1436
J. Phys. Chem. 1995, 99, 1436-1441
Clouding of Nonionic Detergents: Energy Transfer to a Solubilized Probe Gabor Komaromy-Hiller and Ray von Wandruszka" Department of Chemistry, University of Idaho, Moscow, Idaho 83844-2343 Received: July 14, 1994; In Final Form: November IO, 1994@
Both monomeric and aggregated (polymeric) forms of Triton X-114 in aqueous solution are found to sensitize the fluorescence of solubilized perylene. There is evidence of strengthened monomer-acceptor association in the aqueous phase that is formed at the cloud point. In the detergent phase, no monomer excitation is observed, but sensitization by aggregates is strong. However, the emission intensity is only 67% of what is expected from probe distribution effects, indicating the thermal agitation at the cloud point is sufficient to disturb the relatively weak donor-acceptor association. In "artificially" prepared detergent phases, the sensitized emission intensity increases with detergent content and suggests an environmental change around 30% Triton X-114. Donor-acceptor distances between the detergent donor and associated perylene in aqueous solution decrease with increasing concentration, but as the micelles grow, the distance increases again. Monomer-excited sensitization shows that the micellar environment includes detergent species that are relatively loosely associated. The micellar aggregate does not act as a single polymeric donor, but appears to be excited in smaller subsegments.
Introduction Unlike their ionic counterparts, nonionic detergents in aqueous solution have a lower, rather than an upper, consolute temperature. When heated to this temperature, known as the cloud point, the solution becomes turbid and phase separation takes place. A detergent-rich phase is formed which is of smaller volume and higher density than the aqueous bulk. The latter is left with a relatively low detergent Depending on the surfactant, clouding is variously ascribed to an increase in micellar size, intermicellar attraction, or b ~ t h . ~In- a~recent study, Alami et aL9 used fluorescence probes to investigate the effect of concentration and temperature on the micellar aggregation number of some C,E, surfactants. However, not all details of the clouding mechanism are fully understood at this time, and in the present work it is studied by monitoring the energy transfer from a micellar aromatic detergent to a solubilized fluorescent probe at different temperatures. The sensitized fluorescence of the probe serves as an indicator of the micellar structure and the position of the acceptor relative to the donor centers. The rate of nonradiative energy transfer depends on the relative orientation of the donor and acceptor dipoles,I0 and the extent of overlap of the emission spectrum of the donor, and the excitation spectrum of the acceptor.]' In Forster type energy transfer, the rate decreases with the sixth power of the distance between the donor-acceptor pair,I2 making it possible to calculate this parameter (provided that the separation remains constant during the excited state lifetime of the donor). This type of energy transfer, which is effective to about 80 A, comprises two terms:I3 (1) an exchange term that involves an interaction between the electron clouds of the donor and the acceptor and is effective at short distances ( < 8 A); (2) a coulombic term that describes a dipole-dipole interaction and operates over relatively long range. Since the work by Stryer and Haugland,I4 fluorescence sensitization measurements have become a useful tool for estimating intramolecular distances in molecules of biological interest.I5-l7 Fluorescence sensitization can be readily observed in micellar solutions. In most instances the donor and the acceptor are two @
Abstract published in Advance ACS Abstracts, January 1, 1995.
extraneous species located in the same m i ~ e l l e , ~but ~ . efficient '~ energy transfer from a micellar detergent comprising donor groups to a micellized acceptor is also possible. In 1972 Almgren demonstrated that efficient energy transfer takes place from sodium phenylundecanoate micelles to micellized naphthalene.20 Kalyanasundaram and Thomas investigated the energy transfer between Igepal Co-630 and solubilized pyrene.21 Earlier studies in this laboratory dealt with energy transfer from benzyldimethylalkylammonium chloride and Triton X-405 micelles to polyaromatic hydrocarbon^.^^-^^ The efficiency depends on several factors: occupation probabilities, intramicellar energy transfer, orientation of the acceptor and the donor, diffusion of the acceptor, and solubilization site. Thus, fluorescence energy transfer in micelles has been used for determining solubilization sites and statistics, and micellar aggregation number~.*~.~~%~~ Sensitized acceptor fluorescence competes with donor emission and with nonradiative relaxation of both donor and acceptor. There are three steady state methods to calculate energy transfer effi~iency,'~ respectively based on (1) the decrease of donor emission quantum yield; (2) enhancement of the acceptor fluorescence; and (3) a comparison of the donor absorption and excitation spectra via acceptor fluorescence. The last method was adopted in this study. Triton X-114 (TX-114) was chosen as the detergent, since it comprises an aromatic moiety (see Figure 1) that can act as an energy donor. The acceptor (probe) was perylene, which is especially suited for this because of its low absorption at the TX-114 excitation wavelengths and its short fluorescence lifetime.
