Singlet Oxygen Kinetics In Aqueous Micellar Dispersions
at a rate slower than that for BsH,. Hence our original characterization of the (BH), producing reaction being a "faster process" is probably misleading. It is unclear whether or not BBH, is a precursor of (BH),. We found that, by appropriately chopping or pulsing the CW laser, (BH),production could be maximized or minimized (stopped). "Red shifting" was apparent in the (BH), production, but could be explained in terms of simple gas heating. The chopping technique which results in a combination of temperature control and reagent mixing in a phase homogeneous system may have application in thermally steering other reaction systems. Utilizing the unique phase homogeneous flash heating property of the laser, we were able to perform an energy titration and obtain information concerning heating and heat loss for this reactive system by application of a simple, but novel model. To our knowledge this is the first attempt to separate volume- and surface-dependent contributions. Acknowledgment. C.R. expresses his gratitude to the
NSF for the Faculty Development Grant. C.R. also expresses his appreciation to the LIC group in the High Energy Laser Laboratory at Redstone Arsenal, Redstone, Alabama, especially Drs. Richard Hartman and George Tanton, for their cooperation. References and Notes (1) 1977-1978 National Science Foundation Faculty Development Grantee. (2) C. Riley, S. Shatas, and V. Arkie, J. Am. Chem. Soc., 100, 658 (1978). (3) H. R. Bachmann, H. Noth, R. Rinck, and K. L. Kompa, Chem. Phys. Lett., 29, 627 (1974).
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R. Rinck, Ph.D. Disse&tion, Ludwig-Maxilllans-Universitat, Mirnchen, 1976. S. Shatas, D. Gregory, R. Shatas, and C.Riley, Inorg. Chem., 17, 163 (1978). J. Tardieu de Maieissye, F. Lempereur, C.Marsai, end R. K. Ben-Aim, Chem. Phys. Lett., 42, 46 1976. R. N. Zitter and B. F. Koster. J. Am. Ghem. Soc.. 99, 6491 11977). H. R. Bachmann, R. Rinck, H. Noth, and K. L. Kompa, Ghem: F'bys. Lett., 45, 169 (1977). H. Brunet and M. Perez, J. Mol. Spectrosc., 29, 472 (1969). A. V. Nowak and J. L. Lyman, J. Quant. Spectrosc, Radht. Transfer, 15, 945 (1975). C. Riley, R. Shatas, and L. Opp, Inorg. Chem., 18, 460 (1979). R. D. Present, "Klnetic Theory of Gases", McGraw-Hill, New York, 1958, pp 39-42. The Fourier heat transport equation Is generally presented in the form
. .
k at
Solution of this equation maps out temperature contours. Our expression can be generated from this equation through application of the divergence theorem. e.g., I.V. Sololnikoff and R. M. Redheffer, "Mathematics of Physics and Modern Engineering", 2nd ed, McGraw-Hili, New York, N.Y., 1966, pp 423-425. We are interested in surface and volume contributions and not temperature. The reaction temperature required to produce just enough polymer for viewing would be expected to be pressure dependent. For a first-order B,H8 pressure dependence, a factor of 8 exists in the pressure extremes of the experiments. The greater E , (activation energy for (BH), formation) the smaiier the A Trequired to overcome the pressure limits. However, the greater E, the larger Trequired to initiate the process. Ea's of the order of 200 K/J would require only a A T o f 20-40 K which is less than 10% of the average temperature we estimated for reaction. The pulse length of Figure 1 would be expected to increase with decreasing pressure if inAiation temperature variation were significant. We found no upturn for pressures as low as 25 torr. H. L. Lona, Proa. Inora. Chem., 15, 1 (1972). S. W. BeGon, "iherm&hemicai Kinetics"; 2nd &, Wiley, New York, 1976, p 298.
