1905
Micellar Catalysis of Radical Reactions
and entropy being particularly unfavorable in the second ionization stage. As a final remark, the striking borderline behavior of the MAMPe copolymer (especially with Kf counterions) with respect to the conformational transition is to be underlined. Although dilatometry and f l u o r e ~ c e n c equite ~ ~ undoubtedly lend us to propose the occurrence of such a transition, potentiometry and calorimetry alone give weak support to it. This should once more induce one to be very cautious on relying upon one single technique for asserting the presence of a conformational transition for synthetic polyelectrolyte solutions.
Acknowledgment. This work has been sponsored by the Italian Consiglio Nazionale delle Ricerche, Rome. One of the authors (J.C.F.) is grateful to NATO for its financial support. References and Notes (1) On leave of absence from the Laboratoire de Chimie MacromolBculaire, UniversitB de Rouen, Mont-Saint-Aignan, France. (2) Present address: Istituto di Chimica-Fisica, UniversW di Roma, 00185 Roma, Italy. (3) V. Crescenzi, F. Delben, F. Quadrifoglio, and D. Dolar, J. Phys. Chem., 77, 539 (1973). (4) F. Quadrifoglio, V. Crescenzi, and F. Delben, Macromolecules,6, 301 (1973). (5) V. Crescenzi, F. Delben, S. Paoletti, and J. Skerjanc, J. Phys. Chem., 78. 607 (19741. (6) F. Delben, S. Paoletti, V. Crescenzi, and F. Quadrifoglio, Macromolecules, 7, 538 (1974). (7) E. Bianchi, A. Ciferri, R. P a r d , R. Rampone, and T. Tealdi, J. Phys. Chem., 74, 1050 (1970). (8) G. Barone, N. Di Virgilio, V. Elia, and E. Rizzo, J. Polym. Scl., Symp. No. 44, 1 (1974). (9) S. Katz and T. G. Ferris, Biochemistry, 5, 3246 (1966). (10) V. Crescenzi, F. Quadrifoglio, and F. Delben, J . Polym. Sci., Part A - 2 , 10, 357 (1972). (11) V. Crescenzi, F. Quadrifoglio, and F. Delben, J . Polym. Scl., Part C , 39, 241 (1972). (12) F. Delben, V. Crescenzi, and F. Quadrifoglio, Eur. Polym. J., 8, 933 (1972). (13) A. J. Begala and U. P. Strauss, J . Pbys. Chem., 76, 254 (1972).
(14) Similar data for MAE, MAP, MAiB,‘ as well for MA-alkyl vinyl ether copolymersi3 (0 C a C 1) fall in the range -6 to -7 mL/mol of H:’ these data, however, were obtained in the presence of added simple salts. (15) N. Ohno, K. Nitta, S. Makino, and S. Sugai, J. Polym. Scl., Polym. Phys. Ed., 11, 413 (1973). (16) A. Ikegami, Siopolymers, 6, 431 (1968). (17) J. Komiyama, M. Ando, Y. Takeda, and T. Iijima, Eur. Polym. J., 12, 201 (1975). (18) M. Mandel, J. C. Leyte, and M. G. Stadhouder, J. Phys. Chem., 76, 603 (1967). (19) P. L. Dubin and U. P. Strauss, J. Phys. Chem., 74. 2842 (1970). (20) P. L. Dubin and U. P. Strauss; “Hypercoiling in Hydrophobic Polyacids” in “Polyelectrolytes and Their Applications”, A. Rembaum and E. SB!&jgny, Ed., Reidel Publishing Co., Dordrecht, Holland, 1975, pp 3-13. (21) Addition of proper amounts of CU(CIO~)~ to the partially neutralized polyelectrolyte solutions makes it feasible to titrate both carboxyl functions of MAMPe and MA3MPe, as already discussed in the case of MAiB.