J. Phys. Chem. 1993,97, 10498-10503
10498
Electron Spin Resonance and Electron Spin Echo Modulation Studies of the Photoionization of N-AlkyI-N,N’,N’-trimethylbenzidines in AOT Reversed Micelles Chris Stenland and Larry Kevan’ Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received: June I, 1993’
N-Alkyl-N,N’,N’-trimethylbenzidines (C,TMB, n = 1,4,8,12,16) were photoionized in rapidly frozen reversed micelles composed of sodium bis(2-ethyl- l-hexyl) sulfosuccinate or aerosol dioctyl (AOT) in isooctane with 4, 9, 14, 24, 34, and 44 mole ratios of water/AOT. The relative photoyields after irradiation with 300-400-nm light were measured by electron spin resonance. The photoyield decreased as the mole ratio of water to AOT increased. This is interpreted as a more negative surface charge density at the organic-aqueous interface as increased water more completely solvates the sodium counterions. The higher negative surface charge density forms a higher barrier for the photoelectron to surmount to enter the water pool. Electron spin echo modulation spectroscopy was used to probe the local magnetic nuclear environment of the photoproduced cation radicals in these reversed micelles and indicate that the cation radicals are located near the interface. Secondary radicals due to interaction of the AOT headgroup with photoelectrons are observed and assigned. Introduction
and benzene were reagent grade obtained from Fisher Scientific. Methylene chloride was reagent grade from Mallinckrodt. Photoionization of organic molecules with low ionization Absolute ethyl alcohol was purchased from Aaper Chemicals. potential in microheterogeneousmedia has been proposed as model All materials were used as received. systemsfor energy storage and conversion.I4 Recently, reversed In one experiment to determine both the water content and micelles have received attention due to their unique s t r u c t ~ r e . ~ ~ ~influenceof inorganic salts on the photolysis product yields, AOT Reversed micelles are microscopic water pools solubilized by in benzene was washed three times with water, discarding the surfactant molecules in a nonpolar ~olvent.~ This contrasts with aqueousemulsion. The top clear layer was azeotropicallydistilled normal micelles which are like “oil droplets” suspended in an until thedistillate was clear, and then the remainder of the solvent aqueous solvent. Photoionization of a series of neutral aromatic was removed on a rotary evaporator. The AOT was then dried amine electron donors in reversed micelles formed from sodium in vacuo over PzOs at 330 K for several days. bis(2-ethyl-l-hexyl) sulfosuccinate or aerosol dioctyl (AOT) is The N-alkyl-N,N’,N’-trimethylbenzidines (C,TMB, where n investigated. AOT is a well-characterized reversed micellar = 4, 8, 12, 16) were synthesized and purified as previously ~ y s t e m . The ~ structure of the reversed micelle-can be simply described.* CITMB was obtained from Kodak and purified like altered by varying the mole ratio of water to AOT monomer. the synthesized materials. This ratio is denoted WO. Increasing wo increases the water pool All glassware was solvent washed (HzO, alcohol, chloroform, size, which in turn changes the interface structure due to greater alcohol, HzO) and baked dry. Glasswarewas stored in aluminum hydration of the associated counter ion^.^ foil until used to keep it free of dust. In this study, the effect of the water pool size and the electron Sample Preparation. Stock solutions of AOT dissolved in donor alkyl chain length on the relative photoyields of N-alkylisooctane with added D20 with mole fractions of DzO to AOT N,N’,N’-trimethylbenzidinesis investigated by continuous wave surfactant in the range of 0, 5 , 10, 20, 30, and 40 were made and time domain electron spin resonance (ESR) techniques. The separately. The water (HzO) content of AOT was found to be production of secondary radicals due to photoexcitation and wo = 4.3 (see below), making the water/AOT in the range 4-44. electron scavenging by AOT is also investigated. Continuous These AOT stock solutions were made by dissolving 4.54 g of wave ESR at X-band can determine the type and number of AOT in isooctane. The AOT quickly dissolved in isooctane and radicals produced by ultraviolet light irradiation in rapidly frozen was transferred to a clean dry 100-mL volumetric flask, and the reversed micelles at 77 K. Electron spin echo modulation beaker was rinsed several times to wash the remaining AOT into spectroscopy (ESEM) is employed in these reversed micelles to the flask. Finally, the 100-mL volumetric flask was filled to the