Experimental Section Reagents. TX-114 (critical micelle concentration, 2.8 x M; cloud point, 22 "C) and Brij 35 were purchased from Aldrich (Milwaukee, WI) and Sigma (St. Louis, MO), respectively, and used without further purification. Perylene (Aldrich, 99+%) was purified by cold finger sublimation. Quinine sulfate monohydrate was obtained from Aldrich and used as received. Deionized water, treated with 0.22-pm Millipore filter system to 13 MQ cm resistivity, was used for all solutions. Homogeneous Solution and Aqueous Phase Studies. Stock solutions of 1.0 x lop3M perylene in ACS grade chloroform
0022-365419512099-1436$09.0010 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 5, 1995 1437
Clouding of Nonionic Detergents
- - - - 0.01 MTX-I14 ---- 0.WSMTX-I14 0.001MTX-114 - - 0.0005MTX-114
80
/-\
r
/
/
\
h
\
...'./..a
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I
$60
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r"'
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'a
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0
I . . . . l . . . . l . . . . I . . . . l . . . . , . . . . I . . . . I . . . . , . . .
0
250
360
350
Wavelength (nm)
Figure 1. Variation of Triton X-114 excitation with concentration: emission 360 nm.
(Fisher, Pittsburgh, PA) and aqueous 0.01 M TX-114 (-0.6%) were prepared. Solutions of perylene in aqueous detergent were prepared by placing the appropriate amount of the chloroform stock solution in a dry volumetric flask, evaporating the solvent, and redissolving the residue in the detergent stock solution with 5 h sonication. We have found this method more effective than magnetic stirring. A 1.0 x M perylene solution in 0.01 M Brij 35 (prepared similarly) and the TX-114 stock solution were used as spectroscopic blanks. Brij 35 mimics TX-114 in most regards, except in energy transfer, since Brij does not contain an aromatic group like Triton does. The perylene solution with Brij was therefore a suitable blank for sensitization studies. All measurements included two corrections: (1) a detergent blank (TX-114), which corrected for background detergent emission, and (2) a probe blank, which consisted of perylene in a Brij 35 solution and corrected for direct probe excitation that may occur at 291 nm. All solutions were left to equilibrate overnight before use. Fluorescence spectra, measured in a I-cm cell, were taken with a Perkin-Elmer MPF-66 fluorescence spectrophotometer equipped with a thermostated cell housing. This allowed temperature control of f0.5 "C. Monomeric and micellar Triton X-114 were excited at 291 and 312 nm, respectively, and perylene emission was measured at 445 nm. Sensitization in the Micellar Phase. Perylene solutions, prepared as described above, were used for energy transfer measurements. A pure 2% TX-114 solution and a 1.2% Brij 35 solution containing 1.0 x M perylene were employed as spectroscopic blanks. The micellar phases of the samples and the TX- 114 blank were formed at ca. 29 "C and, after separation from the aqueous phase, were transferred to a quartz cuvette and kept at the required temperature for 2 h. All fluorescence measurements were corrected for the inner filter effect by Gauthier's equation:27
where F,,, is the corrected emission intensity, FobS is the observed intensity, A,, and A,, are the absorbancekm of the solution at the excitation and the emission wavelength, respectively, d is the width of the cuvette, s is the beam width, and g is the distance from the edge of the beam to the inner wall of the cuvette. Fluorescence spectra were taken with the Perkin-Elmer MPF66 fluorescence spectrophotometer described above. Absor-
5
10
20 25 30 Temperature(OC)
15
35
40
Figure 2. Temperature dependence of sensitized perylene emission intensity: monomer excitation 291 nm; polymer excitation 312 nm; emission 445 nm.