“ (22) S. Paoletti, F. Delben, and V. Crescenzi, J. Phys. Chem., 80, 2564 (1976). (23) J. C. Fenyo, Eur. Polym. J., 10, 233 (1974). (24) J. C. Fenyo, J. Beaumais, and E. SBIBgny, J . Polym. Sci., Polym. Chem. Ed., 12, 2659 (1974). (25) H. Maeda and F. Oosawa, J. Pbys. Chem., 76, 3445 (1972). (26) G. S. Manning and H. Holtzer, J . Phys. Chem., 77, 2206 (1973). (27) K. Nitta and S. Sugai, J. Phys. Chem., 78, 1189 (1974). (28) K. N b , M. Yoneyama, and N. Ohno, Biophys. Chem., 3,323 (1975). (29) Unreported in Figure 1 to avoid too crowded plots. (30) J. Skerjanc, D. Dohr, and D. LeskovSek, 2.Phys. Chem. (Frankfurt a q Main), 56, 207 (1967). (31) J. Skerjanc, D. Dolar, and D. LeskovSek, Z. Phys. Chem. (Frankfurt am Main). 56. 218 119671. (32) J. SkerJanc,D: Dolar: and 6. LeskovSek, Z. Phys. Chem. (Frankfurt am Main), 70, 31 (1970). (33) N. Ise, K. Mita, and T. Okubo, J. Chem. SOC.,Faraday Trans. 7 , 69,, 106 (1973). (34) J. Skerjanc, Siophys. Chem., 1, 376 (1974). (35) K. Mita and T. Okubo, J. Chem. Soc., Faraday Trans. 7 , 70, 1546 (1974). (36) K. Mita, T. Okubo, and N. Ise, J. Chem. Soc., Faraday Trans. 7 , 71, 1932 (1975). (37) K. Mita, T. Okubo, and N. Ise, J . Chem. Soc., Faraday Trans. 1, 72, 504 (1976). (38) H. Daoust and A. Lajoie, Can. J . Chem., 54, 1853 (1976). (39) The rekheiy hrge AHb of MA3MPe for Q I0.6, a rather u n c o m m feature for the first dissociation of a polycarboxylic acid, might stem at least in part from Bu4N’ counterions binding and/or on Q dependent slightly on the aggregation of MA3MPe. (40) J. C. Fenyo, S. Paoletti, F. Delben, and V. Crescenzi, manuscript in preparation.
Micellar Catalysis of Radical Reactions. A Spin Trapping Study Dennis P. Bakallk and J. K. Thornas* Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received November 15, 1976; Revised Manuscript Received June 1, 1977) Publication costs assisted by the US. Energy Research and Development Administration
The reaction of surfactant radicals and the spin trap 2-methyl-2-nitrosopropanehas been investigated. Sodium dodecyl sulfate and sodium octyl sulfate show no spin adduct when surfactant and spin trap are irradiated with 6oCoat concentrationsbelow their critical micelle concentration. At concentrations above the cmc a nitroxide is formed from the reaction of the trap and a secondary surfactant radical produced via hydroxyl radical attack on the hydrocarbon chain. The micellar catalytic effect and the site of hydroxyl radical attack are discussed. Introduction Radiation induced reactions in micellar systemshave received detailed Over the last few years,i A lively as interest exists in applying such simple systems +Theresearch described herein was supported by the Division of Physical Research of the U.S.Energy Research and Development Administration. This is Radiation Laboratory Document No. NDRL-1721.
for more complex systems such as membranes. However, these systems exhibit many interesting kinetic properties such as catalysis,2which are of direct interest to physical chemists. Several studies (reviewed in ref 1)already exist to show that a sharp change in kinetic properties occurs on micellization of the surfactant molecules. The present work is an to locate the nature and Of reaction of hydroxyl radicals, OH, with anionic surfactants; it has already been established that OH radicals show quite The Journal of Physical Chemistry, Vol. 61, No. 20, 1977
1906
D. P. Bakalik and J. K. Thomas
Y
Figure 1. X band EPR spectrum from y radiolysis of N 2 0 saturated 0.1 M NaLS and lo-' M nitrosc-feH-butane: frequency 9.5 GHz, power 5 mW.
