7900
J. Phys. Chem. 1991, 95, 7900-7907
however, to make conclusions on the effect of H 2 0 replacement by D 2 0 because of the many interactions and the complexities of the side chains. In fact, qualitatively similar results on a per residue basis as obtained here have been interpreted as arising from hydrogen bonding between polar side chains.38 D20 is usually considered to be inherently more structured than HZO2OLe., the correlation among D20molecules is greater than the correlation among H 2 0 molecules. It should be made clear that the statement that D 2 0 is more structured than H 2 0 does not necessarily lead to the conclusion that hydrophobic effect is stronger in D 2 0 than in H 2 0 . Ben-Naim, for example, concurs that D 2 0 is more structured than HzObut believes the hydrophobic interaction to be stronger in H 2 0 than in D20.42 It is not simply the comparison of the relative degree of correlation of the molecules of the two solvents but rather the relative change in the enhancement of the correlation on going from bulk D20 to D 2 0 of hydrophobic hydration with that on going from bulk HzO to H 2 0 of hydrophobic hydration, i.e. (41) For a general reference, please see: Chemistry and Technology of Water-Soluble Polymers; Finch, C . A., Ed.; Plenum Press: New York, 1983. (42) Ben-Naim, A. Hydrophobic Interactions; Plenum Press: New York, 1980.
6D20(structure) =
structure (hydration D20) - structure(bu1k D20)
6H20(structure) = structure (hydration H 2 0 ) - structure(bu1k H 2 0 ) To say that the hydrophobic effect in D20is stronger than in H2,0 means that
6Dz0(structure) > *H20(structure) In other words, the solute-induced correlation enhancement for molecules in the hydration shell can be greater in D 2 0 than in H 2 0 . This enhancement has been found in the computational efforts of Rossky and c o - ~ o r k e r s . ~ ~
Acknowledgment. This work was supported in part by National Institutes of Health Grant HL29578 and Department of the Navy, Office of Naval Research, Contract NOOO14-89-5-1970. Registry NO. poly(VPGVG), 69289-41-4; poly(IPGVC), 106855-576; poly(LPGVG), 1 1 1583-50-7; poly(VPAVG), 89526-94-3; D20, 7789-20-0. (43) Zichi, D. A.;
Rossky, P. J. J . Chem. Phys.
1986, 84, 2823-2826.
Nanosecond Reorganization of Water within the Interior of Reversed Micelles Revealed by Frequency-Domain Fluorescence Spectroscopy Jing Zhang and Frank V. Bright* Department of Chemistry, Acheson Hall, State University of New York at Buffalo, Buffalo, New York 14214 (Received: February 5, 1991; In Final Form: May I. 1991)
The fluorescence properties of 1,8-ANS (1-anilino-8-naphthalenesulfonicacid) in AOT (Aerosol-OT; 1,4-bis[2-ethylhexyl] sulfosuccinate) reverse micelles have been studied. Using steady-state and time-resolved methods we show that water content and temperature play key roles in the photophysics of the emission process. For example, a continuous red shift of the emission spectrum is observed as the water content within the AOT reverse micelle increases. New multifrequency phase. and modulation experiments show clearly that the I,I-ANS emission spectrum is continuously evolving on a nanosecond time scale. From these experiments, we find that the solvation correlation functions ( S ( r ) )are characterized by a pair of solvent relaxation rates when the water content is at or below R = 2.5 (R, molar ratio of water to AOT). Above R = 2.5 (r, = 2R 5 A, r, is the core radius of the water droplet) we observe a single relaxation process. We propose that, at lower water levels, the two solvation rates are a consequence (in part) of interfacial (type 1) and core (type 2) associated water, each characterized by its own relaxation rate. Upon increasing the water content within the reverse micelle, the relative fraction of type 2 water increases and the solvent relaxation process collapses into a single relaxation rate. From temperature-dependent studies we determine the activation barriers for each of these solvent reorganization events.