bance values were measured with a Perkin-Elmer Lambda 4C W / v i s spectrophotometer in the same cells as those used for the fluorescence measurements. Low-Concentration Studies. A dilution series down to 1.0 x M was prepared from the TX-114 stock solution and used to make the corresponding series of 1.O x M perylene solutions. Fluorescence spectra were taken with a SLM AMINCO 8100 fluorescence spectrophotometer equipped with a thermostated cell housing that allowed temperature control to f0.5 "C. High-Concentration Studies. A series of 10-40% (w/v) aqueous TX-114 solutions were prepared. They were used to make 1.0 x M perylene solutions by sonicating at 40 "C for 2 h. Fluorescence spectra were taken with the Perkin-Elmer MPF-66 fluorescence spectrophotometer described above. All measurements were corrected for inner filter effects. Donor Quantum Yields. A 1.024 x M quinine sulfate monohydrate solution in 0.05 M H2S04 was prepared. Its absorption spectrum was measured in a 1-cm quartz cell, and its fluorescence spectra were taken with the SLM AMINCO 8 100 fluorescence spectrophotometer described above. The refractive indices of the TX-114 solutions were measured with a Bausch & Lomb refractometer at 20 OC. All data reported are the mean values of triplicate measurements.
Results and Discussion Fluorescence of TX-114.Figure 1 shows the fluorescence excitation spectra of aqueous TX-114 at different concentrations above the critical micelle concentration (cmc). The intensity of the broad structureless band at approximately 312 nm increases with increasing detergent concentration, indicating that this peak (also referred to as the polymer peak) is due to the excitation of detergent aggregates. The concentration of the detergent monomer is approximately constant above the cmc, but the sharp excitation peak it produces around 291 nm also shifts to slightly longer wavelengths and higher intensities with increasing detergent concentration. This effect, however, can be attributed to progressive overlap with the polymer peak. At TX- 114 concentrations below the cmc, the broad polymer peak disappears completely, and the intensity of the sharper peak varies with the concentration of detergent, confirming that it corresponds to the excitation of the detergent monomer. Homogeneous Solution and Aqueous Phase. The change of sensitized perylene fluorescence, obtained through excitation of both the detergent monomer and the micelle, is shown as a function of temperature in Figure 2. In addition to the
Komaromy-Hiller and von Wandruszka
1438 J. Phys. Chem., Vol. 99, No. 5, 1995
5
10
15
20
25
30
35
40
45
Temperature (OC)
Figure 3. Temperature dependence of perylene fluorescence inten-
o! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
35
40
45
50
55
60
Temperature ("C)
sity: direct excitation at 415 nm; emission 445 nm.
Figure 4. Temperature dependence of sensitized perylene emission in detergent phase: excitation 320 nm; emission 445 nm.
