1763
Langmuir 1990,6, 1763-1768
Fading Phenomena of Azo Oil Dyes in Anionic-Nonionic Surfactant Solutions. 4. Singlet Oxygen Formation in the Mixed Micelle Hirotaka Uchiyama,? Sawako Akao,t Masahiko Abe,**tv* and Keizo Oginotvl Faculty of Science and Technology, Science University of Tokyo, 2641, Yamazaki, Noda, Chiba 278, Japan, and Institute of Colloid and Interface Science, Science University of Tokyo, 1-3, Kagurazaka, Shinjuku-ku, Tokyo, J a p a n Received January 2,1990 The formation of the singlet oxygen in the mixed micelle of anionic-nonionic surfactants in aqueous solution is described. The systems studied are sodium dodecyl sulfate (SDS) and alkyl poly(oxyethylene) ethers (C,POE,: m = 12, 14,16, and 18; n = 10,20,30, and 40). The amounts of oxygen both in the mixed surfactant solution and in the mixed micelle are decreased with increasing the total surfactants concentration above 1.0 X 1k2mol/L. The solubility of the oxygen is independent of the mixed molar ratio of surfactants, alkyl chain length, and the poly(oxyethy1ene)chain length in the nonionic surfactant. The fading rate of 4-(phenylazo)-l-naphthol(4-OH) placed in the dark is as fast as the rate of 4-OH placed under the natural light. No additional effect of the superoxide quencher, superoxide dismutase, is observed. The fading rate of 4-OH is decreased with an increase in the concentration of 1,4diazabicyclo[2.2.2]octane,the quencher for the singlet oxygen. The replacement of H20 by D20 enhances the fading rate of 4-OH. The fading rate of 4-OH is accelerated by the addition of the singlet oxygen sensitizer tetraphenylporphyrin and tetraphenylporphyrintetrasulfonic acid. It is clear that the fading phenomenon of 4-OH is caused by the activated oxygen species, singlet oxygen, even though the amount of dissolved oxygen in the mixed micelle is almost the same as in the mixed surfactant solution.
Introduction Solubilization by a surfactant solution is important to various industrial fields such as detergency, cosmetics, and foods. Lately, micellar solubilization is applied for pharmaceutical and biomedical fields as a model system,1,2 and the catalytic and retarding effects of micelles on the degradation of drugs are being investigated.3 Therefore, research on the interactions between surfactant and solubilizate in the micelle are of great importance. For instance, Nakagaki et al.4*5have reported the effect of micellar environment on the photoreduction of azo dyes sensitized by tetraphenylporphyrin (TPP) derivatives. They concluded that the reaction efficiency was reduced when the sensitizer was solubilized in the nonionic surfactant micelles and that the efficiency of the photooxidation of methyl orange is lower above the critical micelle concentration (cmc) than below the cmc, due to the lower reactivity between singlet oxygen produced by the excited TPP and methyl orange in the less polar environment on the micellar surface in the anionic surfactant solution. Handa and co-workerse have also studied the reactivity of singlet oxygen generated by photosensitization in liposomes and reported that the oxidative degradations of the substrate by singlet oxygen were
* Author to whom correspondence should be addressed at Faculty of Science and Technology, Science University of Tokyo, 2641, Yamazaki, Noda, Chiba 278, Japan. + Faculty of Science and Technology. t Institute of Colloid and Interface Science. (1) Attwood, D.; Florence, A. T. Surfactant systems-Their chemistry, Pharmacy and biology; Chapman and Hall: London, 1985; p 124. (2) Fendler, J. H.; Fendler, E. J. Catalysis in micellar and macromolecular systems; Academic Press: New York, 1975; p 293. ( 3 ) Juliano, R. L. Drug Deliuery Systems; Oxford University Press: New York, 1980; p 189. (4) Nakagaki, M.; Sakai, M.; Handa, T. Chem. Pharm. Bull. 1984,32, 4241. (5) Nakagaki, M.; Inoue, K.; Komatsu, H.; Handa, T.; Miyajima, K. Chem. Pharm. Bull. 1988,36, 1; 1988,36,2742. (6) Handa, T.; Takeuchi, H.; Takagi, H.; Toriyama, S.;Kawashima, Y.; Komatsu, H.; Nakagaki, M. Colloid Polym. Sci. 1988, 266, 745. 0743-7463190/2406-1763$02.50JO