determine the relative distances of the electron donor to the fiducial mark to make 0.1 M AOT. Next, five 1O-mL volumetric deuterated aqueous interface as a function of water pool size and flaskswerefilledtothefiducialmarkwiththe0.1 MAOTsolution. electron donor alkyl chain length. As in normal micelles, a contrast These solutions were deoxygenated by bubbling NZgas through in the local magneticnuclear environment is created for the ESEM the solution. To each flask was added a measured amount of experiments by using deuterated water pools dispersed in protiated deoxygenated D20 from a microliter syringe to make the mole surfactants and hydrocarbon solvent. fractions described above. The mixture initially formed a twophase system, which, on occasionalmixing, eventually formed an Experimental Section optically clear, one-phase solution. M a t e m . Sodiumbis(2-ethyl-1-hexyl) sulfosuccinate or AOT The concentrations of the stock solutions of the C,TMBs were (99%) was obtained from Sigma. The solvent 2,2,4-trimethylchecked just prior to dispensation on a Varian Techtron 635 pentane (isooctane), HPLC grade (99.7%), was purchased from spectrophotometer. Then a measured quantity of the electron Aldrich. Deuterium oxide (99.9% D) was obtained from donor solution was added to make a 3 X 1 V M solution when Cambridge Isotope Labs. The H20 is Millipore MilliQ treated 1 mL of the reversed micelle stock solution was added to the test deionized water with a conductivity of 18 Mil cm. Chloroform tube. Thin films of the electron donors C,TMB were then formed in test tubes from chloroform or methylenechloridestocksolutions Abstract published in Aduance ACS Abstracts, September 1, 1993. by blowing nitrogen gas over the surface of the solution. The 0022-365419312097-10498$04.00/0
0 1993 American Chemical Society
Photoionization of C,TMB in AOT Micelles thin films were stored under vacuum several hours to remove residual solvent. Before addition of the reversed micelle solution the test tube was purged with dry nitrogen gas. After addition of the AOT solution, the tube was tightly sealed with a square of parafilm and allowed to sit 1 day with occasional swirling. All C,TMBs went quickly into solution. The electron donor concentration in the reversed micelle solutionwas checked optically. Thequartzcuvettesused were 1-mmpathlength,and thesolutions were diluted in situ if necessary to make 3 X 1 V M C,,TMB. The correspondingAOT/isooctane/DzO stock solutionwas used in the reference cuvette. Then 100 pL of each solution in duplicate was added to 2-mmi.d. by 3-mm-0.d. quartz Suprasil tubes (Hereaus Amersil) which were previously flame-sealed at one end. The open end of the tube is tightly sealed with a small square of parafilm and the tube is allowed to equilibrate for about 1 h. After equilibration, the parafilm is removed and the tube plunged into liquid nitrogen, where the sample rapidly freezes into a white polycrystalline matrix. Since these solutions are predominately isooctane, the solutionvolumeshrinks on freezing. Thus, the tubes rarely break on rapid freezing by plunging into liquid nitrogen. The samples were stored under liquid nitrogen throughout the experiment, except if annealed, which will be described below. Photoirradiation. Photoirradiation was carried out at 77 K in a quartz finger dewar containing liquid nitrogen. The finger dewar was rotated approximately four times per minute to uniformly irradiate the sample. The light source was a Cermax ILC Technologies 300-W UV-enhanced xenon arc lamp (ILCLX 300 UV). This lamp was energized by an ILC Technology Model PS300-1 power supply operating at a lamp current of 10 A. The light was filtered through a 10-cm water filter and a Corning 7-60 band-pass filter with transmittance in the range 300-400 nm with a maximal transmittance of 70% at 365 nm. Typical light flux intensity at the sample was 1.4 X 102 W m-2 (YSI Kettering Model 65 radiometer). The time of irradiation was controlled by a shutter activated by a Lucas Ledex rotary solenoid controlled by a Gralab 605 timer. To investigate the secondary radicals produced in AOT reversed micelles without C,TMB, a Corning 7-54 band-pass filter was used. This filter passes light from 240 to 400 nm with a maximum transmittance of 80%centered at 3 10 nm. Typical light flux intensity from the 7-54 filter at the sample was 1 X lo3 W m-2. Magnetic ResonanceExperiments. ESR spectra were acquired on a Bruker ESP 300 X-band spectrometer employing a TE102 microwave cavity with 100-kHz Zeeman field modulation with 1.1-G modulation amplitude. The incident microwave