corrections described in the Experimental Section, direct probe excitation at the characteristic perylene excitation wavelength (415 nm) was carried out before and after the cloud point. The purpose of this was to compare the perylene concentrations in the aqueous phase before and after clouding, since the detergent phase would be expected to sequester most of the probe and remove it from the aqueous bulk. With monomer TX-114 excitation (291 nm), the fluorescence of the sensitized probe decreased slowly with increasing temperature until the cloud point was reached. In the vicinity of the cloud point (22-24 "C) the intensity drop appeared somewhat accelerated. Beyond the cloud point, however, there was a sharp rise in the monomersensitized probe intensity. This is especially surprising because of the partition of perylene between the phases. Figure 3 shows the distribution of perylene in TX-114 relative to Brij 35 (which does not cloud at these temperatures) monitored by direct probe excitation. While the TX-114 solution started to become cloudy at 22 "C, the immediate effect of this on directly excited perylene emission was minor. Beyond 25 "C, however, the initially dispersed detergent phase sank to the bottom of the cell and carried an ever increasing amount of probe with it. The emission intensity decreased accordingly. At 40 "C, approximately 78% of the perylene had thus been removed from the aqueous bulk. Despite this substantially lower probe concentration, the TX114 monomer-sensitized perylene emission increased continuously above 25 "C (Figure 2 ) . This is indicative of an increasingly intimate probe-monomer association after phase separation, when the detergent concentration remaining in the aqueous phase had been reduced to approximately the cmc. It is known that with increasing temperature the dehydration of the micelle leads to clouding. This dehydration is due to the reduced polarity of the surfactant oxyethylene moiety at elevated temperature,' which may contribute to the closer probemonomer association. In addition, the slight cloudiness that remained in the aqueous phase after the detergent phase formed a separate layer may have contributed to the enhanced emission intensity through multiple internal reflections of exciting radiation. In the work described above, the system was allowed to stabilize fully before each measurement. Below the cloud point this meant thermal equilibration of 15-20 min per data point, while above the cloud point, readings were taken approximately 2 h after the temperature was adjusted. To further investigate the response of the probe emission to temperature, a fast temperature ramp was implemented. In this, the cell temperature was raised quickly (2-3 min) from 20 to 30 "C and then held steady. The sensitized perylene fluorescence, excited at the TX114 monomer wavelength, was monitored as the system adjusted
to the thermal shock. The initial response was a brief 10% decrease in fluorescence intensity, followed by an approximately hour-long steady increase as the system equilibrated and the detergent phase formed. This progression (data not shown) followed the same profile in the previous set of measurements (Figure 2). The initial decrease in sensitized intensity must be ascribed to the customary thermal effects that prevailed before probe distribution and sequestration could be established. Increased temperature usually leads to decreased fluorescence intensities because of increased Brownian motion and collisional quenching encounters of the fluorophore. The relative insensitivity of micellized probe fluorescence to temperature was confirmed by monitoring the emission of directly excited perylene in micellar TX-114 and Brij 35 solutions. Figure 3 shows that the probe fluorescence up to the cloud point was relatively invariant in both cases, which can be ascribed to improved packing of the detergent residues in the palisade layer of the micelle.28 This renders the micellar interior more viscous, restricting the diffusive mobility of the probe and counteracting increased thermal motion. Perylene fluorescence sensitized by micellar (rather than monomeric) TX- 114 should be expected to respond differently to detergent clouding. When excited at 312 nm, the sensitized intensity decreased with increasing temperature below the cmc (Figure 2). In contrast to 291-nm excitation, however, the intensity declined steeply above the cloud point. This must be attributed to the reduction in perylene concentration, which was not offset by a more intimate donor-acceptor association in this case. As clouding proceeded, an increasing portion of the detergent settled as a separate layer on the bottom of the cell, removing the probe from the aqueous layer. Detergent Phase. Excitation at the monomer wavelength showed no evidence of sensitized perylene fluorescence in the detergent phase. Excitation at the polymer band maximum gave rise to strong perylene emission, which increased approximately 2-fold when the clouding temperature was increased from 30 to 60 "C (Figure 4). This intensity change involved a concentration factor, since the volume of the micellar phase shrank over the temperature range.29 Perylene emission generated by direct excitation, however, showed this concentration increase to be 3-fold, which means that the sensitization increase was actually less than expected. This discrepancy was probably due to reduced energy transfer efficiency in the detergent phase at higher temperatures. It can be rationalized by considering that even in a medium that consists mostly of detergent the very nonpolar perylene molecule will exist in a somewhat structured solvent cage. Detergent molecules surrounding a probe molecule will have their
J. Phys. Chem., Vol. 99, No. 5, I995 1439
Clouding of Nonionic Detergents
were simply mixed in the proportions appropriate to the cloud point phase under scrutiny, and sensitized measurements were taken at 20 “C. This meant that the thermal disruptions described above and demonstrated in Figure 2 did not take place in this case and sensitization intensities consequently increased with detergent concentration. Donor- Acceptor Distances. To calculate the energy transfer efficiency from TX-114 to solubilized perylene, the following relationships were used:I3 lE+OlY 1E-05
. . . . . . ..,
. . . . ....I 1E-04
. . . . ....I
1E-03
I
1E-02
TX-114concentration (M)
Figure 5. Effect of Triton X-114 concentration on sensitized perylene emission intensity: monomer excitation 291 nm, polymer excitation 312 nm; emission 445 nm.
