J . Phys. Chem. 1986, 90, 282-287
282
configuration interaction allowed by CIPSI introduces substantial modifications to this S C F picture. This is, in fact, not the case because the most important configurations of our C I space are basically transitions from the closed d shell of Cu, Le., essentially d-shell relaxation, which allows for a more direct participation of the dzz orbital in a manner that would reinforce rather than contradict the conclusions derived from Tables I and 11.
Conclusions From these theoretical results we can now explain the experCu H2 thermally imental observation of the CuH H induced matrix phase reaction. The computations show that the linear attack of the H atom on the Cu moiety of the copper hydride molecule is energetically completely downhill (see Figure 2) and in complete agreement with the experimental results of Ozin et a1.I This addition of an H atom would form a HCuH complex with a bent structure, although the linear complex is only marginally higher in energy (see Figure 3). By exothermicity considerations the excess energy does not allow for the permanent existence of these species, which explains why they are not detected experimentally.’ To dissociate the HCuH species, only two relative small barriers exist that can easily be surmounted, as is shown in Figure 3 . The reverse reaction is endothermic and has an activation barrier of 28 kcal/mol according to our CIPSI calculations. This last result is in close agreement with the observedI0
+
-
+
heat of dissociative chemisorption of hydrogen on copper surfaces. Regarding the abstraction pathway originally proposed in ref 1, our theoretical results show that there is an energy barrier of several kcal/mol (see Figure 1). At this point the CIPSI results definitely predict that the abstraction is less probable than the addition mechanism discussed above, due to the existence of this nonnegligible barrier for the former while none is present for the latter. Considering however that the rare gas solid matrix cage might conceivably induce an effective lowering of the abstraction barrier, we perhaps should not rule out this alternative pathway altogether. The study of such effects induced by the rare gas matrices implies a completely new approach for theoretical calculations. A pseudopotential perturbative SCF-CI method is being developed for the purpose of evaluating the changes in energy when the surrounding media are taken into account.“ In any case the explanation of the experimental results of ref 1 is quite clear. The potential energy surface shows an optimal energy pathway that corresponds to our so-called addition reaction, and this mechanism allows to justify theoretically all the observations of Ozin and Gracie.’ Registry No. CuH, 13517-00-5: H2, 1333-74-0; H, 12385-13-6 (10) A. Clark, “The Chemisorptive Bond”, Academic Press, New York, 1974. (1 1) Work in progress.
SURFACE SCIENCE, CLUSTERS, MICELLES, AND INTERFACES Water Activity in Reversed Sodium Bis(2-ethylhexyl) Sulfosuccinate Micelles Mario J. Politi* and Hernan Chaimovich Department of Biochemistry, Chemistry Institute, University of Siio Paulo, 01 498 Siio Paulo, SP, Brazil (Received: October 25, 1984; In Final Form: August 30, 1985)
The rate of proton dissociation from the first excited singlet state of aromatic alcohols (8-hydroxypyrene-1,3,6-trisulfonate, ~-naphthol-6,8-disulfonate,@-naphthol-6-sulfonate,and &naphthol) was measured by steady-state fluorimetry in AOT reversed micelles as a function of H 2 0 content. Acid dissociation rate constants (kerf*) of the alcohols were related with apparent water activity (a,’) by comparison with koff*’smeasured in salt solutions of known water activity (uw). The a,’ in the reversed micelles estimated by this procedure depends on the probe positioning in the water pool. The data are consistent with the existence of two types of water in the water pools of reversed micelles.
Introduction Reversed micelles, stable isotropic solutions of the oil/surfactant/water system in the L2 domain, are powerful models that have found several applications ranging from biological cornpartmentalization analysis to chemical catalysis.’s2 Among the surfactants that form reverse micelles the best characterized are the systems derjvd from bis(2-ethy]hexy])sulfosuccinate(AOT).l-S (1) Fendler, J. H. In “Membrane Mimetic Chemistry”: Wiley: New York, 1982. (2) Mittal, K. L., Lindman, B., Eds. “Surfactants in Solution”; Plenum Press: New York, 1984: Vol. 3. (3) (a) Zulauf, M.; Eicke, H.-F. J . Phys. Chem. 1979,83, 85. (b) Eicke, H.-F. Top. Curr. Chem. 1980, 87, 85. (4) Mittal, K. L., Ed. ‘Solution Chemistry of Surfactants”; Plenum Press: New York, 1982; Vol. 2. (5) Douzou, P.; Keh, E.; Balny, C. Proc. Natl. Acad. Sci. U.S.A.1979, 76, 681.
