G. ADLERAND J. H. PETROPOULOS
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have ascribed their Recently, Moscowitz, et results on the temperature dependence of the circular dichroism of camphor to solvation of camphor in the solutions studied. They correlate this with the finding by Pariauda4that at very low temperatures solid phases containing both camphor and solvent separate from certain solutions, with phase characteristics of compounds. With the high temperature coefficients usual to the implied intermolecular complex formation, however, one might question whether such stoichiometric complexes would survive to any significant concentration at room temperature. Any related sort of solutesolvent interaction would obviously be consistent with the regularities in the solvent effects we have pointed
out. It would also seem reasonable to take the view that the solvent effects in this system, and very possibly in other optically active systems, are a more or less quantitative measure of the cumulative effect of various intermolecular interactions in the system. The exact nature or number of such interactions cannot be completely predicted at this time, nor can the point at which they become specific "complexes." Hydrogen-bond formation in the solutions in alcohols and in chloroform is one obvious factor, interaction with the *-electron system of the benzene molecule is another, and other influences also exist. (34) J. C. Pariaud, Bull. soc. chim. France, 103 (1950); Ann. C h i n . (Paris), 6 , 880 (1951).
Electron Spin Resonance Spectroscopy of Irradiated Acrylamide'
by G. Adler and J. H. Petropoulos Nuclear Engineering Department, Brookhaven National Laboratory, Upton, New York (Recsived March 16, 1.966)
It is shown by means of e.s.r. spectroscopy that acrylamide scavenges hydrogen atoms formed during radiolysis. The free radical thus obtained is identical with the one seen in the radiolysis of propionamide. At low temperature the radical is well oriented. Between - 125 and -30" the spectra change in a manner which indicates that the radical reacts with its next nearest neighbor in the direction of the screw axis. Between -30 and -20" the e.s.r. spectrum loses all orientation dependence and becomes that of the propagating radical. This suggests that although the first step of the reaction may be constrained to a given crystallographic direction, polymerization takes place only at those temperatures where the propagating radical can react with near neighbors at random and the polymer becomes amorphous.
Introduction The radical-induced solid-state polymerization of acrylamide has been known since 1954.2 Subsequently, it waa shown that there was a considerable postirradiation reaction; that is, the reaction can proceed for long periods of time after the removal of the material from the radiation s0urce.~-5 It was also demonstrated that the resulting polymer is amorph~us,~J that the polymer nucleated as a second phase early in the reacThe Journal of Physical Chemistry
t i ~ n ,and ~ ~that ~ , ~defects were favored as nucleation sites. Further reaction then occurred at the interface (1) Work done under the auspices of the U. 8 . Atomic Energy Commission. (2) R. B. Mesrobian, P. Ander, D . 8. Ballantine, and G. J. Dienes? J. Chem. Phys., 2 2 , 565 (1954). (3) G.Adler, {bid., 31, 848 (1959). (4) (a) T. A. Fadner, I. Rubin, and H. Morawetr, J. Polymer Sci., 37, 549 (1959); (b) B. Baysal, G. Adler, D. 8. Ballantine, end P. Colombo, (bid., 44, 117 (1960).
E.s.R. SPECTROSCOPY OF IRRADIATED ACRYLAMIDE
between monomer and polymer. The kinetics of this reaction were shown to be unusual and different from those of normal polyrneri~ations.~b~~ The irradiated crystals showed an electron spin resonance spectrum at room temperature which was relatively stable with respect to time in a manner consistent with the reaction kinetics.8 In solid solutions of acrylamide in propionamide the electron spin resonance spectrum is quenched to a large extent by oxygen. Furthermore, the polymerization is also suppressed to a large extent by oxygen in these solid soluti~ns.~For these and other reasons, it was believed that the reaction was free-radical initiated. It was shown, however, that the e.s.r. spectrum seen at room temperature in irradiated acrylamide showed little change of shape with time, was very similar to that seen in other solid state vinyl polymerization,8 and showed no orientation dependence in single crystals, as was consistent with formation of amorphous polymer. These facts indicated that the free radical observed was the propagating radical and not the initiating radical. I n order to understand the reaction more completely, it therefore became important to determine the nature and orientation of the initiating radical, how the first addition steps take place, and the influence of the crystal lattice on this. It is with this end in view that the investigation reported here was undertaken.
