J. Phys. Chem. 1980, 84, 2402-2406
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(11) N. N. Uchtin and M. J. Vignale, J . Am. Chem. Soc., 79, 579 (1957). (12) N. N. Llchtin, B. J. Wasserman, J. A. Wasserman, E. V. Clougherty. and J. F. Reardon, submitted for publication in J . Am. Chem. SOC. (13) N. N. Lichtln and H. Kliman, J . Chem. Eng. Data, 8 , 178 (1963). (14) N. N. Lichtin, B. J. Wasserman, J. A. Wasserman, E. V. Clougherty, and J. F. Reardon, manuscript in preparation. (15) C. W. Davies in “Ionic Assoclatlon”, Butterworth, London, 1961, p 130. (16) C. B. Monk in “Electrolytlc Dissociation”, Academic Press, New York, 1961, p 72. (17) H. S. Hamed and B. B. Owen in “The Physical Chemlsby of Electrolytic Solutions”, Rheinhold, New York, 1958, p 164. (18) Reference 17, p 64. (19) Reference 17, p 68.
(20) Reference 17, p 167. (21) R. Damico, J . Org. Chem., 29, 1971 (1964). (22) R. A. Robinson and R. H. Stokes in “Electrolyte Solutions”, Butterworth, London, 1959, p 396. (23) L. Paullng In “The Nature of The Chemical Bond”, Cornell University Press, Ithaca, NY, 1940, Chapter 10. (24) G. Scatchard and S. S. Prentlss, J. Am. Chem. Soc., 55,4355 (1933). (25) R. P. Seward, J. Am. Chem. SOC.,56, 2610 (1934). (26) R. P. Seward and C. H. Hamblet, J. Am. Chem. Soc., 54,554 (1932). (27) A. W. Scholl, A. W. Hutchison, and G. C. Chandlee, J. Am. Chem. SOC.,55, 3081 (1933). (28) L. J. Nunez and M. C. Day, J . Phys. Chem., 65, 164 (1961). (29) H. Taniguchi and G. J. Janz, J . Phys. Chem., 61, 688 (1957). (30) B. 0. Heston and N. E. Hall, J . Am. Chem. Soc., 56, 1462 (1934).
Light-Induced Redox Reactions of Proflavin in Aqueous and Micellar Solution Marie-Paule Pileni” and Michael Gratzel Instltuf de Chlmie Physlque, Ecole Polytechnlque Fgdgrale, Lausanne, Switzerland (Received: January 18, 1980)
Primary photoprocesses and photochemical behavior of proflavin (PF) were investigated in aqueous and anionic micellar solution. Micellar effects are noted in the pK of the singlet and triplet excited states, which are due to differences in surface and bulk pH. The photoredox behavior is distinguished by (a) a monophotonic photoionization process and T-T annihilation leading to production of oxidized and reduced proflavin radicals, in water solution, and (b) a biphotonic photoionization process in NaLS solution. Because of strong adsorption of PF on the surface of anionic micelles, the T-T annihilation is impaired in the surfactant solutions. Reduced PF radicals are also produced in aqueous solution in the presence of triethanolamine, where reductive quenching of the triplet states is dominant. They are distinguished by a long lifetime and a high reactivity toward colloidal platinum. The kinetics of intervention of this catalyst in the radical reaction is illustrated directly.
