J. Phys. Chem. 1981, 85,2691-2694
.-.
tI
the AOT molecule relatively more flexible, and the polar groups may rotate about the ethanic C-C bond more freely with increasing temperature. Accordingly, the equilibrium mixture of rotamers changes with the decrease of watersolubilizing capacity of AOT, which is evidenced from the rapid change of water proton chemical shifts from downfield to upfield.
c
Flgure 4. Temperature dependence of chemical shifts of water protons In H,O/AOT/CDCI, systems. [H,O]/[AOT] = (0)0.48, (0)1.11, (A) 2.22, and (A) 4.44.
3001
w
d . W
260-
I
-10
2691
0
20
Temperature PCI
40
Flgure 5. Temperature dependence of chemical shifts of water protons in H20/AOT/iCBH18 systems. [H20]/[AOT] = (0)4.0 and (0)40.00.
vs. chemical shift at different [H20]/[AOT]ratios as shown in Figures 4 and 5 , it is reasonable to assume that at low water content (i.e. in the micellar region) the polar groups are highly structured and a particular rotamer is stabilized. At higher water content, i.e., in the microemulsion phase, the surfactant molecules in the interface are bound by hydrogen bonds to a large "pool" of water. This makes
Photooxidation of Ethyl Iodide at 22
Conclusions The proton magnetic resonance spectra of Aerosol OT in different organic solvents reveal that the sodium salt of this compound exists in the form of a temperature-dependent equilibrium mixture of different rotational isomers within the experimental temperature range. Accordingly, the different water-solubilization characteristics of the compound at various temperatures are thought to be due to different compositions of the rotamers present in the equilibrium mixture of AOT in the w/o-microemulsion systems. These compositionsin the equilibrium mixture depend on the soluMolvent interactions between the polar groups of AOT and the solvent molecules. In a polar solvent, the solute-solvent interaction is totally isotropic and the populations of various rotamers depend only on the steric interactions between the bulky substituent groups. In less polar or nonpolar solvents, the polar groups have negligible interactions with the solvent molecules. They are oriented in such a way that the intramolecular interactions of the polar groups as well as interaction with other polar molecules in the system, e.g., hydration, become a maximum. Such a favorable configuration gives rise to optimum amphiphilic character of the AOT molecule.
Acknowledgment. This work is part of project Nos. 2.227.0.79 and 2.623.0.80 of the Swiss National Science Foundation. We are grateful to Dr. V. Arnold, Ciba-Geigy SA, for stimulating discussions.
O C
Paul B. Shepson and Jullan Heicklen4 Department of Chemistty, Center for Air Envlronment Studies, and Ionosphere Research Laboratoty, The Pennsylvads State University, University Park, Pennsylvania 16802 (Received: March 3, I98 1; In Final Form: May 29, I98 1)
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C5H61was photolyzed at 313 nm and 22 "C in the presence of either Iz, (CzH6)zNOH,or 02-He mixtures. We have determined that the quantum yields for the primary processes C2H51+ hv C2HSt+ I (3a) and C2H61 + hv CzH4 + HI (3b) are 9%= 0.31 f 0.01 and +3b = 0.0095 f 0.0005. An upper limit for the rate coefficient for the following reaction has been found to be 1 X cm3/(molecules) at 22 "C: CzH6+ O2 CzH4+ HOz (1).
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Introduction There are two possible competing pathways in the oxidation of ethyl radicals at 22 "C, either the addition of O2 or abstraction to produce CzH& C2H5 + 02 C2H4 + HOP (1) +
C2H6 + 02 (+ M) C2H5O2 (+ M) (2) Several estimates have been made of the relative importance of reaction 1. Knoxl has estimated that kl = 1 X +
(1) J. H. Knox, Adu. Chem. Ser., No.76, 1 (1968).
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cm3/(molecules) at 22 "C based on studies of propane thermal oxidation. Heicklen2 has estimated that kl N 9 x cm3/(molecule s) using an activation energy for reaction 1 of 4.4 kcal/mol in agreement with Benson's3 calculations. McMillan and Calvert4 estimate that k 1 / k 2 I 0.001 making k l I6.9 X cm3/(molecule s), using the value 6.9 X 10-l2 cm3/(molecule s) for k 2 in the second-order limit, obtained from the azomethane flash (2)J. Heicklen, Adu. Chem. Ser., No. 76, 23 (1968). (3)S. W.Benson, J. Am. Chem. SOC., 87,972 (1965). (4) G. R. McMillan and J. G. Calvert, Oxid. Combust. Reu., 1, 83 (1965).
