Langmuir 1992,8, 2417-2421
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Formation and Properties of Reversed Micelles of Aerosol OT Containing Urea in the Aqueous Pool Carmem L. Costa Amaral,+ Oscar Brino,t Hernan Chaimovich,§ and Mario Jose Politi'gg Chemistry Department, Marquette University, Milwqukee, Wisconsin 53233, Departamento de Quimica Fundamental and Departamento de Bioquimica, Laboratorio Interdepartmental de Cinetica Rbpida, Instituto de Quimica, Universidade de Sit0 Paulo, C.P. 20780, Sit0 Paulo, CEP 01498, Brasil Received April 17, 1992. I n Final Form: July 6, 1992 The effect of urea on the properties of reversed micelles was investigatedusing sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in hexane. Using quasielastic light scattering and conductivitymeasurements, we demonstrated the formation of reversed micelles of AOT containing up to 10 M urea in the water pool at low aqueous volume fractions ( c # J ~ ) . Transition to bicontinuous phases are obtained at progressively lower @ Jas ~ the concentration of urea in the water pool is increased. In a particular urea concentration (5.0 M in the aqueous pool) the conductivity of the solution decreases with temperature between 10 and 50 O C . The properties of the urea-containingwater pool of the AOT micelles were probed measuring the rates of excited- and ground-state proton transfer reactions with the prototropic probe 8-hydroxy-1,2,6pyrenetrisulfonate. The proton transfer rates in reversed AOT micelles in hexane containing urea are similar to the rates obtained with the same probe in bulk aqueous solution. Introduction Reversed micelles and water in oil microemulsions permit the investigation of solution properties of highly compartimentalized The properties of compartimentalized water, in an equilibrium domain, can be studied in reversed micelles by varying the water to detergent molar ratio (W). W can be varied from less than 1, where most of the water molecules solvate the detergent head group, to values where most of the water exists as bulklike solvent in the inner pool of reversed micelles. Null or very small interfacial surface free energy maintains these ternary systems within thermodynamic stability boundaries. Other solvents, of polarities similar to water, can fulfill the conditions for reversed micelle formation. For example reversed micelles of AOT in n-heptane form in the presence of glycer01.~Besides their importance as microreactors and models of selected properties of complex biological systems,l0 reversed micelles exhibit several other convenient features such as ease of preparation and a large range of applications.11 Therefore, the description of the effects of cosolutes, cosurfactants, and other additives on reversed micellar properties may broaden their applications. Here we have investigated the effect of urea on the formation, structure, and selected dynamic properties of
* Author for correspondence, FAX 55-11-8155579.
Depto de Quimica Fundamental, Universidade de SHo Paulo. Chemistry Department, Marquette University. Depto de Bioquimica, Universidade de SHo Paulo. (1) Eicke, H. F.; Rehak, J. Helu. Chim. Acta 1976, 59, 2883. (2) Eicke, H. F.; Christen, H. J. Colloid Interface Sci. 1974,46,417. (3) Luisi, P. L.; Giomini, M.; Pileni, M. P.; Robinson, B. H. Biochim. t
Biophys. Acta 1988, 947,209. (4) Langevin, D. Acc. Chem. Res. 1988,21, 255. (5) Ruckenstein, E.; Nagarajan, R. J. Phys. Chem. 1980,84, 1349. (6) Boicelli, C. A.; Giomini, M.; Giuliani, A. M. Appl. Spectrosc. 1984, 38,537. (7) Battistel, E.; Luisi, P. L. J. Colloid Interface Sci. 1989, 128, 7. (8) Bardez, E.; Larrer, B.; Zhu, X. X.; Valeur, B. Chem. Phys. Lett. 1990, 171, 362. (9) Fletcher, P. D. I.; Galal, M. F.; Robinson, B. H. J. Chem. SOC., Faraday Tram 1 1984,80, 3307. (10) Marcel, W. Proteins: Struct. Funct., Genet. 1986, 1,4. (11) Fendler, J. H. In Membrane Mimetic Chemistry; Wiley: New York, 1982.