0022-3654/86/2090-0282.$01.50/0
AOT reversed micelles can dissolve large amounts of water which remain compartmentalized in the organic media as bubbles (water Pools Or droplets) surrounded by the surfactant. Several reports demonstrate that these water pools are spherical with low (size) po1ydispersity.3’6-8 Several features of these systems remain to be solved. One of them pertains to the very debated question of water structure close to the interface9-I4 and the related questions of water activity ( 6 ) Eicke, H.-F.; Kubik, R.; Hammerich, H. J. Colloid Interface Sci. 1982, 90, 27. (7) Magid, L. J.: Daus, K. A,; Butler, P. D.; Quincy, R. B. J . Phys. Chem.
1983,87, 5412. (8) Eicke, H.-F.; Hilfiker, R.; Holz, M . Helu. Chim. Acta 1954, 67, 361. (9) Wong, M.: Thomas, J. K.; Nowak, T. J . Am. Chem. SOC.1977, 99,
4730. (IO) Wong, M.; Thomas, J. K.; Gratzel, M. J . Am. Chem. SOC.1976, 98, 2391.
0 1986 American Chemical Society
Water Activity in Reversed AOT Micelles and internal pH17-23in the aqueous core. The most accepted picture of the reversed micelles is that of Zinslil' which 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,) of AOT reverse micelles results in a discontinuity of several physical properties at Woaround 12. These data are consistent with the hydration of AOT headgroups (and its counterions) at low Woresulting in a highly structured water and by the formation of an aqueous bulklike water core at higher W0.24 Recently Bardez and co-workers'5 have probed the water core of AOT reversed micelles by studying rates of proton transfer of fluorescent probes. Their results were analyzed by using a continuous variation of a, as a function of Worather than a two-state model. By the time that these latter results were submitted one of us was using a similar experimental system26 and our results were similar to those reported by Bardez. Following the same line of thought we decided to extend these results employing fluorescent probes which reside both in the aqueous core and in the interfacial region. (a,)15916
The Journal of Physical Chemistry, Vol. 90, No. 2, 1986 283 TABLE I: Spectral and Emissive Properties of POH,PN68S, ON&, and PN UV-vis compd/ conditions POH HCI. 3 M NaOH, 0.01 M
w,< 1 PN68S MeOH MeOH, HCI MeOH/KOH HCl, 3 M
Experimental Section
Materials. Sodium bis(2-ethylhexyl) sulfosuccinate (AOT), furnished by Prof. 0. El Seoud from this Institute, was further purified following described procedure^.^^-^^*^^ Fresh AOT solutions were made routinely to avoid ester hydrolysis (ref 29 and references therein). Typically AOT solutions were prepared in isooctane to a final concentration of 0.0818 M. Isooctane (2,2,4-trimethylpentane) (Matheson Colleman) was distilled and the fraction distilling around 96-97 O C (760 mmHg) was collected. 2-Naphthol (=&naphthol, PN) (Sigma) was sublimed, recrystallized from E T O H / H 2 0 (70/30, v/v), and dried under vacuum. The absorption and emissive properties of @Nagreed with reported values (Table I). Potassium 2-naphthol-6-sulfonate salt (PN6S) was a gift from Prof. J. Muradian from this Institute. The compound was recrystallized from MeOH/H20 (90/ 10, v/v) after dissolution of the compound in H20 (minimal amount to complete solubilization), treatment with activated charcoal, and filtration. After crystallization the product was extensively washed with MeOH and dried under vacuum. Spectral and emissive data are collected in Table I. Dipotassium 2-naphthol-6,8-disulfonatesalt (PN68S) (Eastman Kodak) was purified by using the same procedure described for PN6S (Table I). Trisodium 8-hydroxy1,3,6-pyrenetrisulfonatesalt (POH) (Eastman, Kodak) was re-
