J . Phys. Chem. 1990, 94, 5337-5341
5337
Polarity of AOT Micellar Interfaces: Use of the Preferential Solvation Concepts in the Evaluation of the Effective Dielectric Constants Michel Belletste, Martin Lachapelle, and Gilles Durocher* DPpartement de Chimie, UniuersitP de Montrdal. C.P. 6128, Succ. A, MontrPal, Quebec. H3C 357 Canada (Received: December 4 , 1989)
Fluorescence lifetimes (q) of 2-[p-(dimethylamino)phenyl]-3,3-dimethyl-3~-indole (1) have been extensively studied in a twwomponent mixture of pdioxanelwater and also in a three-componentmixture of n-heptane/AOT (sodium bis(2-ethylhexyl) sulfosuccinate)/water. We have applied the concept of “dielectric enrichment” to solute/mixed solvents systems in order to evaluate the effective dielectric constants at the AOT interfaces in n-heptane. We have shown that the effective dielectric constant (D)at the interface increases from 2.3 (water molar ratio W = 0) to approximately 9.0 before reaching a plateau at W = 12. This was explained by the solvation of the polar head groups of the surfactant. Equations have been given for the variation of D vs Wand for the variation of D vs [AOT] at various AOT and water concentrations, respectively. The validity of these results has been discussed by taking into account the role played by the probe molecule in the stabilization of the solvation energy (dielectric enrichment effect).
Introduction The influence of solvent dynamics on chemical reactions is a topic that has generated a good deal of experimental and theoretical study over the past decade.’-2 Time-resolved studies of solvation in polar media have recently been shown to benefit from the time-resolved fluorescence and the timedependent Stokes shift of specific molecular probes containing the dimethylamino g r ~ u p . ~ P TICT (twisted internal charge transfer) a* states stabilized in polar solvents are good examples of states having red-shifted fluorescence spectra revealed by dual fluorescence.5 But it was shown recently that specific solute-solvent interactions (complexes and/or exciplexes) are responsible for the rearrangement of the geometry from which the b* to a* state transition can occur.6 We have shown in the past that the spectroscopic and photophysical behavior of the benzylideneaniline molecules and the 3H-indoles like molecule 1 (shown in Figure 1) are drastically influenced by the specific associations with polar molecule^.^-^ Methods for extracting rate constants in the case where single exponentials are observed in the presence of fast decaying exciplexes have recently been proposed1° and applied to molecule 1 dissolved in mixtures of methylcyclohexane (MCH) and ethanol.” The importance of solvation of these potential probes on their ground- and excited-state geometries in homogeneous and heterogeneous media has been emphasized recently.I2J3 We have shown that no dielectric enrichment, as described by Suppan and c o - ~ o r k e r s ,was ~ ~ ltaking ~ place in the system of molecule l/pdioxane/water.I3 This allowed us, in a short communication, to propose this system as a polarity ruler in microenvironments like reverse micelles.l8 Among the amphiphiles capable of forming inverted micelles, sodium bis( 2-ethylhexyl) sulfosuccinate (aerosol OT, AOT) has received particular attention in the past years. Besides its good wetting power, this surfactant has a marked ability to solubilize water in various nonpolar organic solvents, a model feature of great concern to mimic water close to biological ~~ water molecules are membranes or p r ~ t e i n s . l ~ -Solubilized known to be encased by the amphiphile head groups, thus forming the micelle aqueous core of water pool, whereas the hydrocarbon tails protrude into the organic solvent. The literature concerning the polarity of AOT reverse micelles is rather scarce18*2635 in the sense that none of these authors but two1s*35 have proposed a convenient method to follow the dielectric constant of the micelles with water adducts. One of the reasons for this situation was the question of which solvent or solvent mixture to choose in order to mimic the probe microenvironment. Drummond and his group proposed that 1,4-dioxane/water mixtures can be used to mimic the behavior of cationic micelles,36 but they were uncertain whether or not they can be used to mimic ‘To whom correspondence should be addressed.