urea/water/AOT/hexane mixtures. Stablemicroemulsions were formed in the presence of (up to) 10 M urea. By ) a fixed increase of the volume fraction of water ( c $ ~ at value (10.0) of W,percolation, measured by ionic conductance, was observed a t & 1 0.2 in AOT reversed micelles. Addition of urea leads to percolation at lower $J~. Under a particular set of conditions, i.e. water/AOT/ hexane mixtures containing 5.0 M urea in the water pool, we observed a decrease in conductance with increasing temperature. The properties of the urea/water pools, probed using the phototropic probe trisodium 8-hydroxy1,3,6-pyrenetrisulfonate(POH),suggest that addition of urea to the water pool of reversed micelles does not significantly change the rates of acid protonation or deprotonation. Experimental Section Materials. Sodium bis(2-ethylhexyl)sulfosuccinate (AOT) (Sigma)was purified as described,'z stock solutionsof AOT were prepared in n-hexane. Urea (Merck) was triply recrystallized from hot EtOH (99%). The conductance (A) of an aqueous solution of 10 M urea was 6 rS. Trisodium 8-hydroxy-1,3,6pyrenetrisulfonate (POH) (Eastman Kodak)was treated with activated charcoaland recrystallizsd 3times from aqueousacetone (5:95 (vlv)). n-Hexane was distilled prior to use and kept over activated molecular sieves (4 A). Bidistilled water (all glass)was used throughout (A = 2 pS). The values of 4wwere calculated from the volume ratios of water (oraqueous urea) to totalvolume. Measurements. Conductivity was measured with D-21 conductivimeter (Digimed,Brazil) or with Hawwa Instrumenta HI 8333conductivimeter. UV-visible absorptionswere recorded on a Hitachi 2000 spectrophotometer. Fluorescent spectra were obtained in a Perkin-Elmer LS-5 spectrofluorometer. Timeresolved fluorescence data were determined on a single photon counting system (LS-1, PTI) using extra pure Hz to fill the thryratron-drivenpulsed arc lamp. Proton transfer rate constants were calculated by monitoring the emission decay at 440 nm (POH* decay) and 510 nm (PO-* formation and decay) and deconvoluting from the known impulse response (see Scheme I and Table I). Nanosecond laser flash photolysis (Applied Photophysics)kinetics were carried out using the third harmonic line (353 nm) from a YAG laser delivering 8-ns (fwhm) pulses (12) Politi, M. J. PhD Dissertation, Clarkson University, Potsdam, NY. 1984.
0743-1463/92/2408-2417$03.00/00 1992 American Chemical Society
Costa Amaral et al.
2418 Langmuir, Vol. 8,No. 10,1992
0 , I
I
"
0
-i
20
40
W = [H,O]/[AOT]
Figure 1. Hydrodynamic radii (Rh) of AOT reversed micelles as function of W: [AOTI = 0.1 M; [ureal = 0 M (X), 3 M (01, 5 M (01, and 10 M (A). T = 25 "C. having 5 mJ per pulse. Transients were accumulated on a Tektronix 2230digitizer andthedatawereanalyzsdviaaRS232C interface in a PC microcomputer. All the spectra were taken at 30 "C. Light-scatteringmeasurements were performed in a BIC30 autocorrelator(Brookhaven)using a 10m W verticallypolarized He/Ne laser (Hughes).