Introduction Proflavin (PF) has recently attracted attention as a sensitizer in the light-induced hydrogen formation from water.’ Astonishingly high turnover numbers have been obtained with this dye2reflecting chemical stability in the oxidation states of interest. The potential use of P F in light energy conversion systems warrants detailed investigation on the elementary processes induced by light excitation. Although the available literature on the photochemistry of PF analogues is already rich: there are conflicting reports on the role of intermediates in the pathway of PF photoreduction! This has prompted us to undertake a comprehensive study of the elementary photoreactions involving PF in aqueous solutions. The present report focuses on a comparison of PF reactivity in aqueous and micellar solutions. A detailed study of the aqueous photochemistry will be published separatelyS6
Chemicals. Proflavin hemisulfate, PF, and triethanolamine, TEOA, were both BDH products and used as supplied by the vendor. Sodium lauryl sulfate, NaLS (Merck “for tenside investigation”), was purified by multiple recrystallization from ethanol/water mixture. Water was distilled from KMn04 and subsequently twice from a quartz still. The pH was adjusted by HC1 or NaOH addition. Results and Discussion 1. Ground-State pK. Proflavin exists in aqueous solution in three different forms which are related through the protolytic equilibria:
Experimental Section Apparatus. Flash photolysis experiments were carried out by using a JK-2000 Nd laser with a pulse of 15-11s duration and an energy of 10-100 mJ as measured by a bolometer. A small fraction of the laser beam was reflected into an ITT F 4044 54 photodiode to monitor its intensity. Conventional kinetic spectroscopywas employed to detect transients produced by the flash in the sample cell. Details have been given elsewhere.6 Fluorescence spectra were recorded on a Hitachi-Perkin-Elmer MPF-44A spectrofluorimeter. A single photon counting unit, PRA 2000, was used for fluorescence lifetime measurements.
* Address correspondence to this author at Universitg P. et M. Curie, Laboratoire de Chimie-Physique,75005 Paris, France. 0022-3854/80/2084-2402$01.0010
IgCK;2N
H p 2 J a H 2
In aqueous solution the two pK values have been determined as 0.2 and 9.5, respectively.’ In micellar solution of sodium lauryl sulfate (NaLS), the second pK increases to 12.5 as determined by optical absorption spectroscopy. The spectrum of PFH+ in the micellar medium exhibits = a maximum at 455 nm (in pure aqueous solution A,, 445 nm). PF is located at 400 nm in micellar as well as pure aqueous medium. The shift in the pK value is due to an increased local Ht concentration in the Stern layer of anionic micelles.* No absorption changes were noted when the pH was lowered to -1. This is taken as an in0 1980 American Chemical Society
The Journal of Physical Chemistry, Vol. 84, No. 19, 1980 2403
Light-Induced Redox Reactions of Proflavln 1 _.
0.5..
3
5
7
9
11
,PH I-
---
2 L d 3 5 7 9 11 13 PH Flgure 1. Dependence of the relative fluorescence intensity (I,) and fluoroescence lifetimes (T) with the pH of proflavln In NaLS solution (0)and profiavin In water (0). pHz 3
n
41
-r-tii i i l i ,
i
---.
Flgure 3. Decay of transient obsewed at X = 820 and 1160 nm of proflavin in NaLS solution at pH 3 ([PF] = 3 X 10" M, [NaLS] = 5 X lo-' M).
the latter medium, 1ps after the pulse the spectrum displays four maxima located at 560,720,820, and 1160 nm. Forty microsecond after the pulse, only the 720-nm signal and the prominent absorption at 820 nm are observed, the signals at 560 and 1160 nm having totally disappeared. From the kinetic analysis, it is inferred that two transient species are present after the laser excitation. The first absorbing at 560 and 1160 nm has a lifetime of 7 ps in deaerated solution and is identified with the PF triplet state (eq 2). Addition of oxygen drastically reduces the PFH+ -!!L '(PFH+)* 5 3(PFH+)*
triplet lifetime, which is only 0.2 ps in air-saturated solution. The second transient having a pronounced maximum at 820 nm is readily identified with the PF cation radical (PFH2+.).9 The fact that it is produced during the laser pulse indicated that photoionization of PF occurs in parallel with triplet formation (eq 3). The cations thus
k?F%nmd
nrn
Flgure 2. Transient spectra obtained by laser photolysis at pH 3 of proflavin in NaLS and in water ([PF] = 3 X M, [NaLS] = 5 X lo-* M): (a) (-) 1 ps and (.e.) 40 ps after the laser pulse; (b) (-) 500 ns and (.e.) 100 ps after the laser pulse.