0022-3654/90/2094-5337$02.50/0
the anionic micelles and vesicles including the inverted AOT
micelle^.^^-^^
( I ) Rips, I.; Jortner, J. J . Chem. Phys. 1987, 87, 2090. (2) Maroncelli, M.; Fleming, G. R. J . Chem. Phys. 1987, 86, 6221. (3) Simon, J. D. Acc. Chem. Res. 1988, 21, 128. Su, S. G.; Simon, J. D. Chem. Phys. Lett. 1989, 158, 423. (4) Meech, S. R.; Phillips, D. J . Chem. Soc., Faraday Trans. 2 1987,83, 1941. (5) Grabowski, Z. R.; Rotkiewicz, K.; Rubaszewska, W.; Kirkor-Kaminska, E. Acta Phys. Pol. 1978, A45, 767. (6) Jones, A. C.; Phillips, D. Laser Chem. 1988, 9, 317. Weisenbord, P. C. M.; Huizer, A. H.; Varma, C. A. G . 0. Chem. Phys. 1989, 133, 437. (7) Belletite, M.; Durocher, G. J. Photochem. 1983, 21, 251. (8) BelletZte, M.; Lessard, G.; Richer, J.; Durocher, G. J. Lumin. 1986, 34, 279. (9) BelletEte, M.; Lessard, G.; Durocher, G. Can. J . Spectrosc. 1986, 31, 89. ( I O ) Richer, J.; Lessard, G.; BelletZte, M.; Durocher, G. Int. J . Chem. Kiner. 1986, 18, 1163. (1 1) BelletCte, M.; Durocher, G. J. Photochem. Phorobiol. 1988, 44, 275. (12) BelletZte, M.; Durocher, G. J. Phys. Chem. 1989, 93, 1793. (13) BelletEte, M.; Lessard, G.; Durocher, G. J . Lumin. 1989, 42, 337. (14) Midwinter, J.; Suppan, P. Spectrochim. Acta 1969, 25, 953. (15) Suppan, P. J. Lumin. 1985, 33, 335. (16) Suppan, P. J. Chem. SOC.,Faraday Trans. 1 1987, 83, 495. (17) Suppan, P. Faraday Discuss. Chem. SOC.1988,85, 173. (18) BelletEte, M.; Durocher, G. J . Colloid Interface Sci. 1989, 134, 289. (19) Fendler, J. H. Acc. Chem. Res. 1976, 9, 153. (20) Fendler, J. H. Membrane-Mimetic Chemistry; Wiley-Interscience: New York, 1982. (21) Fendler, J. H. Annu. Rev. Phys. Chem. 1984, 35, 137. (22) Luisi, P. L.; Straub, B. E., Eds. Reverse Micelles; Plenum: New York, 1984. (23) Luisi, P. L. Angew. Chem. 1985, 24, 439. (24) OConnor, C. J.; Comax, T. D.; Ramage, R. E. Adu. Colloid Interface Sci. 1984, 20, 2 1. (25) Vos, K.; Laane, C.; Visser, A. J. W. G. Photochem. Photobiol. 1987, 45, 863. (26) Wong, M.; Gratzel, M.; Thomas, J. K. Chem. Phys. Lett. 1975, 30, 329. (27) Wong, M.; Thomas, J . K.; Gratzel, M. J . Am. Chem. SOC.1976, 98, 2391. (28) Rodgers, M. A. J. In Reuerse Micelles; Luisi, P. L., Straub, B. E., Eds; Plenum Press: New York, 1984; pp 165-174. (29) Backer, C. A,; Whitten, D. G. J . Phys. Chem. 1987, 91, 865. (30) Bardez, E.; Monnier, E.; Valeur, B. J . Colloid Interface Sci. 1986, 112, 200. (31) Sahyun, M. R. V. J. Phys. Chem. 1988, 92, 6028. (32) Abdel-Kader, M. H.; Krebs, P. J. Chem. Soc.,Faraday Trans.1 1988, 84, 224 I .
0 1990 American Chemical Society
5338 The Journal of Physical Chemistry, Vol. 94, No. 13, 1990
Belletite et al.