power was 2.0 mW for photoyield studies, which is near the top of the linear portion of the power saturation curve for the alkylbenzidine cation radicals. Each spectrum is a sum of eight scans, and each sample was recorded three times with an average deviation calculated from these measurements. The samples were thermostated in a quartz finger dewar filled with liquid nitrogen. The photoyields were determined by doubly integrating the firstderivative absorption spectra using the ESP 300 software. Two-pulse electron spin echo (ESE) signals were obtained on a home-built spin echo spectrometer operating at X-band, with the samples at 4.2 K in liquid helium using 40-11s exciting pulses. One representative sample of each C,TMB for every wowas used to record the ESE signal. Then, the ESE data were transferred to an IBM PC and analyzed.9 The microwave frequency of the Bruker ESR and the ESE spectrometer was measured by Hewlett Packard Model 5350B and 5342A microwave frequency counters, respectively. The magnetic field was measured using a Varian Model E501 NMR gaussmeter. Determination of Water Content. The AOT particles stuck together, indicating that it contained a small amount of water (H2O). Carefully drying the sample as described above and
The Journal of Physical Chemistry, Vol. 97, No. 40, 1993 10499 running the photoyield experiments showed that the photoyield increases from w o = 1, reaches a maximum in the neighborhood of wo = 5 , and decreasing at higher WO. Comparing these photoyield results with earlier work10 indicates that the initial water content was about 4.3. The water pools are a mixture of H20 and DzO, where wo = 4.3 corresponds to no added DzO. Secondary Radicals. A study of the secondary radicals produced in AOT reversed micelles without added C,TMB was performed. Two procedures were used to prepare samples. The first procedure deoxygenated the samples by nitrogen bubbling followed by transfer to the quartz sample tube unprotected from the atmosphere. The second procedure degassed the samples by five freeze-pumpthaw cycles. Afterward, the sample tube was flame-sealed while immersed in liquid nitrogen under vacuum. The sample was thawed and allowed to equilibrate approximately 1 h as before. The amount of solvent lost by freeze-pumpthaw amounted to less than 6% based on differences in the length of the solution in the tube before degassing and after flame-sealing. After equilibration, the samples were rapidly frozen by plunging into liquid nitrogen. To investigate the contribution of trace sulfite ions to the secondary radical yield, the water-washed AOT in isooctanewas deoxygenated by bubbling argon gas through the solution. The pipets and quartz tubes were also purged with argon before filling, followed by rapid freezing by plunging into liquid nitrogen. To see whether any secondaryradical signal could be enhanced by addition of inorganic impurities that may be present in trace amounts, 9 mM stock solutionsof Na2S04 (Baker reagent grade, 99.6%) and Na2S03 (Mallinckrodt analytical reagent, 98.6%) in D20 were prepared. Then, toseparate test tubes with and without CsTMB, 36 pL of each salt solution was added. Next, 1 mL of the stock AOT solution was added to each test tube and mixed until an optically clear one-phase solution had formed. The test tube was tightly capped and allowed to sit overnight. Then, about 100 pL of each solution was added to a quartz tube, subjected to five freezepumpthaw cycles to remove gases, and flamesealed. The sealed sample was thawed and allowed to equilibrate as above before rapid freezing for photoionization experiments. A 0.1 M Na2SO3 solution in D20 was made and rapidly frozen in a quartz sample tube to confirm the presence of the sulfite radical after photolysis in these frozen systems. Annealing of Samples. Some sampleswere annealed by storing the sample just over a couple of inches of liquid nitrogen in a dewar capped at the top. The temperature was measured to be 100 K a t the sample position using a Tegam digital thermometer, and the samples were held at this temperature for 20 min. Radical Stability as a Function of Temperature. A study of the radical stability as a function of increasing temperature was performed with a Bruker ER4111VT nitrogen flow unit fit into a TElo2 cavity of a Bruker ESP 300 ESR spectrometer. The samples were transferred from liquid nitrogen into the flow unit which was operating at about 130 K, and the temperature was raisedin 20 Ksteps between each ESR measurement. The sample was allowed to equilibrate at each temperature for 5 min before the ESR was recorded.
