J. Phys. Chem. 1988, 92, 2613-2615 by any specific solute-solvent interaction. It is concluded that HOED, HSHQ, and H6HQ can be employed as probes of the electrostatic surface potential of self-assembled surfactant aggregates. However, for each particular system being investigated it is essential to determine whether or not the indicator is complexed with an anion. This determination is necessary in order to ensure that the correct pKa0value is used.
2613
Postgraduate Research Award. Financial support was provided by the Australian Research Grants Scheme. Registry No. HOED, 58346-32-0; HSHQ, 113451-64-2; H6HQ, 113451-65-3; POED, 70850-53-2; PSHQ, 113451-66-4; P6HQ, 113451-67-5; C12E8, 3055-98-9; DM, 69227-93-6; CTAC, 112-02-7; CTAB, 57-09-0; DTAC, 112-00-5; DTAB, 11 19-94-4; Na DBS, 2515530-0; SDS, 151-21-3; Na DHP, 60285-46-3; DDDAB, 3282-73-3; DHDAB, 70755-47-4; DMPC, 18194-24-6; DPPC, 63-89-8; oleic acid, 112-80-1.
Acknowledgment. C.J.D. was the recipient of a Commonwealth
Photoionization of N,N,N’,N’-Tetramefhylbenzldlne in Anionic-Cationic Mixed Micelles of Sodium Dodecyl Sulfate-Dodecyltrlmethylammonium Chlorlde: Electron Spin Resonance and Electron Spln Echo Modulation Studies Elisabeth Rivara-Minten, Piero Baglioni,+and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77004 (Received: November 12, 1987)
Electron spin echo modulation (ESEM) and electron spin resonance (ESR) spectra of the photogenerated N,N,N’,N‘tetramethylbenzidine cation radical (TMB’) in frozen mixed micelles of dodecyltrimethylammonium chloride (DTAC) and sodium dodecyl sulfate (SDS) have been studied as a function of the mixed micelle composition. ESEM effects due to TMB’ interactions with deuterium in DzO show a decrease of the TMB’-water interaction that depends on the SDS-DTAC mixed micelle composition and reaches a minimum for the equimolar mixed micelle. The efficiency of charge separation upon photoionization of TMB to produce TMB’ measured by ESR correlates with the degree of water penetration into the micelle. ESEM effects due to interaction of x-doxylstearic acid nitroxide probes with deuterium in D20show that the decrease of water penetration is due to higher surface packing due to electrostatic attraction among the polar headgroups of the two surfactants.
were obtained from Fisher, and x-doxylstearic acid spin probes (x-DSA), were obtained from Molecular Probes, Inc. All these products were used as received. A stock solution of TMB was prepared in chloroform. Stock solutions of 0.1 M surfactant were prepared in triply distilled and . deoxygenated water and in deuteriated water (Aldrich). The surfactant solution was added to a film of T M B generated by evaporating the chloroform. The film was solubilized by sonicating for 30 min and by stirring the solution for 3 h at 50 OC in a nitrogen atmosphere. The concentrations of the solutions were 0.1 M SDS, 0.1 M M TMB. The samples were prepared by DTAC, and 1 X mixing these solutions according to the different mole fractions of surfactant studied. Stock solutions of x-DSA probes were prepared in chloroform. Films of the spin probes generated by evaporating the chloroform were dissolved in the surfactant solution in a nitrogen atmosphere. The samples having a mole fraction from 0.3 to 0.6, in which a phase separation occurs at room temperature, were heated for several hours at about 50 OC to obtain a homogeneous micellar solution. The other solutions were mixed for several hours at room temperature. All the samples were sealed in 2-mm i.d. X 3-mm 0.d. Suprasil quartz tubes and frozen rapidly in liquid nitrogen. Photoirradiation of the TMB was carried out at 77 K for 6 minutes with a 150-W xenon lamp (Cermax) filtered with 10 cm
Introduction The influence of organized media such as micelles and vesicles on photoinduced charge separation is well documented.’ The photoionization of a solute like N,N,N’,N’-tetramethylbenzidine (TMB) in micellar solution can be controlled by changing the chemical properties of the host assembly. It has been demonstrated that the photoionization efficiency is affected by micellar charge, micellar size, counterion variation, and added cosurfactants.z” In frozen solutions the photoionization in positively charged micelles (dodecyltrimethylammonium chloride, DTAC) has been found to be about twice as efficient as in negatively charged micelles (sodium dodecyl sulfate, SDS). Electron spin echo modulation (ESEM) results have also shown that the photoionization yield correlates, in frozen micellar solution, with the degree of water penetration into the micellar surfacez.6and with the degree of water organization at the micellar In a recent study Bemas et a1.* showed that at room temperature the water-TMB’ interaction amplitude, which is an essential factor governing the decay rate, is not an independent parameter but is correlated with the effective micellar charge. In the present work we study the variation of the photoionization yield of TMB by varying, in a controlled way, the charge density of a mixed micelle that is generated by changing the surfactant mixing ratio of an anionic (SDS) and a cationic (DTAC) surfactant. In this way the net surface charge of the resulting mixed micelle decreases until neutralization for 1:l mole ratio. The results obtained for frozen micellar solutions show how the degree of water interaction of the photoproduced cation and how the degree of water organization of the micellar interface affect the photoionization yield.
