THE J O U R N A L O F
PHYSICAL CHEMISTRY Registered i n US.Patent Office 0 Copyright, 1979, by the American Chemical Society
VOLUME 83, NUMBER 13
JUNE 28, 1979
Low-Power-Pulsed, C0,-Laser-Induced Chemistry of SF,-Sensitized Diborane. Evidence for Thermal Steering
'
Clyde Riley * and Romas Shatas Depatiment of Chemistry, The Universiv of Alabama in Huntsville, Huntsville, Alabama 35807 (Received December 8, 1978; Revised Manuscript Received March 5, 1979)
Recent work involving continuous-wave COz laser irradiation of high pressure diborane sensitized with SF6 indicated that (BH), polymer was produced in two separate processes one of which was relatively fast. We have studied (BH), formation by mechanically chopping and electrically pulsing the laser. These experiments show the process which involves faster (BH), formation to be thermal. An average rate for (BH), formation (assuming n to be 20) was found to be slower than that for BbH9 + B5Hll formation for up to 100 pulses of 100-msduration at 6.1 W. Laser beam chopping experiments showed that polymer formation could be maximized or minimized (stopped) depending upon both the beam on and off time. When the laser's unique feature for phase homogeneous flash heating was applied, the pulse length required to initiate polymer formation was found to increase linearly with partial pressure. On the assumption of a constant T for initiation, energy balance considerations lead to a determination of the energy required to heat the reaction volume of BzH6 to reaction temperature and that involved as heat loss. The laser pulse length required to initiate (BH), polymer formation was also determined for laser lines at the v 3 band of SF6. Red shifting of the minimum required pulse length with respect to the SF6absorption peak at room temperature was observed. This thermally induced shift was used to estimate an average reaction zone temperature for initiation of polymer formation and was found to be -525 K. I
Introduction Recent studies in our laboratory with a chopped continuous-wave (CW) COz laser furnished evidence that neat diborane at pressures of 50-400 torr irradiated with a low-intensity IR laser underwent thermolysis.2 This result was in disagreement with the hypothesis of primarily vibrationally enhanced reactivity invoked to interpret the results of recent experiments dealing with the same system.3i4 In ref 2, we briefly mentioned preliminary work on the CW C02laser irradiation of SF6-sensitized diborane, and reported that we found an apparent alternate path, a faster process generating (BH), polymer, resembling that reported by Bachmann et aL3 and Rinck4 for the production of icosaborane (B2oH16) from neat and SF6-sensitized B2Hs. Previously, we noted, upon opening the beam stop for the CW runs, (BH), polymer could be generated in a visibly faster reaction than in the neat irradi0022-3654/79/2083-1679$01 .OO/O
at ion^.^ Superficially, it also appeared to be faster than the reactions leading to B5H9, B5Hll, and B10H14. The faster process was most pronounced for the P-16-P-30lines and occurred over a BzH6 partial pressure range from 25 to 400 torr. Except for the faster reaction producing (BH),, all of the low power and intensity laser induced chemistry of high pressure diborane (50-500 torr), neat and sensitized, was considered to be consistent with a predominantly thermal process.' Elementary considerations would seem to dictate this outcome with high pressure reactant(s) and low photon intensity conditions. At most only 103-104 photons/ (molecule s) may be absorbed while that molecule is undergoing lo9 collisions/s. The present work was carried out to determine if the apparent faster reaction leading to (BH), formation when SF,-sensitized B?H6 is irradiated with a pulsed and chopped CW C 0 2laser is also 0 1979 American Chemical Society
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The Journal of Physical Chemistry, Vol. 83, No. 13, 1979
TABLE I: B,H, Conversion and B,H, t B,H,,, B,,H,,, and (BH), Yieldsa freq,
Hz 39 open
power t o sample,b W 6.1 6.1
int to sample, W/cmZ 15.9 15.9
yield run time,c s 600 283
B,H, converted, mol 6.0 x 10-4e 1.3 x 10-3
[(BHIn 1Id mg none visible 6.8
[B,OH,,l, mg 0.9 8.8
[B,H,I, mol 2.1 x 10-4e 2.7 x 10-4
Chopped and unchopped CW CO, laser tuned to P-20line irradiating 400 torr of B,H, sensitized by 5 torr of SF,. Laser Dower was 6.6W: 0.5 W was reflected from the entrance window. The actual dutv time or open time was 0.471 times the run time when chopping. d By mass conservation. e Data are average of two runs. a
consistent with a thermal interpretation. This is especially important when considering the conclusions of other work reporting evidence for vibrationally controlled chemistry in high pressure systems irradiated with low power CW 1asersnMSecondly, we wanted to determine if laser pulsing could result in selectivity or a steering of this sensitized system is some particular way regardless of the mechanism since our earlier findings indicated possibility of some controL2 Differences relative to heterogeneous pyrolysis techniques may be attributed to the phase homogeneous flash heating property of the laser. Thirdly, we wanted to obtain information on the rate of (BH), formation in the pulsing mode and compare it with rates for other products. Lastly, because polymer formation is so distinct upon onset, could we deduce energy requirement information for its formation by utilizing the laser as a highly controlled energy input source?