(1 1) Zinsli, p.; J . Phys. Chem. 1979, 83, 3223. (12) Kondo, H.; Miwa, T.; Sunamoto, J. J . Phys. Chem. 1982,86,4826. (13) Pileni, M. P.; Brochette, P.; Hickel, B.; Lerebours, B. J . Colloid Interface Sci.1984, 98,549. (14) Gandin, E.; Lion, Y.; De Vorst, V. J. Phys. Chem. 1984, 88, 280. (15) Bardez, E.; Gouguillon, B. T.; Keh, E.; Valeur, B. J . Phys. Chem. 1984, 88, 1909. (16) Kubik, R.; Eicke, H.-F.; Jbnsson, B. Hela Chim. Acta 1982, 65, 170. (17) Menger, F. M.; Saito, G.; Sanzero, G . V.; Dodd, J. R. J . Am. Chem. SOC.1975, 97,909. (18) Menger, F. M.; Saito, G. J . Am. Chem. SOC.1978, ZOO, 4376. (19) Fujii, H.; Kawai, T.; Nishikawa, H. Bull. Chem. SOC. Jpn. 1979,52, 205 1. Chinelatto, A. M.; Shimizu, M. R. J . Colloid Interface (20) El Seoud, 0.; Sci. 1982, 88, 420. (21) Fujii, H.; Kawai, T.; Nishikawa, H.; Ebert, G. Colloid Polym. Sci. 1982, 260, 697. (22) Smith, R. E.; Luisi, P. L. H e h . Chim. Acta 1980,63, 2302. (23) El Seoud, 0.;Chinelatto, A. M. J. Colloid Interface Sci.1983, 95, 163. (24) Pileni, M. P.; Furois, J. M.; Hickel, B. In *Surfactants in Solution"; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 3, p 1471. (25) Jonsson, B.; Wennerstrom, H. J. Colloid Interface Sci.1981,80,482. Fendler, J. H. J . Phys. Chem. 1985, 89, (26) Politi, M. J.; Brandt, 0.; 2345. (27) Huppert, D.; Kolodney, E. Chem. Phys. 1981, 63, 401. (28) Politi, M. J. Ph.D. Dissertation, Clarkson University, Potsdam, NY, 1984. (29) Delord, D.; Larch&,F. C. J. Colloid Interface Sci. 1984, 98, 277.
NaOH, 0.01 M
w,< 1 PN6S EtOH, 5% HCl. 1 M
absorpn max, nm 403
456
fluorescence, nm c
19900 21700" 2 1600 2 1600n
exc em 403 445 isoemissive 478 456 510 403 430
338.5' 33SC 296.5' 287.0,' 287c 338.5' 287.0' 311' 302b 253' 338, 337, 337 298 288, 288, 290 365
3180' 3020' 5070' 5840,' 5623' 3140' 5650' 99 1Ob 9O6Ob 76700' 3189, 3258, 3276 4760 338 385 5144, 5113, 6034 isoemissive 432 5963
313 300
9320 8700
365 463 338 378
332,c 332 318 280,c 280 332
NaOH, 0.1 M 350 300 PN MeOH 330' 285' 273.5' 263.5' EtOH 285c 274' 265c 320d 286d 275d 265d MeOH/KOH 292.5' 28 1 .5' 272.0' HC1, 1 M 328 284 272 262 NaOH, 0.1 M 345 293 282 27 1
1349,c 1554 1475 5129,c 5018 1450 2988 5115 1870' 3030b 4250' 3680b 331 l C 4677' 389OC 1861d 3301d 4559d 391 Id 4460' 5900' 4470b 1794 2897 4198 3676 2616 4450 5800 4500
280 358 isoemissive 393 350 430
328 350 isoemissive 383 345 417
"From ref 39. bFrom SADTLER. CFrom CRC. dFrom Merck index. SCHEME I: Prototropic Equilibria
crystallized 3 times from acetone/H,O (90/ 10, v/v); no impurities were detected on TLC plates30 (Table I).
284
The Journal of Physical Chemistry, Vol. 90, No. 2, 1986
Politi and Chaimovich
TABLE II: Lifetimes (T,, T,'), ken*, pK., and pK.* of POH, flN68S, flN6S, and flN d a c i d ) , ns ~ t (base), l ns
knm*, S-'
DK,
DK.'