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The present work aims to shed some light on this problem. It will be proposed that the molecular probe should be an integral part of a three-component system including a polar and a less polar solvent like water and p-dioxane. In order that such a system be used to mimic a specific microenvironment, one has to ensure that the preferential solvation of the probe by water in p-dioxane is similar to that in the microenvironment i n ~ e s t i g a t e d . ' ~ JThe ~J~ amphiphilic probe 1 in p-dioxanelwater mixtures will be shown to fit the strict requirements of a mimetic system in a certain range of surfactant molecules (20.5 M) in AOT reverse micelles. It will be shown that no dielectric enrichment of the probe takes place in AOT for this range of surfactant concentration only. The effective dielectric constants at the AOT interface have been obtained and correlated to the size of the water core.
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Experimental Section Synthesis and purification of the compounds and the solvents have been described e1~ewhere.I~ AOT from Aldrich Chemical Co. was purified of the organic and inorganic impurities by the method of El Seoud and da Silva.37 Concentration of molecule 1 is typically between IOd and M in order to avoid selfassociation of the probe. Fluorescence decays were measured with a time-correlated single-photon counting system described in detail elsewhere.38 Lifetimes were obtained by the iterative reconvolution technique (Marquardt algorithm) using the delta function convolution method (DFCM). In order to judge the quality of the fit, plots of weighted residuals and the autocorrelation function have been examined along with the reduced x2 value and the Durbin-Watson parameter (DW). Results Solvatochromic Shifts in Solvent Mixtures. One way to study the nature of the first relaxed singlet excited state of polar probes consists in the plotting of a spectroscopic or photophysical parameter (EP+F, or T F ) vs the mole fraction of the polar solvent added to a less polar solution of the probe s t ~ d i e d . ' ~ -Substantial I~ deviations from linearity are often observed. This behavior has been explained by three main reasons: (1) the nonideal behavior of the solvent mixture, ( 2 ) specific solutesolvent association, and ( 3 ) the dielectric enrichment of the solvent around the dipolar solutes, the last two reasons being referred to as "preferential solvation" of the probe. The preferential solvation is emphasized experimentally by the fact that a nonlinearity does occur even in such ideal solvent mixtures as methylcyclohexane (MCH)/ethanol (EtOH).12 It has already been shown following a thorough photophysical investigation of this system that, assuming the existence of an exciplex between molecule 1 and ethanol, the experimental variations of @'/@ and T ' / T vs the ethanol concentration have been completely recovered kinetically." The dielectric enrichment was then very difficult to dissociate from (33) Fulton, J. L.; Blitz, J. P.; Tingey, J. M.; Smith, R. D. J . Phys. Chem. 1989, 93, 4198. (34) Tamura, K . ; Nii, N. J . Phys. Chem. 1989, 93,4825. (35) Lay, M . B.; Drummond, C . J.; Thistlethwaite, P. J.; Grieser. F. J . Colloid Interface Sci. 1989, 128, 602. ( 3 6 ) Drummond, C . J.; Grieser, F.; Healy, T. W. Faraday Discuss. Chem. SOC.1986, 81, 95. (37) El Swud, 0. A.. Da Silva, M. J. J . Chem. Sm.,Perkin Trans. 2 1980, 76, 127. (38) Rivest-Filion, D.; Thammavong, K.; Durccher, G. Spectrochim. Acra 1981, 39A, 627
0.4
1
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0.0 0.1 0.2 0.3 0.4
0.6
0.5
0.7
0.8
XP
Figure 2. Variation of the fluorescence lifetime ratio of the probe (1) with the molar fraction of added water in p-dioxane (upper curve) and in n-heptane/AOT (0.50 M) (lower curve).