3713
quired length of time. All e.s.r. spectra were measured at -196'. To determine the anisotropy of the spectra, large acrylamide crystals were grown from acetone solution. These were irradiated and observed under the same conditions as the powdered specimens.
Results Acrylamide and Propionamide. Propionamide differs from acrylamide in that the latter contains an CY,p double bond. Therefore, if acrylamide were to scavenge the hydrogen atoms liberated during radiolysis, the resulting radicals would be equivalent to those produced from propionamide by a loss of a hydrogen atom. The e.s.r. spectra obtained from either powdered material when irradiated and measured at -196' are essentially the same as is shown in Figure 1. Both spectra are very similar to that obtained from propionamide at room temperature and quite different from that of acrylamide at 25' where polymerization complicates the p i ~ t u r e . ~These low-temperature spectra show five rather broad main peaks. These peaks and their intensity distribution can be attributed to a system of two overlapping quartets, each with a 1:3:3:1 peak-
I
25 p u r r
A
Experimental Section The acrylamide and propionamide used was recrystallized twice from acetone and then ground. The solid solutions of acrylamide in propionamide were made by shock cooling in the manner described by Fadner and Morawetz, who first demonstrated that these two substances form solid solutions in all proportions.6 The powdered material was then sealed under vacuum torr) in the sample tubes previously describedn8 They were irradiated to the required dose at -196' in a Cow source. The dose rate was 0.75 Mrad/hr. as determined by the Fricke dosimetry method."J In most cases a total dose of about 20 Mrads was used since this gave an e.s.r. spectrum of good amplitude and it was shown that there was no qualitative difference between the spectra at these doses and the lowest ones at which they could be detected. After irradiation, the tip of the specimen tube was annealed while the sample was still kept at - 196'. The samples were then placed in a Varian Model V4500 e.s.r. spectrometer with 100-kc. modulation. The microwave power was sufficiently low so that no pronounced saturation effects were observed. Between readings the samples were stored in a thermostat at the required temperature and for the re-
Figure 1. E m .spectra of acrylamide ( A ) and propionamide (B) irradiated and stored a t -196": radiation dose 20 Mrads. (5) T. A. Fadner and H. Morawetz J . Polymer Sci., 45, 475 (1960). (6) G. Adler and W. Reams, J . Chem. Phys., 32, 1698 (1960). (7) C. Sella and J. J. Trillat, Compt. rend.. 253, 1511 (1961). (8) G. Adler, D. 9. Ballantine, and B. Baysal, J . Polymer Sci., 48, 195 (1960). (9) G. Adler, Proceedings of the International Symposium on Radiation Induced Polymerization and Graft Copolymerization, Battelle Memorial Institute, Columbus,Ohio, Nov. 1962, TID 7643. (10) A.S.T.M. Method D-1671-59T.