Experimental Section The sample preparation and laser irradiation cells have been described p r e v i ~ u s l y . ~The desired SF6 partial pressure was added to the measured diborane sample in the cell from a calibrated volume by condensation at liquid nitrogen temperature in the side vial of the cell. Two methods of pulsing the Coherent Radiation Model 42 COz laser were utilized, electrical (a commercial option) and mechanical by rotating a slotted disk.2 The laser was set on the (P(20), OOol-10°O) line at 944.21 cm-l for all experiments unless otherwise specified. The beam diameter was maintained a t 7 mm with a variable aperture. Beam homogeneity was monitored with an Optical Engineering COz laser thermal image plate Model 22A, No. 3 or 4. The beam diameter was determined with the thermal image plates and by burn spot. Careful adjustments were made to keep the laser operating in the TEMoomode. Pulse shape and energy of the electrically pulsed and mechanically chopped beam were monitored with a Laser Precision Model KT1040 pyroelectric detector and Tektronix Model 564 storage oscilloscope. The onset spike associated with electrical pulsing was found to be ~ 0 . 5 - m s duration and contributed less than 2.5% of the energy associated with a 10-ms pulse. Polymer formation was initiated near the front of the cell and observed visually under illumination as a cloud. The IR absorption spectrum of the v3 band of SF6at room temperature was determined with a Digilab (200B) Fourier transform spectrometer; it agreed very well with that reported by Brunet and Perezg The analytical procedure for determining B5H,, BI0Hl4,and Hz has been discussed p r e v i ~ u s l y . ~(BH), determinations were calculated by difference. Results A chopping experiment with a 0.47 transmission and 0.53 obscuration wheel similar to that performed on neat BzH6 was carried out first to determine if any evidence existed for nonthermal processes. Table I shows the results for 5 torr of SF6 and 400 torr of BzH6 with the P-20 (944.21
PULSE LENGTH FOR POLYMER I N I T I A T I O N
50
-
t I
0
-4
50
100
150
210
250
2.5 TORRSFg 5 , O TORR SFg
300
350
430
B2Hg PARTIAL PRESSURE (TORR)
Figure 1. Pulse length required to initiate polymer formation vs. B2H, partial pressure. Circles and s o l i lines are for 2.5 torr of SFB. Squares and broken lines are for 5.0 tom of SFB. 6.1, 7.8, and 12.2 W correspond to 15.9, 20.3, and 31.7 W/cm2 intensities, respectively.