dication that in the micellar solution the second protonation under very acid conditions occurs less readily than in pure water. Under these conditions of very high ionic strength, double layer effects are minimized. 2. Excited Singlet p K Values. The fluorescence behavior. of proflavin is strongly dependent on the solution pH in the NaLS micellar system. Thus, the maximum in the emission spectrum shifts from 545 to 506 nm when the pH is increased above 3. Concomitantly, one observes significant changes in the fluorescence yield and lifetime (Figure 1). Both parameters increase significantly in the pH domain between 3 and 5, until a plateau is attained. Thereafter, a further increase in the fluorescence lifetime is noted which is associated with a decrease in the fluorescence yield. From this behavior, one would conclude that the pK values for the acid-base equilibria involving proflavin singlet states (eq 1)are 3.5 and 12.5, respectively. '(PFH2+)*
PKi
H+
'(PFH+)*
PKZ
H+
(2)
l(PF)*
(1)
In water, one finds' pK1 = 1.5 and pK2 = 12.5. The more ready protonation of '(PFH+)* on the micelle as compared t o the pure aqueous solution is not yet understood. 3. Time-Resolved Spectral and Kinetic Studies. In Figure 2 are shown laser photolysis results obtained from aqueous and NaLS micellar solutions of PF at pH 3. In
nhv
PFH2+.+ e-
(3) produced have a strikingly long lifetime (7 > 10 ms) which is unaffected by oxygen. The stabilization of cation radicals by anionic micelles has been observed previously1° and is attributed to a prevention of the thermal recombination of PFH2+.and hydrated electron by the repulsive potential field of the anionic micelle. Oscilloscope traces which illustrate these kinetic effects are shown in Figure 3. In NaLS solution we note a fractional decay of the signal at 820 nm which is due to the residual absorption of triplet states. In contrast, at 1160 nm, where the triplet absorption is dominant, the signal returns to the base line with a time constant of 7 ps. A point which has to be dealt with now is whether the photoionization occurs via mono- or biphotonic mechanism. This was checked by investigating the effect of laser intensity on the product yield. Figure 4 shows that in NaLS solution the transient absorption at 820 nm increases in a quadratic fashion with the intensity both in aerated and deaerated solutions. In the former case, the points were taken 40 ps after the laser pulse. If, instead, a time delay of 1ps is selected, then the experimental data fit a straight line with a slope of 1.2. From these results, one infers that the photoionization in anionic micelles obeys a biphotonic mechanism. The "mixed order" of 1.2 observed in deaerated solutions at At = 1 ps arises from contribution of triplets to the absorption at 820 nm. I t is instructive to compare these data to the results obtained in pure aqueous solution. The transient spectrum observed at At = 500 ns in deaerated solutions exhibits PFH+
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The Journal of Physical Chemistry, Vol. 84,No. 19, 1980
Pileni and Gratzel
~-2-m NALS
/ \ I
I5
Flgure 4. Laser intensity effect on transient observed at 820 nm of proflavin at pH 3 ([PF] = 3 X M). (a) In NaLS solution: (0)1 ps, (x) 40 ps after the laser pulse in deaerated solution, and (0)2 ps after the pulse in aerated solution ([NaLS] = 5 X M). (b) I n water solution: (0) 500 ns, (X) 100 ps after the laser pulse in deaerated solution, and ( 0 )17 ps after the laser pulse in aerated solution.
E
10 600, p 3 1 0 0 0 , h n m ~ ,
ARGON
h:820nm pH=3 k96mJ
NALS
I.