(1) Turro, N. J.; Cox, G.S.; Paczkowski, M. A. Top. Curr. Chem. 1985,
129, 57-97.
( 2 ) Narayana, P. A.; Li, A. S.;Kevan, L. J . Am. Chem. SOC.1982, 104, (3) Arce, R.; Kevan, L. J . Chem. Soc., Faraday Trans. I 1985,81, 1025, 1669. (4) Szajdzinska-Pietek, E.; Maldonado, R.; Kevan, L.; Jones, R. R. M. J . Am. Chem. SOC.1984,106, 4675. (5) Malonado, R.; Kevan, L.; Szajdzinska-Pietek, E.; Jones, R. R. M. J . Chem. Phys. 1984,81, 3958. (6) Baglioni, P.; Kevan, L. J . Phys. Chem. 1987, 91, 2106. (7) Baglioni, P.; Kevan, L. J . Chem. SOC.,Faraday Trans. I , in press. (8) Bernas, A,; Grand, D.; Hautecloque, S.;Giannotti, C. J. Phys. Chem. 1986, 90, 6189.
Experimental Section SDS and DTAC were purchased from Eastman Kodak, TMB was obtained from Aldrich, 1-butanol (HPLC grade) and NaCl ‘Current address: Department of Chemistry, University of Florence, Florence, Italy.
0022-365418812092-2613SO1.SO10 , I
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0 1988 American Chemical Society
2614
The Journal of Physical Chemistry, Vol. 92, No. 9, 1988
Rivara-Minten et al.
io2b I
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----_---__ f -----&+-VJ % ----_----__
TMBISDSIDTAC
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Figure 1. Two-pulse ESE spectra at 4.2 K of photogenerated TMB* in SDS, DTAC, and SDS-DTAC 1:l micelles in D,O. The spectral base lines have been offset vertically to avoid overlap.
1
i
Figure 3. Normalized deuterium modulation depth for TMBt in SDSDTAC micellar solutions prepared in D20 as a function of SDS mole fraction. Data points for comparative experiments with added 0.1 M 1-butanol (0)and 0.2 M NaCl (0)are also shown at 0.5 and 1.0 mole fraction SDS. The dotted line represents the systems where heating was necessary to obtain homogeneous solutions. The dashed line shows a linear variation with mole fraction.
=I-
k
0.3
5
0
01
02 05 07 MOLE FRACTION SDS
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MOLE FRACTION SDS
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Figure 2. TMB’ photoionization yield measured by ESR at 77 K in SDS-DTAC mixed micelles versus the SDS mole fraction. The dotted line represents the systems where heating was necessary to obtain homogeneous solutions. The dashed line shows a linear variation with mole fraction. Data points for comparative experiments with added 0.1 M 1-butanol (0)and 0.2 M NaCl (0) are shown at 0.5 and 1.0 mole fraction SDS.
of water and Corning filter No. 7-51, which passes radiation centered a t 370 nm with 80% transmittance. Electron spin resonance (ESR) spectra were recorded at 7 7 K on a Bruker ESP 300 ESR spectrometer. Two-pulse electron spin echo signals were recorded at 4.2 K on a home-built spectrometer by using 40-11s exciting pulses. Results Figure 1 reports two-pulse electron spin echo spectra at 4.2 K of photogenerated TMB+ in pure and mixed micelles of SDS and DTAC in DzO. The spectra exhibit modulation with a 0.5-ps period, which is indicative of electron-deuterium dipolar interaction, and an additional modulation with a 0.08-ps period is due to interaction with hydrogens. The normalized deuterium modulation depths were computed as described previ~usly.~ Figure 2 shows the relative yield of TMB cation at 7 7 K as a function of SDS mole fraction. The TMB’ yields were obtained from doubly integrated ESR spectra. The values reported in this figure are normalized with respect to the DTAC value taken as 1. Figure 3 reports the variation of the normalized deuterium modulation depths of TMB+ in SDS-DTAC micellar solutions prepared in D 2 0 as a function of the SDS mole fraction. Figures 2 and 3 also report the results obtained for solutions of 1:0 and 1:l SDS-DTAC mixed micelles containing I-butanol (0.1 M) or sodium chloride (0.2 M). Figure 4 shows the variation of the normalized deuterium modulation for x-doxylstearic acid spin probes solubilized in SDS and 1:1 SDS-DTAC mixed micelles. Discussion In order to prevent molecular averaging of the electron-nuclear dipolar interactions monitored in ESEM experiments, it is necessary to work with frozen solutions. Previous studies have demonstrated that the micellar structure is retained upon freezing.2 Hashimoto and Thomas9 have recently found similar aggregation
7
IO 12 DOXYL POSITION, X
16
Figure 4. Normalized deuterium modulation depth for x-doxylstearic acid spin probes solubilized in SDS and SDS-DTAC (1:l) micelles prepared in DzO.