the solute-solvent association (exciplex) model since we did not have any spectroscopic evidence of the existence of the exciplex and also because both phenomena are kinetically diffusion controlled.8-'2 Absorption and fluoresence spectra, quantum yields, and lifetimes of molecule 1 in p-dioxanelwater mixtures have also been reported e1~ewhere.l~ The nonlinearity ratio ( p E ) as defined by Suppan and co-workers14-" PE
=
[
s/co)crf - f ( ~ ) l i n ] ,
dx (1)
Y2Af(D)
has been calculated for the lifetime ratio ( T O I T ) , and it was shown equal to 0.8 which is the value obtained for the nonlinearity ratio of the solvent mixture.I3 This was indicating that no dielectric enrichment of the probe molecule was taking place in the coarse of the solvation dynamics. The reason why we used the ratio T O / ? for evaluating the nonlinearity ratios instead of the fluorescence energy maximum EFfor example is only because the experimental precision is much higher on a lifetime than on the fluorescence emission maxima of wide fluorescence bands. Since we have already shownI2 that the solvatochromic shifts of the fluorescence band of molecule 1 in various solvent mixtures follow approximately the fluorescence lifetime variation with the mole fraction of the more polar solvent added, this allows us to choose EF or T ' / T for obtaining a polarity scale. Another way of comparing the nonlinearities in solvent mixtures consists in fitting the parameter examined (Le., T O / . ) with the molar fraction of the polar solvent added to a single exponential TO/T = A exp(-BX,) E (2)
+
where T O is the fluorescence lifetime of the probe in pure pdioxane and Xp is the molar fraction of water added. The parameter B then measures the extent of nonlinearity of the experimental function considered. It is shown in Figure 2 that E = 12 for the p-dioxane/water mixtures. By analogy, B = 45 for 1 dissolved in MCH/EtOH solutions where it is known that "preferential solvation" is playing a major r01e.I~
The Journal of Physical Chemistry, Vol. 94, No. 13, 1990 5339
Polarity of AOT Micellar Interfaces 1.00 I Y
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[AOT] (MI Figure 3. Variation of the nonlinearity ratio ( p ) (upper curve) and of the parameter B (lower curve) with the concentration of the surfactant for probe (1) incorporated into A O T reverse micelles.
The l/p-dioxanelwater system is very sensitive to small dielectric fluctuations ( D in the range of 1-15). In fact, 7-l obeys the following empirical biexponential law T-'
+
= 1002e-3.240+ 0.903e4.364D 0.593
(3)
where D is the dielectric constant of the mixtures as obtained from the work of Critchfield et al.39 Since T " / T has the same nonlinearity ratio ( p and/or B) when water is added to p-dioxane than that of the solvent mixture itself, we expect that this three-component system might be a real representation of multicomponent systems possessing about the same extent of nonlinearity. p and/or B would then become the key parameters to check in order to apply any polarity ruler to heterogeneous systems. Behavior of the Probe ( 1 ) in Reverse AOT Micelles. Fluorescence lifetimes of 1 in various AOT aqueous micellar solutions was measured in n-heptane. Since this probe resides at the micellar interface,I* these results can give information about the polarity in the vicinity of the polar head group of the surfactant. It is shown in the lower part of Figure 2 that the lifetime ratio ( 7 " / 7 ) decreases exponentially with the molar fraction of water incorporated into the micelle (0.5 M AOT) in the same way as the variation of P / T vs XHlo measured in p-dioxanelwater mixtures (same values of B and p ) . The extent of nonlinearity in AOT reverse micelles has been investigated by measuring p and B for various concentrations of the surfactant. These results, which are presented in Figure 3 , show clearly that no dielectric enrichment is taking place in micelles where the concentration of AOT is maintained higher or equal to 0.5 M since p and B have about the same values as those observed for the p-dioxanelwater mixtures ( p = 0.8, B = 12). It suggests that the p-dioxanelwater mixture is a good medium in order to mimic AOT micellar interfaces at least at concentrations around 0.5 M or higher of the surfactant molecule by using our probe molecule. However, for low concentrations of AOT preferential solvation of molecule 1 is taking place, and this suggests that the use of Ilp-dioxanelwater (39) Critchfield, F. E.; Gibson, J . A., Jr.; Hall, J . L.J . Am. Chem. SOC. 1953, 75, 1991.