ReSults The rapidly frozen reversed micellar solution containing the C,TMB electron donors were white polycrystallinesolids with no detectable ESR signal before irradiation. The polycrystalline matrix was observed to shrink some 30% in volume based on the length at 77 K compared to the length at room temperature. All AOT reversed micelles containing C,TMB showed a strong green phosphorescence when the ultraviolet light beam was shut off. After irradiation the samples were light yellow. The light yellow color has been attributed to the C,TMB radical cation.8 For wo = 4, the samples appeared darker yellow, almost orange, indicating the presence of a second light-absorbing species.
Stenland and Kevan
10500 The Journal of Physical Chemistry, Vol. 97, No. 40, 1993
301 1
0
0 0
1 0 2 0 30 4 0 50
wo Figure 1. Relative photoyield at 77 K of C.TMB electron donors in AOT reversed micelles in isooctane versus mole ratio of [D20]/[AOT] equal WO. These samples were irradiated for 10 min with 300400-nm light:
H, CITMB; 0,C4TMB; A, CsTMB; A, CI~TMB; 0, C16TMB.
The ESR spectrum is predominatelya singlet with other radicals present in smaller quantities. The yield of secondary radicals ranges from 20 to 25% of the relative photoyield and is independent of the electron donor alkyl chain length but increases slightly as a function of water pool size. The singlet component has a line width of approximately 23 G and a gfactor = 2.004. This signal has been identified as the C,TMB cation radicalus The optical absorption maximum at room temperature for all C,TMB in AOT reversed micelles is blue-shifted when compared to sodium dodecyl sulfate (SDS) micelles. For example, TMB in SDS micelleshas ,A, = 307 nm while in AOT reversed micelles ,A, = 304 nm. TMB in isooctane gives a similar A,, as in AOT micelles in isooctane. On the same spectrophotometer, TMB in methylene chloride has ,A, = 3 11 nm. The same amount of blue-shifting is observed for all C,TMB. No dependence of A, as a function of wo is measured within a resolution of 2 nm. The optical absorption spectrum of AOT/isooctane reversed micelles without added electron donors shows an absorption tail at 240 nm with a small shoulder near 260-280 nm. Isooctane has an absorption tail at 220 nm with negligible absorbance above 240 nm. Rapidly thawing the frozen polycrystalline matrix in hot tap water (about 50 "C) rapidly bleaches the sample. Slow heating of the frozen polycrystalline matrix briefly forms an optically clear yellow solution which soon fades, becoming colorless at room temperature. ESR at room temperature, or after subsequent refreezing, gives no signal. Photoyields. The photoyields of C,TMB in AOT reversed micelles with wo in the range 0 4 0 are shown in Figures 1 and 2. Figure 1 plots the photoyield of all C.TMB as a function of WO. The general