cm-’) line. For the chopped and unchopped conditions it is noted that the same number of photons irradiating the system does not lead to the same result even though power and intensity remain constant. Polymer formation was stopped completely at 6.1 W power and 15.9 W/cm2 intensity if the beam-on time was less than 12 ms, corresponding to a chopping rate of 39 Hz. The yields of products other than polymer were reduced substantially. Since polymer formation could be stopped with chopping, but yet initiated when the laser beam stop was opened for the CW experiments, we devised an experiment to measure the beam-on time necessary to just initiate polymer pulsing with each pulse. By measuring the length of the pulse required to initiate the formation of (BH), we could determine the energy required to attain the reaction temperature for “fast” (BH), production. This was carried out by utilizing the same CW laser which could be pulsed electrically at a low repetition rate of 1 Hz. The pulse length was increased from the short side until the onset of polymer pulsing was noted. Visual observation under strong light was used for detection. This simple method was reproducible to within 1ms for establishing the laser input energy necessary to initiate fast polymer formation. Concentration was not the question here, only onset. Polymer pulsing appeared as expanding waves from the front of the cell at pulse repetition intervals of 1 s. The waves subsequently penetrated the entire cell as they were carried by convection currents. Figure 1 shows the results of this work. The pulse length required to initiate polymer pulsing is seen to increase with increasing partial pressure of BpH6 and decreasing partial pressure of SF6. The extreme linearity is striking. Figure 2 shows the variation in pulse length needed to initiate polymer formation vs. laser line wavenumber of SF6 and BzH6 at partial pressures of 2.5 and 250 torr, respectively, Power and intensity to the sample were 7.8 W and 20.3 W/cm2, respectively. A definite “red shift” to frequencies lower than the peak of the absorbance of
Laser Induced Chemlstry of SF,-Sensitized Diborane
The Journal of Physical Chemistry, Vol. 83, No. 13,
TABLE 11: Rates as a Function of Number of Pulses and Pulse Lengtha no. of pulse t B,H, rate,b t (BH)n rate,c,d pulses length, ms mol/s mol/s 20 82 101
1.7 x 10-5 9.8 x
60
60 100
1.4 X 1.0 x l o + 4.5 x 1 0 - 6
6.5 X lo-‘
t H , rate, mol/s
- B,H, rate,
1.2 x 10-4 6.7 x 10-5 1.1 x 10-4
5.7 x 10-5 3.5 x 10-5 5.5 x lo-,
1979
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mol/s
Power to sample was 6.1 W with a 7-mm diameter beam. B,H, pressure was 400 torr and SF, partial pressure was 5 torr. Assumes predominately B,H,. Assumes n = 20. B,,H,, was not determined since none was visible.
a
____,_i
IO-.
I
\
4 2
L
the v3 band a t room temperature occurs. Further experiments involving mechanical chopping of the beam with a 47.1%transmission wheel gave additional information concerning polymer formation. As the chopping frequency was decreased from a frequency that produced no polymer (resulting in a longer beam-on time) we noted that only a single wave of polymer was initiated when the chopping open time corresponded to the pulse length of the electrically generated laser pulse of Figure 1. When the frequency of the chopper was decreased further, it was observed that the polymer could be pulsed continually upon each opening of the chopper slot at a frequency of less than 11-13 Hz (6.1 W power and 15.9 W/cm2 intensity to a sample containing 5 torr of SF6and BzH6 at partial pressure of 50-400 torr). Since (BH), appeared to be formed at a faster rate relative to other products under pulsed conditions, several experiments were performed to determine its rate of formation and those of any other products. Table I1 lists these results for 400-torr BzH6partial pressure with 5 torr of SF6at 6.1 W. For the (BH), rate determination, n was assumed to be 20. The only other products identified were B5H9 and B5Hll. The rates listed are average rates calculated for the on time of the laser during the indicated number of shots (yield/integrated open time). Discussion The results of the mechanical chopping experiment on (BH), formation are not consistent with the interpretation offered by Bachmann et aL3 and Rinck4 that the low power CW IR laser irradiation of diborane leads to a predominantly vibrationally controlled chemical process. If this were true, the same results would have been observed for the chopped and unchopped laser beam experiments. The “red shifting” associated with polymer initiation in Figure