0
5 .O
4.0 3.0 I
0.0
. 0.2
I 0.4
0.6
[AOTl (MI Figure 4. Plot of the effective dielectric constant (D)surrounding the probe molecule (1) as a function (top) of the water molar ratio (W) [(--)[AOT] = 0.36 M; (---)[AOT] = 0.5 M, and (-)[AOT] = 1.18 M] and as a function of the concentration of the surfactant for some W values (middle). The bottom part of the figure shows the variation of the maximum dielectric constant value as a function of the concentration of the surfactant. TABLE I: Mathematical Expressions Allowing for the Precise Determination of the Dielectric Constants at a Particular AOT Concentration in n-Heptane/AOT/Water Inverted Micelles lAOTl, M Dvs W 0.09 D = 1.5(1 - exp(-0.25W)) + 2.2 0.18 D = 3.7(1 - exp(-0.20W)) + 2.2 D = 4.4(1 - exp(-0.31 W)) + 1.8 0.27 0.36 D = 4.8(1 - exp(-0.25W)) 2.0 0.50 D = 6.4(1 - exp(-0.28W)) 2.1 0.75 D = 6.1(1 - exp(-0.29W)) + 2.2 D = 6.5(1 - e x p ( 4 2 5 W ) ) + 2.4 1.18
+ +
system as a polarity ruler (see eq 3) might be less significant in these conditions. Measurements of Effective Dielectric Constants. The effective dielectric constants of various AOT reversed micelles have been measured by using eq 3. It is shown in Figure 4 (top) that the D vs W (= [H,O]/ [AOT]) curves obtained follow an associative exponential law with a plateau obtained at around W = 12 for various concentrations of AOT. Mathematical expressions allowing for the precise determination of the dielectric constants at a particular AOT concentration are given in Table I. In Figure 4 (middle) and in Table I1 one can see the variation of the effective
5340
The Journal of Physical Chemistry, Vol. 94, No. 13. 1990
BelletOte et al. 1.00 I
TABLE [I: Mathematical Expressions Allowing for the Precise Determination of the Dielectric Constants at a Particular W Value in n -Heotane/AOT/Water Inverted Micelles u 17 vs [AOT] i D = 1.84(1 -. exp(-2.59[AOT])) + 2.12 D = 5.62(1 - exp(-4.00[AOT])) + I .56 5 D = 6,94(1 .- expi-4.09[AOT])) + 1.50 IO
1
~~
dielectric constant as a function of the AOT surfactant concentrations for various water contents (w).Finally, the maximum values of the dielectric constants as obtained from the curves in Figure 4 (top) and the equations (Table I) with W = have been plotted in Figure 4 (lower part) for various concentrations of AOT. A plateau in D,,, is obtained for higher concentrations than 0.5 M in surfactant.
0.861
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Discussion Concept of ‘Dielectric Enrichment”. The most widely accepted picture of the reverse micelles seems to be that of Zinsli, who describes the water in the pool using a two-state model.@ A very viscous water, close to the interface, would be in equilibrium with that in the center of the pool which exhibits properties similar to bulk water. In fact, increasing the molar ratio of water to surfactant (W) in the AOT reverse micelles results in a discontinuity of several physical properties at Waround 12.2636 These data are consistent with the hydration of AOT head groups and counterions at low W, resulting in a bulky structured water and the formation of an aqueous bulklike water core at higher W. This can explain the variation of the effective dielectric constant experienced by our probe as the water content of reverse micelles is increased (Figure 4, top). Indeed, molecule 1 which resides in the vicinity of the surfactant polar head groups “senses” an increase in polarity to about 10-12 water molecules per AOT molecule after what the excess water goes directly into the water pool and does not change the local polarity at the interface any further. Recently, Tamura and Nii have observed the same kind of behavior.34 They found that the maximum absorption wavelength of ANS molecule incorporated in n-heptane/AOT/water reverse micelles shifts to longer wavelengths with the addition of water and approaches a plateau at a water content of around W = 10. There have been many investigations of solute-solvent systems specificallly designed in order to probe the polarity of heterogeneous media such as aqueous interfaces of micelles and bilaye r ~ . One ~ usually ~ ~ measures ~ , the ~ probe ~ ~molecule ~ property in a series of solvents or solvent mixtures as a function of the dielectric constants. Data are remarkably incomplete though on the question of which solute and solvent combination to choose in order to mimic the probe microenvironment in heterogeneous systems. It appeared to us that the concept of “dielectric enrichment” should first be applied to any solute-solvent combination property in homogeneous systems in order to judge its possibility of application as a polarity ruler in more complex systems. If dielectric enrichment does exist in a two-solvent mixture, it becomes a local property of a three-component system so that the “polarity” as a specific property of the liquid mixture has no meaning whatsoever. We have shown recently that probes with higher excited-state dipole moments than that of 1 give rise to higher degrees of preferential solvation in the same solvent mixture.’