2120
J. Phys. Chem. 1994,98, 2120-2124
Microviscosity in Water Pool of Aerosol-OT Reversed Micelle Determined with Viscosity-Sensitive Fluorescenck Probe, Auramine 0, and Fluorescence Depolarization of Xanthene Dyes Masatoshi Hasegawa,. Tokuko Sugimura, Yukie Suzaki, and Yoichi Shindo Department of Chemistry, Faculty of Science, Toho University, 2-2- 1 , Miyama. Funabashi, Chiba 274, Japan
Ayao Kitahara Department of Industrial Chemistry, Science University of Tokyo, Kagurazaka, Sinjuku- ku, Tokyo 162, Japan Received: May 12, 1993; In Final Form: December 1 , 1993'
Auramine 0 (AuO), the fluorescence quantum yield (@f) of which increases with increasing solvent viscosity, was solubilized in Aerosol-OT (AOT) reversed micellar core (water pool) to determine the microviscosity (q,). This probe is localized at the vicinity of the water/surfactant interface owing to electrostatic association between the polar head groups in AOT and AuO. The microviscosities were determined rapidly as a function of R, (=[HzO]/[AOT]) by measuring the @f of AuO solubilized in the water pool. The AuO fluorescence showed that the water pool is highly viscous in the lower R, region; with increasing R,, qw rapidly decreased below R, = 10 and then gradually decreased until the micellar solution became turbid above R, = 50. Even in the higher R, region, the values of qw were considerably higher than the viscosity of the ordinary bulk water (ca. 1 CPat 25 "C).This indicates that the solubilizing water molecules are tightly bound to the polar head groups in AOT. The values of qw as a function of R, obtained by the AuO method coincided satisfactorily with those determined with the fluorescence depolarization of a cationic dye, rhodamine B. Thus, the results led to the conclusion that the AuO probe is useful for rapid determination of the microviscosity in the water pool of AOT reversed micelle. The fluorescence depolarization for different ionic character of xanthene dyes suggested that the water pool is heterogeneous with respect to the microviscosity.
Introduction Static structures of sodium bis(2-ethylhexyl)sulfosuccinate (Aerosol-OT or AOT) reversed micelle which is capable of solubilizing relatively large amount of water in the core have been well characterized by a variety of techniques. Dynamic light-scattering method (photon-correlation spectroscopy)l4 is useful for direct determination of micellar shape, size, and size distribution. This method indicated that the AOT reversed micelles containing water are essentially~pherical.~ Aggregation number, NaU,is easily calculated from the micellar size and R , if the micelles are monodisperse. Several studieslz showed that the AOT reversed micelles containing water are essentially monodisperse, except for one study.4 Time-resolved fluorescence quenching method'-13 provides information on not only the mean aggregation number but also dynamic structures, i.e., the solubilizate exchange due to intermicellar collision. However, the NaUobtained by the fluorescence quenching method tends to be considerably higher than that measured by the static light-scattering14J5andthevapor pressure osmometry.16 NMR spectroscopy has been applied successfully to study ~onformation~~*18 and molecular motion19.20 of AOT molecule as a function of R,. It is important to examinethe nature of thewater pool as applied to hydrophilic reaction field.21 Near-infrared22923 and 1H NMR spectroscopy19~23~24 (chemical shift and correlation time measurements) are useful for examhiagwater-hcad group interaction. The microviscosity in the wa& pod, as well as pofarity, has been known to change considerrrbfy with increasing R,. The focus of the present work is on the microviscosity in the water pool of AOT reversed micelle as a function of R,, determined with the viscosity-sensitive fluorescence probe, AuO.25 The AuO fluorescence measurement made possible to determine the qwdirectly ,
*Abstract published in Advance ACS Abstracrs, January 15, 1994.