-
Introduction Over the past few decades, surfactant aggregates in nonpolar solvents, termed reversed micelles, have attracted increased attention.I-' Of all the possible systems, reverse micelles formed by the surfactant sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol-OT; AOT) in alkanes (e.g., heptane or isooctane) have been the most widely As a direct result of these types ( I ) Kitahara, A.; Kon-No, K.; Fujiwara. M. J . Colloid Interface Sci. 1976, 57. 391. (2) Mollett, K. J.; O'Connor, C. J. J . Chem. Soc., Perkin Trans. 2 1976, 369. ( 3 ) Masui, T.; Watanabo, F.; Yamagishi, A. J . Phys. Chem. 1977,81,494. (4) Nome, F.; Fendler, J . H. J . Am. Chem. SOC.1977, 99, 1557. (5) Fendler, J . H. Ann. Reo. Phys. Chem. 1984, 35, 137. (6) Vos, K.;Lanne, C. Photochem. Photobiol. 1987, 6, 863. (7) Kalyanasundaram, K. Photochemistry in Microheterogeneous Systems; Academic Press: New York, 1987; Chapter 5.
0022-3654/9 1 /2095-7900$02.50/0
of detailed studies, reverse micelle systems are now used routinely to solve practical problems in biotechnology,I2separation sciences," microparticle synthe~is,'~ and oil recovery technology.Is The AOT-alkane-water system, in particular, is interesting for several reasons. AOT solutions are homogeneous and optically (8) Bruno, P.; Caselli, M.; Luisi, P. L.; Maestro, M.; Traini, A. J . Phys. Chem. 1990, 94, 5908. (9) Luisi, P. L.; Giomini, M.;Pileni, M. P.; Freedman, R. B. Biochim. Biophys. Acta 1988, 947, 209. (IO) Oldfield, C.; Robinson, B. H.; Freedman, R. B. J . Chem. Soc., Foraday Trans. 1990, 86, 833. ( 1 I ) Bardez, E.; Larrey, B.; Zhu, X. X.;Valeur, B. Chem. Phys. Lett. 1990, 171, 362. (12) Luisi, P. L.; Majid, L. J. CRC Crit. Rev. Biochem. 1986, 20, 409. ( I 3) Sheu, E.;Goklen, K. E.; Hatton, T. A.; Chen, S.-H. Biotechnol. Prog. 1987, 4. 175. (14) Lianos, P.; Thomas, J. K. J . Colloid Interface Sci. 1987. 117, 505. ( 1 5 ) Caponetti, E.; Lizzio, A,; Triolo, R.;Compere, A. L.; Griffth, W. L.; Johnson, J. S . Langmuir 1989, 5 , 357.
0 199 1 American Chemical Society
Reorganization of Water within Micelles
The Journal of Physical Chemistry, VoL 95, No. 20, 1991 7901
transparent over a wide range of conditions (temperature, concentration, etc.). AOT reverse micelles form easily in a plethora of nonpolar solvents and can compartmentalize a large amount of water within their central core. The aggregation process is fairly well characterized with respect to size and shape at various water content^.^ Away from phase instability boundaries, the micelle stability is temperature and water droplet (Le., the pool of water surrounded by the AOT micelle) size dependent. At high levels of water loading (R, molar ratio of water to AOT) the stable temperature range is only few degrees, but at low R values the AOT reverse micelle is stable to at least 60 O C . For R < 20, the size of the water pool does not vary significantly over a fairly wide temperature range and the micelles are essentially monodisperse.I6 In liquid alkanes, AOT reverse micelles are completely associated and each micelle contains 23 surfactant monomer^.