2 may be interpreted in terms of the work of Nowak and Lyman.lo They have shown that the v3 spectrum of SF6 changes dramatically with temperature. Hence, laser heating of the SF6 alters the absorption coefficient of the SF6 for the various laser lines causing the apparent “red shift”. Using Figure 2 and the work of Nowak and Lyman,1° we may estimate the average temperature required to initiate the faster polymer formation to be -525 K. This temperature is close to that calculated for the neat laser irradiation^.^ One might expect it to be higher, considering the difference in absorption coefficient between SF6and BzHG.However, we are dealing with an initiation temperature brought about by pulsing which would not be equivalent to the constant heating under CW conditions. In the second part of the mechanical chopping experiments a beam-off time dependence was noted and found to be N 10-13 Hz or 40-50 ms. This off-time requirement may be associated with replenishment of reactant(s) into the reaction zone (volume is related to beam diameter and SF6 absorption coefficient) as opposed to on-time dependence which is related to the reaction initiation temperature. It is apparent from our data that both are important, and by proper adjustment of these variables one could maximize or minimize the amount of polymer formed and in effect steer the reaction. Table I shows no (BH), production with a laser on-time shorter than 12 ms while other products were formed. Table I1 shows measurable amounts of polymer could be formed long before comparable amounts of B10H14 can be produced.ll Since the chopping results support a thermal mechanism, we have interpreted the data in terms of thermal effects. We note that in low pulse rate repetition experiments the initital conditions with respect to the temperature and concentration are effectively reestablished at the beginning of each pulse. The final condition is attained when the temperature needed to initiate fast polymer formation is reached. The temperature of the system is dependent upon a summation of laser energy deposit, energy release in precursor formation, and loss through the surfaces. Integration of the heat conduction equation at fixed end points in time, T , and temperature, 8, yields the energy balance expression: S ‘ d t ( P + AHrxR)=
s“‘s
%p at
dv dt
+
s T ” ~ k V T . fds i dt The left side describes the energy sources, with P denoting the laser power, A“,,the overall reaction enthalpy of any precursors to (BH),, and R their rate of formation. The first term on the right pertains to the heat stored within the reaction volume u with mass density p and specific heat c. The second term describes the heat loss through surface s of the reaction zone. The heat conductivity k is pressure independent,12 fi is a unit vector normal to s and we take that the time evolution of V T is the same for experiments with identical end points 7,8. We apply this conservation
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The Journal of Physical Chemistry, Vol. 83,No. 13, 1979
TABLE 111: Heating Factor Results from Polymer Initiation Pulse Length Study isochron, ms 19 15 19 15 30 19 15
heating factor (Jitorr) x l o 3
power, W 2.5 torr of SF, 6.1, 1 2 . 2 6.1, 1 2 . 2 7.8,12.2 7.8,12.2 7.8, 6.1 7.8, 6 . 1 7.8, 6.1
0.42
av 0.40 20 15
10 10 10
5.0 torr of 6.1, 7.8 6.1, 7.8 7.8, 6 . 1 12.2, 6.1 12.2, 7.8
power, W
* 0.03
SF, 0.25 0.24 0.22 0.23 0.24 av 0.24 t 0.01
of energy relation13 to the analysis of data in Figure 1. The irradiated volume is always at room temperature a t the beginning of a pulse because of the slow 1-Hz laser repetition rate. The outer cell wall is maintained at room temperature for these low power pulses. Since SF6 is the absorber, for our simple model the reaction temperature, 8, is achieved in the length of time, r , along any line of constant time (isochron) in Figure 1 regardless of BzH6 pressure.14 B5H9(predominant) and B5H11 were the only other products found during experiments listed in Table 11. Longls lists the pentaboranes as precursors to (BH), in his proposed reaction scheme for pyrolysis. If B6H, is a precursor to (BH), formation, we would need to include its A“,, on the energy input ledger. We may calculate the energy contribution from B5H, formation (assuming 100% B6H9)during a pulse time necessary to initiate (BH), formation from the rate data of Table I1 and the AHmof B5H9from B2H6 Using a AHmof -21 kJ/mol,16 we find the maximum contribution to be equivalent to less than 6% of the laser input. Since the calculation to follow involves a subtraction at two different pressures for a constant pulse length this contribution will be even less and hence may be neglected. We may now evaluate the first integral by using differences along an isochron (line of constant time) of