(3.2 i 2.1) X IO'O
7.30 7.20
1.2 0.4 0.5
32 27 15
7.25
0.32
26, 28, 39
7.70 7.90
0.5
48, 49
source
POH 5.70 4.10 (HCI04, 5 M) 3.70 ( W , > 2.5) (HC104, 5 M) 5.20a (HCI, 2.8 N) 3.90 (Wo < 1) 5.90-6.00
5.45 5.00 6.00 4.70 5.88
(NaOH, 0.001 M)
(Wo> 2.5) (NaOH, 0.001 M) (NaOH, 0.001 M) (Wo > 10.0)
1.04 X 1O1O 8.9 x 109
1 x 10'0 4 x 109
5.40
44
PN68S 1.38 X IO'O
4.10b
9.2
0.5
37
1.7 1.66 1.94
21 50 51 52
2.5 2.8 2.8 3.0 2.5 2.8
32 53 54 31 37 55
PN6S 1.00 x 109 1 x 109 10" (1.02 i 0.2) x 109
IO
9.5 9.1
PN 5.2
9.3 8.9
1.2
9.43
"These values were used for
T~ in
9.46 9.40 9.1 9.47 3 x 108 7 x 107
9.5
the subsequent manipulations. *Estimated from kOff*(37), the measured Io using eq 1
All organic solvents were distilled prior to use. Water twice distilled in glass was used throughout (conductivity = 2 FS). All other reagents were analytical grade. Special care was taken to avoid light exposure of stock solutions containing fluorescent probes. Methods. UV-vis absorption spectra were recorded in a Cary 14 or Beckman M25 spectrophotometers. Fluorescent spectra were obtained in a Hitachi-Perkin Elmer Model MPF-4at 25 OC (ratio mode), the excitation and emission slits fixed at 2 and 4 nm, respectively. Optical densities were always below 0.1 to avoid inner filter effects. The excitation and emission spectra were corrected only for the optical density of each solution. No further correction was necessary since relative intensities and intensity ratios were obtained at the same wavelength and precalibrated for each experiment.
Results The prototropic equilibria representing the dissociation in the ground and excited states is represented in Scheme I, where RH, R-,RH*, and R-* represent the acid-base conjugated pair in the ground and excited single state, respectively. k f ,k,,, k;, and k,; are the radiative and nonradiative rate constants for the decay of RH* and R-*, respectively. koff*and k,,* are the dissociation and association rate constant in the singlet excited state (K,* = koff*/kon*; pK,* = -log K,*); whereas koffand k,, are those in the ground state ( K , = kOff/kon). The change in pK, (ApK, = pK,* - pK,)31-36can be determined by measuring the emission of one of the (conjugate pair) species in a solution of the other species in the ground state. For our purpose we will restrict the analysis to the singlet excited-state dissociation and for the compounds presently in use pK,* < pK,, with ApK,'s in the order of 5-7 pH units (Table 11). (30) Kano, K.; Fendler, J. H. Biochim. Biophys. Acta 1978, 509, 289. (31) Ireland, J. F.; Wyatt, P. A. Adv. Phys. Org. Chem. 1976, Z2, 131. (32) Weller, A. Prog. React. Kinet. 1961, 1 , 187. (33) Forster, Th.; Volker, S.Chem. Phys. Lett. 1975, 34, 1. (34) Gutman, M.; Huppert, D.; Kaufmann, K. Adu. Chem. Phys. 1981, 47, 643. (35) Schulamn, S.G.Rev. Anal. Chem. 1971, 1, 85. (36) Parker, C.A. In "Photoluminescence of Solutions"; Elsevier: Amsterdam, 1968.