trend for all C,TMB shows a decreasing yield to about half the yield of wo = 4 at wo = 9 or 14 and then remains constant for larger W O , The electron donors CsTMB and (212TMB show a small peak for wo= 24. Figure2 plots the photoyield for all wo as a function of the electron donor alkyl chain length. The photoyield is maximized for CsTMB and C1zTMB. Clearly, the effect that w o has on the photoyield is greater than that of the electron donor alkyl chain length. The electron donor alkyl chain length effect is most pronounced for wo = 9. For wo = 14, 34, and 44, the photoyield as a function of the electron donor alkyl chain length is essentially constant. ESEM. Figure 3 shows the normalized modulation depths of all C,TMB as a function of WO. The normalized modulation depth is defined as the depth of the first modulation minimum from the extrapolated unmodulated curve divided by the distance from the extrapolated unmodulated curve to the base line.9 The electron spin echo decay curve for wo = 4 exhibited no deuterium nuclear modulation as expected. Figure 3 shows that as wo increases from 4 to 9, the normalized modulation depth rapidly
1
0
4
12
8
16
Alkyl Chain Length (n) Figure 2. Relative photoyield at 77 K of C.TMB electron donors versus Cn in AOT reversed micellesin isooctanefor mole ratio of [DzO]/ [AOT] equal wo in the range 040. These samples were irradiated for 10 min with 300400-nm light. Mole ratio of wo: 4 (O), 9 (O), 14 (a),24 (m), 34 (A),and 44 (A).
0.41
wo Figure 3. Normalized deuteron-modulationdepth from two-pulseESEM at 4 K for C,,TMB in AOT reversed micellesversus mole ratio of [D20]/ [AOT]equal WO. These samples are the same samples whose photoyields are shown in Figures 1 and 2: H, CITMB 0, C4TMB A, CsTMB; A,
CizTMB; 0, ClaTMB.
0.41 5 1
0.3
=
i3
0.2
H
0-.
-5
c.
0
4
8
12
16
Alkyl Chain Length (n) Figure 4. Normalized deuteron-modulationdepth from two-pulseESEM at 4 K of C,TMB electron donors versus C. for various [D2O]/[AOT] equal wo in AOT reversed micelles. Mole ratio of WO: 4 (0),9 ( O ) , 14 (0),24 (H), 34 (A), and 44 (A).
increases, becoming approximately linear for larger WO. In Figure 4, the normalized modulation depth decreases as the electron donor alkyl chain length increases for all WO. C,TMB Stabilityas a Functionof Temperahue. Figure 5 shows the ESR intensity as a function of temperature. The decrease in ESR intensityfrom 135to 170 K is due to the decay of secondary radicals. The alkylbenzidine cation radical is stable up to 190 K and then decays rapidly, becoming small at 2 10K and vanishing by 250 K.
Photoionization of C,TMB in AOT Micelles
W
\
I
0 130
The Journal of Physical Chemistry, Vol. 97,No. 40, 1993 10501
180
230
280
Temperature (K) Figure 5. Thermal stability of CsTMB+ in AOT reversed micelles for wo = 24 over the temperature range 130-280 K.