Laser Photolysis Studies of Singlet Molecular Oxygen in Aqueous Micellar Dispersions Barbara A. Llndig and Michael A. J. Rodgers" Department of Chemistry, and Center for Fast Kinetics Research, University of Texas at Austin, Austin, Texas 78712 (Received January 4, 1979) Publication costs assisted by the National Institutes of Health
The kinetics of singlet oxygen in aqueous (DzO and HzO) micellar systems were examined using laser flash photolysis with 1,3-diphenylisobenzofuran(DPBF) as the reactive monitor. The use of two different sensitizer types (2-acetonaphthone, solubilized in the interior of the micelles, and methylene blue, present in bulk aqueous phase) demonstrated that both the natural decay rate of lo2* and its bimolecular rate constant for reaction with DPBF are insensitive to the site of singlet oxygen production in the micellar solution. Singlet oxygen lifetimes in solutions of ionic surfactant (sodium dodecyl sulfate (SDS), cetyl trimethylammonium bromide (CTAB), and sodium laurate) were longer than any values previously reported in DzO: 53 f 5 p s . This value is proposed as a minimum value of the singlet oxygen lifetime in DzO. Lifetimes measured in nonionic surfactant solutions (Brij 35, Igepal CO 630, and Igepal CO 660) were considerably shorter: 21-26 ps. This effect is probably due to the loss of electronic excitation of lo2*(lAg) to vibrational modes of the terminal hydroxyl groups of these nonionic surfactants. This quenching action appears to be related to the aggregation of the surfactant in aqueous media, since quenching by these surfactants was not observed in organic solvents. The bimolecular rate constant for reaction of singlet oxygen with DPBF ( k , ) was approximately 6.5 X los L mol-' s-l for the cationic surfactant CTAB and the three nonionic surfactants. However, h, was found to be approximately 60% higher in the two anionic surfactants SDS and sodium laurate. The lifetime of singlet oxygen in surfactant-H20 solution was estimated by extrapolation from H20-Dz0 mixtures. The values obtained for two surfactants were 4.0 (CTAB) and 3.5 p s (Igepal CO 630). Introduction In recent years considerable attention has been focussed on the solution chemistry of the lowest excited state of
* Author to whom correspondence should be addressed at the Center for Fast Kinetics Research.
molecular oxygen, 02*(lAg)* Stemming from the detailed observations of Khan and Kashal on the red chemiluminescence accompanying the reaction between hydrogen peroxide and hypochlorite ions in aqueous solution and the reports of Foote and wexler2 that such a system caused various organic substrates to be oxidized in an analogous
0022-3654/79/2083-1683$01.00/0 0 1979 American Chemical Society
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way to dye-sensitized photooxidation, an era of tremendous activity in the study of 02*('A,) opened. The result of this activity has been the accrual of a body of knowledge concerning the kinetic and reactive properties of singlet oxygen in chemical, biochemical, and cellular environments. Several thorough reviews of all these aspects have appeared3-12over the past 10 years. The forbidden nature of the 32, transition confers a long radiative lifetime to the 'Ag state in gaseous phase13 (2700 s). The natural lifetime in condensed phase, however, is shortened to the 10"-10-3-s range depending on the medium. This is due to the ability of the IA, state to interact with solvent vibrational modes in an energy-loss process14 with an efficiency governed by the absorption coefficient of the medium at the 02*(lAJ energy level (ca. 1 eV). This effect has been shown most clearly by the dramatic differences between the natural lifetimes in protiated and deuterated s01vents.l~These differences are especially large in water where 02*(la,)lifetimes in H 2 0 and D20 are separated by approximately an order of magnitude. Although this ratio of lifetimes is widely accepted there remains confusion over the absolute values in aqueous systems based on light and heavy water.14-16 It is this question which is addressed by the work presented here. Our approach has been to generate 02*(lAg) in aqueous systems by energy transfer from triplet states of photoexcited sensitizer molecules and follow its disappearance by spectrophotometric observation of the time evolution of the loss of optical absorption of a monitor species. Following others14-19 we have used 1,3-diphenylisobenzofuran (DPBF) as the monitor. The insolubility of the furan and one sensitizer in water led us to utilize surfactant micelles as a means of solubilizing reactants, a method originally developed20for steady-state experiments. This technique has been used by others21i22and extended to the time-resolved situation.15J6 The micellar medium has advantages beyond those of practicality; the combination of hydrophilic and hydrophobic elements in the amphipathic surfactant aggregates has been widely regarded as an accessible working model of biological assemblies. +