Results The formation of reversed micelles in urea/water/AOT/ hexane mixtures was evidenced from the hydrodynamic radii (Rh)obtained from dynamic light-scattering measurements (Figure l). The values of &, obtained for reversed micelles of AOT containing only water (Figure 1) are in good agreement with published data.lg-16 Addition of urea to the (reversed micellar) water pool does not significantly affect the increase in Rh with W up to a value of 10.0(Figure 1). The increase of Rh with W, above W = 10,is a function of the urea concentration in the aqueous pool (Figure 1). Urea does not measurably dissolve in hexane; therefore, these effects are not related to urea solubilization in hexane. On the other hand urea solubilizes, to a limit of ca. one molecule per surfactant, in water-free hexane containing 0.1 M AOT.ls Increasingthe volume fraction of water (or aqueous urea) (#w) in reversed micellar solutions leads to a change in the aggregate structure resulting in percolation, easily demonstrated by measuring electrical conductivity Addition of urea to the aqueous pool of reversed micelles sharply affected the onset of increased conductance as #w increased (Figure 2). For 3 M and 5 M urea, percolation began when #w exceed 0.05 and 0.02, respectively. It is important to note that the R h values presented in Figure 1were measured under conditions where no percolation is measurable by conductance. The effects of temperature (0 < T < 50 O C ) on the X of selected urea/water/AOT/hexane mixtures are presented (13) Zulnuf, M.; Eicke, H. F. J. Phys. Chem. 1979,83,480. (14) Maitra, A. J. Phys. Chem. 1984,88, 5122. (15) Huang, J. S.; Sung, J.; Wu, X. L.J. CoZZoid Interface Sci. 1989, 132,34. (16) The solubility of urea in pure hexane (A) and in AOT/hexane mixture (B)was determined with the following procedures: 0.1245 g of urea wae added to 10mL of A and B and the suspensionswere magnetically stirred for 48h. Undiesolved urea was filteredand the solvent wae stripped off. For solution A we did not detect any residue. Evaporation of unfiltered A quantitatively yielded the added urea. For solution B we obtained a residue containing the initial AOT maw and an excess of about 15%. Elemental analysis of the vacuum-dried residue (PerkinElmer elemental analyzer Model 2400 CHN) yielded 50.05% C, 8.31% H,and4.49%N. Fromthenedataweestimatedthattheresiduecontained 1 mol of urea per mole of AOT. As a control for solution B, a solution of AOT in hexane (without urea) was submitted to the same procedure and we quantitatively recovered the expected mass of AOT. (17) Eicke, H. F.; Borkovec, M.; Das-Gupta, B. J . Phys. Chem. 1989, 93, 314.
-I ,001 0.00
1
0.I O
t
I 0.20
$w
Figure 2. Electrical conductivity (A) as a function of the water volume fraction (&,) (see Methods) for urea/water/AOT/hexane mixtures: [urea] = 0 M (O), 1.4 M (a),3 M (A), and 5 M (+). T = 25 O C .
in Figure 3. For water/AOT/hexane (W= 10)and for 2.5 M aqueous urea/AOT/hexane ( W = 10)percolation starts at c$w Y 0.15(above 20 OC)and at #* = 0.08 (above 30 "C), respectively (Figures 3a,b). For 5 M aqueous urea/AOT/ hexane (W= 10)percolation occurs from #w H 0.03 but decreases with temperature in the range of #w 0.08-0.03. The study of the behavior of excited-state prototropic probes has been used to probe several features of the aqueous pool of reversed micelles.1B-20We and others have used the triply negatively charged fluorescent probe &hydroxy-1,3,6-pyrenetrisulfonate (POH)to analyze the properties of AOT reversed micelles.19-22 The spectra of protonated (POH) and deprotonated (PO-) species can be described using a Forster cycle (Scheme I). In Scheme I, POH is the protonated ground-state form and PO- the corresponding conjugated base. The deprotonation and protonation rate