Figure 1at two different laser powers and the same SF6pressure. For example, if we draw a horizontal line at 19 ms it intersects the 6.1-W least-squares fit at 90.6 torr of B2H6 and the 12.2-W data line at 366 torr of BzHG for 2.5 torr of SF,. Neglecting the small AHrx contribution, we now have the sum of two integrals equal to Pr at each of the intersected points. Subtraction leaves P1r - P2r = (gas heating) p1 - (gas heating) p2where P1 and Pz are the laser powers associated with the intersected points at pressure p1 and p2. The heat loss integrals have cancelled a t the same T and 8 due to the pressure independence discussed previously. Dividing by the difference in pressure p1 - p 2 ,we obtain a factor which we define as the gas heating factor for BzH6 in joules per torr. The heating factor is a function of the SF, partial pressure because it depends upon the SF6 volume necessary to absorb all the energy, but the SF6heating cancels in the subtraction. For this example it has a value of 0.42 X J/torr. This same 19-ms isochron could have been used to intersect the 7.8- and 12.2-W data or the 6.1- and 7.8-W data combinations for the same SF6pressure. Table I11 lists the gas heating factors in J/torr for a number of different isochrons and power combinations for the two SF, partial pressures. They should be constant for a given partial pressure of SF6 if the model has validity. The
B,H, partial press., torr
7.8 7.8
5 torr of 50 200 400 50 200
7.8
400
12.2 12.2 12.2
50 200
6.1 6.1 6.1
0.40 0.45 0.42 0.38 0.37 0.36
TABLE IV: Fraction of Energy Absorbed by System Reauired to Heat B,H, to Reaction TemDerature
6.1 6.1 6.1
7.8 7.8 7.8 12.2 12.2 12.2
400
%
%
heating
loss
SF, 19 41 52 23 46 56 20 52 63
2.5 torr of SF, 50 20 200 48 400 62 50 22 200 52 400 65 50 26 200 53 400 65
81 59
48 I7 54
44 80 48 37
80 52 38
78 48 35 14 47 35
average heating factors for BzH6were 2.4 X and 4.0 X J/torr at 5 and 2.5 torr of SF,, respectively. The value of -2 for the ratio of the heating factors is close to the ratio of reaction volumes which is related to the SF6 partial pressure. A greater energy is required to attain the reaction temperature for the larger volume of B2HGassociated with the lower partial pressure of the sensitizer. The heat storage intergral for (BH), formation can now be evaluated at its end points for any initial pressure of BzH6, for a given SF6sensitizer pressure, by multiplying by the initial BzH6 partial pressure. The fraction of pulse energy for gas heating of the BzHGreaction volume and the fractional heat loss were then determined from the total pulse energy for the partial pressure in question at a given partial pressure of SF,. Table IV lists the results for the fraction of the absorbed laser pulse energy expended into heating the reactant to reaction temperature and that lost through various cooling processes. Although there is about a factor 2 difference in the amount of total energy required for the two different SF6partial pressures there is only a small difference in fractioning between gas heating and heat loss for the two different SF6 partial pressures. This may be related to the small change of the surface-to-volume ratio of the illuminated envelope for the two different sensitizer partial pressures. At the end of the pulse the calculated s / u ratio for the P-20 lines was 7.5 and 6.6 cm-’ for 5 and 2.5 torr of SF,, respectively. Consistent with the model, the amount of energy required to reach the reaction temperature increases as the B2HG partial pressure increases. It bears mentioning that all the laser energy is absorbed in a small reaction volume for both SF6 partial pressures. The strong excitation gradient created along the optical path will have no effect upon the analysis. We are measuring the minimum energy to attain a reaction temperature. We may summarize our results in the following manner. The mechanism of the reaction leading to (BH), which was qualitatively characterized in our earlier work2as a “faster process” has been found to be compatible with a thermal interpretation. We believe this “faster process” leading to (BH), is the same as that reported previousl$p4 leading to B20H16 since no B2oH16 was found in our work.2 We found no evidence supporting an explanation that (BH), is produced by laser induced vibrationally controlled chemical processes under our experimental conditions. The (BH), was found to be produced (at least initially)
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).
The Journal of Physical Chemistry, Vol. 83,
No. 13,
1979
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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'hys. 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