numbers for SDS micelles in liquid and in frozen solutions. Baglioni and KevanIo measured the partition coefficient of several alcohols in frozen SDS micellar solutions and found values in good agreement with those obtained by N M R and thermochemical analyses in liquid solutions. Good agreement is also found for the partition coefficient of 15-crown-5 ether at the SDS micellar interface.I0 Mixed micelles formed from anionic-cationic surfactants show strong deviations from ideal behavior. This is largely due to the electrostatic interactions among the polar head groups. These interactions lead to a decrease of the micellar surface charge, and for a 1:1 mole ratio of the surfactants, the micelles behave, from a macroscopic point of view, as if they were neutral.” The surface “neutralization” produces a gradual increase in the aggregation number of the mixed micelleI2 and a strong increase of the microviscosity of the micellar interface.I3 Figure 2 reports the TMB’ photoionization yield as a function of the mixed micelle composition. The photocation yield, for all the mixtures, is lower than expected from the ideal behavior of the pure components, and it is constant for the mixtures with compositions from 1 .O to about 0.4 SDS mole fraction. For the other mixtures from 0.4 to 0.0 SDS mole fraction the TMB’ photoionization yield increases almost linearly to the pure DTAC value. A first consideration from the previous results is that the photoionization yield depends on the mixed micelle composition, and in particular “neutral” surfaces and “negative” surfaces have similar photoionization yields while the yield increases with a net positive micellar surface charge. Rather similar results to the TMB’ photoionization yield are obtained for the variation of the normalized deuterium modulation depths as a function of the mixed micelle composition; see Figure 3. Negative deviations from ideal behavior for the deuterium (9) Hashimoto, S.; Thomas, J . K. J . Phys. Chem. 1984, 86, 4044 (10) Baglioni, P.; Kevan, L. J . Phys. Chem. 1987, 91, 1516. ( 1 1) Schwuger, M. J. In Anionic Sur/acrants; Lucassen-Reynders, E. H., Ed.; Marcel Dekker: New York, 1981; pp 267-316 and references therein. (12) Malliaris, A,; Binana-Limbele, W.; Zana, R. J. Colloid Interface Sci. 1986, 110, 114. (13) Baglioni, P. In Sur/uctants in Solurion; Mittal, K. L., Bothorel, P., Eds.; Plenum: New York, 1986; Vol. 4, pp 393-404. Baglioni, P ; Dei. L., Ferroni, E , to be submitted for publication.