Secondary Radicals. As noted above, the ESR signal was not simply due to C,TMB+ alone; other radical species were present. A power saturation study indicated the presence of three or more radicals produced by 300400-nm light irradiation of C,TMB in AOT reversed micelles. Among the secondary radicals is one with a narrow line width and another with axial symmetry. The narrow singlet overlaps the C,TMB+ signal with a nearly equal gfactor. At low power, 2 pW, this signal strongly resembles the ESR spectra shown in Figures 5 and 6 in ref 10. This very narrow signal dominates the ESR signal at low microwave power. Its line width is about 5 G and has a g factor of 2.003. The fact that there is an SO3functionality in the AOT monomer suggests that it is a sulfite radical, Photolysis of Na2S03in aqueous polycrystalline solutions also gives this narrow ESR line with the same g factor. An attempt to detect the 33Scouplings was made, but the low natural abundance of 33s (0.74%) presumably precluded detection. The small axially symmetric signal was enhanced when the sample was annealed. Due to the complexity of the ESR signal, an effort to generate these secondary radicals directly by irradiation of AOT micelles without added electron donor was carried out. Irradiating AOT reversed micelles in isooctane with various amounts of D20 with 300400-nm light containing no electron donor at 77 K for 1 h produced no radical species. Yet irradiating with 240400-nm light for 10 min at 77 K produced a weak ESR signal. For better signal-to-noise the samples were irradiated for 1 h and scanned 128 times. Again, a complex ESR signal is produced that consists of several radicals according to microwave power saturation studies. The ESR signal has a broad singlet character with a spectral extent of over 100 G. This broad signal has distinct hyperfine features and is probably an AOT headgroup radical. The sulfite radical is again observed. In addition to the broad singlet with hyperfine structure, another ESR signal with axial symmetry is observed. Annealing these samples causes quantitative conversion of the AOT radicals into an axially symmetric ESR signal. The gfactors are gl = 2.0340 and 81= 2.0017. Several facts suggest that this is a peroxy radical. The first evidence is its axial symmetry, with g l > 8 1 . l ~Most importantly, removing oxygen eliminated this signal. Samples of AOT reversed micelles without added electron donors that were degassed by the freeze-pumpthaw technique and irradiated for 1 h 240-400-nm light showed no traceof theaxially symmetric signal, even after annealing. This signal is not due to 0 2 - since the g factors measured in this system are reversed to what is expected for 02-, which has ~1 > gL > g,.l5 This supports the assignment of the axially symmetric signal as a peroxy radical. The broad signal with the distinct hyperfine features is tentatively assigned to an AOT headgroup radical. This result contrasts with the secondary radicals reported in the photoionization of alkylphenothiazines in AOT.16 The g factor relative to the sulfite radical is 2.002. There is literature evidence for
3250
3350
3300
3400
3450
3500 [GI
Figure 6. ESR spectraof photoproducedradicals in AOTreverdmicelles without CnTMB,irradiatedwith24O40O-nmlightfor1h. Eachspectrum is 128 accumulations. The AOT concentration in isooctane is 0.1 M; (a) wo= 4, (b) wo = 24. Radical species I, 1’, and I1 are electron attachment products to the AOT carboxylic acid headgroups; see text. H
I RO-
SO3
I d
I H
C
O
R
H
so3-
I
I
HI
H
R-C----COOR
H I
I
(111)
Figure 7. Two inequivalent carboxylic headgroups on AOT give two electron attachment products, I and 11. The existence of SO,’ as a
photolysis product in washed AOT indicates that this species arises primarily due to C-S bond scission. The associateddecompositionproduct I11 may also be present, but its presence by ESR, in this system, has not b d unambigously identified. AOT reversed micelles acting as an electron scavenger.” The ESR spectrum shown in Figure 6 has at least three radicals present. Since the two carboxylic acid headgroups are inequivalent, two different ESR signals are expected. Radical I is expected to yield a doublet, while radical I1 will give a doublet of doublets; these radicals are shown in Figure 7. Radical I1 resembles the carboxylic acid headgroup radical in X-irradiated single crystals of succinic acid at 77 K,18but the coupling constants are different. The coupling constants of radical I is 40 G with relative intensity of 1:l. The coupling constants of radical I1 are 40 and 22 G with line intensities of 1:l:l:l. For AOT with wo about 24, another signal I’ with the same g factor as the AOT headgroup radical is apparent. It is a doublet with approximate intensities of 1:l and a coupling constant of only 13 G . The overlap due to the SO3- signal obscures the peak intensities of radical 1’. This I’ signal is assigned to a different conformation of radical I. The secondary radicals in deoxygenatedAOT reversed micelles with electron donors are slightly different than the radicals
10502 The Journal of Physical Chemistry, Vol. 97, No. 40, 1993
3250
3300
3350
3300
3450
3500 [GI
Figure 8. ESR spcctra of photoproduced radical species (a) CsTMB+ and secondary radicals in AOT reverscd micelles with wo = 24 deoxygenated by freeze-pupthaw cycles, irradiated 10 min with 300400-nm light. (b) Secondary radical signal obtained by subtractingthe
CsTMB+component. Thissignalisacombmationofelectronattachment to the AOT sulfosuccinate headgroup and a sulfite radical.