0022-3654/81/2085-2691$01.25/00 1981 American Chemical Society
2892
The Journal of Physical Chemistry, Vol, 85, No. 18, 1981
photolysis studies of Dingledy and C a l ~ e r t .At ~ temperatures between 440 and 540 “C, Baldwin et have found kl = 1.42 X exp(-1949/TJ cm3/(molecule s). This extrapolates to a value of 2.0 x cm3/(molecule s) at 22 “C. However, recent unpublished work done in our laboratory on the photolysis of propionaldehyde in the presence of O2 has indicated that C2H4 is produced. Thus, reaction 1 might be important at low total pressure if reaction 2 is third order. Howard’ has used reaction 1in a discharge flow system as a source of HOz radicals and has observed them by laser magnetic resonance detection but indicated that the H 0 2 yields were somewhat low. Ryan and Mulcahya have reported that a t l-torr total pressure 16% of C2H5 radicals react via reaction 1and that this value drops to 8% at 5 torr, indicating that reaction 2 is in the transition from third- to second-order regime at these pressures. In order to resolve this apparent controversy, we have undertaken this study to determine experimentally the ratio k l / k 2 by photolysis of C2H51in the presence of 02.
The photolysis of C2H51is generally accepted, but based on little evidence, to proceed with a quantum yield of approximately unityg via process 3a:
+
CzH51 hv
-
+
C2H5+ I
(34
where CzH5t represents “hot” C2H5 radicals. Since the photon energy at 313 nm corresponds to 91 kcal/mol and the C-I bond strength DZ9&C2H5-I)= 53 kcal/mol, the photofragments of reaction 3a will contain 38 kcal/mol excess energy, partitioned among translational and the various internal energies. The energy distribution of photofragments from CzH51photolysis at 266.2 nm has been studied by Riley and Wilson.1o Their results indicate that -79% of the I atoms produced are excited I(2P1/z) (21.7 kcal/mol excess energy), the remainder being I(V3 z), although this ratio may be a function of wavelength. &a. 40% of the total available excess energy appears as internal (vibrational, rotational, and electronic) energy of the ethyl radical, the remainder being translational energy. It has also been suggested that process 3b may occur, which C2H5I
+ hv
+
CzH4 + HI
(3b)
would give even more highly excited reaction products. Thrush1’ observed HI absorption bands in the flash photolysis of C2H51. Schindler and Wijned2 found that C2H4 formation could not be completely suppressed by radical scavengers in the photolysis of C2H51. They found that, with full arc irradiation, the residual C2H4 was -5% of the C2H5radical production. This residual C2H4 was attributed to reaction 3b. The photooxidation of CzH51can lead to CzH4 production via three processes. The first is in the primary process 3b. The other two processes involve the oxidation of either “hot” or thermalized CzH5 radicals via reaction 1. It is the ~~~
~
(5) D. P. Dingledy and J. G. Calvert, J. Am. Chem. Soc., 85,856 (1963). (6) R. R. Baldwin, I. A. Pickering, and R. W. Walker, J. Chem. SOC., Faraday Trans. 1 , 76,2374 (1980). (7) C. J. Howard, J. Chem. Phys., 67, 5258 (1977). (8) K. R. Ryan, I. C. Plumb, J. R. Steven, and M. F. R. Mulcahy,
presented at the American Society for Mass Spectrometry Conference, New York, 1980. (9) J. G. Calvert and J. N. Pitts, Jr., “Photochemistry,” Wiley, New York, 1966. (10) S. J. Riley and K. R. Wilson, Discuss. Faraday Soc., 53, 132
(1972). (11) B. A. Thrush, Proc. R. SOC.London, Ser. A , 243, 555 (1958). (12) R. Schindler and M. H. J. Wijnen, Z. Phys. Chem. (Wiesbaden), 34, 109 (1962).
Shepson and Heicklen
TABLE I: Photolysis of C 2 H J in the Presence of I, [C,H,I], torr
[I2], torr
15.3 20.9 30.4
0.228 0.234 0.231
Ia 9 mtorr irradiation min-’ time, min
@(C2Hd)
0.265 0.362 0.526
0.00947 0.00979 0.00945
405 90 180
purpose of this work to measure the importance of these three processes for CzH4 production. Experimental Section The photolysis of CzH51was carried out in a 500-cm3 Pyrex bulb by irradiation from a Hanovia medium-pressure mercury lamp. The 313-nm line was isolated with a Corion SM-3130-2interference filter. All experiments were performed at 22 O C . The contents of the reaction mixture were bled continuously through a pinhole into an Extranuclear Type I1 quadrupole mass spectrometer providing for continuous product analysis. Mass-spectrometric determinations were performed by measuring product ion current peaks relative to the m / e 84 peak from a small amount of Kr which was added to the reaction mixture in accurately measured amounts previous to irradiation. The CzH4 and CH3CH0 produced were determined by expansion to a gas chromatograph sample loop after irradiation. These products were then separated on a 9 f t X 1/4 in. 0.d. stainless-steel column packed with Porapak QS a t an isothermal operating temperature of 130 “C and a He flow rate of 35 cm3/min. C2H50Hwas determined both by gas chromatography and by mass spectrometry. CzH6 was analyzed by gas chromatography on the above column operating a t 23 OC. C2H51,obtained from Aldrich, was purified by trap-totrap distillation from -63 to -131 OC. Matheson extra dry grade Oz (99.6% minimum) was used without further purification. Gas-chromatographic analysis of the Oz indicated a concentration of
