J. Phys. Chem. 1981, 85, 2687-2691
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Effect of Rotational Isomerism on the Water-Solubilizing Properties of Aerosol OT as Studied by ‘H NMR Spectroscopy A. N. Maitrat and H.-F. Elcke’ InstirUte for phvsical Chemistry, Universiry of Basel, CW4056 Basel, Swltzerbnd (Received: February 12, 198 1; In Final Form: May 1, 1981)
Proton magnetic resonance spectra of aerosol OT (AOT) in three different solvents, viz., methanol, chloroform, and isooctane, have been studied within the temperature range of -10 to +60 “C in order to investigate the phenomenon of rotational isomerism of the compound and its effects on the stability of water/oil (w/o) microemulsions. The temperature dependence of various proton-proton spin-coupling constants was investigated, and the relative energies of the rotamers were determined in the above solvents. In methanolic solution the solute-solvent interactions with the polar head group of AOT were found to be totally isotropic, and the composition of the equilibrium mixture of the rotamers was dependent only on the steric interaction between the two bulky ester groups of the AOT molecule. In less polar or nonpolar solvents like chloroform or isooctane, the energetically most favored rotamer is that in which all of the polar groups are spatially confined in such a way as to lead to a more pronounced amphiphilic character of the AOT molecule. In this way a favorable situation gives rise to maximum hydrogen-bond formation with water molecules and a strong intramolecular ion-dipole interaction with the counterions. This particular property of AOT is believed to enhance its surface activity and, hence, its aggregational and solubilizing tendency in apolar systems.
Introduction Aerosol OT or AOT (sodium bis(2-ethylhexyl) sulfosuccinate) is an important anionic surfactant preferentially used in nonaqueous systems. It forms reversed micelles and thermodynamically stable w/o microemulsions in apolar solvents.l-s The formation and the nature of such molecular aggregations in isooctane and other apolar media have been extensively studied by Eicke et al.316v7and othersal0 from both experimental and theoretical viewpoints. They have shown a clear distinction betwen micellar solutions and microemulsions according to the degree of hydration or the amount of solubilized water, respectively. Shinoda et al.4 have pointed out that the water solubilizations and phase regions of w/o microemulsions, as well as inversed micellar solutions, are distinctly temperature dependent. EickelO has observed that, up to certain amounts, benzene, cyclohexane, or nitrobenzene cause a considerable increase in the maximum amount of solubilized water of a w/o microemulsion (H20-AOT-i-C8HI8) a t a specified temperature, apparently due to the “cosurfactant” properties of these solvents. Reversed micellar systems formed by AOT can solubilize relatively large amounts of water, and the w/o microemulsion consists of droplets with a free water core and a monomolecular surfactant film. The sizes and thermodynamic stabilities of these droplets with water pools are temperature dependent, and above certain temperatures these microemulsion systems become unstable and phase separation takes place. On a molecular level, there exists no satisfactory explanation of the surface-active properties of AOT, particularly in reversed micellar systems, and an up-to-date literature survey reveals that no study has yet been made on whether the above physicochemical characteristics of w/o microemulsion systems of AOT are related to the stereochemical configurations of the AOT molecule. AOT is a diester derivative of succinic acid with two long hydrocarbon chains from the ester groups, the sulfonate and ‘Onleave from the Department of Chemistry, Delhi University, India. 0022-365418 112085-2687$01.25/0
two carbonyl groups remaining within the skeleton of the succinic acid part of the molecule. Micellar aggregates are formed through hydrogen bonds between polar parts of AOT molecules and water. These polar groups are stereochemically directed along the various bonds of the ethane skeleton of the succinic acid part of the molecule and can rotate about the ethanic C-C bond. Thus, if the AOT molecule is brought to a polar-apolar interface (or even if self-aggregationor homoassociation is possible), the polar groups experience an anisotropic solute-solvent interaction, such that the interfacial phenomena in a reversed micellar system would be governed by the composition of various rotational isomers present in the equilibrium mixture of AOT in that system. Under these circumstances it appears worthwhile to study the rotational isomerism of AOT in different solvents where it exists in various aggregational states. For this purpose spin-spin coupling constants and chemical shifts of the three protons bonded to the ethane skeleton of the succinic acid part of the AOT molecule were studied by proton magnetic resonance spectroscopy at various temperatures. The NMR spectra are suitable for this kind of investigations since they are determined by factors like the relative energies of the various rotamers, the extent of hindrance toward internal rotations about the C-C bond, electronegativities of -COOR and -SOs groups, shielding and deshielding (1)K. Shinoda, J. Colloid Interface Sci., 48,281 (1974). (2)K.Shinoda and S. Friberg, Adu. Colloid Interface Sci., 4, 281 (1975). (3)H.F. Eicke, Micellization, Solubilization, Microemulsions [Proc. Int. Symp., 19761,1,429(1977);J. ColloidInterfuce Sci., 59,310 (1976); Pure Appl. Chem., 52,1349(1980);H.F.Eicke and J. Rehak, Helu. Chim. Acta, 59, 2883 (1976). (4)H.Kunieda and H. Shinoda, J. Colloid Interface Sci., 70, 577 (1979). (5)B.Tamamushi and N. Watanabe, Colloid Polym. Sci., 258,174 (1980). (6)H.F.Eicke and H. Christen, Helu. Chim.Acta, 61,2258 (1978). (7)P.Ekwall, L. Mandell, and K. Fontell, J. Colloid Interface Sci., 33,215 (1970). (8)I. Lundstrom and K. Fontell, J . Colloid Interface Sci., 59, 360 (1977). (9)M.Zulauf and H. F. Eicke, J. Phys. Chem., 83,480 (1979). (10)H.F.Eicke, J. Colloid Interface Sci., 68,440 (1979).