^ The structure of such an aggregate is slightly asymmetric and is best represented by a round cylinder. The degree of asymmetry is highly reduced in the presence of water which forms a spherical pool in the micelle center. Several methods have been used to study the microenvironment and transition properties of AOT reverse micelles. For example, 'H N M R spectroscopy was employed to determine the site of interaction between several solvents and the tail regions of the AOT m i ~ e l l e . ' ~ JIn~ an effort to understand the structure and state of water within the micelle, attempts have been made to correlate the liquid microstructure of AOT microemulsions with their colloidal By use of photon correlation spectro~copy~'-~~ and small-angle neutron ~ c a t t e r i n g , ' ~ .geo~~-~~ metric features of the water pool have been studied. The effect of cosurfactants and location of probe molecules in these same systems have been detected by means of ESR spectro~copy.~~ Fluorescence spectroscopy has also been used to study the AOT reverse micelle system. For example, fluorescent probes have been used to determine viscosity, binding site, rigidity, and proximity within the water pool.33334 Various properties such as reaction quantum yield^,'^,^^ water activity,40 and polarity of micellar interfaces4' have been recovered by using fluorescence methods. Furthermore, fluorescence has been employed to study (16) Robinson, B. H.; Toprakciogh, C.; Dore, J. C. J. Chem. Soc., Faraday Tram. 1 1984,80,13. (17) Fendler, J. H.; Fendler, E. J.; Medary, R. T.; Woods, V. A. J . Am. Chem. SOC.1972,94,7288. ( I 8) Elseoud, 0. A.; Fendler, E. J.; Fendler, J. H. J . Chem. Soc., Faraday Trans. I 1974,70,450. Elseoud, 0.A.; Fendler, E. J.; Fendler, J. H. J. Chem. Soc., Faraday Trans. I 1974,70,459. (19)Wong, M.; Thomas, J. K.; Nowak, T. J . Am. Chem. SOC.1977.99, 4730. (20) (21) (22) (23) (24)
Martin, C. A.; Magid, L. J. J . Phys. Chem. 1981,85, 3938. Maitra, A. J . Phys. Chem. 1984,88,5122. Llor, A.; Rigny, P. J . Am. Chem. SOC.1986,108, 7533. Zulauf, M.: Eicke, H.-F. J . Phys. Chem. 1979,83, 480. Day, R. A.; Robinson, B. H.; Clarke, J. H. R.; Doherty, J. V. J . Chem. SOC.,Faraday Trans. I 1979,75, 132. (25) Gulari, E.; Bedwell, B.; Alkafaji, A. J . Colloid. Interface Sci. 1980, 77.. ~ 202. . (26) Hilfiker, R.; Eicke, H. F.; Geiger, S.; Furler, G.J . Colloid. Interface Sci. 1985, 105, 378. (271 Chatenav. D.; Urbach. W.: Cazabat. A. M.: Vacher. N.: Waks. M. Biophys. J . 1985, 48,893. (28) C a b s , C.; Delord, P. J. Phys. Lett. 1980, 41,455. (29) Robinson, B. H.; Toprakcioglu, C.; Dore, J. C.; Chieux, P. J . Chem. SOC.,Faraday Trans. I 1984,80, 130. (30) Fletcher, P. D. 1.; Robinson, B. H.; Tabony, J. J . Chem. Soc., Faraday Trans. I 1986,82,231 I . (31) Howe, A. M.; Toprakcioglu, C.; Dore, J. C.; Robinson, B. H. J . Chem. SOC.,Faraday Tram. I 1986.82.241 1, (32) Hu, M.; Kevan, L. J. Phys. Chem. 1990,94,5348. (33) Brand, L.; Gohlke, J. R. Annu. Rev. Biochem. 1972,41,843. (34) Wehry, E. L. Modern Fluorescence Spectroscopy; Plenum Press: New York, 1976. (35) Fendler. J. H. Arc. Chem. Res. 1976,9, 1953. (36) Menger, F. Acc. Chem. Res. 1979,12, 1 1 I , (37) Atik. S. S.; Thomas, J. K. J. Am. Chem. Soc. 1981, 103, 3543. (38) Howe, A. M.; McDonald, J. A.; Robinson, B. H. J . Chem. SOC., Faraday Trans. I 1987,83,1007. (39) Saze, M.; Abuin, E. A.; Lissi, E. A . Langmuir 1989,5 , 942. (40) Politi, M. J.: Chaimovich, H. J . Phys. Chem. 1986,90,282. (41) Belletete, M.: Lachapelle, M.: Durocher, G. J . Phys. Chem. 1990,94, 5337.