1h:lIW nm
l=YS
1
h.820nn
/--
H20
Flgure 5. Transient decays observed at 820 nm of proflavin in NaLS M, and in water in deareated and aerated solution ([PF] = 3 X [NaLS] = 5 X lo-* M).
several maxima of which the peak of the cation radical at 820 nm is noteworthy. The latter is the only one which persists after At = 100 ps (Figure 2). We attribute these optical effects to the formation of cations and triplet s t a h . In an attempt to verify the mechanism of photoionization, we examined again the effect of laser dose on the transient absorbance at 820 nm. In deaerated solutions, the slope of the logarithmic is 1 at At = 500 ns and 0.75 at At = 100 ps respectively. The value of 0.75 is also observed in the presence of air after At = 17 1s. As the absorbance observed shortly after the laser pulse contains both contributions from triplets and cations, it is difficult to assess the order of photoionization from these data. The prompt appearance of cation radicals after the laser pulse can be due to monophotonic photoionization eq 3. Consecutively there is a second and slower pathway of ion formation. It is apparent from Figure 5 that the initial steep rise of the 820 nm absorbance followed, at low laser intensity, by a slower growth which attains a plateau within a period of 20-30 ps. As the kinetics of this process match those at the triplet decay, we attribute it to the formation of cation via T-T annihilation reaction 4. Reaction 4 is suppressed 23(PFH+)* PFH2+*+ PFH. (4) upon admission of oxygen to the solution. Interestingly, it is also not observed in NaLS solution where PFH+ is strongly associated with the micellar aggregates. Here the intermicellar repulsion prevents effectively the eventual encounter of triplet states. I t is concluded that in aqueous solution two pathways for proflavin cation radical formation are available: monophotonic ejection of electron and T-T annihilation. Only the former can occur in NaLS micellar solution where T-T interactions are impaired by repulsive electrostatic
-
1
1
,
,
? 3 5 7 9 PY2 4 6 8
,
I . .
pH24 6 8
pd
Figure 7. pH effects ( A ) on the triplet decay, (B)on the triplet optical density at 1100 nm, and (c) on cation radical decay.
interactions. Our data agree with the conclusions derived from low-temperature studies in rigid glasses where a monophotonic photoejection of electrons was postulated." 4. Flash Photolysis Studies in Alkaline Solutions. Laser photolysis results obtained from alkaline proflavin solutions are shown in Figure 6. In micellar solutions, the transient spectrum observed 1 KS after the laser pulse distinguishes itself from the one shown in Figure 2 through the virtual absence of the cation radical absorption at 820 nm. A more detailed analysis of the pH effects showed that the cation radical decay is sharply enhanced upon increasing the OH- concentration (Figure 712). At pH >9, this process is so fast that it occurs already within the laser pulse. Apparently the deprotonation reaction 5 is rePFH2+* PF+* H+ (5) sponsible for this observation. Changes in pH were also found to affect the triplet spectrum and the lifetime. Thus,when the pH is increased from 3 to 8, the maximum is shifted abruptly from 1160 to 1080 nm with a concurrent augmentation in the lifetime by a factor of 5 (Figure 7, a and b). These effects may be rationalized in terms of an acid-base equilibrium of the TI state, i.e.
+
-+
pK
4
3(PFH22+)* e3(PFH+)*+ H+ (6) In aqueous solutions, we note the peak of PFH2+.at 820 nm together with the triplet absorption around 1080 nm and a broad maximum at 720 nm. The latter is assigned to hydrated electrons formed via reaction 3. Within 400 ps after the pulse, all three species have practically disappeared, leaving behind a transient with a maximum at
Light-Induced Redox Reactions of Proflavin
Triplet State Singlet state
'
'[PFHy)* l(PFHfl*
Ground State
k's o 0 6 0 0 ..i.o 0.600 - - .
......
I
'IPF)'
PF
. .-
Flgure 8. Transient spectrum obtalned by laser pulse (-) 500 ns and (- - -) 400 ps after the laser pulse of proflavin in water with TEOA (2 X lo-* M) at pH 8. Insert: transient decay observed at 550 nm (A) without catalyst and (B)i with PVA catalyst.