considered to be much faster in the polar environment than in the less polar environment. However these studies were not done in a mixed surfactant solution but in a single surfactant solution. Previously, we have reported on the fading phenomena of azo oil dyes in the anionic-nonionic mixed surfactant ~ystems.~-'OThe remarkable fading of 4-(phenylazo)-lnaphthol (4-OH) was a t 1.0 X mol/L of total surfactant concentration and 0.5 mixed molar ratio of SDSS7 Furthermore, the fading rate of 4-OH accelerated with increasing alkyl chain length or with decreasing oxyethylene chain length in the nonionic surfactant molecule. The effect on the fading behavior of 4-OH was larger for a system which can easily form a mixed micelle than for a system in which two kinds of micelles coexist.8 We have also reported the effect of bubbling oxygen gas and/or nitrogen gas on the fading rate of 4-0HS7t8The mixed surfactant solution after aeration with oxygen gas for 10 min showed an accelerated fading rate, while that after bubbling nitrogen gas showed a decelerated rate. Therefore, it is clear that this fading phenomenon of an azo oil dye is related to the oxygen dissolving in the mixed surfactant solution. We have explained that this fading phenomenon of 4-OH might be caused by the singlet oxygen in the hydrophilic part of the mixed micelle, In this paper, we report the amount of oxygen both in the surfactant solution and in the mixed micelle and the effect of some quenchers and sensitizers for the active oxygen species on the fading behavior of 4-OH. We have reconfirmed the possibility of the singlet oxygen formation in the anionic-nonionic mixed micelle in detail. (7) Ogino, K.; Uchiyama, H.; Ohsato, M.; Abe, M. J. Colloid Interface Sci. 1987, 116, 81. (8)Ogino, K.; Uchiyama, H.; Abe, M. Colloid Polym. Sci. 1987,266, 52. (9) Uchiyama, H.; Abe, M.; Ogino, K. Colloid Polym. Sci. 1987,265, 838. (10) Ogino, KO;Abe, M. In Phenomena in Mixed Surfactant Systems. ACS Symp. Ser. 1986,311,68.
Q 1990 American
Chemical Society
1764 Langmuir, Vol. 6, No. 12, 1990 Experimental Section Materials. Anionic Surfactant. Sodium dodecyl sulfate (SDS) was the purest grade product of Tokyo Kasei Kogyo Co., Ltd. (Tokyo), more than 99.7% pure. It was extracted with ether and recrystallized from ethanol. Nonionic Surfactant. Alkyl poly(oxyethy1ene)ethers (CmPOE,, C,Hzm+10(CHzCHz0)20H; m = 10, 12, 14, 16, and 18; C16H330(CH2CH~0)nH, n = 10,20,30,and 40) were supplied by Nihon Surfactant Industries Co., Ltd. (Tokyo). They have a narrow molecular weight distribution. Purities of these surfactants were ascertained by surface tension measurement and differential scanning calorimetry. Azo Oil Dye. The synthesis and purification of 4-(phenylazo)-1-naphthol (4-OH)were described in our previous paper.11 Additions. Pyrene was purchased from Wako Pure Chemical Industries Co., Ltd. It was recrystallized twice from ethanol and passed through silica gel (WakogelC-200 of Wako Pure Chemical Industries Co., Ltd.) in hexane and obtained by evaporation. Superoxide dismutase (Cu, Zn type SOD; SOD), 1,4diazabicyclo[2.2.2]octane (DABCO)from Wako Pure Chemical Industries Co., Ltd., deuterium oxide from Merck Co., and tetraphenylporphine (TPP) and tetraphenylporphine tetrasulfonic acid (TPPS) from Tokyo Kasei Kogyo Co., Ltd., were used as received. Water used in this experiment was twice distilled and was deionized by an ion-exchange instrument (NAN0 pure D-1791 of Barnstead Co., Ltd.) and then distilled again just before use; its resistivity was about 18.0 mQcm, and its pH was 6.7. Method. Preparation of Surfactant Solutions Including Additions and Azo Oil Dye. Into several 100-mL glassstoppered Erlenmeyer flasks, 25-mL portions of a given concentration of anionic surfactant solution were placed, followed by addition of a given concentration of nonionic surfactant solution. The concentrations of the mixed surfactant solution having various mixed molar ratios were above the critical micelle concentration of mixed surfactants. The mixtures were left for 1 h in a thermostat at 30 "C in order to establish their equilibria. A measured amount of some additions was added to each solution, and each solution was stirred by a shaker (Model SS-82D of Tokyo Rikakikai Co., Tokyo) for 24 h and allowed to stand for 24 h thermostated at 30 "C to establish a solubilization equilibrium. 