R
0 10 20 30 40 50 60 70 80 90 100 7X content (%)
Figure 6. Effect of detergent phase composition on sensitized perylene emission: excitation 320 nm; emission 445 nm.
hydrocarbon chains oriented “inward”, while the polyoxyethylene moieties will point toward the detergent bulk. This would constitute a favorable energy transfer scenario, were it not for the elevated temperature needed to create the detergent-rich phase. The associated thermal motion should be expected to disrupt the relatively weak solute-solvent interactions described, leading to less favorable donor-acceptor orientations and reduced emission intensities. This was borne out by the fact that a similar detergent-water phase prepared by a simple mixing of solvents at room temperature did not show this reduction in sensitization (see discussion below). Concentration Effects. Sensitized perylene fluorescence excited at the wavelengths of both monomeric and micellar TX114 varied with detergent concentration as shown in Figure 5. In both instances there was a rapid rise of intensity in the vicinity of the cmc, followed by a leveling out of the signal. The monomer-sensitized emission decreased again at higher detergent concentrations, suggesting that the monomer concentration declined as more and larger micelles were formed. Figure 6 shows the variation of polymer-sensitized emission intensity in the detergent phase with increasing TX-114 content. The sensitized intensity increased slowly in the 10-30% concentration range. Above 30% it rose rapidly, suggesting a change in the donor-acceptor environment at that point. The nature of this change is not clear at this point, although the effect is probably related to the dispersal of water in the detergent phase. It should be noted that the data shown here were generated with “ d i c i a l ” detergent-water mixtures, rather than with detergent phases obtained after clouding. The latter were derived from solutions containing 2% TX-114, and after warming and clouding they gave detergent phases with concentrations up to 40%. In the present case, detergent and water
(3) where @T is the energy transfer efficiency; the subscripts a and d refer to the acceptor and donor, respectively; A is the absorbance of the solution in which both the donor and the acceptor are present; A d is the absorbance of the donor alone; A, is the absorbance of the acceptor alone; Fa and F d are the fluorescence intensities of the donor and the acceptor in the presence of the other; and F d o is the donor fluorescence in the absence of the acceptor at the excitation wavelength indicated in parentheses. Equation 2 is used to calculate the transfer efficiency from the decrease in donor fluorescence. While it is the simpler expression, it cannot be used when the donor quantum yield is low. The quantum yields of the surfactant at the two different excitation wavelengths (291 nm for the monomer and 312 nm for the polymer) were calculated using the relationship introduced by Marsh and L ~ w e y : ~ ~
ad= (area under emission spectrum of TX- 114) 0.70Aex, (area under emission spectrum of quinine sulfate) Aex2
(4) where 0.70 is the quantum efficiency of quinine sulfate in 0.05 M sulfuric acid, Aext is the absorbance of quinine sulfate at the donor excitation wavelength, and Aex2 is the absorbance of TX114 at the donor excitation wavelength. The quantum yield of the TX-114 monomer was found to be 2-7%, while it was 1215% for the polymer. Thus, for monomer excitation eq 3 was used, and for the micellar eq 2 was used to calculate energy transfer efficiencies. The quantum efficiency was used to calculate the Forster radius, Ro, Le. the critical distance at which spontaneous decay of the donor and energy transfer to the acceptor are equally possible:
R,6 =
9(ln 10) K2@dJ
12 8 n 4 d N A
(5)
Here N A is Avogadro’s number, n is the refractive index of the medium, @d is the donor quantum yield in the absence of the acceptor, and K is the orientation factor ( K ~is taken as 2/3). J is the overlap integral (in centimeters‘? mole) given by
where €,(A) is the absorptivity of the acceptor in centimeters2/
1440 J. Phys. Chem., Vol. 99, No. 5, I995
Komaromy-Hiller and von Wandruszka
TABLE 1: Energy Transfer Efficiencies and Donor-Acceptor Distances in Perylene Sensitization by Monomeric and Polymeric TX-114 DetergenP conc of TX-114 (M) 5 x 10-5 10-4 3 x 10-4 5 x 10-4 10-3 2 x 10-3 5 x 10-3 a
monomer excitation @T R (A)
polymer excitation
0.020 0.026 0.300 0.574 0.417 0.239 0.072
0.055 0.371 0.358 0.257 0.218 0.116 0.034
32.1 31.6 22.1 18.5 20.9 22.9 26.1
@T
R (A)
43.0 28.4 28.1 29.8 31.0 34.6 41.9
Temperature 10 “C.