Tetramethylbenzidine in Mixed Micelles
The Journal of Physical Chemistry, Vol. 92, No. 9, 1988 2615
modulation depths are present for all of the mixtures indicating, for the mixed micelle, a decrease in the TMB+-water interactions with respect to the pure micellar components. As for the TMB+ photoionization yield, little change of the modulation depths is seen for the mixed micelles with compositions from 1.0 to about 0.4 SDS mole fraction, whereas the modulation depths increase for DTAC-rich mixed micelles. As previously stated, the TMB' photoproduction yield in positively charged micelles (DTAC) is almost double that in negatively charged micelles (SDS).S This is due to the larger TMB'-water interactions present in DTAC micelles and to the positive surface potential of DTAC micelles that assist the escape of the photoelectron from the micelle interior into the aqueous m a t r i ~ .A~ decrease of the positive surface potential produces a decrease of the TMB' yield.s Therefore for the SDS-DTAC mixed micelles a decrease in the TMB' yield is expected as the amount of SDS is increased in the mixed micelles (charge effect). For the mixtures of compositions from 0.0 to 0.4 SDS mole ratio a decrease of the TMB+ yield is found as expected from the decrease of the positive surface charge of the mixed micelle, but this decrease is coupled to a decrease of the deuterium modulation depth. Therefore, for these mixtures it is not possible to separate the "charge effect" from the effect of a decrease in the TMB'-water interactions as measured by the deuterium modulation depth. For the other SDS-DTAC mixtures, the photoionization yield as well as the normalized deuterium modulation depth is almost constant over the micellar composition range. Here the decrease of the net negative micellar charge is expected to produce an increase in the TMB' yield since the partial neutralization of the negative surface charge by addition of the cationic surfactant reduces the repulsive barrier for electron escape from the micelle. The invariance of the TMB'-water interaction and the micellar surface charge with respect to the micellar composition strongly supports that at least for these mixtures, the main contribution to the TMB' yield is due to the water-TMB' interactions, while the charge effect seems to have seyondary importance. It should be stressed that this result is not in contrast with other results at room temperature. In fact, as Bernas et aL8J4showed, the decay rate of the generated photocation depends on the micelle effective charge and on the water-TMB+ interaction. We showed that, at least for the system studied, the most important contribution is, at 77 K, due to the water-TMB' interaction. At room temperature, the decrease of the TMB' effective lifetimes correlates fairly well with the increased TMB+-water interaction as deduced from ESEM data. Therefore, the overall yield of TMB' a t room temperature depends at least on two important contributions: (1) the water-TMB' interaction that assists the photocation yield and (2) the activation energy barrier8 due to the micellar interfacial electrostatic potential that assists the stabilization of the photocation. Another consideration emerges from an analysis of Figures 2 and 3. If we consider the percentage deviation from the ideal
behavior (dotted line in Figures 2 and 3), we find that the maximim deviation occurs for the 0.5 mole fraction mixture, possibly due to higher surface packing associated with the electrostatic attractions among the polar headgroups. This has been further investigated by addition of sodium chloride or 1-butanol to the equimolar mixtures. Salt addition is known to affect the surface charge and the interfacial water density of SDS micelles,5 while alcohols6J0~1s have been found to open up the interfacial region by intercalating between the surfactant headgroups and by partial penetration into the micellar core depending on the alkyl chain length. The lack of an effect on the TMB+ yield and deuterium modulation depth of 1-butanol addition to the 0.5 SDS mole fraction solution supports a picture in which the strong electrostatic interactions between the oppositely charged SDS and DTAC headgroups inhibit the increase of the TMB+ yield and TMB+-water interaction due to intercalation of the alcohol. Furthermore, the lack of an effect of sodium chloride addition seems to confirm that the mixed 1:l SDS-DTAC micelle behaves like a %eutral'' micelle. In this case the addition of sodium chloride is expected to have little effect on the micellar interface and therefore on the TMB' yield or deuterium modulation depth. These results suggest that as the micellar surface charge is decreased by addition of a surfactant of opposite charge, the micellar surface becomes more rigid. Water that is known to penetrate below the polar headgroup region16is excluded to some extent, and the deuterium modulation depth decreases. This picture is fully confirmed by the analysis of the deuterium modulation results obtained with x-doxylstearic acid probes. A decrease in deuterium modulation depth (see Figure 4) is found for the 1:l SDS-DTAC mixture with respect to the micelles formed from the pure components. Conclusions
The photoionization yield of TMB' in mixed, frozen micelles of DTAC and SDS mainly depends on the photocation-water interaction. It is shown that the TMB'-water interaction is a function of the composition of the mixed micelle and in particular decreases and reaches a minimum for the 1 :1 SDS-DTAC mixture where the surface packing is higher due to the electrostatic attraction between the oppositely charged polar headgroups of the surfactants. The analysis of the deuterium modulation of x doxylstearic acid probes and the lack of an effect by the addition of 1-butanol or sodium chloride to the equimolar mixture support the presence of high surface packing that probably occurs with the expulsion of some water molecules present among the polar headgroups.
Acknowledgment. This research was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, US.Department of Energy. P.B. thanks MPI for partial financial support. Registry No. TMB, 366-29-0; TMB', 21296-82-2; DTAC, 112-00-5; SDS,151-21-3.
~~
(14) Bernas, A.; Grand, D.; Hautecloque, S.;Chambauded, A. J . Phys. Chem. 1981.85, 3684. Grand, D.; Hautecloque, S.; Bernas, A,; Petit, A. J . Phys. Chem. 1983,87,5236. Hautecloque, S.; Grand, D.; Bernas, A. J. Phys. Chem. 1985, 89, 2705.
(15) Szajdzinska-Pietek, E.; Maldonado, R.; Kevan, L.; Jones, R. R. M.
J . Am. Chem. SOC.1985, 107, 6461. (16) Lindmann, B.; Wernnerstrom, H. Top. Curr. Chem. 1980,87, 53-56.