The relationship between koff*and steady-state fluorescence intensities is given by eq 1,32,36where Io represents the fluorescence
-- 1 I intensity for the acidic specie in the absence of proton transfer and I the fluorescence intensity under conditions which permit proton dissociation, and Tf and T ( are the fluorescence lifetimes of RH* and R-*, respectively. At low proton concentration (pH's smaller than but close to pK, - 1 and higher than pK,*) the second term of eq 1 can be neglected and the following approximation holds:
Although excited-state proton transfer occurs adiabatically, we decided not to use the fluorescence emission relative to R-* for the calculations due to peak overlap and incomplete spectral resolution. Since Io was obtained by employing a concentrated acid solution (see Experimental Section), the Io/I ratios are directly proportional to the quantum yield ratios (the emission and excitation wavelengths were fixed, or showed a small shift of 10 nm at the most). Molar absorptivity changes as a function of the water content were corrected by using the absorbances of the same solution. With this procedure random analytical errors were also compensated. In cases where the values of Io were doubtful we used known koff*values and eq 2 for a given I to estimate I,,. Usually the agreement between the experimental and calculated Io was satisfactory. The Tf values of the probes, essential for the calculation of koff*, are presented in Table 11; Tf for POH varies with Wofrom 3.70 to 4.80 ns.15,26328 However, no such data are available for PN, /3N6S, and PN68S. Provided the variation in 7;s for the naphthalene probes is of the order of that of POH the error in koff* is, at maximum, comparable to our experimental error. Considering, for example, the two T~ values of POH (3.7 ns, Wo < 1 and 4.7 ns, Wo > lo), koff*values for Wo 0.9 and 12 would become 9.4 X lo7 and 6.5 X lo9 s-', instead of 7.1 X lo7 and 5.8 X lo9 s-l, for T f = 5.2 ns (see Figure 2). Since the error estimated in the determination of koff*is of the order of 10-1 5% (see Figure
The Journal of Physical Chemistry, Vol. 90, No. 2, 1986 285
Water Activity in Reversed AOT Micelles TABLE 111: Na' Free Ions and Extent of AOT Dissociation Rfl" 31 32 33 36 38 39 57 96 102 106
WO 4.74 5.86 6.98 9.20 11.40 13.59 24.34 32.69 34.75 54.60
total Na' (max), M pool dim (2)
dilutn (1) 11.62 9.38 7.70 5.85 4.72 3.96 2.21 1.65 1.55 0.99
112.0 8.90 7.82 7.57 6.33 6.22 3.38 1.80 1.60 0.98
a/b 0.78 0.82 0.86 0.91 0.94 0.96 0.97 0.97 0.97 0.97
[NaCl], M
UIC
%b
5.0 f 0.8 4.3 f 0.8 3.5 f 0.8 2.5 f 0.8 1.7 f 0.8 1.2 f 0.8 0.8 f 0.8
0.43 f 0.07 0.46 f 0.08 0.43 f 0.09 0.43 f 0.13 0.36 f 0.15 0.30 f 0.20 0.36 f 0.36
0.42 f 0.07 0.48 f 0.08 0.43 f 0.09 0.33 f 0.10 0.27 f 0.12 0.19 f 0.15 0.20 f 0.20
0.05 0.05 0.05 0.05 0.05 0.05 0.05 f 0.05 f 0.05 f 0.05 f f f f f f f
" Micellar hydrodynamic radii. Values estimated from ref 3a for T = 30 O C . from Figure 1A; estimated errors were obtained from the data scatter in Figure 1A. c a I and cy2 = ratios of [Na'],,,,,/[Na+],,,,, maximum dilution and pool dimensions, respectively. IO
9
* ) ' c .
0 -Y
cn 0
8
J
7 0
x
9.0
0
er
IO
20
30
40
WO
0
1
Figure 2. Dissociation rate constants (kofI*)as a function of W, (AOT = 0.082 M) of (0) POH 3 X M; (a) PN68S 2 X M; ( 0 ) PN6S. kOff*values obtained as in Figure 1.
B 0.0
- 0.1
0
10.0
C
Figure 1. Salt effect on the excited-state dissociation rate constants (kOff*)of 3 X 10" M POH (A), 2 X M PN6S (B); 2 X M PN68S (C) koff*values were obtained from fluorescence intensity ratios
according to eq 2 and TI values from Table 11. (A) POH, A,, = 403 nm, A,, = 445 nm; (B) PN6S, A,, = 280 nm, A,, = 350 nm; (C) PN68S, A,, = 338 nm, A,, = 390 nm (slits 2 nm (exc), 4 nm (em); T = 25 "C). a, values were obtained from reported molal osmotic coefficients'* from NaCl solutions and MgCI2solutions: 0 , NaCl; 0,MgCI,. Dotted lines in A are estimates of the error in the method. lA), the effect of varying T { S can therefore be neglected. Water Activities (a,) us. Salt Concentration. The variation of koff*with a, (molal scale) for the probes is presented in Figure 1. The linear variation of koff*and a , was independent of the nature of the electrolyte (Figure 1A). Parts B and C of Figure 1 show the variation of koff*with a, for PN6S and PN68S. These results agree with those of Huppert and give support to our methodology (compare intercepts in Figure 1A-C with Table 111). [This agreement implies that the proton does not recombine with the excited anion before it decays to the ground state.37 Fur-