Volume 69,Number 11 November 1966
G. ADLERAND J. H.PETROPOULOS
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height ratio. This is the spectrum that would be expected from a radical having the structure CHa-cHCONH2. We shall hereafter refer to this as the P radical. The double quartet nature of the spectrum and the structural assignment of the radical have been confirmed by single crystal data. The latter data also show that the radical is well oriented in the lattice. The details of the single crystal experiments will be reported in full in a subsequent publication. Deuteration of the amide group in propionamide showed no observable change in the e.s.r. spectrum thus indicating that the amide group is not significantly involved. There seems to be evidence of very small amounts of some component other than the P radical but this spectrum was too weak to get any clear-cut indication of its nature. Thus it appears that the P radical, the principal one seen in acrylamide irradiated at - 196O, is produced by hydrogen scavenging. This scavenging is consistent with the low temperature (-78') hydrogen yields reported by Adler, et al." The changes in the e.s.r. spectra of acrylamide and propionamide were followed by warming the samples to various temperatures, usually for 16 hr., and then recooling to - 196'. Upon warming to 145' both spectra show similar but minor changes which can be explained most easily by assuming the decay of a minor radical other than the P radical (Figure 2). On further heating the spectrum of propionamide remains the same, except for intensity, until it decays completely upon being kept for protracted periods. In contrast to this, the acrylamide spectrum shows very marked differences in behavior. Thus, after warming above - 125O, the spectrum slowly changes to a broad triplet, very similar to that seen in acrylamide and other vinyl polymerizitions at room temperature. [This change becomes most clear-cut and easily seen at about -80' (Figure 2).] The most important difference between this and the room temperature spectra is that the former shows pronounced orientation effectsin single crystals though the orientation does not appear to be as precisely defined as that of the P radical. We believe the radical responsible for this spectrum to be A. It is the species obtained
zsoourr
A
I
I
I
C
I
D
Figure 2. E.s.r. spectra of acrylamide irradiated at - 196" and stored for 16 hr. a t (A)-145", (B)-go", (C) -40', (D) -30', and ( E ) -20': radiation dose 20 Mrads. The e.s.r. spectra of propionamide at - 145' and above are essentially similar to that of acrylamide a t 145".
-
H H
H
I
I I
C-NHtH
/I
C-NH2
I/
0
0 A The J o u r d of Physical Chemistry
by adding an acrylamide molecule to the P radical. The disappearance of the two outermost peaks of the P radical spectrum, the general broadening, and the directional dependency of the A radical spectrum indicate (11) G. Adler, D. 8. Ballantine, T. Davis, and R. Ranganathan,
J. Phys. C h m . , 68, 2184 (1964).
E.s.R.SPECTROSCOPY OF IRRADIATED ACRYLAMIDE
that at least dimerization can occur above - 125'. The crystal is monoclinic space group P21,,. The monomer addition seems to be most probable with its neighbor in the direction of the screw axis. Addition across the center of symmetry seems to have a lower probability. Crystallographic considerations similar to those of Adler and Reams12 and the fact that no polymer formation has ever been reported at these temperatures indicate this cannot proceed much beyond dimerization at this stage. One would normally expect a free radical such as A to have a more complex spectrum than is shown in Figure 2 and the single crystal spectrum bears this out. Figure 3 shows the spectrum of the A radical in three different orientations 15' apart. This also shows that the spectra has a marked dependence on direction. It is only because the spectra seen in powdered samples are averaged over all orientations that the spectrum seems as simple as in Figure 2. The data in Figure 3 were taken at - looo but the spectra are essentially the same at -80'. The A radical exhibits only a decrease in intensity until -60". Between -60 and -40" some small new peaks become apparent. These are probably due to small amounts of another minor radical which we shall call B. At present we can say little about the B radical since its spectrum cannot be separated from that of the A radical in a clear-cut fashion. It could have arisen from the conversion of a small amount of A to B or it could have been present initially but masked by the much more intense P and A spectra. We feel the latter is more probable since the B radical appears to be very precisely oriented whereas a small amount of disorientation has already taken place in the A radical. I
I
3715
Between -30 and -20' the e.s.r. spectrum changes somewhat and, more importantly, loses all orientation dependence. It becomes essentially the same radical spectrum as that observed after room temperature irradiation (Figure 2). The upper part of this temperature range corresponds to that at which polymer yields can first be measured.'* We shall call this new spectrum A1 and attribute it to the propagating polymer radical. The loss in directional dependence may be taken as an indication that the radical can now react with near neighbors in directions other than that of the screw axis. One would expect the A1 radical to have the same e.s.r. spectrum as the A radical. However, the loss of directional dependency should make the single crystal and powder spectrum at - 20' almost identical and very similar to the powder spectra of the A radical at, say, -80". We have indeed found this to be true. Figure 2 shows the powder spectra. Above room temperature the e.s.r. data we obtained are similar to that of Ueda and Kuril* and need not be described here.