without any assumption. The q, values obtained by the AuO method coincided satisfactorilywith those measured by thesteadystate fluorescence depolarization technique, indicating the availability of the AuO method. There are several reports on the microviscosity estimation for the AOT reversed micellar system by steady-state2630 and transient31s32fluorescence depolarization. These techniques are frequently used to examine qualitativelyviscosity change against Rw and heterogeneity in the water pool. However, in the fluorescence depolarization measurements for the AOT micellar system, the microviscosity cannot be easily estimated quantitatively because the effect of not only rotational diffusion of a dye solubilized in the water pool but also rotational motion of the micelle itself are included in the depolarization. We discuss the relation of qwvs R , obtained by using a data analysis method by Keh et ala2*and also describe heterogeneity of the water pool from the results of the fluorescence depolarization.
Experimental Section Samples. AOT (Nacalai Tesque) was purified as reported previ0usly.~3No fluorescence was detected for the purified AOT in n-alkanes (C6-CI2)by irradiationat the excitation wavelengths for all the probes. AuO (Tokyo Chemical Industry) was recrystallized twice from ethyl acetatemethanol mixture and then vacuum-dried at 50 "Cfor 24 h (mp 265 "C). Rhodamine B (RhB) and rhodamine 6G (Rh6G) (Tokyo Chemical Industry, fluorometricgrade) were used without further purification;sodium fluorescein (Na-Fl) (TokyoChemical Industry) was recrystallized from ethanol-THF and then vacuum-dried at 50 "C. All the probes were insolublein n-alkanes used. Figure 1 showschemical structures of AOT and the probes. n-Alkanes (C6-CIz) (Koso Chemical) dried with molecular sieves (4A) and deionized water were distilled. Fluorometric grade glycerol (Nacalai Tesque) was used without further purification.
0022-365419412098-2120$04.50/0 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 8, 1994 2121
Rhodamine 6G (Rh6G) Sodium Fluorescein (Na-FI) Figure 1. Chemical structures of surfactant and fluorescence probes.
-
IA I
Na-FI
1
probes in the water pool. We took 1.5 1153~for RhB in water, 4.8 1153~for Rh6G in ethanol, and 4.6 8935 for Na-F1 in water as the 70 values. It is believed that the +f of AuO correlates with a quenching phenomenon induced by internal rotation of the dimethylaniline group, the internal rotation is affected by solvent viscosity.36 In order to prepare the calibration curve for the microviscosity determination,the relationship between the 9fof AuO in glycerolwater mixtures and the viscosity of the mixtures was examined a t 25 f 0.2 OC; the viscosities of the commercially available fluorometric grade glycerol are in fact dispersed somewhat. Furthermore, even if one used only one kind of commercially available glycerol, the viscosities of glycerol-water mixtures with the same composition are not always the same because of the intractability of highly viscous glycerol. Then, we measured the densities of glycerol-water mixtures to determine the viscosities of the mixtures. Theviscosities of glycerol-water mixtures widely changed by varying the mixing ratio were determined by interpolation of the viscosity-density data;37the densities of the glycerol-water mixtures were measured at 25 O C with a pycnometer. The critical micelle concentration (cmc) for the AOT-water system was determined a t 25 O C from the plot of the surface tension vs log[AOT] with a Wilhelmy-type automatic surface tensionmeter (Kyowa Interface Science, CBVP-A3). Data Analysis. Fluorescence anisotropy ratio, r, is given by
Rh6G
400
500 600 Wavelength I nm
700 400
500
600
700
Wavelength I nm
Figure2. Fluorescenceandexcitation spectra of probes in aqueoussolution
