J. Phys. Chem. 1980, 84, 1491-1495 (47) Kano, K.; Fendler, J . H. Chem. Phys. Lipids 1979, 23, 189. (48) Kano, K.; Fendler, J. H. Biochim. Biophys. Acta 1978, 509, 289. Cafiso, D. S.;Hubbell, W. L. Biochemistry 1978, 77, 3871. (49) Hansen, J. R. J. Phys. Chem. 1974, 78, 256. (50) Quina, F. H.; Toscano, V. G. J. Phys. Chem. 1977, 81, 1750. (51) Infelta, P. P.; Gratzel, M. J . Chem. Phys. 1979, 70,179. (52) Aimgren, M.; Griesser, F.; Thomas, J. K. J. Am. Chem. SOC.1979, 707, 279. (53) Yekta, A.; Aikawa, M.; Turro, N. Chem. Phys. Lett. 1979, 63,543. (54) Turro, N. J.; Yektai, A. J. Am. Chem. SOC. 1978, 700, 5951. (55) Infelta, P. P. Chem. Phys. Lett. 1979, 67, 88. (56) Thomas, J. K., unpublished results. (57) See the following for kinetic treatments of permeabilitiesand tansport in liposomes: Johnson, F. H.; Eyrlng, H. “The Theory of Rate Processes in Biology and Medicine”; Wiley: New York, 1974. Tomkiewicz, M.; Corker, G. A. Chem. Phys. Lett. 1970, 37,537. Roomans, 0. M.; Borst-Pauweis, G. W. F. H. J. Theor. 5\01.1978, 73, 453. Schullery, P. E. Biochem. Biophys. Acta 1977, 468,238. (58) Andresko, J.; Forsen, S. Biochem. Biophys. Res. Commun. 1974, 60, 813. Blttmani, R.; Blau, L. Biochemistry 1972, 7 1 , 4831. Tosteson, D. C.; Ovohlnnikov, Y. A,; Latorre, R. “Membrane Transport Processes”, Raven Press: New York, 1978. Chan, W. K.; Pershan, R. S. Biophys. J. ‘1978, 2 3 , 427. (59) Fendler, J. H. I n “Liposomes in Biological Systems”, Gregoriadis, G.; Allison, A. C., Ed.; Wiley: New York, 1980.
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(60) McLaughlin, S. Curr. Top. Membr. Transp. 1977, 9 , 71. (61) For a kinetic analysis of the electron transfer from N-methylphenothiazlne to the photosensitized long chain derivative of tris(2,2’-bipyridine)ruthenlum perchlorate In surfactant vesicles as a functlon of added NaCi, see Infelta, P. P.; Gratzel, M.; Fendler, J. H. J. Am. Chem. SOC.I n press. (62) Smith, G. D.; Garret, B. B.; H o t S. L.; Barden, R. E. J. Phys. Chem. 1970, 80, 1708. Smith, G. D.; Garrett, B. B.; Holt, S. L.; Barden, R. E. Inorg. Chem. 1077, 76, 558. Smith, 0. D.; Barden, R. E.; Holt, S. L. J. Coord. Chem. 1978, 8 , 157. (63) Letts, K.; Mackay, R. A. Inorg. Chem. 1975, 74, 2990; 2993. (64) Holt, S.,to be submltted for publication. (65) Hermansky, C.; Mackay, R. A. In “Solution Propsrties of Surfactants”; Mlttal, K. L., Ed.; Plenum Press: New York, 1979; p 723. (66) Fletcher, P. D. I.; Robinson, B. H. I n “Proceedings of the NATO Summer School In Chemical and Bioiogical Applications In Relaxatlon Spectroscopy”; Abersytwyth, 1978. (67) Lowe, M. B.; Phillips, J. N. Nature, (London)1961, 790, 262; 1902, 794, 1058. (68) Masslni, P . Voorn, G. Biochim. Biophys. Acta 1968, 753,589. (69) Kiwi, J.; G;atzel, M.; Fendler, J. H.; unpublished results. (70) Infelta, P. P.; Fendler, J. H., Photochem. Photobiol. I n press. (71) Gregoritch, S. J.; Thomas, J. K. J. Phys. Chem. In press. (72) Escabi-Perez, J.; Nome, F.; Fendler, J. H. J. Am. Chem. Soc. 1977, 9 9 , 7749.