8. A. Lindig and M. A. J. Rodgers
I I
1
SEGUENSE
'$
Experimental Section The following compounds were commercial products and used as supplied: methylene blue (J. T. Baker), 1,3-diphenylisobenzofuran (Aldrich), 2-acetonaphthone (Aldrich), sodium dodecyl sulfate (BDH), cetyl trimethylammonium bromide (Sigma), Igepals CO 630 and CO 660 (GAF), and Brij 35 (Aldrich). Sodium laurate was prepared and donated by the laboratory of W. H. Wade, Department of Chemistry. D 2 0 was supplied by Merck Sharpe and Dohme (99.7 atom %), Sigma (99.8 atom %), or Sci-graphics (99.8 atom %). Light water was double distilled, once from potassium permanganate. Toluene was Fisher, purified, and methanol was MCB, spectroquality. Individual stock solutions of DPBF and 2-acetonaphthone in aqueous surfactant were prepared by stirring and warming the solids in stock surfactant solution. Solutions used in these experiments were prepared by diluting aliquots of these solutions with the stock surfactant solution. Sensitizer concentrations were made sufficiently high to minimize excitation of DPBF. Typically absorbances between 1.0 and 2.0 at the excitation wavelength were used. DPBF concentration was determined by measurement of absorbance at 415 nm immediately prior to photolysis, using a Zeiss PM Q11 spectrophotometer. The absorption coefficient of DPBF at 415 nm was taken to be 2.2 X lo4L mol-l cm-l in all surfactant
II
II
Flgure 1. Schematic of laser photolysis experiment: SS, slow shutter: FS, programmable fast shutter: F, filter; L, lens; S, sample: A, attenuator; MC, Monochromator; P, photomultiplier tube; D, photodiode energy monitor; M, mirror: G, quartz slide.
solutions. Laser flash photolysis was performed with a N2-laser built at CFKR and a frequency-doubled Quanta-Ray Nd/YAG laser. The former was used to excite acetonaphthone at 337 nm; the latter to excite methylene blue at 530 nm. Transient absorptions were monitored at right angles in both cases with an Oriel xenon lamp, a programmable shutter, a Bausch and Lomb monochromator, and a Hamamatsu Corp. R928 or RCA IP28 photomultiplier tube. The output waveforms were digitized by a Biomation 8100 digitizer and read into a DEC PDP 11/34 computer for kinetic analysis and storage (Figure 1). The photolysis cell had a monitoring path length of 0.5 cm. A filter cutting off below 400 nm was interposed between the lamp and sample to minimize direct photolysis of DPBF by the monitoring lamp during the time (less than 10 ms) that the programmable shutter was open. In experiments using 530-nm excitation, a filter cutting off above 500 nm was added. In all experiments with DPBF, photolysis by the lamp was negligible with these precautions, as shown by the fact that the transmitted light intensity at 415 nm showed no significant change during the period the shutter was open without the laser switched on. Neutral density filters were used to attenuate the laser beam in order to limit the laser-induced photobleaching of DPBF to less than 10% of its initial absorbance in the excitation volume. All solutions were prepared and all experiments conducted in darkened rooms. Solutions were photolyzed in contact with room air a t room temperature except where stated. Each shot was initiated by a pulse from the computer to the trigger sequence generator, which, in turn, triggered the shutters, the digitizer, and the laser. Shot-to-shot normalization of laser intensity was accomplished by scattering a portion of the laser beam into a photodiode detector for measurement of intensity on each shot. The amplified output was read by a digital voltmeter and read into computer memory. The solution was replenished for each shot. Computer fits to the decay curves were performed by a least-squares analysis program on the computer.
Results Anionic Surfactants. (a) Acetonaphthone as Sensitizer. A D 2 0 solution containing SDS (0.1 M) micelles was prepared with 2-acetonaphthone solubilized in the micellar aggregates such that the ketone absorbance was 2.0 at 337 nm (1.1X M). Laser photoexcitation was carried out under a constant stream of N2 gas and a transient absorption spectrum was recorded point-by-point over the
The Journal of Physical Chemistry, Vol. 83, No. 13, 1979
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TABLE I : Kinetic Parameters for Singlet Oxygen in Micellar D,O Solutionn
a
surfactant sodium dodecyl sulfate
type anionic
sodium laurate cetyl trimethylammonium bromide
anionic cationic
Igepal C0630 Igepal C0660
nonionic nonionic
Brij 35
nonionic
All surfactants 0.1 M, except sodium laurate, 0.05 M.