constants in the excited and ground states are k*,ft, and k,,, respectively. The radiative rate constant of (POH*)and that for (PO-)* are kfand k(, respectively. X,b, Xf, and A( represent the absorption and emission bands maxima for POH, PO-, POH*, and (PO-)*,respectively. Since the fluorescencequantum yields for POH* and PO-* are almost unitary,a nonradiative decayhates were not included. Absorption spectra of POH incorporated in AOT reversed micelles, under our experimental conditions, 403 exhibited only the band relative to POH (X,bnm). This condition simplifies the analysis of the Forster cycle and ensures that our measurements are not complicated by prior existence (and subsequent excitation of) of ground-state PO-. Steady-state fluorescence spectra of POH in water/AOT/hexane reversed micelles showed two main emission bands centered around 440 and 510 nm, corresponding to the radiative decay of POH* and PO-*,respectively.20 As previously observed the intensity of the emission of PO-* increases with W, while that of POH* decreases.20*21The same spectral behavior was observed with reversed AOT micelles containing 5 M or 10 M urea in the aqueous pool (Figure 4). These observations suggest that urea does not change appreciably the prototropic behavior of POH in the water pool. The
-
(18) Bardez, E.; Monieur,E.; Valeur, B. J. Phys. Chem. 1986,89,5031. (19) Politi, M. J.; Chaimovich, H. J. Phys. Chem. 1986,90, 282. (20) Politi, M. J.; Brandt, 0.; Fendler, J. H. J.Phys. Chem. 198S, 89, 2345. (21) Bardez, E.; Goguillon, B. T.; Keh, E.; Valeur, B. J . Phyr. Chem. 1984,88, 1909. (22) Perez, V. C.; Beddard, C. S.;Fendler, J. H. J . Phyr. Chem. 1981, 85,2316. (23) Gutman, M.; Huppert, D.; Nachliel, E. Eur. J. Biochem. 1982, 121, 637.
Effect of Urea on Reversed Micelles
-
Langmuir, Vol. 8,No. 10, 1992 2419
3.0-
r
'E v
-
v)
a
A 1.001 2
I
*
47.93
,.-
0.0
40.0
20.0
60.0 0.00
oc
t
t
////
-cL / 400
U M O
450
550
WAVELEffiTH INMI
Figure 4. Fluorescence spectra of 6 X 1od M POH in 5 M aqueous
urea/AOT/hexane at room temperature aa a function of W ( W = [water]/[AOT]): (a) W = 1; (b) W = 2; (c) W = 3; (d) W = 4; (e) W = 5; (0 W = 6; (9) W = 7; (h) W = 8; (i) W = 9; (j)W = 10; (k) W = 11. [AOT] = 0.1 M; &,,, = 403 nm.
-1.0 I 0.0
,20.0
40.0 O
D
30
60.0 E
C
I+---20.0
0.0
40.0
60.0
OC
Figure 3. Effect of temperature, water volume fraction (4w),
and ureaconcentrationson the electrical conductivity(A) of AOT/ hexane mixtures: (a, top) urea 0 M, = 0.01 (*), 0.05 (X), 0.10 (A),0.15 (Oh0.2 (+); (b,middle) urea 2.5 M, 4w= 0.01 (*I, 0.05 (X), 0.076 (e),0.15 (O),0.2 (+); (c, bottom) urea 5 M, I # J ~ = 0.02 ( 0 ) ,0.03 (W), 0.04 (A),0.076 (O),0.1 (A),0.2 (+). was fixed at 10.0 by adding the appropriate amount of AOT. C#J~
w
Scheme I. Forster Cycle
k."
POH Xbs = 409 nm
+
."ll
H20 km
It
PO+ H30' Labs= 455 nm
increase in the 510-nm band is a function of the amount of protonable solvent reaching a plateau at around W 1 12, where POH* dissociates freely as in bulk ~ a t e r . 2 ~ The values of k*off of POH (Scheme I) are presented in Table I. The excited-state deprotonation rate constant (k*oed was not sensitive to urea in AOT reversed micelles (Table I). However the average value of k*offwas smaller than that in AOT micelles containing no urea.20,21 (24) Politi, M. J.; Chaimovich, H. J. Sol. Chem. 1989, 18, 1055.
Ground-statereprotonation rate constants (koJ of POH in the urea-containing reversed micelles of AOT, determined by laser pH jump, are presented in Figure 5. The values of kon decrease with (the increase of) W in AOT micelles with or without urea. The recombination process (Scheme I) is controlled by H+ diffusion;25therefore,these results suggest that the rate of proton diffusion in the water pool of reversed AOT micelles is unaffected by urea.