(for AuO,measured in glycerol). The aqueous solutions of the probes and pure water were added in the n-alkane solutions of AOT (0.1 mol dm-3) with a digital micropipet (Nichiryo, Model 800) and solubilized by vigorous agitation for 5 min with an automatic mixer (Taitec S-100). Absorbances at the peak wavelengths of the probes in the AOT micellar solutions were maintained within 0.13418. All the probes solubilized iq the water pool below R, = 4 or 5 showed absorbances (molar extinction coefficients) smaller than that in ordinary bulk water; anionic Na-Fl was difficult to solubilize in the water pool in comparison with the cationic dyes (AuO, RhB, and Rh6G) beckuse the absorbance of the solubilized Na-Fl was too small to measure in very small R, region (R, < 2), although the amount of Na-Fl introduced in the solution is enough. On thecontrary, cationicAuO could be easily solubilized in the water pool even in R, < 1. Above R, = 5, the peak wavelengths of both the absorption and the fluorescence spectra for all the probes in the water pool were nearly consistent with those in bulk water. The microviscosities were therefore measured above R, = 4 for ali probes. Measurements. UV-Vis absorption and fluorescence spectra of the probes were recorded at 25 f 0.2 O C on Jasco Ubest-30 spectrophotometer and Hitachi F-2000 fluorimeter equipped with thermocontrolled cell holder, respectively. The band-passes for excitation and emission monochromators are 10 nm, respectively. Figure 2 shows fluorescence and excitation spectra of the probes in aqueous solution (glycerol solution for AuO). Fluorescence quantum yield, Cpf, of AuO was measured by comparison with 1 N sulfuric acid aqueous solution of quinine sulfate as a standard sample (+f = 0.55). For the microviscosity determination by the depolarization method, the fluorescence lifetime of the probes in the water pool, Tf, was calculated from the relation that 7/70 N Zf/Zm; 70 is the fluorescence lifetime taken from the literature, Zm the fluorescence intensity measured under the condition common with the 70 measurement, and Zf the measured fluorescence intensity of the
where Z, and zvh represent the vertically and horizontally polarized emission intensity by the excitation with vertically polarized light, respectively, and G (=Zhv/zhh) is a correction factor for detector sensitivity to the polarization direction of emission. According to Keh et a1.,2*the I)and , the hydrodynamic micellar volume, V,, are determined from the expression
where ro is the intrinsic anisotropy ratio in the absence of both rotational diffusion and excitation energy migration, k Boltzman's constant, Tthe absolute temperature, and q, the solvent viscosity. Thevaluesof q, are taken from ref 37. D is the rotational diffusion coefficient of the probe solubilized in the water pool and is written from the Stokes-Einstein relation as
D = kT/6Vpq, (3) where V, is the hydrodynamic volume of the probes and was determined from the slope of the Perrin-Weber plot (rlvs Tv-1). This data analysis is based on the assumption in which the micellar volume and the nature of the water pool are essentially unchanged by solvents (n-alkane series). Indeed, good linear relation between rl and q,-l in eq 2 was also observed in our experiments; this indicates that the assumption in eq 2 is reasonable.
Results and Discussion Microviscosity Determined by AuO Fluorescence. It is accepted that the 9fof AuO increases with an increase in viscosity of the surrounding medium.25J-2 The nonfluorescent nature of AuO in water is attributed to destruction of coplanar structure due to internal rotation of the dimethylaniline moieties. Oster et al.36 showed that tbe AuO fluorescence enhances markedly in a highly viscous medium such as glycerol, and the reciprocal fluorescence yield of AuO in glycerol or in dextrose-glycerol-water mixtures is proportional to Tq-1 in very narrow range (0.2 < Tq-1 < 1.3; T in K,q in cP). In order to measure the .7, we prepared the calibration curve plotted as 9f-1 vs T9-l over wide range (0.32 < Tq-1 < 60) as shown in Figure 3.
2122 The Journal of Physical Chemistry, Vol. 98, No. 8, 1994
Hasegawa et al.
3000 50
/,'
2000
1
e AuO
RhB
X
x
2
3
&-
i
1000
10 2o 0' 0
0 0
20
40
60
80
Tq" / K c P " Figure 3. Calibration curve for microviscosity determination,plotted as of AuO vs 7'q-I. 2.0 I
1.5
1
1
.
"
10
'
x
X
'
.