0 1981 American Chemical Society
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effects of these substituents, as well as spin-spin interactions of various protons in different isomers. Methanol, chloroform, and isooctane were selected as organic dispersion media in order to vary the degree of solute-solvent interactions of AOT. Experimental Section Materials and Purifications. Sodium bis(Bethylhexy1) sulfosuccinate, i.e., AOT (Fluka AG), of pharmaceuticalgrade purity was additionally purified as follows. Twenty grams of the AOT (treated with active charcoal) was dissolved in 300 mL of methanol, into which 300 mL of petroleum ether (bp 60-80 OC) was added. After the mixture was shaken thoroughly, the petroleum ether phase was separated and the methanol phase was shaken 2 times each with 100 mL of petroleum ether. The methanol phase was then evaporated, and the residue was dissolved in ca. 400 mL of benzene and mixed with ca. 200 mL of cooled water. Finally, the benzene phase was separated and evaporated (yield, ca. 75%), and the solute was dissolved in benzene. The clear benzene solution was then azeotropicallydistilled 2-3 times to remove water. The final product was then evaporated and dried under vacuum (0.01 torr) for 12 h. All of the solvents were dried by distilling over calcium hydride. Residual water of hydration in AOT was determined by the Karl Fischer method. Determination of the Association Number of AOT in Different Soluents. Methanol, chloroform and isooctane were used as solvents for studying the rotational isomerism of AOT. The aggregation numbers of AOT in the respective solutions were determined by vapor-pressure osmometry. N M R Spectra. All spectra were recorded on a 90-MHz Varian EM 390 high-resolutin spectrometer. Tetramethylsilane was added as an internal reference standard for chemical-shift measurements. The instrument was locked at the MelSi frequency for field/frequency stabilization. Spectra were taken in CD30D, CDC13, and isooctane as solvents. The probes were heatedfcooled by nitrogen (preheated/precooled) at the desired temperatures which were controlled within an accuracy of f l "C. Analysis of N M R Spectra. The partial NMR spectra of AOT (AB and X parts) were retraced by digitilizing and replotting on an expanded scale. The doublets, the separation between the outer parts of the triplets, and the separation between the averages of the two pseudoquartets were measured from the splitting of the lines in the spectra on the expanded scales. Further calculations of J M and JBxwere done for the spectra of AOT-methanol systems in which all of the eight distinct lines are available. All of the coupling-constant values and the chemical shifts have been reported in the text in Hz. Analysis of Data. Proton NMR spectra of various substituted succinic acid esters have been investigated by Ericksonll and Brink12in terms of Karplus' ~alculati0ns.l~ It was observed that the substituents have a remarkable effect on the JZ,"coupling constants in the case of monosubstituted succinic acid esters. The data obtained for different solutions of AOT, as reported in the present paper, are based on the assumption that the observed spectra are those of the equilibrium mixtures of various rotational isomers at particular temperatures. The energetically favored configurations of AOT are staggered, and the different rotamers may be represented by the struc(11)L.E.Erickson, J. Am. Chem. Soc., 87,1867 (1965). (12)M.Brink, Tetrahedron, 24, 7005 (1968). (13)M.Karplus, J. Chem. Phys., 30,ll (1959);J.Am. Chem. SOC.,85, 2870 (1963).