Photochemistry in Microemulsions. Photophysical Studies in Oleate/Hexanol/Hexadecane, Oil in Water Microemulsion‘ S. J. Gregoritch and J. K. Thomas” Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556 (Received September 26, 1979)
Laser flash photolysis and spectroscopic techniques have been used to investigate the nature of reactions in oleate/hexanol/hexadecane microemulsions (ME). The gradual evolution of a microemulsion system from a micelle via progressive addition of a cosurfactantlong-chain alcohol and an oil tends to provide a more hydrophobic environment for solutes such as pyrene, etc. Other slightly polar probe molecules such as N-phenylnaphthylamine and pyrenecarboxaldehyde also tend to locate in the ME interior, in contrast to micelle systems where they lie on the surface. Kinetic studies show that pyrene moves freely within the confines of the ME to react with a variety of molecules located in the ME interior or surface. Confining the chromophore to the surface as with pyrenebutyric acid or pyrenesulfonic acid shows that this decreases the efficiency of reaction of these probes with other quencher molecules, compared to similar reactions with other quencher molecules, compared to similar reactions with pyrene. Photoionization studies show that most of the pyrene is located 150 A from the ME surface which leads to low photoionization efficiency. PBA and PSA photoionize efficiency in ME from the surf,sceregion.
Introduction Although the prelcise nature of micellar catalysis is uncertain, experimental data illustrate that reactions are often catalyzed by rnicellar systems with increases in rate of some 1000X.2-5 I[t is suggested that the catalysis is in part due to u crowding together of the reactants at the micelle particles or 1% proximity effect. It is also suggested that the ionic nature of the micellar surface influences the transition state of tlhe reactions6 The influence of surface electrostatic effects in micellar systems has been illustrated in the case of radiation-induced ionic reaction^.^?' Micelles tend to be small with radii in the 10-20-A region, and it is desirable to increase the size of a micelle-like particle over a wider range in order to study in more detail the effect of reactant separation. Microemulsions provide such a possibility as these particles can be several hundred angstroms in radius. These entities are better solvents for many reactions as they accomodate larger molecules more easily than micelles, where in the latter case the dimensions 0022-3654/80/2084-149 1$01.OO/O
of solute and micelle are comparable and distortion of the micelle may occur. The early work of Schulman and co-workersa utilizing electron microscopy showed that microemulsions are spherical particles of radii 200-500 A. Further details of microemulsion systems have been published recently,“l’ and details of photoinitiated reactions ,on these systems also have been p ~ b l i s h e d . l ~The - ~ ~larger size of microemulsions is due to two factors: (a) the cosurfactant (long-chain alcohol) inserts itself into the head group region of the micelle, thereby increasing their separation and leading to a larger structure where (b) the oil provides stability for the larger structure by decreasing the curvature of the assembly. It is apparent that reliable experimental conditions for forming various microemulsion systems are available in the literature. In the present studies we have utilized potassium oleate/hexanol/hexadecane/water microemulsions as established by Shah and co-~orkers.’~ These systems 0 1980 American Chemical Society
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The Journal of Physical Chemistry, Vol. 84, No. 12, 1980
Gregoritch and Thomas
form large oil in water microemulsions containing a larger percentage of hydrocarbon and provide the ideal extreme from small micellar systems. Accordingly we have initiated several photophysical studies in these systems and compared them with similar studies in micellar systems. The nature of solubilization of a molecule in the microemulsion, its approximate location, and the effect of these properties on photoinduced reactions are also studied and discussed.