mole, F&) is the spectral distribution of the donor fluorescence in arbitrary unit, and A is the wavelength in centimeters. The values of J was calculated by integrating the fluorescence spectrum using Simpson’s rule of integration. The donoracceptor distance, R, was calculated from
I
1.25 & .
Figure 7. Molecular dimensions of donor and acceptor molecules from
(7) The results, obtained well below the cloud point, are summarized in Table 1. They show that for probe sensitization by both monomer and polymer excitation the donor-acceptor distance decreased with increasing detergent concentration. This was the case up to, or slightly beyond, the cmc of TX-114 (which was determined to be 2.8 x by surface tension measurements). For polymer excitation this can be understood by considering that in detergent solutions below the cmc premicellar aggregates exist,31whose formation is promoted by the presence of hydrophobic nucleation centers (the probe in this case). These aggregates have a loose structure, reflected in the relatively large donor-acceptor separation. As the detergent concentration approached the cmc, the organization of the aggregates became tighter, and the perylene acceptor was forced into closer proximity to the donor centers (benzene rings) of the mutually associated TX-114 species. Micellar growth above the cmc produced larger aggregates with a less viscous interior, giving the micellized probe the opportunity to diffuse away from the palisade layer at the phase boundary. This led to a larger average donor -acceptor distance. In the case of monomer excitation, the energy donor was a detergent species that was not very closely associated with its neighbors. Since the data show that this donor-acceptor distance also decreased as the micelles aggregated, it must be inferred that monomer-excited sensitization took place within the micelle. This means that at least some of the detergent monomers that comprised the micelle were not sufficiently closely packed to appear as a polymeric species in a spectroscopic sense. Moreover, the data show that the donor-acceptor distances for monomer sensitization were consistently smaller than for polymer sensitization. This indicates that the monomerprobe approach within the micellar environment was closer than the corresponding polymer-probe approach. While this latter phenomenon cannot be fully rationalized, it is reasonable to consider that a micelle that consists of more than 100 monomers will not be excited as a whole. Longwavelength (polymer) excitation will be limited to a variable number of closely packed detergent molecules that are part of the micellar surface. The proximity of such a moiety to a micellized probe depends on their relative sizes. In an idealized minimum-size detergent aggregate, derived from the bondlength values shown in Figure 7, overlap between the probe and some of the micellar phenyl groups was clearly unavoidable.
standard bond lengths.
However, this did not necessarily lead to polymer-excited sensitization of the probe, because of the possibility of partial excitation of the micelle. As the micelles grew, the average distance between the micellized probe and the polymeric “skin” of the micelle increased. In contrast, monomeric detergent molecules that were part of the micelle probably had more freedom of movement while they remained associated with the probe.