Maitra and Eicke COOR
COOR
COOR
Flgure 1. Different conformations of monosubstituted succinic acid esters.
tures shown in Figure 1. In these systems the experimentally observed average resonance frequencies (or chemical shifts) for the ith nucleus are given by the relation and the spin-coupling constants for the spin-spin interaction between the nuclei i and j as where the subscripts refer to rotamers 1-3 and xi designates the mole fraction of the ith rotamer present in the equilibrium mixture. Relative populations of rotamers 1, 2, and 3, and their relative order of energies, can be determined from the temperature dependence of J by using typical vicinal coupling constants for gauche and trans as Jgauche < Jtr,,. Since a temperature increase would be expected to lead to an increased contribution of rotamers with higher energy, a change in the coupling-constant values with temperature would indicate directly whether the rotamers with gauche or trans orientations are stable at lower temperature in a particular AOT-olvent system. Although exact values of the energy differences cannot be derived from the experimental data, it can be shown that the data are consistent with a model which considers whether the steric interaction between the bulky substituent groups or the orientation of the hydrophilic groups is in such a direction so as to confer maximum amphiphilic character to the molecule. This would mean that the amphiphilicity of the molecule is determined by an equilibrium rotamer mixture which corresponds to the most stable state of the AOT-solvent system. Results and Discussion General Characteristics of the Partial Spectra of AOT. The molecular structure of AOT may be represented by an asymmetric succinic acid derivative with three asymmetric carbon atoms, one of which is in the ethane skeleton of the succinic acid part of the molecule. The three energetically favored staggered configurations of the nonequivalent rotamers, represented by the structures shown in Figure 1,exhibit NMR spectra of the ABX type. Because of the presence of the asymmetric carbon atom in
The Journal of Physical Chemistty, Val. 85, No. 18, 1981 2889
Water-Solubilizing Properties of Aerosol OT
h
-10
0
20 Temperature
PcI
40
60
Figure 2. Temperature dependence of coupling constants (JAx JBx) of AOT in different solvents: (A)CDBOD with [H20]/[AOT] = 0.11; (0)CDCI, with [H,O]/[AOT] = 0.11; (A) CDCI, with [H,O]/[AOT] = 0.48; (0)i-C,H,, with [H,O]/[AOT] = 0.48.
the succinic acid part of the molecule, the spectra become even more complicated. To study the rotational isomerism, we had to consider only the partial AI3X spectra.14J5 The partial spectra can be separated into two parts, viz., CHAHB(CO0R) and CHx(S03-Na+)(COOR). The appearance of the spectra of both parts differs with the nature of the solvent. In methanol, AOT exhibits a welldefined partial AB spectrum containing eight distinct lines of two pseudoquartets. The splitting of each doublet within these pseudoquartets is defined by I JHH I l6 which can be easily recognized on symmetry grountim in methanol, the IJkEl coupling constant was found to have a temperature-independent value of 18.25 f 0.20 Hz in the temperature range between -10 and +60 “C. The separation between the average of the pseudoquartets, defined + J B X ~was , ~ measured ~ at different temperaas ‘/zIJAx tures. A plot of lJ- + JBxl vs. temperature, shown in Figure 2, demonstrates that with increasing temperature the average coupling constants of the equilibrium mixture of rotamers decrease. The partial spectra of AOT in chloroform solution are more complicated than in methanol. The spectra exhibit a strong doublet in the AB part and a distinct triplet in the X part of the partial spectra. The resonances of the CHx(SO;Na+)(COOR) part, which appear downfield with respect to the CHAHB(CO0R) part of the spectra, consist of an asymmetric triplet ( 1:2:1) with the center line as an unresolved doublet. The separation of the two outer lines of this triplet is equal to ~ J A X+ JBX1,l6 which was shown to be about twice the separation of the doublet in the AB partial spectrum. The observed spin-couplingconstant values lJAx + JBxl for AOT in chloroform at different temperatures have been plotted as shown in Figure 2. The partial spectra of AOT in isooctane, however, exhibit only broad and unresolved peaks, and, hence, the triplets and pseudoquartets are not distinctly recognizable, particularly in the lower-temperature range. A partial plot of IJobsdl vs. temperature in the higher-temperature range shows that the AOT in isooctane system has the same trend as in chloroform; Le., observed coupling-constant values increase with increasing temperatures. Because of different conformations, the rotamers have the possibility of both cis and trans couplings between HA N
(14)H. S. Gutowsky, G. G. Belford, and P. E. McMahon, J. Chem. Phys., 36, 3353 (1962); (15) S. Sternell in “Dynamic Nuclear Magnetic Resonance SDectroscopv”, L. A. Jackman and F. A. Cotton, Eds., Academic Press, New York,-i975, pp 163. (16) H. S. Gutowsky, “Techniques of Organic Chemistry”, Vol. 1, part IV, 3rd ed., A. Weissberger, Ed., Interscience Publishers, New York, 1960, Chapter XLI; J. A. Pople, W. G. Schneider, and H. J. Bernstein, “High Resolution Nuclear Magnetic Resonance”. McGraw-Hill. New York. 1959. Chapter 6, p 134.
