear, without any effort to relate amount of product to energy input. However, even when such data appear, they are an inadequate criterion of the degree of decomposition. This situation is particularly true when mechanisms are present for the absorption of free radicals and a t o m s - a s in the decomposition of unsaturated compounds. The work of Schoepfle and Fellows, already cited, also showed that, as far as gas production is concerned, unsaturated hydrocarbons appear more stable than saturated, and aromatic compounds appear far more stable than aliphatic. Aliphatic side chains on aromatic rings increased the gas yield. In table 2, which comes from project data (2), these facts are very clearly shown, but it is a t the same time apparent that many more molecules react than are indicated by gas production alone. In cyclohexene, toluene, and benzene the number of TABLE 2
Effecf of fast-electron irradiation on liquid hydrocarbons (J. V. Flanagan, C. J. Hochanadel, and R. -4.Penneman (2)) EYDPOCALBOS
Benzene, ................................... n-Heptane .................................. Cyclohexane. ............................... Cyclohexene. .............................. Methylcyclohexane. ........................ Toluene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0.04 4.2 4.0 1.0 4.5 0.09
0.5 1.i 1.2 4.2 4.2
0.7
molecules which enter into reaction exceeds the number of molecules of gas produced by a factor ranging from 4.2 to 12.5.
C. E$ect of liquid state Franck and Rabinonitch have pointed out that, in certain photochemical cases, where the same decomposition reaction can be studied both in the gas and in the liquid, the quantum yield of the reaction is generally lower in the liquid ( 5 ) . Two factors contribute to this decrease in yield: a collisional deactivation effect and a cage effect. The former occurs because the period betveen sec. (Le., about one vibration collisions in the molecules in a liquid is about period) and thus, unless reaction occurs in the interval between collisions (as by rupture), there is a good chance that the excitation energy may be picked off before it becomes effective. The more the excitation energy exceeds the energy required for the reaction, the less is the probability of this process. In the cage effect, radicals produced by rupture collide with the surrounding molecules while they are still within each other’s spheres of influence; there is consequently
EFFECTS OF R.4DI.4TlOK O S ORGASIC COYPOUXDS
793
a probability of primary recombination. The cage effect is enhanced where product radicals and substrate molecules are both large; it would be smaller when a hydrogen atom is a primary product (12). I t might be expected that the cage effect would also be reduced when the primary product is travelling with high energy. The cage effect should be without influence on the chance of split into ultimate molecules (11). On the whole, excessive excitation energy will increase yield mainly because of decreased importance of leaking-off of energy rather than because of decreased cage effect. Thus, decomposition yields in radiation-induced reactions in the liquid state should be generally high (as compared with photochemical reactions) and the ultimate-molecule mechanism should be favored. As for the first part of that conclusion, no evidence is readily available now, although we may expect that it nil1 be subjected to test. The conclusion that the ultimate-molecule mechanism should be favored by the liquid state has received support in photochemistry. Some data on the radiation chemistry of fatty acids by Sheppard and Whitehead (151, cited in section III,D,(3), also agree with that conclusion.
D. Reactivity factors Any factor which increases the lifetime of the excited molecule formed in the act of ionic discharge serves to decrease both decomposition yield and ratio of free radicals to ultimate molecules formed in the primary act. There are several such factors. (1). Among isomers, greater complexity means longer life of the excited states. Complexity increases the probability of internal conversion of energy from an initial excited state (where energy may be localized in a single part of the molecule) to one of a number of equivalent states in which the energy is fairly well distributed throughout the molecule. While in such a relatively stable state the molecule may lose a critical amount of energy by collision before bond rupture (but not necessarily decomposition by rearrangement) has a chance to occur. The only experimental data available on this point are those of table 1, relating to the isomeric octanes. The marked decrease in hydrogen yield in going from n-octane to its isomers cannot be accounted for on the basis of competition with C-CHP splits. An alternative explanation can be based on the notion that the number of C-H splits is reduced from another cause and that the increased yield of methane is not necessarily via free CHI radicals. According to the point of vieiv of the previous paragraph, the explanation could be that, because of the successively increased complexity of the isomeric octanes, the chance of internal conversion to a state in which the energy is non-localized is increased. Thus, that portion of the decomposition resulting from bond splits is markedly decreased, while rearrangement processes (in n hich, perhaps, some hydrogen is primarily formed) are not greatly affected. The decrease in hydrogen yield represents then a substantial decrease in C-H splits. Incidentally, small “smearing-out” of excitation energy as in internal conyemion should theoretically be reflected first in a decrease In splits of bondi of the greater strcngth; i e., C-H bonds as compared nith C-C bonds.