References and Notes (1) Lindman, B. In Surfactants; Tadros, Th. F., Ed.; Academic Press: Orlando, FL, 1984; pp 102-104. (2) Hinze, W. L.; Pramauro, E. Crit. Rev. Anal. Chem. 1993, 24 (2), 133. (3) Siebert, E. D.; Knobler, C. M. Phys. Rev. Lett. 1985,54 (8), 819. (4) Nilsson, P.; Wennerstrom, H.; Lindman, B. J . Phys. Chem. 1983, 87 (8), 1377. ( 5 ) Aveyard, R.; Binks, B. P.; Clark, S.; Fletcher, P. D. I. J . Chem. Tech. Eiotechnol. 1990, 48, 161. (6) Valaulikar, B. S . ; Manohar, C. J . Colloid Interface Sci. 1985, 108 (2), 403-406. (7) Corti, M.; Minero, C.; Degiorgio, V. J . Phys. Chem. 1984, 88 (2), 309. (8) Binana-Limbelt, W.; Zana, R. J . Colloid Interface Sei. 1988, 121 (11, 81.
(9) Alami, E.; Kamenka, N.; Raharimihamina, A,; Zana, R. J . Colloid Interface Sei. 1993, 158, 342. (10) Haas, E.; Katchalski-Katzir, E.; Steinberg, I. Z. Eiochemistry 1978, 17 (23), 5064. (11) Haugland, R. P.; Yguerabide, J.; Stryer, L. Proc. Natl. Acad. Sei. U S A . 1969, 63, 23. (12) Forster, Th. In Modern Quantum Chemistry, Part I I I Action of Light and Organic Crystals; Sinanoglu, O., Ed.; Academic Press: New York, 1965; pp 93-137. (13) Valeur, E. In Molecular Luminescence Spectroscopy, Methods and Applications: Part 3; Schulman, S . G., Ed.; John Wiley & Sons: New York, 1993; pp 65-69. (14) Stryer, L.; Haugland, R. P. Proc. Natl. Acad. Sei. U.S.A. 1967, 58, 719. (15) Stryer, L. Ann. Rev. Biochem. 1978, 47, 819. (16) Lakowitz, J. R. Principles of Fluorescence Spectroscopy; Plenum: New York, 1983; pp 303-336. (17) Cheung, H. C. In Topics in Fluorescence Spectroscopy, Volume 2: Principles; Lakowitz, J. R., Ed.; Plenum: New York, 1991; pp 127-176. (18) Singhal, G: S.; Rabinowitch, E.; Hevesi, J.; Srinivasan, V. Photochem. Photobiol. 1970, 11, 531. (19) Usui, Y.; Gotou, A. Photochem. Photobiol. 1979, 29, 165. (20) Almgren, M. Photochem. Photobiol. 1972, 15, 297. (2 1) Kalyanasundaram, K.; Thomas, J. K. In Micellization, Solubilization, and Microemulsion; Mittal, K. L., Ed.; Plenum: New York, 1977; Vol. 2, pp 569-588.
Clouding of Nonionic Detergents (22) Ndou, T. T.; von Wandmszka, R.Anal. Lerr. 1989,22 (8), 1997. (23) Ndou, T. T.;von Wandruszka, R. T u h t u 1989,36 (4), 485. (24) Ndou, T. T.; von Wandruszka, R.Photochem. Photobiol. 1989,50 (4), 547. (25) Koglin, P. K. F.; Miller, D.J.; Steinwandel, J.; Hauser, M. J. Phys. Chem. 1981.85 (16), 2363. (26) Rothenberger, G.; Infelta, P. P.; Gratzel, M. J . Phys. Chem. 1979, 83 (14), 1871.
J. Phys. Chem., Vol. 99, No. 5, 1995 1441 (27) Gauthier, T.D.;Shane, E. C.; Guerin, W. F., Seitz, W. R.; Grant, C . L. Environ. Sci. Technol. 1986,20, 1162. (28) Parthasarathy,R.; Labes, M. M. Langmuir 1990,6 (3), 542. (29) Komaromy-Hiller, G.; von Wandruszka, R. Tuluntu, in press. (30) Marsh, D. J.; Lowey, S. Biochemistry 1980,19 (4), 774. (31) Loran, C. P.; von Wandruszka, R. Tuluntu 1991,38 (3,497.
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