--
02*('A,)
+M
kr 4
-M
where S represents a sensitizer, M a micellized monitor molecule (DPBF), and Q an unspecified quencher which may or may not be in micellar phase. When the percentage removal of DPBF is limited to small amounts (in all our experiments the incident laser intensity was adjusted to ensure less than 10% conversion) the following rate equation is readily deduced: -d[M]/dt = k,[M] [O,*] exp(-k't) where k' = k d + Iz,[M] + k,[Q]. Thus an exponential bleaching rate is anticipated and measurements of k'with either [&I or [MI constant allows kd and k, to be evaluated. The singlet oxygen lifetime, rA,is given by l / k d . Figure
7 ,p s
h,, L m o l - ' s - '
1.86 x 104 53.7 10.9 x 1.85 X l o 4 54.0 10.0 x 2.07 x 104 48.3 9.84X 57.4 6.22 X 1 - 7 4 x 104 1.86 X I O 4 53.7 6.74 X 4.20 x 104 24.0 6.28 X 4.65 X l o 4 21.5 6.48 X 4.26 X l o 4 23.5 6.63 X 3.78 X l o 4 26.4 6.61 X ACN = 2-acetonaphthone, MB = methylene blue.
range 370-500 nm. This spectrum showed a A,, of 440 nm in reasonable agreement with that reported for the T-T* triplet of 2-acetonaphthone in benzene.23 The rates of decay of the absorption at 430 nm were measured in nitrogen-saturated, air-saturated, and oxygen-saturated solutions. The decay was kinetically first order in all cases and proportional to oxygen concentration. The bimolecular rate constant for oxygen quenching was measured at 2.0 X lo9L mol-l s-l. This compares favorably with the values of 1.5 X lo9 and 1.6 X lo9 L mol-l s-l measured in homogeneous benzene solution^^^^^ allowing the conclusion that the introduction of hydrophilic and hydrophobic regions produces no unusual kinetic features. D20solutions containing SDS (0.1 M), 2-acetonaphthone (1.1X M), and diphenylisobenzofuran in the range 1.0-7.0 X M were prepared and photoexcited at 337 nm. The transient absorption spectrum of a typical solution is shown in Figure 2. The spectrum of the initial absorbance (curve A) is identical with that of the triplet-triplet spectrum of acetonaphthone in the absence of DPBF. The bleaching (negative absorbance) portion of the spectrum (curves B-D) is identical with the ground state absorption spectrum of DPBF. This bleaching of the absorption band of DPBF was complete after several tens of microseconds. The rate of bleaching was exponential with an observed rate constant (k? which was proportional to the concentration of furan. This parallels behavior in homogeneous systems where bleaching of DPBF by 02* (la,) has been investigated and is consistent with the following scheme for generation and decay of singlet oxygen in micellar media: hu s+ IS* + 3s* 3S* + O2 (32,) S + 02*(lA,) 3s* s
k , , s-'
sensitizerb ACN MB ACN ACN MB ACN ACN MB ACN
10' 10' 10' 10' 10' 10' 10' 10' 10'
0.12
0.08
0.04
c
VI
I
380
~
~ 400
l
~ 420
~ 440
I 460
I
I 480
I 500
WAVELENGTH ( n m )
Flgure 2. Transient absorption spectrum of a solution containing 1.1 X M DPBF, in 0.1 M SDS M 2-acetonaphthone, and 2.8 X in D,O, excited by a N, laser. Tlme after pulse: A, 1.75 ps; B, 5.63 ps; C,17.4 ps; D, 32.5 ks. Each shot is corrected for variation in laser intensity.