different reactivities with the monomeric and micellized form of sodium dodecyl sulfate.3 The reaction of surfactants with OH radicals leads to surfactant radicals which are suggested to reside in the hydrocarbon portion of the molecule. In the present experiments OH radicals are generated by @Coy radiolysis of N20 saturated aqueous solutions of surfactants. The solutions also contain a "spin trap" 2-methyl-2-nitrosopropane, which reacts with the surfactant radical to give a stable nitroxide radical.* EPR and NMR measurements of this latter radical are used to discuss the reaction of OH radicals with surfactant. Experimental Section Sodium dodecyl sulfate (NaLS, Matheson Coleman and Bell, technical grade) was purified by recrystallization from ethan01,~however it was noted that purified and technical grade gave identical results in this set of experiments. The spin trap 2-methyl-2-nitrosopropane (nitroso-tert-butane, N-t-Bu) was purchased from Aldrich Chemical and used without further purification. Sodium octyl sulfate (NaOS, Eastman) was used without purification. All samples were saturated with nitrous oxide and irradiated with a 6oCoy source (dose rate 6.5 X 1017eV mL-l rnin-l). Radiolysis of water leads to the formation of hydrated electrons ea; and OH radicals. Nitrous oxide captures all hydrated electrons produced in the radiolysis of water converting them to OH radicals. Thus the radiolysis of water saturated with NzO leads substantially to the formation of OH radicals. Total doses for all samples were 6.5 X l0ls eV mL-'. EPR spectra were recorded on a Varian Model V 4500-10A spectrometer with fieldial and 100-kHz modulation unit. Proton spin-lattice relaxation times were measured by the inversion-recoverymethod6 ( 180°-7-900 sequence) on a Varian XL-100 NMR spectrometer equipped with Transform Technology 1010 Fourier transform unit. Relaxation times were obtained from linear plots of In (1- A / A , ) vs. 7 where A was measured from peak height. Recovery times of 7X the longest TI were used and each T1 represents data from 157 values. Results and Discussion EPR Observations. Figure 1 shows the EPR spectrum resulting from the 6oCoy irradiation of NzO saturated 0.1 M NaLS in the presence of M nitroso-tert-butane. This spectrum is typical of a nitroxide radical. The nitroxide radical is produced via reactions l and 2 and is C,,H,,SO,-
+ OH-
C,,H;,SO,-
+ H,O
(1)
0
C,,H;,SO,-
I
H
I
t t-Bu-N=O + ~-Bu-N-(C,,H,,)SO,-
(2)
attributed to nitroso addition to a radical associated with the hydrocarbon chains of the surfactant. These statements will be discussed subsequently. The Journal of Physlcal Chemistry, Vol. 81, No. 20, 1977
I
I 10-2
5 x IO-' to- I [SURFACTANT]
5x10-I
Flgure 2. Yield of nitroso spin adduct from y radiolysls of surfactant in the presence of lo-' M nitroso-fert-butane. Critical micelle concentrations are 8 X and 1.4 X lo-' M for NaLS (0)and NaOS (O), respectively. The experimental variation in the data is k5%.
Figure 2 represents the yield of trapped radicals (measured from the height of the ESR signal) as a function of the NaLS and NaOS concentration. The line widths of all spectra were identical, hence the signal height gives a good measure of the radical concentrations. The effects noted are at least an order of magnitude change in radical concentration and well outside the approximation introduced by using signal height as a measure of radical concentration. The yield of trapped radicals was independent of nitroso-tert-butane concentration from 3 X low3 to 1.5 X lo-' M, and was linear with dose from 2 X 1Ol8 to 1019eV mL-l. It is interesting to note that radicals are not trapped below the critical micelle concentration (cmc) in these systems, and that there is a rather sharp increase in the concentration of nitroxide radical after the onset M NaLS and 1.4 X 10-1 of micelle formation (cmc 8 X M NaOSh7 It should also be pointed out that the cationic surfactant cetyl trimethylammonium chloride and the neutral Brij 35 polyoxyethylene dodecyl ether exhibited similar behavior to the NaLS and NaOS systems. Direct reaction of OH with nitroso-tert-butane is unimportant, as the measured rate of this reaction is 4.0 f 0.5 X lo8 M-I s-lS8This is at least an order of magnitude lower than the rate of reaction of OH with the surfactant. Experimental conditions were also such that surfactant concentrations were in tenfold excess over the nitroso trap. This indicates that the data show a trapping of a secondary radical produced in the micellar aggregate. There are several possible explanations for the preferential trapping in the micellar state: (a) preferential