Solid Solutions E.s.r. studies of solid solutions of acrylamide and propionamide provide interesting confirmation of most of the above points. Two solid solutions, containing 65 and 92% acrylamide, were studied over a wide range of temperatures. Their spectra were compared to a physical mixture of the two components used as a control. The e.s.r. spectra of the solid solutions at -196' are indistinguishable from that of the P radical as obtained from the individual components. As the temperature is increased, the distinction between A and P radicals can most easily be seen by observation of the outermost peaks of the solid-solution spectra. The relevant portions of the spectra are shown in Figure
4.
I
I
The behavior of the 92% solid solution closely resembles that of pure acrylamide and that of the physical mixture. This is not unexpected since there was not enough propionamide present to produce easily observable effects. The 65% acrylamide solid solution presents a different story, as shown in Figure 4. The following paints should be noted. Firstly, at - 50' the P radical peak is more prominent relative to the A radical in solid solutions than in the physical mixture. The P radical ~~~
Figure 3. E.s.r. spectrum of a single crystal of acrylamide showing the A spectrum. Three orientations 15" apart are shown: temperature -100".
~~
(12) G.Adler and W. Reams, J. Polyter Sci., A2, 2617 (1964). (13) P. Ander, Thesis, Polytechnic Institute of Brooklyn, 1954. (14) H. Ueda and Z. Kuri, J . Polymer Sci., 61, 333 (1962).
Volume 69,Number 11 November 1966
'3716
G. ADLERAND J. H. PETROPOULOS
1
I
SOLID SOLUTION, 65 % ACRY L A M l D E MIXTURE
Bz
65% A C R Y L A M I D E SOLID SOLUTION, 92% ACRYLAMIDE
I !
MIXTURE 92% ACRYLAMIDE
\
\
TEMPERATURE
Figure 4. Outer, low-field side of the spectrum of solid solutions (subscript 1 ) and physical mixtures (subscript 2) of acrylamide and propionamide: acrylamide content 65%; storage temperature (A) -50", ( B ) -30", (C) -20'; radiation dose 20 Mrads.
therefore persists to a greater extent than is explainable simply by the ratio of the two components of the solid solution, and this persistence occurs at temperatures above that at which the P radical disappears in pure acrylamide. Secondly, the P peak disappears from the solid solution between -30 and -20'. The first observation apparently indicates that if the P radical does not find an acrylamide molecule in the b-axis direction a t low temperature, it will not react. The second observation shows that at the temperatures at which the unoriented A radical appears in pure acrylamide, the restriction implied by the first observation is no longer important. The spectrum now corresponds to that obtained at room temperature where no P radical is observed until the acrylamide content of the solid solution is less than 10%. This indicates that the radicals now have sufficient mobility to allow reaction with near neighbors in directions other than the screw axis. Similar arguments accounted for the previously noted change of A to AI radicals in the same temperature region. The solid solution data are therefore consistent with and amplify that obtained from the pure materials. Relative radical populations were estimated for the solid solutions a t various temperatures. Below 4 O 0 , the results are not sufficiently satisfactory from a quantitative viewpoint to permit definitive conclusions to be drawn However, above -60' the data are reasonThe J o u m l of Physical Chernietry
Figure 5. Decay of the e.s.r. spectrum of solid solutions and physical mixtures of acrylamide and propionamide as a function of temperature: time of storage at a given temperature, 16 hr. except where noted.
ably consistent and reproducible and are plotted in Figure 5. Considerable radical decay occurs in the 65% acrylamide solution up to -30', the temperature at which the P radicals disappear. The corresponding physical mixture shows somewhat less decay. During prolonged storage a t the -30 to -20' temperature range the 65% solid solutions and mixture show appreciable radical decay. The 92% acrylamide solution, however, shows very little. Furthermore, the observable difference in behavior between the solid solution and the physical mixture is very small at this composition. The over-all radical yield upon irradiation at - 196' is greater in the solid solution than in pure acrylamide. However, the radical decay in the solid solutions is greater so that after prolonged storage at room temperature pure acrylamide exhibits a more intense spectrum. The mechanism for the decay of the radical spectrum is not known, but it is probably similar to that of pure propionamide at room temperature which apparently decays by a second-order process.