* This was also shown on other systems before.I6J7 This Zinsli. P.J. Phys. Chem. 1979, 83, 3223. Mukerjee, P.;Ray A. J . Phys. Chem. 1966, 70, 2144. Funasaki, N . J. Colloid Interface Sci. 1977>60. 54. Funasaki, N. J . Phys. Chem. 1979.83, 1998. Fernandez, M. S.;Fromherz, P. J . Phys. Chem. 1977, 81, 1755. (45) Mackay, R. A.; Jacobson, K.; Tourian, J. J . Colloid Interface Sci. 1980, 76, 515. (46) Vaz, W. L. C.: Nicksch, A.; Jahnig, F . Eur. J . Biochem. 1978, 83, (40) (41) (42) (43) (44)
299. (47) Podzimek, J. M.; Friberg, S. E. J . Dispersion Sci. ’Technol. 1980. I , 341. (48) Zachariasse, K. A.; Phuc, N . V.; Kozawklevicz, B. J . Phys. Chem. 1981, 85. 2676.
,
,
,
1
0.05
0.10
0.15
0.20
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Figure 5. Variation of the chemical shifts ratio (taken from Maitra50) with the molar fraction of water incorporated in cyclohexane/AOT/ water reverse micelles.
proves that the probe should be an integral part of the ruler system used in evaluating physical properties in heterogeneous media. Effective Dielectric Constant of the Oil/ Water Interfaces. There are quite a few papers in the literature dealing with the dielectric constant of AOT reverse micelles. Rodgers28using Rose Bengal as a fluorescence probe proposed that at W = 0 the polarity at the interface is already equivalent to that in tert-butyl alcohol (D = 9.9) for AOT (0.1 M) in many solvents. They also assumed that as more water molecules are included into the swelling reverse micelles, the polarity continually increases as a result of the hydrogen-bonding power of the interfacial region. Backer and Whitten using the vibronic ratio ( ] , / I 3 )of the pyrene monomer fluorescence, which is well-known to be sensitive to the solvent polarity, found a polarity which compares to that of tert-amyl alcohol (D = 5.9) for 1 M AOT micelle at W = 15.29 Our evaluation for [AOT] = 1.18 M is approximately 9. However, these authors concluded that their value might be underestimated because of fl/13 ratio represents an average for emitting singlets in two or more solubilization sites. Recently, Lay et al. have measured the effective dielectric constant of hexane/AOT/water reverse micelles by using 2,6-diphenyl-4-(2,4,6-triphenyl-lpyridini0)phenoxide (ET(30) betaine reagent) as a probe.35 Relatively high dielectric constants were determined which reach a superior limit of D = 47 at Waround 12. They have interpreted their results by suggesting that ET(30) molecules are solubilized more or less into the aqueous region of the dispersed drops. Moreover, the same research group had already concluded before that some uncertainties were associated with the use of ET(30) in anionic systems.36 Dawber et al. have also demonstrated recently that in aqueous organic solvent mixtures the behavior of ET(30) is biphasic, Le., that in regions of high mole fraction of water the betaine is preferentially solvated by the organic component, whereas in mixtures of low water concentration the betaine is preferentially solvated by water.49 All these results indicate that the use of ET(30) reagent as an amphiphilic probe in heterogeneous media is in jeopardy to say the least. On the other hand, Maitra measured the variation of the water proton chemical shifts in cyclohexane/AOT/water reverse micelles with the mole fraction of water ( bt9.50The observed chemical shift variation has been interpreted as a sum of the weighted average shifts of the bound water protons at the interface (upfield direction) and free water protons (downfield direction). If one calculates from these results the nonlinearity ratio by plotting the ratio of the chemical shift (6,) of the interface water protons extrapolated to W = 0 to that (6) with various amounts of water added, one gets a value of B = 9 (see Figure 5 ) , which is in excellent agreement with the values obtained for our lifetime data in [AOT] 1: 0.5 M (see Figure 3, lower part). It thus seems that our probe does not perturb the interfacial structure of AOT at least at concentrations of the surfactant equal to or greater than 0.5 M. This might not be the case for larger probes like ET(30)
(49) Dawber, J. G.; Ward, J.; Williams, R. A. J . Chem. SOC.,Faraday Trans. I 1988, 84, 713. ( 5 0 ) Maitra. A . J . Phys. Chem. 1984, 88, 5122.