thermore the determination of koff* by simple steady-state fluorescence measurements can be extended to other indicators (Figure lB,C).] koff*Measurements in Reversed Micelles. The variation of koff* for the three probes with Woin reversed micelles of AOT is presented in Figure 2. Even at W,as low as 0.3 the most hydrophilic probe (POH) dissociated in the reversed m i ~ e l l e . ' ~ ~ ~ ~ For PN68S and PN6S deprotonation was observed beyond Wo 3 2. Since these latter probes are insoluble in the organic phase it must be concluded that below Wo= 2 they reside exclusively in the interface where they cannot dissociate. The value of koff* for POH increased with the increase of Woup to a plateau at Wo 12, where its value was close to that observed in pure water. For PN6S a similar dependence of koff*with Wowas obtained; however, the plateau is reached at a much higher Wo. When a less charged probe, PN6S, is used, the value of koff*increased with Woand did not reach a maximum. The value of koff* in water for PN6S would only be reached beyond the L2 phase domain. We also attempted to measure the dissociation of PN as function of W, but none was observed up to Wo 50. It is well established that excited-state acids with a pK,* > 0 use only water as proton acceptors.27 Thus, failure to observe dissociation of PN in reversed micelles suggests that this probe is located in a surrounding such that no water is available for deprotonation. In order to obtain an experimental support about the PN location we measured the partitioning of PN between isooctane/H20 for both acidic PN and basic PN. For protonated PN the partition coefficient ( K )
-
(37) Huppert, D.; Kolodney, E.; Gutman, M.; Nachliel, E. J . Am. Chem. SOC.1982, 104, 6949. (38) Robinson, R. A,; Stokes, R. H. 'Electrolyte Solutions"; Butterworths:
London, 1959. (39) Politi, M. J.; Fendler, J. H. J . Am. Chem. SOC.1984, 106, 265.
286
The Journal of Physical Chemistry, Vol. 90, No. 2, 1986
Politi and Chaimovich
was K(H20~,H-6,0)/isooct) N 0.8. For the conjugated base K(H20(pH,2)/isooct) >> lo4. These data indicate that protonated PN could be located at the interface, whereas the conjugate base will reside at the aqueous pool. It is interesting to note that even M N a O H solution instead of pure H 2 0 to by addition of obtain up to Wo 50 no ground-state or excited-state base was observed. [The dissociation of PN in normal micelles (SDS) is ~ i g n i f i c a n t . Since ~ ~ BN resides in the interface in both normal and reversed micelles the difference in dissociation behavior points out the magnitude of structural difference between the interfaces of both aggregates.] In order to check salt effects upon koff*’swithin the water pools we attempted to introduce concentrated NaCl solutions instead of pure H 2 0in the reversed micelles. We started by introducing NaCl solutions around -2 M which resulted in phase separation. The highest salt concentration which we were able to solubilize in the AOT reversed micelles was around -0.2 M (compare, for example, with ref 42 and 43), which is too small to affect koff* in water (Figure 1).
I .o
-
Discussion The extent of proton transfer in excited-state prototropic equilibria is dependent on the properties of the acceptor (solvent) and donor (excited-state acid). For compounds with pKa* > 0 typically the probe used in the present study H 2 0 is the only solvent capable of accepting the proton.27 On the other hand compounds with pKa* < 0 can dissociate in pure hydrophilic solvents such as low alcohols.56 Thus at least in principle this set of probes (pKa* > 0) should allow the correlation between the rate of dissociation and the amount, or properties, of HzO. Let us consider first the behavior of POH as a function of H 2 0 content in the AOT reversed micelles (Figure 2). Since POH is negatively charged and highly hydrophilic it is expected that the probe will reside in the center of the water pool. It is noteworthy that POH seems to report an aqueous pool even under conditions where the radius of the (unperturbed) reversed micelle (Table 111) barely exceeds the effective radius of the probe (Debye radius of POH = 30 A).33 Several experimental observations support this expectation. The absorption maxima of POH, as well as its fluorescence emission maxima, are blue-shifted with respect to bulk water below Wo 2.15*28Time-dependent polarization anisotropy experiments show no anisotropy of POH at around Wo 1.26,28 This behavior is opposed to that of pyrenetetracarboxylate where the probe was found to be locked within the micelle at low W,46,47This different behavior indicates the higher degree of hydration of the -SOY when compared with the