ground- and excited-state proton transfer in AOT reversed mic e l l e ~ . ~ * ~In~ all * these experiments the authors often explained their results on the basis of a model in which the water pool contained two types of water: interfacial (type 1) and core (type 2).49 In a very recent work, Jain et al. have, however, reported the existence of a third type of water, trapped between the polar head groups of the individual surfactant molecules.5o Thus, while the actual model describing the number of water domains within the AOT micelle is currently under debate, there is clear evidence that the interior of the reverse micelle is not at all homogeneous. Therefore, in such a complex system, one would expect a polar fluorescent probe to distribute throughout these domains and not just remain in a single environment (e.g., type 1 water). To improve our understanding of the dynamic properties within AOT reversed micelles, we have chosen to use the fluorescent probe 1-anilinonaphthalene-8-sulfonicacid (1,8-ANS). 1&ANS was chosen because its spectral and temporal characteristics are extremely sensitive to the local solvent environments'"' and it has been studied previously in the AOT reverse micelle In this report, we confirm the original steady-state results reported by Thomas and co-workers6* and Tamura and Nii.69 However, we find that the time-resolved decay kinetics are not accurately described, as reported previously,68 by a simple single-exponential-decay law. In fact, we find that there is clear evidence for nanosecond solvent reorganization occurring, around 1,8-ANS, within the water pool of the AOT micelle. This nanosecond reorganization manifests itself in the observation of multiexponential decays of fluorescence. To study the kinetics of these processes, time-resolved fluorescence spectra have been constructed by using frequency-domain fluorescence techn i q u e ~ .Using ~ ~ ~these ~ time-resolved emission spectra we then
(42) Kondo, H.; Miwa, I.; Sunamoto, J. J . Phys. Chem. 1982,86,4826. (43) Bardez, J. E.; Goguillon, B. T.; Keh, E.; Valeur, B. J . Phys. Chem. 1984,88,1909. (44) Bardez, J. E.; Monnier, E.; Valeur, B. J. Phys. Chem. 1985,89,5031. (45) Politi, M. J.; Brandt, 0.;Fendler, J. H. J . Phys. Chem. 1985,89, 2345. (46) Bardez, J. E.; Monnier, E.; Valeur, B. J . Colloid Interface Sci. 1986, 112,200. (47) Pileni, M. P.; Hickel, B.; Ferradini, C.; Pucheault, J. Chem. Phys. Lett. 1982,92,308. (48) Petit, C.; Brochette, P.; Pileni, M. P. J . Phys. Chem. 1986,90,6517. (49) Zinsili, P. E. J . Phys. Chem. 1979,83,3223. (50)Jain, T. K.; Varshney, M.; Maitra, A. J . Phys. Chem. 1989,93,7409. (51) Worah, D. M.; Gibhoney, K. M.; Yang, S. S.; York, S . S . Eiochemistry 1978,17, 4487. (52) Kosower, E. M.; Dudiuk, H. J. J . Phys. Chem. 1978,82,2012. (53) Mock, D. M.; Langford, G.; Dubois, D.; Criscimanga, N.; Horowitz, P. Anal. Biochem. 1985,151, 178. (54) Seliskar, C. J.; Brand, L. J . Am. Chem. SOC.1971,93,5405. (55) Huang, C.-H.; Charlton, J. P. Biochemistry 1972,1 1 , 735. (56) Grellman, K. H.; Schmitt, U. J . Am. Chem. SOC.1982,104,6267. (57) Jobe, D. J.; Verral, R. E.; Palepu, R.; Reineborough, V. C. J . Phys. Chem. 1988,92,3582. (58) Seliskar, C. J.; Brand, L. Science 1971,171, 799. (59) Cramer, F.; Saenger, W.; Spatz, H.