produced by UV irradiation without electron donors present (Figure 8). These secondary radical spectra were deconvoluted using spectral analysis techniques with software supplied on the Bruker ESP 300 data system. The ESR spectrum was zero filled and Fourier transformedto a dual space. Here the high-frequency components in both the real and imaginary domains were zeroed, and this dual representation was back-transformed to the magnetic field domain. These high-frequency components are due to the secondaryradicals and not the benzidine cation singlet. Comparing the back-transformed singlet to a bona fide C,TMB+ signal indicates that the singlet component in this system can be reliably extracted by Fourier spectral analysis. A difference spectrum, obtained by subtracting the derived cation radical singlet, indicates the presence of at least two radicals, the sulfite radical and what is suspected to be predominately the AOT headgroup radical(1). Photolysis of Aqueous NnpSOs Irradiating Na2SO3 in rapidly frozen polycrystalline D2O produces the sulfite radical. This radical has been observed by UV photolysis in aqueous solution at 296 K.19 Photolysis of Washed AOT. Irradiating washed AOT gave similar results to the unwashed AOT with one significant difference. The intensity of the SO3*-signal relative to the AOT headgroup radical was slightly smaller than in the unwashed AOT system. Addition of SO& and SO3%to Reversed Micelles. The effect of added S042-and S O Q on the sulfite radical production was investigated. Rapidly frozen solutions of AOT/isooctane with D20 containing 9 mM Na2S04 or Na2S03 were irradiated with and without CnTMB. No difference in the spectra with added SO42- and Sop2-was observed. The spectra with added S042and Sop2-and no added salts, i.e., pure D20, were essentially identical within experimental uncertainty. Discussion
ESEM. ESEM is a time domain pulsed ESR technique that is sensitive to weak electron-nuclear anisotropic hyperfine
Stenland and Kevan interactions that are not easily resolvable by continuous wave ESR. ESEM can determine the type and distribution of magnetic nuclei by the analysis of the oscillations found in the electron spin echo decay curve. In disordered systems, like reversed micelles, a relative measure of the distanoe from the free radical to nearby nuclei is made by measuring the normalized modulation depth.9 Since this interaction is dipolar in nature, the rangeof interaction is limited to about 0.60 nm. This measurement is possible when a contrast in the local nuclear environment is created by dispersing D20 in reversed micelles composed of hydrocarbon surfactants in hydrocarbon solvents. The ESEM nuclear modulation depth in the spin echo decay curve is proportional to the number of interacting nuclei and inversely to the averagedistanceto the nearby nuclear distribution. The average distance may be defined from the center of the spin distribution to the center of the nearby nuclear distribution of interest. It has been demonstrated* that the spin distribution in C,TMB for n 1 2 is not significantly different from that for CITMB. So the comparison of the normalizedmodulationdepths between different C,TMB’s is valid. The deeper the deuteron modulation, the closer the cation radical is to the deuterated aqueous interface. It is concluded that the reversed micellar structure is retained upon rapid freezingbased on related work. It has been established that fast freezing of normal micelles and vesicles retains the solution statestructure.2’J-23 A study using freezefractureelectron microscopy shows that the reversed micellar structure is retained when the samples are rapidly frozen.24 The electron spin echo modulation data indicates that, for wo 1 9, the radical cation is near the water pool and not solubilized deeply into the organic phase. The decrease in ESEM deuteron modulation as a function of C,TMB alkyl chain length is interpreted in terms of the benzidine moiety being pulled more into the organic phase, away from the water pool. This is a small effect compared to the influence of the water pool size. The increase in the radical cation to deuteron interaction may also be due, in part, to increased water contact with the organic phase since the packing density of AOT headgroups decreases as the water pool grows in size.5 Thus, the ESEM data versus the water pool size is not so much a change in relative position of the C,TMB radical cations with respect