Experimental Section Materials. Oleic acid was purchased in sealed vials as pure grade from Sigma and used as received; KOH was certified ACS grade from Fisher Co. Hexanol was Baker pure grade, and this sample was also distilled from acidic dinitrophenylhydrazine solution to remove ketones and aldehydes; the original sample gave results identical with the purified one. Hexadecane was purified by passage down a 3-ft-long activated alumina column; water was doubly distilled. The microemulsion was prepared by mixing together 1 g of potassium oleate, 2 mL of hexanol, and 5 mL of hexadecane and diluting with water. These samples were clear, exhibited light scatter, and were found to be stable for several weeks. Solutions were deoxygenated by bubbling with purified N2. Fluorescence measurements were carried out in 1-cm cells on an MPF44 Perkin-Elmer spectrofluorimeter. Optical densities of solutions were measured on a a Varian Cary 219 spectrophotometer; microviscosites were measured on an Elscint polarimeter. The laser photolysis studies were carried out with a Korad KIQ switched ruby laser. The fundamental was doubled giving a 0.1 J lo-* s pulse of 3471-A light. The laser photolysis arrangement has been described.16 Experimental Data An estimate of average size of the microemulsion particles was obtained via triplet quenching and via light scattering. In the triplet quenching experiments a low concentration of methylanthracene, MA ([MA] < [particles]), was placed in the microemulsion and the MA was excited by the 3471-A laser beam; the subsequent decay of triplet MA was monitored at X 4250 A. The MA triplet transfers energy to azulene at a diffusion-controlled rate in ethanol, k = 7 X lo9 M-l s-l. In the microemulsion system an initial rapid decay of MA triplet was observed followed by the normal decay rate in the absence of azulene. Extrapolation of the data back to t = 0 measured the MA triplets that decay via reaction with azulene and those that decay via other means, presumably via impurities. These effects result from a Poisson distribution of azulene among the microemulsion particles, particles containing MA and azulene exhibiting fast decay while particles containing MA only give the slower rate of decay. A similar approach has been used by Turro and Yekta in NaLS rnicel1es.l' The destruction of azulene among the particles may be calculated from the Poisson equation p, = e-(Q)
(1)
where P,, is the probability that a particle contains n molecules of solute at concentration [SI and ( Q ) is the ratio of [particles]/ [SI. The probability of particles with [SI = 0 is given by P = e-(@and is a measure of the ratio of intercepts of MA triplets decaying via the normal slow processes to the total. For [azulene] = 2 X lo4, 4 X lo4, and 8 X lo4 M, the ratios are 0.72, 0.63, and 0.37, respectively. If the [particles] is 7.0 x lo4 M, then these ratios calculated from eq 1 are 0.75, 0.60, and 0.31. The
OLEATE
/=
ME.
HEXADECANE
I
0
100
% HEXANOL
Figure 1. Fluorescence spectrum of pyrene in various media. Bottom half: variation of III/I ratio with hexanol-hexadecane content.
TABLE I: Pyrene Fluorescence Spectrum. 'Ratio of Peak III/I solvent 11111 oleic acid 0.1 M oleate micelle 0.1 M hexanol, 0.1 M oleate 0.4 M hexanol, 0.1M oleate 50%hexanol/50%hexadecane hexadecane 1%oleate ME 1%stearate ME
1.09 0.985
1.07 1.12 1.21 1.56 1.40 1.44
agreement is good and indicates [particles] = 7.0 X lo4 M in the 1% oil in water microemulsion used. If the microemulsion particle is considered to be a spherical oil drop covered by a layer of surfactant, then the radius r of the particle is r (oil drop) + length of the surfactant chain (-25 A). By utilizing the measured value for the [particles] in a 19'0 ME, the r of the oil drop is calculated to be 100 A, thus giving an r particle of -125 A. The light scattering data were obtained using conventional turbidity measurements, and a molecular weight of 7 X lo6 was obtained, i.e., y = 158 A.
Location and Environment of Pyrene Pyrene is located in the microemulsion particle and as shown later it moves freely within the particle. The average environment of the pyrene is measured by observing the fluorescence fine structure. Figure 1 shows the fluorescence spectrum of pyrene under various conditions of the present system, and for convenicence the peaks are marked 1through 5. I t was established earlier that the III/I peak ratio is sensitive to pyrene environment,l' an effect which is also seen in the present measurements. Figure 1also shows the variation in III/I ratio in mixtures of hexadecane and hexanol. Table I gives the III/I ratio for various components of the microemulsion system. It is seen that the environments in oleic acid or potassium oleate micelles are similar, becoming progressively more hydrophobic, i.e., increasing III/I ratio, as the cosurfactant hexanol and then the oil are added.
The Journal of Physical Chemistty, Vol. 84,
Photochemlstry in Microemulsions
,
,
,
,
No. 12, 1980 1493
I C l M / L Bulk-+
,
,
T-~
t t
IC1 M/L. ME.-
Figure 3. Formation of excimers of pyrene and PBA in a hexadecandhexanol mixture and in microemulsions.
t
TABLE 11: Rate Constants for Quenching of Excited Pyrene h (M-l s-l)
L-,
0
(1.1 M / L
10-2
Flgure 2. Effect on iodide ion on the fluorescence of pyrene (P), pyrenesulfonlc acid (PSA), and pyrenebutyric acid (PBA) in a microemulsion. Data plotted as inverse fluorescence vs. [I-].