79-1:
MILTON BURTON
In future work it will be desirable to obtain information on the actual quantities of isomeric compounds decomposed as well as on the fractions of products formed uia free radicals (or atoms) and rearrangements t o ultimate molecules. (2). The more resonance in a molecule (as in benzene), the more likely it is that the state immediately ensuant on ionic discharge will be one in which the excitation energy is distributed throughout the molecule. Thus, the probability is high that the excited molecule may endure without bond rupture until it becomes stabilized by loss of energy in collision processes (or decomposes by rearrangement into ultimate molecules). (3). The closer the correspondence between ionic and normal-molecular configurations, the less is the molecule ultimately affected by radiation. Benzene may be cited as a possible illustrative extreme case. If, as suggested in Section 11, the missing orbital is %meared" over the whole ion, the ionic and normal-molecular configurations may be so closely alike that the excitation in the molecule caused by discharge of the ion cannot be said to be localized preferably in any single bond. Such a molecule would be rather unreactive, for it would be unlikely that energy so thoroughly distributed among various degrees of freedom would concentrate sufficiently at one bond between collisions (Le., in about one vibration period in the liquid state) to cause rupture at that bond. Thus, the relative probability either of deactivation or of decomposition by rearrangement would be increased. For complete understanding of the effects of any radiation on a substance, it would be desirable in the first instance to know what ions are produced by that radiation. Data are in general lacking. There is some information which can be obtained from low-voltage mass-spectroscopicdata. It is improper of course t o apply such data without reservation or modification in considerations of the effect of high-voltage electrons on liquids, but hints arc thereby obtained and in such a way the data are qualitatively instructive. For example, benzene has been bombarded with electrons at 72 v. in the mass spectrograph and ion abundance ratios have been obtained. Hustrulid, Kusch, and Tate (10) report that, on the scale CeH6+= 100, only seven other species have abundances exceeding 4.0. They are
CeH6'
15.2
C4H3'
15.7
CsHaf
4.6
CaHl'
13.3
CeH2'
4.0
C3H3'
0.6
CaH,'
13.5
An easy hypothesis 011 the basis of which to account for the low decomposition yield in benzene is that neither the species CeHe', on discharge, nor the simply excited benzene molecules (which might be expected to make the ordinary "photochemical" contribution) lczd to decomposition. On the basis of the ideas suggested in this and the previous section, this hypothesis does not appear too strange.