Discussion The main goal of the present investigation was to demonstrate the formation of discrete reversed micelles containing urea in the water pool and to investigate the limits where these discrete aggregates begin to percolate and form higher order structures. Our secondaryobjective was to initiate a study of the properties of the dimensionally-restricted water pools containing urea. Dynamic light scattering has been used to determine the hydrodynamic radius of several discrete supramolecular aggregates, including reversed micelles.26 Our data for R h of the systems containing AOT/water in hexane were in good agreement with published ~a1ues.l~The values of Rh, increase with W , specially at high urea concentration, suggesting that the presence of urea in the water pool enhances the micellar growth promoted by augmenting W. Urea increases the critical micelle concentration (cmc) of aqueous ionic micelles, possibly by increasing the solubility of the monomer without changing the solvents' hydrogen-bonding ~ a p a b i l i t y . l ~ ~Urea ~'-~~ preferentially binds at the micelle/water interface32and induces a larger degree of counterion dissociation from ionic micelle^.^^^^^ The charge density a t the aqueous micellar surface does not necessarily change, since the increase in counterion dissociation produced by urea could be compensated by an increase in the micellar size and binding of urea to the interface. These findings indicate that for reversed micelles containing urea in the aqueous pool, the condition of zero (or vanishingly small) interfacial (25)Pines, E.;Huppert, D. J. Chem. Phys. 1986,84, 3576. (26) Day, R. A.; Robinson, B. H.; Clarke, J. H. R.; Doherty, J. V. J. Chem. Soc., Faraday Trans. 1 1979, 75, 132. (27) Muller, N. J.Phys. Chen. 1990, 94, 3856. (28)Abu-Hamdiyyah, M.;Kumari, K. J . Phys. Chem. 1990,94,6445. (29)Causi, S.; De Lisi, R.; Milioto, S.;Terone, N. J.Phya. Chem. 1991, 95, 5664. (30) Baglioni, P.; Minten, E. R.; Dei, L.; Ferroni, E. J . Phys. Chem. 1990,94, 8218. (31)Mukerjee, P.; Ray, A. J.Phys. Chem. 1963, 67, 190. (32)Sarkar, N.;Bhattacharyya, K. Chem. Phys. Lett. 1991,180,283.
2420 Langmuir, Vol. 8, No. 10, 1992
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Costa Amaral et al.
Table I. Excited-State Deprotonation of POH (6 X M) in Water/Urea/AOT/Hexane Mixtures. [AOT] = 0.1 M urea, 3 Ma urea, 5 M urea, 10 M hb TI(ns) Tz (ns) k*orf (lo9 fl TI (ns) TZ(ns) k*ofr (lo9S-l) fl TI (ns) TZ(ns) k*off (109 S-l) X = 440 nm
3 4 5 6 7 8 9 10 12
0.38 0.64 0.57
0.49 0.25 0.79
3.0 2.7 2.7
0.64 2.30 0.61
0.71 0.10 0.86 0.83 0.83
0.69 0.58 0.51 0.81 0.71
2.4 5.0 2.7 4.1 4.8
3 4 5 6 7 8 9 10 12
0.92 0.96 0.89 0.72 0.67 0.51 0.61 0.53 0.67
0.10 0.62 0.20 0.59 0.60 0.11 0.53 0.66 0.41
4.6 4.8 4.8 4.8 4.6 4.9 4.9 4.9 5.2
0.76 0.15 1.39 0.83 0.99
0.42 0.31 0.68 0.75 0.39 0.83 0.88 0.88
0.26 0.54 0.63 0.55 0.32 0.76 0.66 0.54
2.52 2.19 2.61 2.61 1.71 3.64 4.66 5.15 X = 510 nm
2.67 1.19 4.20 1.06 0.97 4.13 1.00 0.65 1.40
0.91 0.81 0.76 0.73 0.69 0.79 0.76 0.30
0.34 0.45 0.57 0.33 0.38 0.26 0.18 2.30
5.11 4.93 5.21 5.11 4.53 4.79 4.58 4.67
1.44 0.43 0.83 1.09 1.01 0.87 1.14 1.46
0.45 0.57 0.65 0.62 0.71 0.73 0.76
0.48 0.44 1.32 0.32 1.23 1.03 1.14
2.92 2.75 3.23 2.27 3.23 3.47 4.71
0.78 1.08 0.29 1.63 0.35 0.49 0.50
2.50 1.59 1.17 2.04 1.63 2.79 3.96
a Urea concentrationfrom stock aqueous solutions. Decay curves were normalized. Calculatedusing standard two interconvertiblestates.% Values determined using as the impulse response the emission decay function at 440 nm.42
Figure 5. Proton association rate constants (ken) of PO-in AOT reversed micelles at various water and urea/water contents ( W). [AOT] = 0.1 M and [urea] = 0 M (XI,3 M (n),5 M ( O ) ,and 10
M
(A).