Discussion The foregoing results in which P-type radicals are formed in the radiolysis of acrylamide clearly indicate the ability of acrylamide to scavenge hydrogen. It has recently been demonstrated that acrylic acid15 and -~
~~
~~
(15) Y. Shioii, S. Ohnishi, and I. Nitta, J . Polymer Sci., (1963).
AI, 3373
E.s.R. SPECTROSCOPY OF IRRADIATED ACRYLAMIDE
3717
barium methacrylateL6 can also scavenge hydrogen will occur preferentially near those defects and at those atoms in the solid state. Similarly, Klein and Scheer temperatures where greater mobility is possible. In a have shown that frozen terminal olefins exhibit this crystal there is likely to be a range of different defects same scavenging ability. l7 The phenomenon, therewhich, in turn, imply a limited range of localized enfore, appears to be fairly common and not too surprisvironments and energies, all within the possibilities aling in view of the high diffusive capability of the small lowed by the crystal structure. The broad temperahydrogen atom. An analogous diffusion capability has ture range of the P + A transition becomes easily explainable in these terms. been previously reported in acrylamide-propionamide solid solutions where the much larger oxygen molecule These latter comments regarding a spectrum of dewas shown to be an effective polymerization i n h i b i t ~ r . ~ fects and their possible effect on solid-state reactions This efficient scavenging process helps explain the depoint to considerations that are sometimes overlooked pressed hydrogen yield in the radiolysis of propionand indicate that activation energies for solid-state amide-acrylamide solid solutions at low temperature. processes should be treated with caution. FurtherThe P radical which results from the scavenging procmore, collisional factors should be considered more carefully than is usual in kinetic studies. ess is relatively stable since it can be stored indefinitely at - 196’. It is also very precisely oriented within the It has been pointed out that after the first addition lattice. The stability of the P radical below -125’ the nearest neighbors to the radical are no longer in the strongly suggests that the mobility in this temperature screw-axis direction. The second transition from A to range is severely restricted. AI radicals reflects again the attainment of sufficient Above approximately - 125’ the P radical begins to thermal motion for further reaction. There are three change to an A-type radical. This transition requires pronounced characteristics associated with this temperthe onset of sufficient mobility so that the P radical can ature region. Firstly, it is the temperature range approach the nearest double bond on a neighboring where measurable polymerization first occurs. Secmolecule with sufficient energy so that dimerization can ondly, it marks the loss of all orientation dependence take place. This A radical still exhibits a pronounced of the free-radical spectrum. Thirdly, it is the range orientation dependence which requires this first addiwhere the P spectrum finally disappears in the solid tion step to be nonrandom in direction. It is not unsolution. The loss of orientation dependence and P radical in reasonable to assume, and indeed the evidence indithe solid solution is the consequence of further addicates, that the first addition is in the direction of the tion in directions other than the b axis. It is at this twofold screw (b) axis of the crystal. The addition of another molecule to form a trimer radical is not expoint that as we have previously suggested the polymer “nucleates as a second phase.” The random direccluded. However, it has a much lower probability than tional addition also accounts for the acrylamide polydimerization. mer being completely amorphous. The fact that this The net shrinkage which accompanies the monomer temperature range appears to be the threshold for obaddition process (about one-third of a unit cell in the bservable polymerization is also explainable in these axis direction for every two monomer units added) terms. At least a limited amount of mobility is requickly puts the radical out of phase with the original quired for the addition reaction. It is only when the crystal lattice.12 At this point, the nearest neighbors thermal motion reaches a stage where it allows the are no longer in the b-axis direction and further addiradical and monomer molecules to approach within tions must take pIace in other directions and this would reaction