I
, -10
, 0
,
.
,
10 20 30 Temperoture
,
,
10
50
L“cI
,
c
60
Flgure 3. Temperature dependence of coupling constants (JM, Jsx) of AOT in methanol.
and Hx as well as between HB and Hx. Because of the asymmetry of the molecule, these coupling constants are not identical; i.e., # Accordingly, the coupling constant of rotamer 2 is different from that of rotamer 3, and both of them are unequal to those of rotamer 1. This is also true because of the bond angles and other aspects of the electronic structures of the rotamers which, in principle, would cause small differences in the overall coupling-constantvalues, i.e., Jl” # Jzm # J3”. Energies and Configurationsof the AOT Rotamers in Methanolic Solution. In the temperature range between -10 and +60 “C, the partial AB spectra of AOT in methanol exhibit eight distinct lines which were numbered consecutivelyfrom low to high field, and the doublets were recognized as 5-7, 8-11, 6-9, and 10-12. The central splittings of the AB quartets and of the outer pair of X lines were measured as a function of temperature. Jm and J B X were calculated from the splittings of the AB quartets, and the values were plotted as a function of temperature as shown in Figure 3. The coupling-constant values for the AOT-methanol system decrease with the rise of temperature. If one uses the order of the typical vicinal coupling constants for gauche and trans as JgaUhe C Jt,,, the decreasing trend of J A X and J B X with increasing temperature indicates that the energetically least-favored configuration is rotamer 1 in the AOT-methanol system, showing that the energy of the rotamer in this system is governed by the steric hindrance between the two bulky ester groups and the sulfonic group. Since the steric interaction is expected to be minimal in rotamer 3 because of the trans orientation of the two bulky ester groups, it is quite reasonable to expect that, in the AOT-methanol system, rotamer 3 is the energetically most favored configuration. In a polar solvent like methanol, the AOT molecule forms hydrogen bonds between its polar groups and the alcohol molecules. In this solvent AOT does not exhibit any amphiphilic character, and thus the rotations of the two ester and the sulfonate groups about the ethanic C-C bond are isotropic and they experience an isotropic solute-solvent interaction during rotation. Under these circumstances the energy of a rotamer is governed by the steric hindrance between the substituent groups, particularly the two bulky ester groups. Energies and Configurationsof the AOT Rotamers in ChloroformSolution. Attempts to calculate the JAx v d J B X values from the observed partial spectra of AOT in chloroform solution have not been particularly successful because some of the eight lines in the partial AB spectra are completely overlapping and indistinguishable. If one uses typical vicinal couplings for gauche and trans as Jgauhe C Jtr,,which follow the minimum values of J a + J B T for rotamer 1, the trend in the change of the equilibrium
Jg$)vic JFx)vic.
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The Journal of Physical Chemistry, Vol. 85, No. 18, 1981
distribution of rotamers with temperature can, however, be predicted from the change of the sum of JAx and JBx values. The observed increase for JAx + JBxwith temperature implies that rotamer 1 has the lowest energy. Moreover, from Figure 2 it is seen that, with increasing temperature, the coupling constants of AOT decrease in methanolic solution but increase in chloroform. From the above data it is apparent that the factors which determine the relative stabilities of various rotamers are different in chloroform compared to those in methanol. While rotamer 3 is the energetically most favored configuration in methanolic solution, it is the least favored in chloroform. In the latter solvent, the polar groups of AOT do not experience any appreciable interaction with chloroform molecules but can interact very strongly with water molecules, viz., hydrogen bonds, and with counterions via ion-dipole interactions. Very feeble interactions between the polar groups and chloroform and strong interactions between the intramolecular polar groups and water molecules have directional characteristics with respect to the orientations of polar groups of the AOT molecule. Because of this fact, the polar groups of the AOT molecule during the rotation about the ethanic C-C bond experience an anisotropic nucleophilic interaction in the chloroform system. It appears that such an anisotropic interaction becomes highly favorable if all of the polar groups of the AOT molecule are spatially confined in such a way as to lead to a more pronounced amphiphilic character of the molecule with maximum separation of the polar head group and the apolar tail. These favorable interactions within the polar groups, and with water molecules, overcome the steric hindrance between the substituents so that the corresponding stereochemical configuration becomes energetically most favored. This is possible with rotamer 1 in which both the carbonyl and sulfonate groups are localized in such a way in space that the typical amphiphilic properties of AOT molecules are retained. Within the molecule, the polar groups of appropriate functionality (sulfonate and carbonyl groups) behave