surface free energy should be attained at a higher detergent concentration, i.e. higher cmc. Under most of the experimental conditions used in this work the surfactant concentration was fixed (ca. 0.1 M). A higher degree of counterion dissociation from the surface of the reversed micelle, caused by urea, should increase the interfacial area in order to maintain thermodynamic stability decreasing charge-charge repulsion. Binding of urea a t the (reversed micellar) interface is equivalent, on the average, to a change in the form of the monomer from a typical trapezoid format to a conelike structure.33 The increase in the interfacial area permits an increas-e in the average reversed micellar aggregation number (N). In the separated particle regime, Le. low r#JW's, the R h values increase with urea concentration (Figure 1). It is likely, therefore, that as a consequence of the presence of urea in the aqueous pool of reversed micelles, there is an increase in the interfacially bound additive. The solubilization of urea in AOT/hexane solutions16also suggests that the interface is a preferential binding site for urea in the reversed micelles. Definitive evidence for the existence of a domain consisting of discrete aggregates, in the presence of high (33) Israelachvili,J. N.; Mitchell, D. J.; Ninham, B. W. J.Chem. SOC., Faraday Trans. 2 1976, 72, 1525.
urea concentrations in the water pool of AOT reversed micelles, was obtained by conductivity studies (Figure 2). Under conditions where the R h suggests the existence of small discrete particles, the conductance was low. The system begins to percolate above r#Jw L 0.064 and r#Jw L 0.02 for AOT (reversed micelles containing 3 and 5 M urea, respectively). This effect is typical of a transition from a phase containing discrete aggregates to a bicontinuous phase.4,34-36The reversed temperature effect (Figure 3c) leading to loss of percolation with increasing temperature can be rationalized by assuming that, at this particular concentration, the relative amount of urea associated with the interface is particularly sensitive to temperature. The solubility of urea in water increases with temperature.37 Therefore, the interface/pool distribution partition coefficient of urea should decrease with temperature. With increasing temperature the relative proportion of urea at the interface should decrease and so should the interfacial area. The effective monomer shape changes back to trapezoidal and the minimum free energy of the mixture lies in the separate particle domain. This effect, although reproducible, is subtle, since for the other mixtures studied no decrease in X with temperature was observed. The reverse conductivity effect (Figure 3c) can be rationalized, alternatively, by simply considering thermal motion of the particles in a particular condition of the phase diagram. The conductance values for the mixtures exhibiting an inversetemperature effect are relativelysmall (Figure 3c) (for r#Jw = 0.076, X = 209 p S at 10 "C and X = 135 p S at 45 "C; for rpW= 0.04,X = 60 p S at 10 "C and X = 2.6 at 45 "C; for dw= 0.03, A = 20 p S at 10 OC and X = 0.8 $3 at 45 "C). These conductance values suggest that the increased conductance, at these particular &'s, may arise from micellar clustering rather than bicontinuous phases. If the increased conductivity observed for AOT reversed micelles containing 5 M urea is due to the existence of clusters of aggregates, these may separate as the thermal energy increases. This explanation has been ~~~~
(34) Safran, S. A.; Webman, I.; Grest, G. S. Phys. Rev. A 1985,32,50. (35) Talmn,Y.; Prager, S. J. Chem. Phys. 1978, 69, 2984. (36)Zana, R.; Lang, J.; Canet, D. J. Phys. Chem. 1991,95, 336. (37) Merck Index, 8th ed.; 1968; p 1094.