Surface Probe The probe molecules pyrenecarboxaldehyde (PCHO) and N-phenylnaphthylamine normally reside in the surface of solute particles because of the polar -CHO or amine components of these molecules. The fluorescence of the molecules is also quite dependent on environment showing red shifts with increasing solvent p ~ l a r i t y . *The ~~~ A,, ~ of the PCHO fluorescence is 4400 A in the ME and that of NPN is 4050 A. These indicate quite a hydrophobic environment for these molecules and suggest that they may lie away from the particle surface. The rigidity of the NPN environment was determined by measuring the degree of the NPN fluorescence depolarization in the ME on excitation with polarized light. Almost no polarization of NPN fluorescence was detected; similar data were obtained with the hydrophobic probe diphenylhexatriene. These data indicate a very fluid interior of the ME with a microviscosity not more than 3 cP. Pyrenesulfonic Acid and Pyrenebutyric Acid Pyrenesulfonic acid (PSA) and pyrenebutyric acid (PBA) are expected to reside in the surface region of the ME particle. The absorption spectra of these molecules are solvent dependentz1and have been used previously to argue the location of the probe molecule in a reversed micellar system. The spectral differences are small, however, but the data indicate that these molecules lie in a polar environment. Figure 2 shows a Stern-Volmer-type plot of the quenching of fluorescence of PSA, PBA, and pyrene by I-. The efficiency of the quenching reaction is greatest with PSA and smallest with pyrene. These data are readity interpreted with the condition that I- does not penetrate the hydrophobic core of the ME but does have access to the surfacle region. The data then indicate that most of the pyrene molecules are located well away from the ME surface, that PBA is located closer to the surface, while PSA is in a surface environment where I- gains ready access. The plots are not linear as required for a simple quenching mechanism and indicate more than one type of environment for the probes. This is in accord with a picture where the probes are moving in the ME structure. However, as indicated, the probes tend to distribute more uniquely to a selected region of the ME, and an average location is indicated. The fluorescence spectrum of dimethylaniline (DMA) shows a solvent dependency with maxima at 3600,3285,
probe/ quencher
TI+
CHJ,
DMA
Oleate Microemulsion pyrene 2.3 X 10" 3.4 X 10'' 2.0 X 10" PBA 2.8 x 109 3.0 x 109 1.4 x 109 PSA 1.3 x 1010 6.5 x 109 1.9 x 109 PDA 8.4 X 10'" 5.2 X 10"' 8.2 X 10"
CH,NO,
1.0 X lo9 6.5 x 109 1.8 X
lo9
Oleate Micelle pyrene 8.0 X loio 2.0 X 10"' 1.5 X 1O'O PBA 4.2 x 109 8.5 x 109 4.3 x 109 pyrene PBA
Stearate Microemulsion 1.5 X 10" 3.7 X 10" 3.9 x 109 6.3 x 109
3375,3390, and 3300 A, in water, hexadecane, ME, hexanol, and 2 5 hexanol/hexadecane mixture, respectively. Once again this polar probe was found to reside in a hexanol environment at the ME surface. The polar molecules or ions, e.g., T1+, methylviologen ( M F ) , etc., were found to reside in the ME surface, the ions replacing Na+ ions in the Stern layer.
Excimer Formation It is well established that pyrene and PBA form excimers in most solvents. A typical dependency of excimer yield I e / I mis plotted in Figure 3 vs. solute concentration. The ratio Ie/I,,, is the ratio of the fluorescence intensit at the excimer maximum, X 4650 A, to that a t X 3700 in the monomer fluorescence. The figure variation in Ie/Imfor pyrene and PBA in 1%ME. The excimer of pyrene in ME forms at [pyrene] l/looththat in hexadecane. However, no PBA excimers could be detected in 1%ME at the highest [PBA] used, although excimers form readily in hexadecane and hexanol. These data indicate that pyrene is free to move in an environment similar in viscosity to that of hexadecane; however, PBA experiences more restriction in its movement at the ME surface.
K
-
Quenching Reactions The molecules pyrene (P),pyrenebutyric acid (PBA), pyrenesulfonic acid (PSA), and pyrenedodecanoic acid (PDA) were placed in the microemulsions and in components of this system. The rate of reaction of various quenchers with the excited pyrene chromophore was then observed and the data are shown in Table 11. Photoionization It has been shown previously that pyrene is photoionized by 2 quanta of light of wavelength 3471 A giving the pyrene cation and solvated electron.z2-26 The yield of ions is
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The Journal of Physical Chemistry, Vola84, No. 12, 1980
TABLE 111: Yield of Photoionization of Pyrene and PBA in Various Systems
a
system/probe
pyrenea
PBAa
0.1 M sodium lauryl sulfate 0.1 M oleate micelle 0.5% microemulsion
1.0 0.98 0.38
1.0 1.0 0.9
Observation of yield of e a i at h 600 nm.