3a shows a representative experimental curve of absorbance a t 415 nm vs. time, together with the best fit obtained by computer fitting the log(absorbance) vs. time plot by a least-squares technique (Figure 3b). A plot of k 'against diphenylisobenzofuran concentration for 0.1 M SDS in D20with 2-acetonaphthone as sensitizer is shown in Figure 4. Values of kd and k, were obtained from the least-squares fit to the data (Table I). Exactly analogous experiments were performed with sodium laurate (0.05 M) in place of sodium dodecyl sulfate as solubilizing agent. Again excellent first-order behavior and linear k'vs. DPBF concentration plots were obtained. Kinetic parameters are presented in Table I. ( b ) Methylene Blue as Sensitizer. To vary the location of the sensitizer from the lipid region of amphipathic aggregates to the aqueous phase we performed experiments with the water-soluble dye methylene blue as sensitizer (2 X M, absorbance at 530 nm = 1.0) and with exciting pulses of 530-nm light from a frequency-doubled Nd/YAG laser. The k' VB. DPBF concentration plot for SDS is shown in Figure 4 and k d and k, values are collected in Table I. Cationic Surfactants. The experiments described above were repeated with cetyl trimethylammonium bromide (CTAB), a surfactant with a cationic head group, as solubilizing agent, at 0.1 M. Both 2-acetonaphthone and methylene blue were utilized as sensitizers. Representative kinetic data for methylene blue are presented in Figure 5 and the relevant parameters collected in Table I.
L
l
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B. A. Lindig and M. A. J. Rodgers
Flgure 3. Experimental curve of absorbance monitored at 415 nm for a solution c 1 M SDS in D20. [DPBF] = 1.84 X M, 2acetonaphthonel = 1.1 X lop3M. Excitation at 337 nm. (a) Experimental decay curve. Dots are experimental points and solid line is least-squares fit. (b) log (absorbance) vs. time for curve (a), Solid line is least-squares fit.
C
*
’I-
P
Q
I
s DS
-.-A.
1
2
3
4
5
lo5 CDPBF], 4
-
--X
1
2
3
4
2-wcetonaphthone
6
7
8
6
7
8
9
M
Flgure 5. First-order rate constant for bleaching of DPBF, k’, as a function of DPBF concentration for three surfactants (Igepal CO 660, SDS, and CTAB), each at 0.1 M in D20, with methylene blue as sensitizer. Each point is the mean of at least six individual determinations. The error bars indicate the overall spread of measured values.
methylene blue
5
- CTAB
9
IO5 [DPBFI,M
Flgure 4. First-order rate constant for bleaching of DPBF, k’, as a function of DPBF concentration, with %acetonaphthone and methylene blue as sensitizers, for 0.1 M SDS in D20. Each point is the mean of at least six individual determinations. The error bars indicate the overall spread of measured values.
Nonionic Surfactants. To fully examine the effect of micelle charge on the dynamic properties of singlet oxygen we performed a series of experiments with Igepal CO 630 (polyoxyethylene(9)-nonylphenol), Igepal CO 660 (polyoxyethylene(10)-nonylphenol), and Brij 35 (polyoxyethylene(35)-dodecanol) as solubilizing materials, each at 0.1 M. The concentration of furan monitor was varied over M and both 2-acetonaphthone and the range 1.0-8.0 X methylene blue were examined as sensitizers with the appropriate excitation wavelengths. The results obtained
for CO 660 with methylene blue as sensitizer are shown in Figure 4, and Table I presents data for all three surfactants. Mixtures of D 2 0 and H20.By using the time-resolved techniques described here we found it difficult to obtain precise kinetic data for H20-based solutions since the natural lifetime of 02*(la,) in H20 is much shorter than in D20,causing the energy transfer from sensitizer triplet to dissolved oxygen to be close to rate determining in solutions under O2 at 1 atm. Our method requires the 02* (la,) reaction to be the overall rate-limiting process. We decided to carry out experiments with two of the above surfactants (CTAB and Igepal CO 630) in HzO-DzO mixtures in which the H20:D20 ratio was varied. 2Acetonaphthone was used as sensitizer (337-nm excitation) and the DPBF monitor concentration was held constant. The values of k’were plotted against D 2 0 concentration
Singlet Oxygen Kinetics in Aqueous Micellar Dispersions
1
"
0.7
"
"
l
0.8 0.9 MOLE FRACTION D20
1.0
Figure 6, First-order rate constant for bleaching of DPBF, k', as a function of D20 mole fraction in H,O-D20 mixtures (constant [DPBF]). M; for Each surfactant is 0.1 M. For CTAB, [DPBF] = 4.77 X M. Each point is the mean of at least CO 630, [DPBF] = 3.95 X six individual determinations. The error bars indicate the overall spread of measured values.