4-OH (5.0 X 10" mol/L) was added to the mixed surfactant solution. Determination of Maximal Absorption Wavelength (A,,,=) and Optical Density of Each Solution. The maximal absorption wavelength and absorbance were measured by a multipurpose recording spectrophotometer (Model MPS-2000 of Shimadzu Co., Tokyo) with a quartz cell (10.0 mm in light pass length) at 30 "C. Determination of the Lifetime of Monomeric Pyrene Fluorescence. The lifetime of monomeric pyrene fluorescence was observed by the time-resolved fluorescence spectrophotometer (Model NAES-1100, Horiba Ltd., Co., Kyoto). The excitation wavelength was 335 nm, and the emission one was 394 nm. Determination of the Amount of Dissolved Oxygen in the Surfactant Solution. The amount of dissolved oxygen in the surfactant solution was determined by an ultra-DO meter (Model UD-1 of Central Kagaku Co., Ltd, Tokyo). Results Dissolved Oxygen in t h e Surfactant Solution. The concentration of dissolved oxygen and the lifetime of pyrene in the SDS/ClsPOEzo mixed surfactant solutions are plotted against the total surfactant concentration a t 30 "C in Figure 1. As the total concentration of mixed surfactants increases, the DO values are kept constant until around 1.0 X mol/L and decreased above t h a t concentration. The lifetime of pyrene is also kept constant until 1.0 X mol/L and increases with an increase in the total surfactant concentration. (11) Abe, M.; Suzuki, N.; Ogino, K.J . Colloid Interface Sci. 1984,99, 226.
Uchiyama et al.
4t
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Mixed molar ratio ; SDS/C,,POE,,=l/l
31 '
'
"
L
'
'
I
'
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'
f
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Concantratlon of surfactants (mol/l)
Figure 1. Amount of dissolved oxygen (DO value) and the lifetime of pyrene in the SDS-CgOEQ mixed surfactant solutions plotted against the total surfactant concentration at 30 OC. 8
I
I
I
I
1
0.2
0.4
0.6
0.8
1.0
0 - 2 0
Mole fraction of SDS Figure 2. DO value and lifetime of pyrene versus the mole fraction of SDS in SDS-ClsPOh mixed surfactant system (total concentration 1.0 x 10" mol/L) at 30 "C.
Figure 2 shows the DO value as well as the lifetime of pyrene versus the mole fraction of SDS. The DO value is increased gradually with an increase in the mole fraction of SDS. On the other hand, the lifetime of pyrene decreased little by little. The effects of alkyl chain length in the nonionic surfactant on the amount of dissolved oxygen and on the lifetime of pyrene are shown in Figure 3. No difference of the lifetime of pyrene is recognized in the mixed surfactant system; the DO value is slightly decreased with an increase in the alkyl chain length in the nonionic surfactant. Figure 4 depicts the changes in the DO value and the lifetime of pyrene in the mixed surfactant system with the poly(oxyethy1ene) chain length in the nonionic surfactant a t 30 O C . The DO values are kept constant, and the lifetime of solubilized pyrene is sparingly increased with the number of ethylene oxide molecules in the nonionic surfactant. As can be seen from Figures 3 and 4, the amount of dissolved oxygen in the mixed surfactant solution and the lifetime of pyrene solubilized into the mixed micelle are almost constant for all mixed surfactant systems.
Langmuir, Vol. 6,No. 12, 1990 1765
Fading of Dyes in Mixed Surfactant Solutions I
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Geiger and Turro'8 have measured the pyrene fluorescence lifetime in various solutions such as water, cationic and anionic surfactant solution, and cyclohexane, which were saturated by oxygen, nitrogen, or air and also degassed. Pyrene fluorescence lifetimes in the SDS solution saturated with oxygen, air, and nitrogen gas were about 58,158, and 314 ns, respectively. The same solution degassed has a lifetime of 314 ns. Geiger and Turro have concluded that the fluorescence lifetime of solubilized pyrene has been used t o show t h a t micelles can be oxygenated and deoxygenated. Their results suggested that oxygen is a t least as soluble in micelles as in water and that oxygen moved across the micelle-water interface. The fluorescence lifetimes of pyrene solubilized into the anionienonionic mixed surfactant solution were measured. The fluorescence lifetime of pyrene in the mixed solution bubbling through the oxygen or nitrogen gas showed a lifetime of 75 and 300 ns, respectively. The lifetime of pyrene fluorescence in the mixed surfactant solution without some treatment was 192 ns. The pyrene fluorescence lifetime is increased with an increase in the concentration of oxygen in the micelle. T h a t is, the fluorescence lifetime of pyrene can be dealt with as one of the reasonable parameters for studying the solubility of oxygen in the micelle since pyrene is solubilized into the micelle in the surfactant solution.