-Ch. J. Am. Chem. Soc. 1%7,89, 14. (60) Kinoshita, T.; linuma, F.; Tsuji, A. Chem. Pharm. Bull. 1974,22, 2735. (61) Kondo, H.;Nakatani, H.; Hiromi, K. Carbohydr. Res. 1976,52,I . (62) Park, J. W.; Song, H.J. J. Phys. Chem. 1980, 12,29. (63) Kosower, E.M.; Kanety, H.; Dodiuk, H.; Striker, G.; Jovin, T.; Boni, H.; Huppert, D. J. Phys. Chem. 1983,87,2479. (64) Catena, G. C.; Bright, F. V. Anal. Chem. 1989,61, 905. (65) Bright, F. V.; Catena, G . C.; Huang, J. J . Am. Chem. Soc. 1990,112, 1343. (66) Huang, J.; Bright, F. V. J . Phys. Chem. 1990,94,8457. (67) Ebbcrson, T. W.; Ghiron. C. A. J . Phys. Chem. 1989,93, 7139. (68) Wong, M.; Gratzel, M.; Thomas, J. K. J . Am. Chem. SOC.1976,98, 2391. (69) Tamura, K.; Nii, N . J. Phys. Chem. 1989,93,4825.
7902 The Journal of Physical Chemistry, Vol. 95, No. 20,1991
calculate the solvation correlation f ~ n c t i o nand ~ ~determine .~~ the nanosecond reorganizational rates within the interior of the AOT micelle as a function of water content and temperature.
Experimental Section Materials. NaAOT and 1 ,&ANS were purchased from Sigma and Molecular Probes, respectively. n-Heptane (99%, spectrophotometric grade) was from Aldrich and distilled-deionized water was used throughout. The AOT was checked by UV-visible absorbance and the spectra agreed with those reported for purified preparation^.'^ We performed a few studies on purified preparation~'~ and observed no difference between these and as received samples. Thus, all reagents were used as received. Sample Preparation. 3% (w/v) AOT/heptane stock solutions were used for all experiments reported here. Prior to analysis, samples were kept in the desiccator over CaCI,. During analysis all samples were placed into quartz cuvettes with sealed Teflon stoppers. A lCr5 M I,8-ANS in 3% AOT/heptane stock solution was prepared by pipetting 500 pL of IF3M 1,8-ANS (in ethanol) into a 50-mL volumetric flask, evaporating off residual ethanol with a gentle stream of N,, diluting to volume with the 3% AOT/heptane solution, and sonicating for 30-40 min to ensure all 1,8-ANS was solubilized. This solution was sealed and stored in the refrigerator. All experiments were performed with M I,8-ANS and there were no signs of primary or secondary interfilter effects. Samples for analysis were prepared by mixing appropriate amounts of the stock solutions, adding small volumes of water (as needed) directly to the cuvette with a micropipet, and effectively mixing the cells. Background levels were always a t or below 1%. Water incorporation into the micelle was instantaneous and we noticed no appreciable difference between freshly prepared and several day old samples. Instrumentation and Methodology. All steady-state and frequency-domain data were obtained with an SLM 48000 M H F spectrofluorometer. A CW argon ion laser (Innova Model 90-6; 35 1.1 nm) served as the excitation source for all frequency-domain studies. Sample temperatures were controlled with a Lauda RLS-6 temperature circulator ( i O . 1 "C). In order to ensure accurate temperature measurements, a thermocouple was placed directly in a local sample cell. Following temperature adjustments the sample was allowed to equilibrate for 25-30 min. For all fluorescence lifetime measurements, the repetition rate of the Pockel's cell was 5 MHz and the modulation frequencies were obtained for the harmonic content of this pulse train as described e l ~ e w h e r e . ~ ~Emission -~~ was observed through band-pass filters covering the range from 421 to 560 nm. To eliminate polarization bias, magic angle polarization was used throughout all lifetime measurements. We used Me2POPOP and POPOP in ethanol as reference lifetime standards.*O To ensure against artifacts or biases we employed the methodology described by Litwiler et Multifrequency phase and modulation data were fitted by various theoretical decay models using commercially available software (Globals Unlimited). At present our time resolution for this system is 20 ps. Thus, any processes occurring