solubilization of the nitroso trap in the micelle; (b) increased lifetime of radical in micelles; (c) increase in encounters of Re and N-0 in micelle; and (d) a change in the dimer/monomer ratio of the spin trap in micellar solution. The partitioning of N-t-B between the micellar and aqueous phase is difficult to assess, however, it is noted that N-t-B is solubilized more readily in surfactant solutions than in pure water. Hence, at least partial solubilization of the trap does occur in the micelle. A radical may be produced by OH attack directly in the micelle. Hydroxyl radical attack on the monomer will produce a radical which may also eventually associate with the micelle as the micelle is in dynamic equilibrium with the
1907
Micellar Catalysis of Radical Reactions
monomer. A radical associated with a micelle will have a longer lifetime, thus facilitating the interaction of the nitroso compound and the surfactant radical in the micelle. Further considerations on this point are as follows. The mean diffusion distance X in a time t of a species with diffusion coefficient D is given by X = (2Dt)’i2. Hence, if two reactants exist in a solvent cage of 10 8, in water ( D = 2 X 10” cm2d),then the duration of such an encounter s. If reaction does not occur‘during this is 2.5 X period of time then the reactants diffuse apart. The residence time of a solute such as a spin trap in a micelle varies from 5 psgato 2 msgbin micelles. The residence time for reaction of a radical and spin trap in a micelle is then >5 ps. The probability of reaction in a micelle over that in water is then increased by more than 2 X lo4. This effect could explain the data of Figure 2. It is well known that nitroso compounds form dimers in solution.lOJ1The monomer-dimer equilibrium may be moved toward the monomer by solubilization of the spin trap in the micelle. Since the monomeric form of the spin trap is more reactive than the dimeric form, then data shown in Figure 2 may be explained. It is not possible to immediately discount the possibility of a shift in equilibrium toward the reactive monomer in the micellar environment as being responsible for the increased trapping in the micellar state. However, it is also reported that the trap nitroso-tert-butane exists substantially in the monomeric formll in aqueous solution and that the monomer concentration would not increase substantially in micellar solution. It is therefore suggested that the latter effect is not a determining factor in explaining the observed effects of micellization. It seems more likely that spin-trap partitioning into the micelle, increased radical lifetime, and an increase in the efficiency of reaction 2 are the determining factors in explaining the observations. The EPR spectrum (Figure 1)indicates that the trapped radical is of the structure HO
c C-N-t-BU ’ 1 c> since the nitrogen hyperfine lines are split by a single proton. The magnitude of the coupling constant ( 1.5 G) is approximately the value to be expected for a proton coupling.1° We cannot determine from the spectrum the position or positions on the hydrocarbon chain for nitroso addition. The line widths are broad due to the decreased rotational mobility of this radical, hence coupling due to ,8 protons is unresolved. Dilution of the radiation produced nitroxide product below the cmc does not affect the line shapes of the spectrum in Figure 1. The nitrogen hyperfine constant (16 G ) is suggestive of a polar environment for the spin adduct. Subsequent NMR data will indicate, however, that the nitroxide radical has to be partly associated with a micelle. To accomodate the data it is suggested that the polar nature of the nitroxide radical tends to locate it at the polar micellar surface. The radical will hence experience little restriction of movement and an environment similar to water. The spectrum above and below the cmc will hence be identical. At this stage it is worth commenting on the relative points of attack of the OH radical. The rate constants of reaction of OH radicals with monomer and micellar NaLS are k, = 6 X lo9 M-I s-l and kmi = 2 x lo8 M-l s-l , respectively.12 The relative extent to which OH radicals will react with micelles is given by the rate of this reaction k,i[OH][micelle] divided by the total rate of reaction Iz,i[OHl [micelle] + k,[OH] [monomer]. The concentraN
TABLE I: Spin-Lattice Relaxation Times for N a L S in the Presence
of Paramagnetic Probesa H3-C(CH,)-
0.1 M N a L S 0.1 M N a L S M N-t-Bb 0.1M N a L S ’ M Mn2+
H3C-(CH,),
H,C-SO,-
0.76 0.65
0.41 0.31
0.60 0.38
0.34
0.18
0.06
a TI(s) measured at 30 C. B, 10 min, 6oCoirradiation.