TABLE IV: Quenching of Pyrene Chromophore Fluorescence by Oxygen probe/ rate of decay of fluorescence, s-l condition air-saturated [ O,] c , M a 0, saturated [ O,] pyrene
PBA PSA a
2.1 x lo7; 1.8 x 9.8 x 106; 1.9 x 10-4 1.0x 107; 3.0 x 10-4
[O,]M calculated using k ,
c,
Ma
5.3 x 10'; 5 x 1.8 x 107; 10-3 3.5 x 107; 2.2 x 10-3
= 1O'O M-ls-l.
similar in all solvents at short time periods, but owing to rapid ion recombination recombination, the observed yield s varies with solvent polarity. Increasing of ions at solvent polarity leads to more efficient ion separation and is most efficient in anionic micelles.25 In several systems studied the photoionization of pyrene is least efficient in ME. Table I11 shows the yield of photoionization relative to that observed in NaLS and in several components of the ME and the ME itself for pyrene and PBA. The efficiency of photoionization is comparable for PBA in all systems but is low for pyrene in a ME. The observed yield of photoionization in these systems depends on the relative efficiency of escape of the photoproduced ion pair. The anionic micellar surface aids this process. However, if in the ME pyrene is situated at a large distance (150 A) from the polar ME surface, then the photoproduced electron is thermalized in the oil drop interior of the micelle and rapid ion neutralization neutralization leads to low photoionization yield.
Oxygen Quenching Table IV shows R, the rate of decay of fluorescence of pyrene, PBA, and PSA, in ME. These reactions are diffusion controlled in molecular solvents with K = 1O1O 2 X 1O1O M-l s-l for alcohols, alkanes, and water. As the ME particle is fluid, a rate constant for reaction of 1O1O M-l s-l is assumed for all three molecules and hence the [O,] experienced by the molecules is calculated at R/lO1° = M. In the case of pyrene which is in the ME interior the [O,] is much larger than that experienced by PSA and PBA which are at the ME surface and experience [O,] approaching that in water. This observation is in accord with the O2solubility in micelle^^^^^' and NaLS/pentanol/dodecane MEs which are similar to alkanes.
-
Discussion The ME size measured by both turbidity and fluorescence techniques is in keeping with the the large radii normally assigned to these systems. Accordingly, the conventional picture of a ME particle is retained for discussion of the present data, Le., a spherical particle with an oil core covered with a layer of surfactant and cosurfactant. The particle has a fluid structure as indicated by the NPN and diphenylhexatriene fluorescence depolarization data. The kinetic data are more readily discussed by considering two regions of the ME: (a) the interior and (b) the surface. Very polar molecules such as Tl', DMA, M Y , and Ru(bpy) should reside at the ME surface. Indeed the fluorescence spectrum of DMA in a ME supports the location assigned for this molecule. The I- quenching
Gregoritch and Thomas
data indicate that PSA and PBA are primarily located in the ME surface region presumably with the acid group in the ME head group region. The pyrene chromophore of PBA will be located further into the ME then that of PSA. Hydrophobic molecules such as pyrene, CH212,etc., will be located in the the hydrophobic region of the ME, and III/I ratio of the pyrene fluorescence confirming this viewpoint. It has been shown previously that CHJ, partitions into micelles in preference to water.B Oxygen and CH,N02 tend to distribute themselves throughout the aqueous and ME systems, 0, being more concentrated in the ME lipid or oil region, and CH3N02being more concentrated in the water and ME surface region. The kinetics of Table I1 are discussed with the above solute locations in mind. In a 1% ME solution, Le., 1% oil in water, if both reactants are associated with the ME, then the rate of reaction should be -1OOX that in a simple molecular liquid. This demands that little or no restrictions are imposed on movement of the reactants other than those experienced by the reactants in a simple liquid. This type of concept is borne out for pyrene excimer formation which occurs in a ME at 1/100 [pyrene] in hexadecane, in the pyrene quenching by 0, which is rapid in the ME, and in loll M-l s-l quenching with CHJ2 and DMA where k is observed. In homogeneous solution the rate constants 10'O M-l s-l for are close to diffusion control with K solvents of low viscosity (of