'
Triplet State
520 nm. This absorption arises from PF semiquinone radicals (PF-a) produced via reaction 3 and the reduction of PF by hydrated electrons (eq 7). The latter process PFH+ ea; PFH(7)
-
may only involve part of the hydrated electrons, as their recombination with cation radicals, Le., the inverse of reaction 3, is also possible. PFH. is known to dissociate in neutral or basic solution, the pK value of the equilibrium PFH. ir PF-* Hf (8) being 4.5.' The absorption band at 520 nm is due to the anion PF;, the neutral form PFH. having no significant absorption in the investigated wavelength region. This explains the finding that in Figure 2b only the cation radical peak appears despite the fact that the dismutation of triplets produces both PFH2+. and PFH.. 5. Reductive Quenching of Proflavin Triplets. From the reduction potential of proflavin, E,' = -0.78 V," and its triplet energy (2.14 eV)I2 one predicts for the redox potential of the triplet state a value of 1.36 eV. Hence, in the presence of trilethanolamine in aqueous solution one observes reductive quenchingldaccording to eq 9. Reac3PFH+ 'I'EOA PFH. TEOA' (9) tion 9 is irreversible because of the subsequent rapid deprotonation of TEOA+. It provides a means to selectively produce PF semiquinone and study its spectral and kinetic properties. Laser phlotolysis data obtained with 1.5 X M TEOA are shown in Figure 8. Because of reaction 9, the triplet lifetime is here significantly reduced, the spectrum at At = 10 ps displaying essentially the features of PF-0. These results confirm the assignment of the 520-nm band in Figure 6 to the PF anion radical. We note, however, from the inserted oscillograms that PF- is much more stable in TEOA-containing solution than in pure water. This is explained by the fact that, in the former medium, there is only one decay route for PF-, i.e.
+
-
+
-
2l?F-. products (10) whereas, in the latter, two channels are available, via reaction 10 and recombination with cation radicals (reaction 11). The surprisingly high turnover numbers found for PF-e 4- PFH?+- ZPFH+ (11) proflavin in hydrogen-producing systems may be rationalized in terms of the longevity and stability of P F anion radical in water. Recently, it has been shown in continuous photolysis experiments that PF, apart from sensitizing the initial electron-transfer event, can also act as an intermediate promoting hydrogen evolution in the presence of a Pt catalyst. A prerequisite for such an intervention is that the electron transfer from reduced PF, Le., PF-a to the colloidal particles of platine occurs efficiently and at
-
'
(PFH~+I*
Singlet stat*
"(pFH~+)*
Ground State
ppHf
(PFH'I'
'IPF)*
(?Xi*)'
i 0
+
'IPFH+I'
PFH+
600 1000 Anrn
+
I
IPFH*)*
PFH+
2
4
?F
6
8
1 0 1 . 2
PH
a high speed. Indeed, the second oscillogram inserted in Figure 8 clearly illustrates that the lifetime of the 520-nm absorption is drastically reduced if finely dispersed colloidal platine is added to the solution. We attribute this to the charge transfer from reduced PF to colloidal Pt which, as has been shown earlier,ld involves quantitative formation of hydrogen from water.