0.1 M N a L S ,
M N-t-
tions of micelle and monomer are calculated from the total surfactant concentration and the critical micelle concentration. Hence, in Figure 2 at 2 X M NaLS only 5 % of the OH radicals react directly with the micelle. However, a large increase ( N O ) in the yield of nitroxide M NaLS. radical is noted between and 2 X Increasing the (NaLS) further to 0.1 M increases the nitroxide yield by 1.6. However, 28% of the radicals now react directly with the micelle. It is concluded that surfactant molecules other than those found directly in the micelle produce nitroxide radicals. Association of monomeric surfactant radicals with the micelle then leads to the observed data, NMR Observation. NMR experiments were carried out in the micellar system with and without the radiation induced nitroxide. These experiments describe the nature and site of OH attack on the hydrocarbon chain more completely. It is well kn0wn13that the presence of a strong magnetic dipole, such as a paramagnetic center, provides a strong relaxation mechanism for and greatly enhances the spin-lattice relaxation times (TJ of neighboring nuclei viz. protons. The dipolar interaction which exists between the electron and nucleus has been used to estimate interaction distances in many systems.14 This principle has been applied to the trapped nitroxide radical in the present system in order to locate the position of attack of OH radicals on the hydrocarbon chain. Table I shows proton Tl values for micellized NaLS in the presence and absence of Mn2+ions and for the irradiated NaLS/nitroso system. The Mn2+ data where Mn2+ is located at the micelle surface show that the largest effect of the paramagnetic probe is on the relaxation of the protons of the carbon atoms CY to the sulfate group. The magnetic effect falls off as l/r6 so little effect should be noted for protons at the center of the micelle. The fact that some effect is noted for CH, groups shows that some of these groups reside at the surface of the micelle. The nitroxide gradient effects are also interpreted in this way. The radical is located at the surface of the micelle and the major relaxation effect is in the protons of the C atom a to the sulfate group. A point to note, however, is that the observations of Mn2+ and nitroxide radicals are similar. If there was no association of the radical with the micelle then these effects would not be noted. The data show that the nitroxide radical is close to the micellar surface and not in the interior of the micelle. The data do not show precisely which C atom of the NaLS is attacked by OH radicals, either in monomeric or micellar form. The data do, however, illustrate a marked catalytic effect of micelles on the rates of reaction of radicals with nitroso spin traps. References and Notes (1) (a) M. Gratzel and J. K. Thomas, in “Modern Fluorescence Spectroscopy”, Vol. 2, Plenum Press, New York, N.Y., 1976, p 169. (b) J. H. Fendler and E. J. Fendler, “Catalysis in Micellar and Macromolecular Systems”, Academic Press, New York, N.Y., 1975. (2) E. H. Cordes and R. B. Dunlop, Acc. Chem. Res., 2 , 329 (1969). (3) K. M. Bansal, L. K. Patterson, E. J. Fendler, and J. H. Fendler, Int. J . Radlat. Chem., 3 , 321 (1971). The Journal of Physical Chemistw, Vol. 81, No. 20, 1977
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K. Iwasaki and T. Fujiyama
(4) For review of spin trapping, see E. G. Janzen, Acc. Chem. Res., 4, 31 (1971). (5) E. F. J. Dugnstee and J. Grunwold, J . Am. Chem. Soc., 81, 4540 (1959). (6) R. L. Vold, J. S. Waugh, M. P. Klein, and D. E. Phelps, J . Chem. Phys., 48, 3831 (1968). (7) P. Mukerjee and K. J. Mysels, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 36 (1971). (8) Measured by pulse radiolysis competition with potassium thiocyanate. G. E. Adams, J. W. Boag, and B. D. Michael, Proc. Chem. SOC., 411 (1964).
(9) (a) N. M. Atherton and S. J. Starch, J. Chem. SOC.,374 (1972); (b) P. P. Infelta, M. Gratzel, and J. K. Thomas, J . Phys. Chem., 78, 190 (1974). (10) S. Forshult, C. Lagercrantz, and K. Trossell, Acta Chem. Scand., 23, 522 (1968). (1 1) C.Lagercratz, and S. Forshalt, Nature(London),218, 1247 (1968). (12) J. K. Thomas and D. P. Bakalik, to be submitted for publication. These numbers are in essential agreement with ref 3. (13) See, for example, Abragam, "The Principles of Nuclear Magnetism", Clarendon Press, Oxford, London, 1961, p 378. (14) P. E. Godlci, and F. R. Landsberger, Biochemlstry, 13, 362 (1974).