distances that polymer will be formed. It lead to a loss of orientation dependence of the obwould seem, therefore, that one of the important roles served radical signal. of the crystal lattice in influencing a chemical reaction, It should be noted that this P + A transition takes place over a broad temperature range. This is not aside from controlling the pure geometry, is determining the type and extent of mobility possible in a given too surprising since the transition requires mobility and situation. the mobility will be affected by the immediate crystallographic environment of the radical. The achieveWe also wish to call attention again to the possible contributions of annealing processes to the reacment of the required mobility will occur more readily at tion. Increasing the temperature will promote annealdefects such as edge dislocations or mosaic boundaries. One should also consider that the shrinkage consequent (16) H. Morawetz, 148th National Meeting of the American Chemiupon monomer addition necessarily introduces strain cal Society, Chicago, Ill., Sept. 1964. within the crystal lattice. Further reaction can be en(17) R. Klein and M. D. Soheer, J. Am. Chem. SOC.,80, 1007 (1958); hanced by an annealing process and again annealing J. Phy8. Chem., 6 2 , 1011 (1958). Volume 69.Number 11 November 1966
M. F. EMERSON AND A. HOLTZER
3718
ing and the annealing temperature will have a range of values. It has been noted that in some solid-state polymerizations the polymer yield reaches a limiting value which is temperature dependent. It is possible that in many cases this is due to the reaction being carried out in a temperature range which is insufficient for annealing out most of the lattice strain induced by the reaction and/or sufficient to provide adeauate mobility over only a limited range of sites. In a recently published papert8 the powder spectra
from irradiated propionamide were determined and the radical was taken to be CH&H-CONH2, the same as our P radical. Acknowledgment. The authors wish to thank Dr. D.
s. Ballantine for his help in writing the manuscript and Mr. w.Reams for preparing the samples used in these experiments. (18)M.T.Rogers, 8.Bolte, and P.9. Reo, J . Am. Chem. SOC.,87, 1875 (1965).
On the Ionic Strength Dependence of Micelle Number’.’
by M. F. Emerson and A. Holtzer Department of Chemistry, Washington University, St. Louis,Missouri
(Received March $9, 1966)
Examination of the thermodynamics of micelle formation reveals that the quantity RT In (c.m.c.) is equal to the standard free energy change of addition of one more detergent molecule to a micelle that already contains the number of molecules most probable at the c.m.c. This result is independent of the usual assumption about phase separation. This standard free energy can be conceptually divided into a hydrocarbon part and an electrical part. The magnitude of the electrical part can be estimated from numerical (computer) solutions to the nonlinearized Poisson-Boltsmann equation assuming that the micelle is a sphere with smeared charge and using published values of the radius (determined by low angle X-ray scattering) and of the micelle number (determined by light scattering). Subtraction of the electrostatic contribution from the total provides the hydrocarbon part of the standard free energy. Applying this method to published data for sodium dodecyl sulfate and dodecyltrimethylammonium bromide, we find that the hydrocarbon contribution is roughly independent of detergent and ionic strength, having an approximate value of 8000 cal./reaction unit (infinitely dilute reference state, molality basis).
Introduction I n a germinal paper by Debyeathe model of a micelle adopted is that of a platelet-shaped aggregate of detergent ions limited in size (Le., in the number of detergent molecules per micelle or “micelle number,” N ) by the electrostatic repulsion of the charged head groups and stabilized by the hydrophobic adhesion of the hydrocarbon tails, Debye further assumed that in the abThe Journal of Physical Chemistry
sence of added electrolyte the electrical (repulsive) part of the free energy of formation of the platelike (1) This investigation waa supported by Research Grant RG-5488 from the Division of General Medical Sciences, Public Health Service. (2) Support for some of the computation was provided by National Science Foundation Grant G 22296 to the Washington University Computer Center. (3) P. Debye, Ann. N. Y. A d . Sei., 51, 575 (1949).