as very reactive nucleophiles or proton transfer agents. Hence, in a micellar system with a negligible amount of water, attainment of the above configuration takes place not only via strong hydrogen-bond formation with water molecules but also via intramolecular interactions between the polar groups and counterions. This particular configuration enhances hydrogen-bond formation, thereby increasing the watersolubilizing capacity of AOT. Although it was not possible to measure the coupling constants of AOT from its spectra in isooctane throughout the entire temperature range studied, it appears from the partial plot of some of the measurable points (cf. Figure 2) that AOT in isooctane behaves in the same manner as in chloroform. Perhaps because of the more pronounced apolar nature of the solvent, rotamer 1 of AOT is more populated in isooctane than in chloroform. This leads to a stronger micellar network structure of AOT in isooctane and its ability to dissolve a larger amount of water. Rotation of AOT in w / o Microemulsions. With growing amounts of water a transition takes place from the highly structural micellar network to microemulsion aggregates containing large water P O O ~ S . ~The sizes of the microemulsion aggregates are very sensitive to temperature, since their behavior is now determined by the surface free energy of the AOT surfactant monolayer. The AOT molecules in the microemulsion systems (w/o) are relatively flexible, and the polar groups are more easily susceptible to free rotation about the ethanic C-C bond. If the equilibrium distribution of AOT rotamers is respon-
Maltra and Eicke
sible for hydrogen bonding between water molecules and polar groups of AOT, it is interesting to study the water binding of AOT with increasing temperatures at various [H20]/[AOT]ratios. The possibility of applying proton magnetic resonance measurements to study hydrogen-bond formation between water and polar groups of AOT in w/o microemulsions became apparent from the fact that the signals of water protons in the above systems are temperature dependent. It is customary to consider four different contributions to the chemical shifts, viz., the van der Waals forces between the solute and solvent molecules (vJ, electric fields caused by the dipole moments of the dissolved molecules (Q), hydrogen-bond formation due to solute-solvent interaction (uH), and the effect due to molecules with highly anisotropic magnetic susceptibilities (v,).16 Hydrogen-bonded solute-solvent complexes yield a pronounced shift to lower field (vH).16 A temperature dependence of proton resonance signals may be observed if there are alternative molecular states with energy gaps of the order of kT. Since water forms strong hydrogen bridges with the polar head groups of AOT, these water protons should experience different magnetic shieldings for the bound and free states of water. The shieldings of the bound states depend on the nature and mole fractions of the various rotamers of AOT present in the equilibrium mixture. Since a change of temperature would alter the populations of the various rotamers of AOT, the chemical shift of water protons would change correspondingly. In a w/o microemulsion system, water is distributed in a thermodynamic equilibrium between the aggregates and the bulk oil phase. If the chemical shifts of the water protons of these two different states are not very far from each other, and the exchange of protons between these two states of water is rapid, then the observed chemical shift can be represented by the equation = xmVm + xoilVoi1
(3) where the subscript m designates the aggregational pseudophase. Within an aggregational droplet with a core of water, the water is distributed in bound and free states which are responsible for the average chemical shifts of water protons as = xbVb + xfvf (4) where the subscripts b and f designate bound and free water, respectively. Bound water molecules are distributed among various rotamers in the equilibrium mixture of AOT, and thus the chemical shifts of water protons bound to AOT may be described by the equation Vb = XIVbl + x2Vb2 + x3vb3 (5) where Vbr designates the chemical shift of water protons bound to the ith rotamer shown in figure 1. Combining eq 3-5 one obtains Vobsd = xm{xfVf + xb(x1Vbl + x2VbZ + x3Vb3)) + xoilVoil (6) From eq 6, it is apparent that the observed chemical shift is directly related to the mole fractions of the rotamers x i present in a particular equilibrium mixture of AOT. Rotamer 1 has a favorable configuration for maximum hydrogen bonding, but with increasing temperature the equilibrium is shifted toward rotamers 2 and 3. Hence, hydrogen bonds are loosened, and the chemical shifts are expected to appear at higher fields. Gradual upfield shifts of the water proton resonance with increasing temperature depends, thus, upon the rotation of the polar groups about the ethanic C-C bond. From the slopes of the temperature Vobsd
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 compositions in 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).
-
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).
-
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). 87,972 (1965). (3)S. W.Benson, J. Am. Chem. SOC., (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