1
100 30 40 POE chain W t h Figure 4. DO value and fluorescence lifetime of pyrene vs the number of ethylene oxide molecules as the hydrophilic part of the nonionic surfactant in SDS-ClePOE, mixed surfactant systems (mixed molar ratio 1/1; total surfactant concentration 1.0 X 10-* mol/L) at 30 OC.
Discussion Examination of Oxygen Solubility in the Mixed Micelles. Some methods are applied for determination of the solubility of oxygen in water or surfactant ~olution.'~-'5 It is, however, difficult to determine the amount of oxygen dissolving in the micelle directly. The dissolved oxygen is well-known to be a typical quencher of fluorescence.16J7 Especially the lifetime of a solubilized fluorescence probe such as an aromatic hydrocarbon like pyrene is decreased by the oxygen dissolving in the solution. (12) Winkler, L. W. Ber. 1889,22, 1964. (13) Battino, R.; Clever, H. L. Chem. Rev. 1966, 66,395. (14) Ben Naim, A.; Baer, 5. Tram. Faraday Soe. 1963,69, 2735. (15) Kouaaka, K.; Kise, H.; %no, M. J . J p n . Oil Chem. Soe. 1980,29, 177. _ .,.
(16) Hautula, R. R.; Schore, N. E.; Turro, N. J. J. Am. Oil Chem. SOC.
1973,95,5508. (17) Wallace, S . C.; Thomas, J. K. Radiat. Res. 1973,54, 49.
of Dissolved Oxygen. As mentioned above, the fading rate of 4-OH was accelerated by the bubbling of oxygen gas through the mixed surfactant solution and decelerated by that of nitrogen gas. Therefore, the fading rate of 4-OH may be proportional t o the concentration of oxygen dissolved in the surfactant solution.7~~ The absorbance a t 480 nm corresponding to the hydrazo form of 4-OH showed a minimum at about 1.0 X mol/L, and it was increased with an increase in the surfactant concentration ,above 1.0 X mol/LS7v8As can be seen from Figure 1, the dissolved oxygen in the surfactant solution was decreased, and the pyrene fluorescence lifetime was increased above 1.0 X mol/L. In other words, the amount of oxygen in the mixed micelle was decreased by increasing the total concentration of surfactants above 1.0 X mol/L. Therefore, the fading phenomenon of 4-OH was not remarkable due to decreasing the amount of oxygen in the mixed micelle. Next, the effect of mixed molar ratio on the amount of oxygen in the mixed surfactant solution and/or mixed micelle was discussed. The absorbance of 4-OH after 48 h becomes a minimum in the vicinity of a SDS/C,POE, molar ratio of l/l,a t which the fading phenomenon was exceptionally observed."v8 As can be seen from Figure 2, the oxygen dissolved in the surfactant solution increased with an increase in the mole fraction of SDS. The amount of oxygen in the mixed micelle seems to be increased slightly as the pyrene fluorescence lifetime decreases with an increase in the mixed molar ratio. However, no minimum appears in the curves of the concentration of dissolved oxygen in the surfactant solution or in the mixed micelle. The clear difference of the dissolved oxygen concentration between the single surfactant system and mixed one is not observed. Figures 3 and 4 represent the dissolved oxygen in the surfactant solution and the lifetime of pyrene plotted against the alkyl and/or poly(oxyethy1ene) chain length in the nonionic surfactant. As shown in Figures 3 and 4, the dissolved oxygens in the mixed surfactant solution and/ or in the mixed micelle are almost constant and in(18)Geiger, M. W.; Turro, N.J. Photochem. Photobiol. 1975,22,273.
Uchiyama et ai.