"
30
20
40
'
50
Rw F i p e 5. Microviscosity determind with AuO fluorescence and with
the fluorescence depolarization of RhB. 1v
8
(a)
0 " I 0 2 0.00 0.02 0.04
0.06 0.08 0.10 0.12
[EtS03Na] /
1.5
r
(b)
I
I
I
0.10
0.15
0.20
I
0.05
"
l
6
8
.
l
1
'
"
t
0
1
2
[AOT] / lo3mol d ~ n ' ~ Figures. Microviscosity at the vicinity of AOT aqueous micellar surface, determined with AuO fluorescence.
1
0.01 0.00
*
4
[EtS03Na] / moldmm3 Figure 4. Normalized fluorescenceintensities of AuO in (a) aqueous and (b) glycerol solutions as function of the EtSO3Na concentration.
Each micelle includes less than one probe molecule (roughly one probe per 500 micelles) on the average. Therefore, the concentration (self)-quenchingof fluorescencefor all the probes is negligible. When the glycerol solution of RhB was solubilized in the AOT reversed micelles ([glycerol]/[AOT] = 4), no fluorescence depolarization was observed, showing that no excitation energy migration occurs; this suggests that the solubilized probe is also isolated in the water pool as well as in the glycerol pool. Before the microviscosity measurement by AuO, we examined whether the AuO fluorescence is subjected to quenching by sulfonate group in AOT when AuO is solubilized in the water pool. Low molecular weight sodium ethanesulfonate,which forms no micellar aggregates and does not enhance the solutionviscosity by its addition, was added to the glycerol and aqueous solutions of AuO. Figure 4 shows the Stern-Volmer plots for the AuO fluorescence intensity. Neither quenching nor enhancement of the AuO fluorescence was observed for both solutions within
[EtS03Na] = 10-44.1 or 0.175 mol dm-3. The results means that the AuO fluorescenceyield reflects directly the microviscosity in the water pool of the AOT reversed micelles. In the fluorescence probe method, the probe position is an important point to notice. Since AuO is a cationic dye, the solubilized probe is most likely expected to be localized at the vicinity of the anionic sulfonate head groups in AOT owing to an electrostatic attractive for~e.3*~40~41 The microviscosity determined with the AuO fluorescence is shown in Figure 5 as a function of R,. With increasing R,, the microviscosity rapidly decreases below R, = 10, and gradually decreases until the micellar solution becomes turbid above Rw= 50 because of the limit of water solubilization. The considerably high viscosity at low R, is expected to be attributable to water molecules tightly bound to the sulfonate head groups in AOT. Near-infrared ~pectroscopy~~**~ also demonstrated the presence of the bound water. In addition, Figure 5 shows that the microviscosity at the vicinity of water/surfactant interface is considerably higher than that of bulk water (ca. 1 CPat 25 "C) even in a fully expanded state (microemulsion). If AuO molecules were added to the aqueous micellar solution of AOT (the measured cmc is 4 X mol dm-9, the cationic AuO probe presumably associates with the negatively charged surface of the aqueous AOT micelles. Figure 6 shows the microviscosity at the vicinity of the AOT aqueous micellar surface. The results indicate that the viscosity values are slightly higher than that of bulk water. This suggests the effect of binding of AuO to a negatively charged site on the AuO fluorescence enhancementis small,coinciding with Wang's results in the AuO/ water/poly(methacrylic acid) system.'0 In addition, the values of viscosities in Figure 6 are smaller than the values of qwin AOT W/O microemulsion in Figure 5 . This supports the presence of the bound water. A couple of invcstigatorsaJ9~~1 pointed out that water localizing around the center of the water pool in AOT W/O microemulsion
The Journal of Physical Chemistry, Vol. 98, No. 8, 1994 2123
Microviscosity of AOT Reversed Micelles 0.25
oRh6G oRhB A Na-Fl
0.23 0.21 h
\
\
0.19 0.17 0.15
0
20
5
25
30
Figure 7. Changesin the fluorescence anisotropyratio for RhB solubilized in AOT reversed micelle.