EFFECTS O F RADIATIOK ON 0RG.LYIC COhlPOUSDS
795
An alternative suggestion which has been offered for the stability of aromatic hydrocarbons is rather special. It finds the explanation in the behavior of the primary products after decomposition of excited CBH~*.The suggestion is that the hydrogen atom initially produced reacts with an adjacent C6H6molecule to give C&, and that that radical back-reacts 11-ith the parent C& to form two C& molecules. h difficulty with this interpretation is that work of Bonhoeffer and Harteck (1) indicates that atomic hydrogen reacts with benzene vapor (not liquid) to split the ring with formation of methane, ethylene, and acetylene-all of which, incidentally, were found in the studies summarized in table 2. However, the early work of Schoepfle and Fellows on mixtures of benzene and cyclohexane indicates a decrease in gas yield best explained in terms of free-atom acceptance by the benzene. More work is clearly required.'" Another factor Tyhich can decrease reactivity (under irradiation) in the way discussed in this section is molecular size. If there is a tendency for spreading of the excitation energy. then the .larger the molecule the more nearly will the ionic configuration and molecular configuration correspond. In such a case insufficient energy will be localized a t a bond for rupture in that locale and the time required for such localization to occur may, in general, be so large that the decomposition takes place preferentially via an ultimate molecule (i.e., rearrangement) mechanism. The data of Sheppard and Whitehead (15) on the decomposition of fatty acids by radon alphas and high-roltage deuterons have already been cited (2) as illustrative of this principle. The facts are that hydrogen, carbon dioxide, carbon monoxide, and methane are all produced in approximately equal amount in the decomposition of acetic acid but that, when palmitic acid is reached, only the first two are among the major gaseous products and the liquid product is principally pentadecane. Off-hand, without a principle such as that here offered, one would expect a wider diversity of products with increase of molecular size, particularly if a freeradical mechanism is assumed for the decomposition. The difficulty is well appreciated by Honig (7), who assumes the free-radical mechanism.8 He points out that, on his assumption, for every 100 C-H bonds broken, about 45 C-COOH bonds are ruptured, while probably no more than 5 C-C bonds are broken elseiyhere in the chain. Honig suggests an explanation of the ratios 73 S o l e added in proof: Dr. A. 0. Allen (private communication, February 24, 1947) calls attention to old work of Linder and Davis (1931) and of Mund (1935), in which substantial decomposition of benzene vapor is reported. Although this result is consonant with the notion t h a t collisional deactivation is a n important factor in the non-reactivity of liquid benzene, i t does not eliminate the other explanation based on the special cage effect, i.e., t h a t an adjacent CsH, hack-reacts with a parent CsHd within a period of about one vibration. 8The data of figure 2 are no proof of an exclusively free-radical mechanism Increased yield of hydrogen with more hydrogen in the molecule can equally well occur via an ultimate-molecule mechanism. However, the important point is the manner of production of pentadecane itself. Even if hydrogen were produced via free atoms, the pentadecane could still be formed exclusively via rearrangement.
796
MILTOX BURTOX
based on relative bond strengths; he concludes that the C-COOH bond is the weakest C-C bond. Evidently, any straightforward free-radical-mechanism interpretation of the data in the way described leads to a complication resident in the fact that the C-H bonds are certainly stronger than any of the C-C bonds. There is also, however, the difficulty that the excitation energy is far more than sufficient to break any of the bonds in the molecule. If it is spread out over the molecule, then the weakest bonds (i.e., the C-C bonds) should be preferentially broken and the C-H bonds should be barely affected. The free-radical mechanism attempts to account for the pentadecane by the successive reactions: RCOOH COOH
--+
+ R -+
R
+ COOH
CO1f RH
If this is the mechanism, some explanation is required. In Section I1 it is mentioned that the chances are approximately equally good for ionization in the primary act of any one of a large number of electrons in the molecule. If the locus of excitation corresponds precisely to the point of initial ionization, it seems most probable that there should be a wide diversity of products. However, we have no information on the behavior of the ion. It may very well be that, following the ionization step, the charge of the ion (Le., the missing orbital) will shift t o a preferred position; e.@;.,in palmitic acid the COOH end of the molecule. Thereafter, the break (if rupture is assumed to be the most reasonable process) can occur preferentially in that region. There is one difficulty with this explanation other than the question of why hydrogen is among the products a t all. It is in the question: why should the ultimate ionization tend to be more localized in a large molecule like palmitic acid than in a small one like acetic acid? For the time being, an explanation of the results on fatty acids based strictly on the matter of molecular size does not appear too unreasonable; but much more quantitative information on a diversity of compounds is necessary before this principle can be considered established. IV. SUMMARY
All the processes which occur in photochemical reactions of organic compounds occur also in radiation-chemical processes. In addition, there are reactions resultant from the pebuliar sequence characteristic of radiation chemistry: Le., ionization, discharge, and decomposition. In general, any electron in the molecule is equally susceptible to ionization in the initial act; this fact must be constantly recalled in any interpretation of radiation-chemical mechanisms. Since, in general, the excitation energy lies in any part of the molecule, the yield of a particular product is closely related to the number of parent groups in the molecule. Gas production, particularly in unsaturated compounds, is an inadequate criterion of the resistctnce of a compound to high-energy radiation.