1766 Langmuir, Vol. 6, No. 12, 1990
dependent of the alkyl chain lengths and poly(oxyethy1ene) chain lengths. The fading rate of 4-OH in the mixed surfactant solution increases with an increase in the number of carbon atoms in the alkyl group of the nonionic surfactant.8 As the oxyethylene chain length in the nonionic surfactant increases, its fading rate decreases.8 Although the fading rate is dependent on the alkyl and poly(oxyethy1ene) chain lengths, the concentration of dissolved oxygen in the micelle is independent of both chain lengths in the nonionic surfactant. In spite of the same concentration of dissolved oxygen in the mixed micelle, the fading rate of 4-OH depends on the mixed molar ratio and chain length in the nonionic surfactant as mentioned above. This fading phenomenon is related to the mixed micelle formation; that is, this fading reaction take place in the hydrophilic part of the mixed micelle because the hydrazo form of 4-OH is decomposed by the oxygen molecules. Existing as much in the pure surfactant solution as in the mixed surfactant solution, the oxygen molecules can be turned into the activated species only in the mixed micelle. Determination of Active Oxygen Species i n t h e Mixed Micelle. The oxygen molecule usually exists as the triplet oxygen of the ground state. This triplet oxygen can be converted into the active oxygen species such as the hydroxyl radical, hydrogen peroxide, superoxide anion radical, and singlet oxygen, which are more reactive than the triplet oxygen. Then, we have studied the possibilities of each active oxygen species formation contributing to the fading phenomena in the mixed micelle. First of all, the possibilities of hydrogen peroxide and hydroxyl radical forming in the mixed micelle are studied. The hydrogen peroxide molecules have a low ability to oxidize with some compounds. It is, however, noted that the hydrogen peroxide is a good source of production of the hydroxyl radical or hydroperoxyl radi~a1.l~ Therefore, the formation of the hydroxyl radical has only to be considered. The hydroxyl radical is the most reactive compound of the active oxygen species.lg In general, irradiation with light is required to form the hydroxyl radical from hydrogen peroxide or water molecule. If 4-OH would be decomposed by the hydroxyl radical, the fading phenomena could not be observed without light. Therefore, it was investigated whether this fading phenomena of 4-OH occurs in a dark place. Figure 5 shows the time dependence of absorbance a t 480 nm of 4-OH in the mixed surfactant solution when placed in the dark after adding 4-OH. Optical densities a t 480 nm decreased with time. The fading rate of 4-OH placed in the dark place was, however, as fast as that under the natural light. The fading phenomenon of 4-OH is ultimately independent of the hydroxyl radical formation in the mixed surfactant solution. Secondly, the possibility of superoxide formation in the mixed surfactant solution is investigated. Superoxide dismutase (SOD)is well-known as a typical quencher for ~uperoxide.'~The additional effect of SOD on the fading rate of 4-OH is examined in the mixed surfactant solution. Figure 6 shows the changes in the absorbance a t 480 nm with time in the presence of SOD in SDS/ClsPOElo mixed surfactant systems a t 30 O C . The fading rates of 4-OH in the presence of SOD are the same as that without SOD and are independent of SOD concentration. There is no additional effect of the superoxide quencher on the fading (19) Yagui, K.; Nakano, M. Kassei Sanso; Ishiyaku Shuppan: Tokyo, 1987.
I
1
o ; under natural light
0.8
0
; in the dark
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l
c
m
20 30 Time ( hour ) Figure 5. Time dependence of absorbance at 480 nm of 4-(phenylazo)-l-naphthol(4-OH) in the SDS-C16POElo mixed surfactant solution when placed in the dark or after not adding 4-OH. 0
0.8
10
}
0; Ounitslml 0 ;0.5unitslml Q;
Punits/ml
e;lOunits/ml
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20
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lime ( hour ) Figure 6. Effect of superoxide dismutase on the fading behavior of 4-OHin SDS-ClePOElo mixed surfactant systems at 30 OC.
behavior of 4-OH. It can be concluded that the superoxide would not be formed in the mixed micelle of SDS/
C,POE,. Next, the possibility of the fading reaction by the singlet oxygen is investigated. The singlet oxygen has two types: one is the lAg type, with electron spins of opposite direction in the same orbital; the other is the lZg type, with spins in a different orbital. As the lifetime of the lZg type is, however, extremely short and decay of the '2, type into lAg or ground state occurs, the existence of 12, could be negligible. DABCO is well-known as one of the typical quenchers for singlet oxygen.18120 The quenching effect on the singlet oxygen related to the fading phenomenon of 4-OH was studied in Figure 7. Figure 7 depicts the time dependence of absorbance a t 480 nm in S D S / C d O E l o (=1/ 1)mixed surfactant solution containing DABCO. The change in the fading phenomenon of 4-OH is observed by the addition of DABCO; the fading rate of 4-OH is decreased with increasing the concentration of DABCO (20) Kuramoto, N.; Kitao, T. J . SOC.Dyers Colour. 1982,98,334.