(asmole fraction) and are shown in Figure 6, together with the least-squares fit to the data. A value for k' in surfactant-HzO was obtained by extrapolating the leastsquares fit to 100% HzO. The singlet oxygen lifetime, 7 p (= l / k d ) , was calculated by k d = k'- k,[DPBF], where the value used for k, was that listed in Table I for each surfactant. The resulting lifetime values were 4.0 (CTAB) and 3.5 ps (CO 630). Discussion Consideration of the data in Table I shows the following: (a) The values of 7 p obtained from ionic micellar solutions fall between 48 and 57 ps. This range can probably be accounted for by random error and we therefore conclude that in such systems the natural lifetime of Oz* (la,) is independent of ionic surfactant type and sensitizer and equal to 53 f 5 ps (mean). (b) These 7 p values are significantly larger than those reported earlier.l4-I6 (c) The values of 78 in all three nonionic surfactants are independent of surfactant and sensitizer, but are significantly lower than the values measured for the ionic micellar systems. (d) The bimolecular rate constants for the reaction between 02*(la,)and DPBF are very nearly constant for CTAB and the nonionic surfactants. For SDS and sodium laurate, both anionic surfactants, K, is significantly higher. These observations are considered in order. (a)and ( b ) 7 p in Ionic Micellar Systems. A preliminary publication from this laboratory15reported that r p for SDS micelles in DzO was near 30 ps, in agreement with a contemporary determination by Matheson, King, and Lee.16 This value was itself larger than the 20 ps obtained by Merkel and Kearns14 by extrapolation from a value measured for neat methanol and a 5050 vol % DzO: methanol mixture. We now understand why the 3 0 - h ~ value obtained by Gorman and RodgerP differed from our current value; NMR determination of the proton content of the DzO used at that time showed it to be contaminated by nearly 10% (volume) HzO which, according to our experiments with the H20-Dz0 mixtures, would fully account for the discrepancy. In fact, using the contaminated D20 batch as solvent we were able to reproduce their reported 7 A value. (DzOused in the present work was found to contain up to 0.7% vol HzO after exposure to air
The Journal of Physical Chemistry, Vol. 83, No. 13, 1979
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and transfers between glass vessels.) These considerations are unhelpful in understanding the 32 ps found by Matheson, King, and Lee.16 However, close inspection of their k'vs. DPBF concentration plot shows only a limited range of furan concentration was studied (up to 2.5 X M). We submit that the large intrinsic imprecision of the bleaching experiments (see Figure 2 of ref 15 and Figures 4 and 5 of this work) requires exhaustive replicate experiments over as large a concentration range as possible to closely define the intercept of the k' against [DPBF] plots. We believe that the present work, in which each experimental point on the k'plots was the mean of at least six separate determinations, gives the closest approach to the value of 7 p for DzO solutions of ionic micelles. Whether the mean r p (53 f 5 ps) determined here is applicable to micelle-free DzO is not certain. It is, however, likely to represent a minimum value of that quantity since it is unlikely that diffusion through a micellar medium is able to protect Oz* (la,)from natural decay (singlet oxygen in hydrocarbon solvents has a natural lifetime much lower than 50 ps). The finding that the singlet oxygen lifetime is invariant over the SDS concentration range 0.02-0.45 M in D 2 0 further supports this view.15J6 The use of methylene blue as sensitizer shows that 7 p is insensitive to the site of production. Methylene blue is water-soluble and lipid-insoluble and will therefore reside in the dispersion medium in a micellar system. Since it is cationic it will be located in bulk water phase in the presence of cationic micelles and probably within the Stern layer of anionic micelles. That 7 p is independent