Conclusions The present study illustrates the importance of acidbase equilibria and redox processes in the photochemistry of proflavin in aqueous solutions. The formation of oxidized species (cation radicals) is associated with mono- or biphotonic photoionization according to the medium. In assigning transient absorptions, one has to take extreme care in the interpretation of pH effects, since a multitude of acid-base equilibria involving excited states and ion radicals are to be considered. For the excited states, these have been summarized in Table I, which lists pK values for triplets and singlets in aqueous and micellar systems. Anionic micelles facilitate this task considerably, as they suppress the T-T annihilation process and stabilize the cation radical drastically. In water solution a monophotonic ejection of electron occurs, whereas in NaLS solution the process is biphotonic. The relay function of PFH+ in the hydrogen-generating system deserves particular attention and demonstrates the potential of this sensitizer in light-energy conversion systems. Acknowledgment. We thank D. Duonghong for the preparation of the drawings. We are grateful for support of this work by Swiss National Foundation (grant No. 4.061.076.04) and Ciba Geigy, Basel, Switzerland, for financial aid. References and Notes (a)B. V. Koriakin, T. S. Dzhabiev, and A. E. Shllov, Ookl. Mad. Nauk. SSSR,233, 620 (1977); (b) M. Kirch, J. M. Lehn, and J. P. Sawage, Helv. Chim. Acta, 62, 1345 (1979); (c) A. I. Krasna, Photochem. Photobiol. 29, 267 (1979); (d) K. Kalyanasundaran and M. Gratzel, J . Chem. SOC. Chem. Commun., 1137 (1979). See ref 1, b and d. (a) R. M. Danzinger, K. H. Bar-Eli, and K. Weiss, J . Phys. Chem., 71, 2633 (1967); (b) A. Kellmann, Photochem. Photobiol., 14, 85 (1971); 20, 103 (1979); J. Phys. Chem., 81, 1195 (1977); (c) R. Bonneau, J. Joussot-Dubien, and J. Faure, Photochem. Photobbl., 17, 313 (1973); (d) R. Bonneau. P. Fornier de Violet, and J. Joussot-Dubien, ibM., 19, 129 (1974); (e) R. Bonneau and J. Pereyre, ibid., 21, 173 (1975); (f) I. C. Ferreira and A. Harriman, J . Chem. Soc., Faraday Trans. 7,73, 1085 (1977); (9) U. Steiner, 0. Winter, and H. E. A. Kramer, J . Phys. Chem., 81, 1104 (1977); (h) E. Vogelmann, W. Rauscher, and H. E. A. Kramer, Photochem. Phofobiol., 29, 217 (1979); (i) A. Kellmann and Y. Lion, ibM.. 29, 217 (1979); (j) B. Soap, A. Kellmann, M. Martin, and L. Llnquist, Chem. Phys. Lett., 13, 241 (1972). (a) J. S. Bellin, R. Alexander, and R. D. Mahoney, Photochem. Photobb/.,17, 17 (1973); (b) K. Kikuchi and H. Koizumi, Bull. Chem. SOC.Jpn., 40, 736 (1967).
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(5) K. Kabyanasundaranand D. Duonghong, to be submitted to J. Phys. Chem . (6) G. Rothenberger, P. P. Infelta, and M. Gratzei, J . Phys. Chem., 83, 1871 (1979). (7) N. Mataga, Y. Kaifu, and M . Koizurni, Bull. Chem. SOC. Jpn., 29, 373 (1956). (8) (a) P. Mukerjee and K. Banerjee, J. Phys. Chern., 88, 3567 (1964); (b) M. S. Fernandez and P. Fromheiz, /bid.,81, 1755 (1977); (c) K. L. Mittal, Ed., "Micellization, Solubilization and Microemulsions", Plenum Press, New York, 1976.
(9) (a) E. C. Lirn, C. P. Larzara, M. Y. Yang, and G. W. Swenson, J . Chem. Phys., 43, 970 (1965); (b) E. C. Lirn and W. Y. Wen, /bid., 39, 847 (1963); (c) E. C. Lim and G. W. Swenson, lbM., 36, 118 (1963). (10) S. A. Alkaitis and M.Gratzel, J . Am. Chern. Soc., 98, 3549 (1976). (11) R. C. Kaye and H. I. Stonehiil, J. Chem. SOC.,2638 (1951). (12) S. P. McGlynn, T. Azurni, and M. Kinoshka, "Mdecular Spectroscopy of the Triplet State, Prentice-Hall, Englewood Cliffs, NJ, 1969. (13) K. Kalyanasundaran, J. Kiwi, and M. Gratzel, Helv. Chirn. Acta, 61, 2720 (1978).