Langmuir, Vol. 6, No. 12, 1990 1767
Fading of Dyes in Mixed Surfactant Solutions
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20 30 Time ( hour ) Figure 9. Changes of absorbance at 480 nm of 4-OH in the mixed surfactant solution solubilized with TPP at 30 "C. 0
0.8
0 H2O
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-
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0
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Time ( hour ) 0 0
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Figure 8. Effect of repalcement of D20 on the fading rate of 4-OH in SDS-ClsPOElomixed systems at 30 "C. solubilized in the mixed surfactant solution. Therefore, this fading phenomenon of 4-OH could be caused by the singlet oxygen. Next, the following experiment was performed in order to confirm this result. The lifetime of the singlet oxygen However, being in H20 is approximately 2-4.2 ps.19v21+22 55-68 ps in D20123-25the lifetime of the singlet oxygen in D2O is about 10 times longer than that in H2O. This can be explained in terms of intermolecular electric-tovibrational energy transfer:2s the transition energy of the singlet oxygen is exchanged to the internal energy of the water, mainly t o the vibrational energy of the water molecule. The effect of replacement of H2O by deuterium oxide on the fading rate of 4-OH was studied. Figure 8 (21) Wasllerman, H. H.; Murray, R. W. Singlet Oxygen:Academic Press: New York, 1979. (22) Rodgere, M. A. J. J . Am. Chem. SOC.1983,105,6201. (23) Foote, C. S. In Biochemical and clinical aspects of oxygen; Caughey, W. S., Ed.: Academic Press: New York, 1979; pp 603-626. (24) Ogibly, P. R.: Foote, C. S. J. Am. Oil Chem. SOC.1983,106,3423. (25) Merkel, P. G.; K e a m , D. R. J . Am. Oil Chem. SOC.1972,94,7244.
Figure 10. Effect of TPPS on the fading rate of 4-OH in the mixed surfactant solution. shows the absorbance a t 480 nm for SDS/ClsPOEl,-, mixed surfactant solution containing D2O plotted against time. As can be seen from the figure, the decrease in the optical densities for the solution including D2O is faster than that without D2O. The fading rate of 4-OH is increased as the mixed molar ratio of D2O increased. This may be attributed to the fact that the lifetime of the singlet oxygen is lengthened in the D2O surfactant solution. Furthermore, the effect of the singlet oxygen sensitizer on the fading behavior of 4-OH is investigated in the mixed surfactant solution. The singlet oxygen sensitizers used in this study are TPP and TPPS, which are water insoluble and soluble, respectively.4~6The TPP must, therefore, be solubilized into the mixed micelle, and TPPS would be adsorbed a t the mixed micelle surface. Time dependence of absorbance in the mixed surfactant solution solubilized with TPP is shown in Figure 9, and the relationship between absorbance corresponding to the hydrazo form of 4-OH and time in the TPPS solubilized mixed surfactant solution is described in Figure 10. As shown in both figures, the fading rate of 4-OH is accelerated by the addition of the singlet oxygen sensitizer and increases with an increase in the concentration of sensitizer. Concerning how the singlet oxygen is formed in the
1768 Langmuir, Vol. 6, No. 12, 1990
mixed micelle, it might be due to self-sensitized or dyesensitized oxidation in the surfactant solution, as reported in the case of photofading in the organic solvent.20.26 In addition, this fading phenomenon of 4-OH can be accounted for by the lengthened lifetime of singlet oxygen, as reported previo~sly.~-l~ It can be explained that the water molecules in the hydrophilic parts of the mixed micelle are mechanically trapped in the locations between the oxygen atoms of the hydrophilic group of SDS and ethylene oxide in the nonionic surfactant molecule, which depress its vibrational motion; the water molecules’ (26) Griffiths, J.; Hawkins, C. J . Chem. SOC.,Perkin Trans. 2 1977, 747.
Uchiyama et al. vibrational motion is blocked in the hydrophilic parts of the mixed micelle. The singlet oxygen lifetime lengthens, since energy transfer from the singlet oxygen to water molecules has not occurred. Consequently, even though the amount of dissolved oxygen in the micelle is almost the same in each surfactant solution, the activated oxygen species, singlet oxygens, are formed in the mixed micelle, and it causes the fading phenomena of 4-OH.
Acknowledgment. We appreciate the cooperation of Drs. Ayao Kitahara and Kijiro Kon-no, Faculty of Technology, Science University of Tokyo, in measuring the fluorescence lifetime.