Langmuir, Vol. 8, No. 10, 1992 2421
Effect of Urea on Reversed Micelles
used to explain dielectricrelaxation studies in AOT/water/ heptane mixtures.% In order to obtain information on the solvent state in the urea/water pool reversed micelles, we investigated the behavior of a highly hydrophilic triply negatively charged prototropic probe. The typical behavior of POH prototropic steady-state luminescence previously found for ureafree reversed AOT micelles was observed in the presence of urea19120(Figure 4). The extent of proton dissociation for prototropic probes such as POH, having a pKa* of 0, is a result from the relative values of k * , ~vs [solvent] (where [solvent] stands for the waterlike concentration of solvent and its capability to hydrate the departing H+ before the excited state decays) and the magnitude of k*,"* [H+l (see Scheme I). In bulk aqueous solution, urea (up to 10M) has no effect on the dissociation extent of POH.19 It is still unclear whether this (lack of) effect is due to an increase in k*,nor to an enhanced protonation capabilities of the solvent coupled to dielectric effects on the geminate ion pair.394 A similar (lack of) effect was found here with high concentrations of urea in the aqueous pool of reversed micelles (Table I). Therefore it is evident that even in the restricted dimensionality of the reversed micellar pool the hydrogen bond capability of the ure4water solvent was maintained. The same behavior was found on the reprotonation kinetics of PO- (Figure 5 ) suggesting that urea does not affect H+ diffusion in reversed micelles. At this time it is not evident why k,, is not dependent on urea concentration. Urea was shown to decrease H+ activity and increase acid ionization (which is due to increased (38) Van Dijk, M. A.; Casteleijn, G.;Joosten, J. G.H.; Levine, Y. K. J . Chem. Phys. 1986,85,626. (39) Woldan, M. Ber. Bunsen-Gee. Phys. Chem. 1989,93, 782. (40) Bateman, J. B.; Gabriel, C.; Evans, G. F.; Grant, E. H. J. Chem. SOC.,Faraday Tram. 2 1990,86, 321.
dielectric constant).41 It appears that these contradictory factors cancel out in such a manner that the overall effect is a k,, equal to that of free urea water/AOT pools. Conclusions The formation of reversed micelles and microemulsions in AOT/hexane in the presence of up to 10 M aqueous urea has been demonstrated. A preferential solubilization of urea at the interface is proposed to account for earlier discrete to bicontinuous phase change. The anomalous behavior of decreased electrical conductivity with increasing temperature for AOT in hexane containing 5 M urea in the aqueous pool can be attributed to interfacial solubilization of urea or to breakage of micellar aggregates. The behavior of a prototropic probe suggests that the solvation properties of the hydrophilic core of ureacontaining reversed micelles are similar to those in bulk aqueous urea. A detailed photophysical study is currently being undertaken to provide more information on the properties of urea-containingmicroemulaions. We are also studying the effect of urea in reversed micelles on slower thermal reactions and enzyme-catalyzed reactions. Acknowledgment. We are very grateful for enlightening discussionswith Professor C. Tran from Marquette University and to the followingBrazilian funding agencies, CNPq, FAPESP, FINEP, PADCT, and CAPES. M.P. acknowledgestravel support from the BID-USP Program. We thank one of the reviewers for an extremely detailed and useful review of an earlier version of this work. Registry No. AOT,577-11-7; POH, 6358-69-6; urea, 57-13-6; hexane, 110-54-3. (41) Bull, H. B.; Breese, K.; Ferguson, G. L.; Swenson, C. A. Arch. Biochem. Biophys. 1964, 104,297. (42) Lakowicz, J. R.; Balter, A. Biophys. Chem. 1982, 16, 223.