of such considerations of whether the sensitizer is lipid-bound or water-bound infers that we are monitoring the intrinsic lifetime of 02*(la,)in a micellar environment and that it has no dependence on any fortuitous pairings of sensitizer and monitor at a single micelle. On the assumption that the diffusion coefficient of 02*is not different from that of the ground state species it was earlier calculatedz0 that the rate constant for SDS micelle-02* encounter is M micelles the 5.8 X 1O1O L mol-l s-l. Thus at 2 X frequency of encounters is near 1.2 X lo8 s-l. Thus during its natural lifetime a singlet oxygen molecule can encounter, on the average, some 6000 micelles. Therefore, if ionic micelles (e.g., SDS) do quench Oz* (l$) and if r p for DzO is in fact longer than the 53 ps reported here for the micellar case, then the quenching efficiency is very low. It is interesting to note here that the values of 7 p for CTAB in DzO and HzO are in the ratio of 13:l as required from measured relative rate constant ratios (0values) obtained from continuous illumination studies.z5 (c) rAin Nonionic Micellar Systems. As Table I shows, the values of 7 p measured with DPBF dispersed in the nonionic aggregates (Igepals and Brij 35) are less than 50% of the values in the ionic surfactants and that they are independent of sensitizer type. Since HzO is one material which could contribute to such a lowering (cf. Figure 6), we measured the water content of these nonionic surface-active materials by Karl Fischer titration. In all cases less than 2% (by weight) HzO was found in the surfactant. The gravimetric amount of surfactant in the experimental solutions was thus far too small for the HzO content to be responsible for the low 7 p values. A possible quenching by other unknown impurities was examined by performing experiments in which high concentrations (0.1 M) of Igepal CO 660 were added to toluene for a determination. The kinetics of DPBF bleaching were unaffected by the added surfactant. A similar experiment was carried out with high Brij 35 concentrations (0.08 M) in methanol, with the same result. These experiments indicate that when the sur-
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The Journal of fbysical Chemistry, Vol. 83, No. 13, 7979
factants (and impurities) are dispersed more homogeneously than in water, no quenching occurs. Thus the quenching efficiency of these surfactants is enhanced by the aqueous micellar system. It is probable that the observed low values are due to the high local concentrations of terminal alcohol groups in micelles of these nonionic surfactants. Merkel and Kearns have reported14 that solvent quenching of 02* (la,)results from excitation transfer from electronic levels of O2 to vibrational modes of the solvent and is greatest where the solvent has large molar extinction coefficient at 1269 nm, which corresponds to the energy of the 32, lAg transition in oxygen. All the nonionics used here have significant absorbances in this region and it may be conjectured that a diffusing singlet oxygen species, exploring large numbers of micelles during its natural lifetime, therefore spends significant periods in the neighborhood of pools of surfactant to which energy transfer can occur. These considerations lead us to speculate that the lack of quenching by the nonionic surfactants in organic media is a consequence of the differences in aggregation between the aqueous and organic systems. Either the surfactants are monomerically dispersed or if supramolecular assemblies are present then their quenching regions are less accessible. The aggregation number of Brij 35 has been reported to be 4026and the critical micelle concentration is 6-9 X M.26 Thus at 0.1 M Brij 35, the concentration of micelles is 2.5 X M. Assuming that the value of rAfor neat D 2 0 is at least 53 ps, then the rate constant for quenching reaction be(lA,) is at least 8 X lo6 L mol-l tween Brij micelles and 02* s-l (expressed for micelle concentration). Attempts were made to measure this quenching rate constant