PHASE BOLXDARIES I S SODKErI OLEATE-WATER SYSTEMS
797
In the liquid state, the excessive excitation energy tends to minimize the FranckRabinowitch effect (Le., decrease in yield due to collisional deactivation and cage effect). Factors which increase resistance of organic compounds to radiation (and ratio of ultimate molecules to free-radical processes) are molecular complexity, resonance in the molecule, and all properties of the molecule which tend to increase the correspondence between ionic and molecular configurations. ilmong the latter are molecular symmetry (cj. benzene) and molecular size (cj. palmitic acid). Apparently, increase of molecular size tends to channel the decomposition along a particular path rather than to diversify the products. REFEREX'CES (1) BOKHOEFFER, K. F., AND HARTECK, P.: 2. physik. Chem. 139 (Haber Band), 64 (1928). (2) BURTON, M.: J. Phys. Colloid Chem. 61, 611 (1947). (3) BURTON, M., ASD ROLLEFSON, G. K.: J. Chem. Phys. 6, 416 (1938). (4) EYRING,H., HIRSCHFELDER, J. C., ASD TAYLOR, H . S.: J. Chem. Phys. 4 , 479, 570 (1936). (5) FRANCK, J . , A K D RABISOWITCH, E.: Trans. Faraday Soc. 30, 120 (1934). (6) GARRISON, W.>I., A N D BCRTOS,h l . : J . Chem. Phys. 10, 730 (1942). (7) HOKIG,R . E.: Science 104, 27 (1946). (8) HONIG,R. E., A N D SHEPPARD C. W.: J. Phys. Chem. 60, 119 (1946). (9) HIPPLE,J. A , , Fox, R . E., . ~ N DCONDOX, E. E.: Phys. Rev. 69, 347 (1946). (10) HUSTRULID, A , , K c s c a , P., ASD TATE,J. T.: Phys. Rev. 64, 103i (1938). (11) KORRIRH, R . G. W.:Trans. Faraday Soc. 33, 1521 (1937). (12) ROLLEFSOK, G. K.. A V D BPRTOS,M.: Photochemistry and the Mechanism of Chemical Reactions, p. 367. Prentice-Hall, Inc., New York (1939). (13) SCHOEPFLE, C. S., . ~ N D FELLOWS, C. H . : Ind. Eng. Chem. 23, 1396 (1931). (14) SHEPPARD, C. W . , . ~ N DHONIG,R . E . : J. Phys. Chem. 60, 144 (1946). (15) SHEPPARD, C. W.,ASD WHITEHEAD, W. L.: Bull. Am. Assoc. Petroleum Geol. 30, 32 (1946).
PHBSE BOUNDARIES I N COSCESTRATED SYSTEMS OF SODIUM OLEATE A S D WATER', * ROBERT D . VOLD Department of Chemistry, l'niversity of Southern California, Los Angeles 7. California Received December S, 1946
Despite extensive investigation (lj), knowledge of the behavior of concentrated systems of soap and water is still incomplete. There are three important aspects to the problem. First is the search for phase changes with the object of establishing their occurrence as unambiguously as possible. Second is the ques'Presented before the Division of Colloid Chemistry a t the 110th Meeting of the Amerioan Chemical Society, Chicago, Illinois, September, 1946. *Most of the experimental work on which this,paper is based was carried out at Stanford University in 1940-41.