to the water pool as a change in the local nuclear distribution caused by the water pool growing in size. This suggests that ESEM may be used as an indirect probe of the water pool size in AOT reversed micelles. Photoyield. The photoyield as a function of C,TMB alkyl chain length is essentially constant for all water pool sizes except wo = 9. The photoyield trend as a function of the alkyl chain length is not well understood. The average photoyield of C,TMB for each wo places wo = 9 lower than wo = 4 but higher than wo 1 14. The observed trend for wo = 9 may represent the region where the photoyield is most sensitive to small changes in WO. Neutral N-alkylphenothiazinesor PC,have been photoionized in AOT reversed micelles and studied by ESR and ESEM techniques.16 As is expected, the similarities are numerous. In both systems, the photoyield decreases as a function of WO; the largest alkyl chain length effect on the photoyield is seen for small WO; the photoyields for wo 1 14 are essentially constant as a function of the alkyl chain length of PC,;the radical cation of PC, has proximity to the water pool and the strength of the electron-deuteron interaction is also dominated by the size of the water pool. In both C,TMB and PC., the photoyield is observed to anticorrelate to the ESEM normalized modulation depth data. This means that the degree of water contact does not significantly influence the photoionization process in AOT reversed micelles as it does for vesicles.Zs.26 Instead, the interface charge has been suggested to be the dominant factor governing the charge separation process in this system. It has been proposed that the negative interface charge should increase as the water pool grows
Photoionization of C,TMB in AOT Micelles in size due to increasing hydration of the sodium counterion to separate it from the AOT surfactant headgroup.I6 This makes the interface charge more negative and provides a higher energy barrier to electron transfer, thus lowering the photoyield. Source of the Sulfite Radical. A question arises as to the origin of the S O j radical. Is it produced by photoionization of trace salt impurities present in commercial AOT, or is it produced by C S bond scission? If the C S bond is broken, then radical (111) in Figure 7 might be expected. Radical I11 has been observed in single crystals of succinic acid and is very stable.2’ The ESR spectrum of irradiated polycrystalline AOT reversed micelles may contain this radical, though attempts to unambiguously identifyit havenot been successful. The powderpatternofradical I11 has a spectral extent of about 100 G and is complicated due to anisotropic hyperfine interaction with the a-proton.27 The data from washed AOT seem to suggest that most of the sulfite radical comes from C S bond scission due to decay or rearrangement of the AOT carboxylic headgroup radical, with some contribution of the sulfite radical due to direct photoionization of trace amounts of sulfite ions. Addition of S042- and S0j2- does not affect the intensity of the sulfite radical signal in these systems. This further suggests that the sulfite radical is produced by C S bond scission and not from ionization of trace quantities of S032- or S042-.
Conclusions The photoyield of C,TMB in AOT reversed micelles is maximized for wo = 4-9. This is attributed to the partial neutralization of the sulfite anionic headgroup by bound sodium ions, which lowers the barrier to passage of the photoelectron into the water pool. The decrease in yield as a function of increasing water pool size is interpreted as due to an increase in the anionic surface charge density by more complete solvation of the sodium counterions in the water pool. Secondary radicals due to electron attachment to the AOT headgroup have been proposed, and several secondary radicals have been tentatively assigned. Electron spin echo modulation spectroscopy shows that the alkylbenzidine cation radicals are located near the water pools. The major influence on the ESEM deuteron modulation depth is the size of the water pool. There is only a small deuteron modulation depth change as a function of C,TMB alkyl chain
The Journal of Physical Chemistry, Vol. 97, No. 40, 1993 10503 length. The interpretation of the alkyl chain length effect is that the benzidine moiety locates more into the organic phase, away from the water pool as the alkyl chain length increases.
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