Partitioning of Aromatic Alcohols between a Water Solution and Cationic Micelles Eduardo Llssl,' Elsa Abuln, and Ana M. Rocha Departamento de Qdmica. Facultad de Clencb, UnlversMsd T6cnica del Estado, Santlago, Chile (Received: December 31, 1979)
The distribution constants of alcohols of general formula HQ(CH2),CeH5(n = 0-3) between water (0.5 M in sulfate ions) and cetyltrimethylammonium micelles have been obtained from fluorescence measurements by employing nickel ions as selective quenchers of the fluorescence arising from the water phase. The results obtained show that the addition of the first and the second methylene groups decreases the fraction of probe incorporated into the micelle while the third group increases it. This anomalous behavior is not observed for the distribution of the probes between n-heptane and water (0.5 M in zinc sulfate). The results are interpreted by taking into account the tendency of both the hydroxyl and the phenyl groups to be located in the micellar surface.
The distribution of a probe molecule between the aqueous and micellar phases is a matter of current interest.*-3 Similarly, the location of the probe inside the micelle is also a matter of relevance since it can determine the accessibility of the probe to reactants present in either the aqueous or micellar phases. Fluorescence measurements have been extensively employed to determine both the probe distribution at low occupancie~~-~ and the position of the probe inside the micelle.gg In the present work we have applied this technique to determine the distribution of probes of general formula HO(CH2),,C6H5(n = 0-3) between water and cetyltrimethylammonium bromide micelles. These molecules were considered of interest since they contain a hydrophobic chain between a polar head (the OH group) and a group (the benzene ring) that in micelles is known to have a strong tendency to be located near the surface.l0
Experimental Section Materials. Cetyltrimethylammonium bromide (CTAB) (Merck, p.a.) was purified by recrystallization." The phenols (Fluka, p.a.) were purified by sublimation. Benzyl alcohol, 2-phenylethanol, and 3-phenylpropanol (Merck p.a.) were used as purchased or purified as described in the next section; nickel sulfate (Merck, p.a.) and n-heptane (Fluka p.a.) were used without further purification. Distribution Studies. The experimental method employed to determine the probe distribution between the aqueous and micellar phases was similar to that previously d e ~ c r i b e d . ~ JAll ~ measurements were carried out at (probe)/(CTAB) below 0.01 and employing a 0.5 M sulfate solution as aqueous phase. This low (probe)/(CTAB) assures a low level of occupancy (less than 20% double occupancy if the aggregation number is considered to be nearly 100). In agreement with this predominantly single occupancy, only monomeric emission was observed for all the compounds considered. When phenol or p-ethylphenol was the probe considered, the pH of the solution was kept a t 3.5 by adding concentrated HCl acid. Quenching experiments were done at fixed sulfate concentration (0.5 M) 0022-3654/80/2084-2406$0 1.OO/O
and variable amounts of Ni2+and Zn2+ions. The zinc ion was inert toward the excited probes in the aqueous and micellar phases. The distribution between the aqueous and n-heptane phases was determined spectrophotometrically by measuring the W absorption of both phases after equilibration. These measurements were carried out a t low probe cow centration (less than M) in order to obtain the partition constant in the highly diluted region. The results obtained were independent of the probe concentration. When only a small fraction of the probe is present in one of the phases, the results could be influenced by the presence of small amounts of impurities selectively extracted and with large extinction coefficients and/or high fluorescence quantum yields. In order to test this possibility some distributions were carried out employing solutions of the probe that had been previously washed several times either with water (when the probe was introduced in the n-heptane phase) or with n-heptane (when the probe was introduced in the aqueous phase). The results obtained were independent of the previous treatment of the probe solution.
Results Distribution of the Probes between Water (0.5 M in Zinc Sulfate) and n-Heptane. The values of the distribution constant defined by eq 1 are given in Table I. K H j W = (probe)n-heptane/(probe)watar (1) These values were obtained with the concentrations expressed in mol/L. The use of concentrations instead of activities is justified by the low concentrations employed. In this table are also included the values of the molar free energies of transfer obtained by employing the diluted solutions as reference state. The trend in KHp, as well as the increment in the free energy of transfer introduced by each methylene group, is similar to those obtained in other systems and can be considered as n0rma1.l~ Distribution of the Probes between Water (0.5 M in Sulfate) and Micelles. The fluorescenceof water solutions 0 1980 American Chemical Sociefy