Light-Scattering Study of Clathrate Hydrate Formation in Binary Mixtures of ferf-Butyl Alcohol and Water Kenji Iwasakl and Tsunetake Fujlyama" Depaflment of Chemistry, Faculty of Science, Tokyo Metropolitan University, Setagaya, Tokyo 158, Japan (Recelved March 11, 1977) Publication costs assisted by the Tokyo Metropolltan University
Light-scattering spectra were observed for binary mixtures of tert-butyl alcohol and water at various concentrations. The Rayleigh intensities were reduced to concentration fluctuations. The observed concentration dependencies of the concentration fluctuation were well explained if we consider the existence of the clathrate hydrates in binary mixtures. The structures of the clathrates were (H20)21-TBAfor XTBA 1/22, and (H20)105-(TBA)B for XTBA 2 1/22, where XTBA is the mole fraction of TBA.
Introd uc tion Anomalous physical properties of tert-butyl alcohol (TBAI-water mixtures have been studied by many w o r k e r P and their results suggest that TBA-water mixtures are in a particular state of mixing. The Rayleigh ratios observed by the conventional light-scatteringmethod for TBA-water mixtures have recently been reported by two authorsgJOwithout any further analytical studies. In our preceding report,ll the method of observing the concentration fluctuation by light-scattering spectra12J3 has been discussed in detail. We have also shown the relation between the concentration fluctuation and formation of local structures in a liquid mixture.14J5 In this report, we will discuss the formation of local structure in TBA-water mixtures on the basis of the quantitative analysis of the concentration fluctuation measured by the light-scattering spectra. Furthermore, we will discuss the relation between the solid clathrate hydrate1q16,17of TBA and the local structure formed in TBA-water mixtures. Experimental Section Light-scattering spectra were recorded with a spectrometer designed and constructed in our 1ab0ratory.l~ The spectrometer is composed of a He-Ne gas laser source (NEC, GLG 108) and a pressure scanning Fabry-Perot interferometer. The spacer between the interferometer etalons is 6 mm thick, giving a free spectral range of 0.82 cm-l and an overall instrumental half-width of about 0.042 cm-l. The temperature inside the sample cell was maintained at 24.0 f 1.0 O C . tert-Butyl alcohol from Wako Pure Chemical Industries, Ltd. was used without further purification and water was triply distilled. The binary mixtures of TBA and water were made dust-free by the use of a Nuclepore filter with a pore size of 0.1 wm. The dust-free state was certified by observing the L-P ratio of water to be zero (see Figure 1). The preparation (filtration and injection of a sample into a cell) was repeated three The Journal of Physical Chemistry, Vol. 81, No. 20, 1977
times and the sample which showed the least L-P ratio was used for further experiments. The refractive index of the sample was measured by means of a ShimadzuBausch-Lomb Abbe refractometer 3L. The composition derivative of the refractive index at the mole fraction x was estimated from the difference between refractive indices measured at x f 0.012. Results and Discussion Light-Scattering Spectra. The observed spectra for TBA-water mixtures are given in Figure 1. The intensity of a Rayleigh line for water is seen to be nearly zero. The intensities of the Rayleigh lines begin to increase abruptly as the concentration of TBA increases. On the other hand, the intensities of the Brillouin lines are constant from water to TBA-water mixtures. Concentration Fluctuation. In the previous report,ll we have shown a procedure for estimating concentration fluctuations from Rayleigh intensities of light-scattering )~, spectra. The Rayleigh ratio for a Rayleigh line, ( R g O is related to the concentration fluctuation, V*( (Ax)?-),by the equation 1
where V* is the fluctuation volume, x the mole fraction, Xi the wavelength of incident light, and n the refractive index. (R& is the Rayleigh ratio corresponding to entropy fluctuation and is given by
where k is the Boltzmann constant, T the temperature, C, the isobaric heat capacity per gram, and p the density. The values of (Rw)s at each mole fraction were calculated from