-
n
-
15 20 25 30 Rw Fipre 8. Hydrodynamic volume of AOT reversed micelle as function
of R,.
0
5
10
resembles bulk water. The connection with the present results suggests that the water pool has a heterogeneous structure composed of bound and not bound water. The heterogeneity is discussed later. Microviscosity Determined by Steady-State Fluorescence Depolarization. As far as we know, there is no work in which the microviscosity at the vicinity of water/surfactant interface of AOT reversed micelle was quantitatively measured with cationic dye by the fluorescencedepolarization technique. The values of qw obtained by the AuO method were compared with those determined with the steady-state fluorescencedepolarization of a typical cationic dye, RhB. The fact that the cationic xanthene dyes such as RhB are widely known to be used as an ion association reagent for the quantitative analysis of anionic compounds43 demonstratesthat this dye associatespresumablywith the anionic head groups in AOT as well as in AuO. Figure 7 shows the changes in the fluorescence anisotropy ratio, r, against R, in n-alkanes (C&*). It is clearly shown from eq 2 that an increase in V, due to an increase in R, contributes to an increase in r. On the contrary, as R, is increased, D increases presumably because of an increase in local fluidity in the water pool; the increase in D contributes to an decrease in r. Therefore, the changes in r against R, becomes nonmonotonous because of the total effect of the opposite contribution to r. According to eq 2, the plots of rl vs qS-l as a function of R, showed a linear relationship and provided the values of V, and v,. The V,, as illustrated in Figure 8, is proportional to R,. This correspondsto an acceptable assumption in which all amount of the added water is solubilized in the AOT reversed micelles. The hydrodynamicradii of the AOT reversed micelles determined by this method were about 13% smaller than the data of Keh et al. probably owing to the difference of the purity of AOT. Figure 5 shows the change in the qwobtained with the RhB fluorescence depolarization together with that measured by the AuO method. Good agreement is found in the experimental range. The result led to the conclusion that the AuO method is very useful for rapid determination of the water microviscosityat thevicinity of water/
0
10
20
30
40
50
R, Figure9. Microviscosity determined with the fluoresccncedepolarization
of various xanthene dyes.
surfactant interface in AOT reversed micelle. Additionally, the AuO method is promising for application to other reversed micellar systems. Heterogeneous Structure in the Water Pool of AOT Reversed Micelle. As mentioned in the previous section, the depolarization measurement using different ionic character of xanthene dyes provides information on the heterogeneity of the water pool becausethe probes should be site-selectively localized in the water pool, dependingon an electrostatic interaction between the probes and anionic head groups of AOT. Then, we compared the qw-R, curves obtained with the fluorescence depolarization of the xanthene dye series. In comparison with RhB, Na-Fl should be localized in the water pool center owing to electrostatic repulsion, and Rh6G having the ester group instead of carboxyl group associates more tightly with the head groups. Figure 9 indicates that the water pool has a heterogeneous structure with respect to the microviscosity even in a fully expanded state. Thus, our quantitative results on the heterogeneity in the water pool of the AOT reversed micelles supports a model in which the water pool consists of bound and free water, as proposed by Kondo et al.29 and Zinsli.31