The Journal of Physical Chemistry, Vol. 94, No. 13, I990 5341
Polarity of AOT Micellar Interfaces
I 6.0
8.0
10.0
12.0
14.0
16.0
18.0
20.0
W r
Figure 6. Plot of the effective dielectric constant ( D ) surrounding the probe. molecule 1 as a function of the water core radius (r, in A) (taken from Keh and Valeurs’) for n-heptane/AOT/water reverse micelles.
and/or Rose Bengal. These probes might perturb their environment because of their size and their high polarity and could tend to probe the water core instead of the interface because of their ionicity. This could explain the high values of effective dielectric constant obtained by using &(30) and Rose Bengal as polarity probes. The value of the effective dielectric constant at the aqueous micellar interface can be correlated to the size of the water core (radius). Abdel-Kader and Krebs32have recently shown that the optical absorption spectrum of localized excess electrons solubilized in water clusters of AOT/n-heptane solutions is red-shifted when the water cluster size increases. Bardez and co-workers also showed that the microenvironment of their interfacial probe becomes more and more polar as the micelle is swollen.30 By using the values of the water pool radius (r,) for the heptane/AOT/ water system published by Keh and Valeur,sl we have reproduced the variation of the effective dielectric constant against r, in Figure 6 . It is quite obvious that the polarity of the interfacial region of AOT reverse micelles increases with the size of the water cluster. As we know, a slight increase in droplet size (r,) increases the (51) Keh,
E.;Valeur, B. J . Colloid Interjace Sci.
1981, 79, 465.
interfacial area (4rrw2)considerably such that the inside curvature of the droplet increases also, allowing for much more water molecules to come by the solvation shell of the probe at the interface. This phenomenon would be mainly responsible for the fast increase in the interface polarity at low Wvalues. At the same time some AOT molecules residing out of the interface can come to it making new hydrogen bonds with the head groups at the interface.50 Finally, it was shown in Figure 4 (lower part) that D,,, reaches a top limit for [AOT] = 0.5 M and remains approximately constant after that. This phenomenon has to be correlated with the fact that preferential solvation of the probe by water occurs at lower concentrations of the surfactant as shown in Figure 3. This is corroborated in Figure 5 where the nonlinearity ratio of the AOT (0.1 M) interface without any probe does not exceed the value of 9. We then conclude that the effective dielectric constants measured with our probe in reverse micelles where [AOT] < 0.5 M might be underestimated. If this happens, D,,, might possibly be nearly constant throughout the surfactant concentration range. We have shown that the three-component system l/p-dioxane/water is very sensitive to small dielectric fluctuations (D = 1-12) and can mimic the solvation of the interfaces of AOT reverse micelles in n-heptane at least at concentrations equal to or higher than 0.5 M of the surfactant. We have proposed that the nature of the probe has to be an integral part of the ruler investigated. The fact that the excited state responsible for the fluorescence emission of probe 1 in polar environments is not very does not give rise to any dielectric enrichment in p-dioxane and in higher concentrations of AOT so that the photophysical parameters measured in these media are real functions of the nonideal solvation phenomenon. Also, the fact that the fluorescence lifetime of the probe is single exponential in various polar environment~’*’~ makes the interpretation of the results much easier.
Acknowledgment. We are grateful to the Natural Sciences and Engineering Research Council of Canada (NSERC) and to the “Ministire de 1’Education du Quebec” (FCAR) for support of this research. M.L. is grateful to the NSERC for Research Scholarships.