directly by measuring k’ for varying surfactant concentrations a t constant DPBF concentration. It was evident from k’ measurements over the limited available range 0.045-0.15 M Brij 35 that the extent of quenching increased with increasing surfactant concentration. However, the relationship between k’ and Brij 35 concentration was not linear, which may be due to variation in the aggregation number at these high surfactant concentrations, causing nonlinearity in the actual micelle concentrations. ( d ) Bimolecular Rate Constant Differences. All of the bimolecular rate constants, k.,, found in this work are within the range of those determined in homogeneous organic solvents. (Typical values are, for toluene,276.6 X lo8 L mol-l s-l, and for benzene,I9 9.4 X lo8 L mol-’ s-l.) Reference to Table I shows that k, values are very close to 6.5 X lo8 L mol-l s-l for CTAB and the nonionics, but that the SDS and sodium laurate k, values are some 60% higher. The reasons for this difference are not clear, although it may be that addition of oxygen to the diene-type moiety is assisted by favorable orientation of the furan in
-
the micellar body. SDS and sodium laurate differ from the other surfactants in this study in two aspects: they are anionic, and they have a shorter chain length (and presumeably smaller micelles). It is unlikely that energetic effects are important since it has been shown that the reaction between DPBF (like other furans) with singlet oxygen has essentially zero activation energy.28
Acknowledgment. The CFKR is supported by NIH Grant RR-00886 from the Biotechnology Branch of the Division of Research Resources and by the University of Texas at Austin. One of us (B.A.L.) thanks Marathon Oil Co. for a fellowship which helped make this work possible. We are also grateful to Dr. W. H. Woodruff for the use of his Nd/YAG laser. The N2 gas laser and kinetic spectrometry equipment were designed and built by Dr. Z. A. Zimek. Dr. D. C. Foyt of the CFKR wrote the experimental control and data processing software. References and Notes A. U. Khan and M. Kasha, J. Cbem. Pbys., 35, 2105 (1963); 40, 605 (1964); Nature (London), 204, 241 (1964). C. S.Foote and S.Wexler, J . Am. Cbem. SOC.,86, 3879 (1964). C. S.Foote, Acc. Cbem. Res., 1, 104 (1968). K. Golinick, Adv. Pbotocbem., 8, 1 (1968). J. W. Hastings and T. Wilson, Pbotopbyslology, 5, 49 (1970). R. P. Wayne, Adv. Pbotocbem., 7, 311 (1969). M. Kasha and A. U. Khan, Ann. N. Y . Acad. Sci., 171, 5 (1970). D. R. Kearns, Cbem. Rev., 71, 395 (1971). W. Adam, Cbem. Z.,99, 142 (1975). A. P. Schaap, Ed., “Singlet Molecular Oxygen”, Dowden, Hutchinson and Ross, New York, N.Y., 1966. B. Stevens, Acc. Cbem. Res., 6, 90 (1973). V. Ya. Schlyapintokh and V. B. Ivanov, Russ. Cbem. Rev., 45, 99 (1976). R. M. Badger, A. C. Wright, and R. F. Whitiock, J . Cbem. Pbys., 43, 4345 (1965). P. B. Merkel and K. R. Kearns, J. Am. Cbem. Soc., 94, 7244 (1972). A. A. Gorman and M. A. J. Rodgers, Cbem. &ys. Lett.,55, 52 (1978). I. B. C. Matheson, A. D. King, and J. Lee, Cbem. fhys. Lett., 55, 55 (1978). D. R. Adams and F. Wilkinson, J. Chem. Soc., Faraday Trans. 2, 68, 586 (1972). R. H. Young, D. Brewer, and R. A. Keller, J. Am. Cbem. Soc., 95, 375 (1973). A. A. Gorman, G. Lovering, and M. A. J. Rodgers, J . Am. Cbem. Soc., 100, 4527 (1978). A. A. Gorman, G. Loverlng, and M. A. J. Rodgers, fbotocbem. Pbofobiol., 23, 299 (1976). Y. Usui, M. Tsukada, and H. Nakamura, Bull. Cbem. SOC.Jpn., 51, 379 (1978). N. Barboy and I. KraljiE, J. Pbotocbem., 9, 322 (1978). R. Bensasson and E. J. Land, Trans. Faraday Soc., 87, 1904 (1971). A. Garner and F. Wilklnson, Cbem. Pbys. Lett., 45, 432 (1977). C. S.Foote in “Singlet Oxygen Reactions with Organic Compounds and Polymers”, B. Ranby and J. F. Rabek, Ed., Wiley, New York, N.Y., 1978, p 135. J. H. Fendler and E. J. Fendler, “Catalysis in Micellar and Macromolecular Systems”, Academic Press, New York, N.Y., 1975. A. A. Gorman and M. A. J. Rodgers, unpublished data. A. A. Gorman, G. Lovering, and M. A. J. Rodgers, J. Am. Cbem. Soc., in press.