Conclusions Viscosity-sensitive AuO made possible to determine rapidly the microviscosity at the vicinity of water/surfactant interface in the water pool of AOT reversed micelles without any assumption. As R, is increased, the microviscosity rapidly decreased below R, = 10, and then gradually decreased until the solution becomes turbidabove R, = 50. The qrR,curveobtained by the AuO method coincided satisfactorily with that measured with the fluorescencedepolarization of RhB. The fluorescence depolarization measurements using different ionic character of xanthene dyes elucidated a heterogeneousstructure with respect to the microviscosity. Acknowledgment. We are indebted to Miss N. Sonta for technical assistance. References and Notes (1) Day, R. A.; Robinson, B. H. J. Chem. Soc., Furuduy Trans. 1 1979, 75, 132. (2) Zulauf, M.; Eicke, H.-F. J . Phys. Chem. 1979, 83, 480. (3) Sein, E.; Lalanne, J. R.;Buchert, J.; Kielich, S.J . Colloid Inrerface Sci. 1979, 72, 363. (4) Gulari, E.; Bedwell, B. J . Colloid Inrerfuce Sci. 77, 202. ( 5 ) Cacazabat, A. M.; Langevin, D. J . Chem. Phys. l w , 74, 3148. (6) Clarke, J. H. R.; Nicholson, J. D.; Regan, K. N. J. Chem. Soc., Furuday Trans. I 1985, 81, 1173. (7) Atik, S.S.;Thomas, J. K.J. Am. Chem. Soc. 1981, 103, 3543. (8) Bridge, N. J.; Fletcher, P. D. I. J . Chem. Soc., Faruduy Truns. 1 1983, 79, 2161. (9) Gelade, E.; De Schryver, F. C. Reverse Micelles;Plenum: New York, 1984; p 143. (10) Gam, A.M.; Boeger, B.E. J . Colloid InrerfoceSci. 1986,109,504.
2124 The Journal of Physical Chemistry, Vol. 98, No. 8, 1994 (11) Howe, A. M.; McDonald, J. A.; Robinson, B. H. J . Chem. SOC., Faraday Trans. 1 1987,83, 1007. (12) Lang, J.; Jada, A.; Mlliaris, A. J . Phys. Chem. 1988, 92, 1946. (13) Modes, S.;Lianos, P. J. Phys. Chem. 1989.93, 5854. (14) Peri, J. B. J . Colloid Interface Sci. 1969, 29, 6. (15) Kitahara, A.; Kobayashi, T.; Tachibana, T. J . Phys. Chem. 1962,615, 363. (16) (17) (18) (19) 4730. (20) (21)
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Eicke, H.-F.; Zinsli, P. E. J . Colloid Interface Sci. 1978, 65, 131. Photochemistry in Microheterogeneous Systems; Academic Press: New York, 1987. (22) Sunamoto, J.; Hamada, T.; Seto, T.; Yamamoto, S.Bull. Chem. SOC. Jon. 1980. 53. 583.
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Chem. 1988,92, 759. (33) Sugimura, T.; Shindo, Y.; Hasegawa, M.;Kitahara, A.; Masuda, Y. J . DisDersion. Sci. Technol. 1992. 13. 251. (34) Webb, J. P.; McColgin,-W. C i Peterson, 0. G.; Stockman, D. L.; Eberly, J. H. J . Chem. Phys. 1970, 53, 4227. (35) Ware, W. R.; Baldwin, B. A. J . Chem. Phys. 1964,40, 1703 (36) Oster, G.; Nishijima, Y. J . Am. Chem. Soc. 1956, 78, 1581. (37) Handbook of Chemistry, 2nd ed.; The Chemical Society of Japan; Maruzen: Tokyo, 1975. (38) Oster, G. J . Polym. Sci. 1955, 16, 235. (39) Nishijima, Y.; Midorikawa, T.; Taki, F. Rep. Progr. Polym. Phys. Jpn. 1965,8, 139. (40) Wang, Y.; Morawetz, H. Macromolecules 1986, 19, 1925. (41) Chu, D. Y.; Thomas, J. K. Photophysics of Polymers; Hoyle, C. E., Torkelson, J. M., Us.; ACS Symposium Series 358; American Chemical Society: Washington, DC, 1987; p 434. (42) Meyer, E. F.; Jamieson, A. M.; Simha, R.; Palmen, J. H.M.; Booij, H. C.; Maurer, F. J. H. Polymer 1990, 31, 243. (43) Fogg, A. G.; Burgess, C.; Burns, D. T. Talanra 1971, 18, 1175.