Zhang and Bright faster than 20 ps are beyond our experimental time window. Kinetic Analysis. The time evolution of an emission spectrum gives one insight into how the local solvent environment affects the emission of the probe.. Generally, such spectra are determined from the time-resolved fluorescence decay, if one acquires these decay curves at several emission wavelengths (&). In the simplest situation, the recovered fluorescence decay times ( T i ) should remain constant across the spectrum; however, the preexponential amplitude factors ( a i @ ) )are expected to be wavelength dependent.70q7' The systematic variability of these preexponential terms arise from (1) the individual component emission spectra; (2) their quantum yields; and (3) the interconversion rates between speci e ~ . As ~ ~a result, , ~ ~ the wavelength-dependent time course of the fluorescence decay can be written I(A,t) =
C q(A)
exp(-t/ri)
(1)
where ai(A)and T~ are recovered from frequency-domain minimization of the wavelength-dependent In the frequency domain, the experimentally measured quantities are the frequency ( w ) - and wavelength (A)-dependent phase shift (e,(A,w)) and demodulation factors (M,(A,o)). For any assumed decay model these values are calculated from the sine (S(A,w)) and cosine (C(A,w)) Fourier transforms of the impulse-response function:7679 S(A,w) = j Z ( A , t ) sin ut dt
(2)
C(A,w) = j Z ( A , t )
(3)
For any set of ai(A) and modulation factors are
7 i the
COS wt
dt
calculated phase shift and de-
e,(X,w) = arctan [(S(A,w)/C(X,w)]
(4)
+ C(A,U)2]'/*
(5)
M , ( X , W ) = [S(A,w)2
The apparent decay parameters (ai(A)and T ~ are ) recovered from the experimentally measured data by the method of nonlinear least square^.^^-^^ The goodness-of-fit between the fitting model (c subscript) and measured (m subscript) experimental data is judged by the reduced chi-squared (x2)
where D is the number of degrees of freedom, ue and u,, are the uncertainties in the measured phase angle and demodulation factor, respectively. Typically, we perform experiments at eight emission wavelengths, and 25 modulation frequencies are used to interrogate the sample so D is in excess of 350 and ge and u,,, are at or below 0 . 1 5 O and 0.002, respectively. If the assumed uncertainties truly reflect the uncertainty in the measurements then an ideal fit will yield a x2 value of unity. In addition, the residual error terms will randomly oscillate about zero and no systematic deviation of the residuals will be e ~ i d e n t . ~ ~ - ~ ~ Time-resolved emission spectra are formed from the normalized impulse response f u n ~ t i o n s given ~ ~ ~ by ~~~*
= N(X) l(X,t) (7) where N(A) is the wavelength-dependent normalization factor I'(X,t)
(70) Lakowicz, J. R.; Cherek, H. Chem. fhys. Lett. 1985, 122, 380. (71) Lakowicz. J. R.; Gratton, E.; Cherek, H.; Maliwal, B. P.; Laczko, G . J . Eiol. Chem. 1984, 259, 10972. (72) Betts, T. A.; Bright, F. V. Appl. Spectrosc. 1990, 44, 1203. (73) Maroncelli, M.; Maclnnis, J.; Fleming, G. R. Science 1989, 243, 1674. (74) Simon, J. D. Arc. Chem. Res. 1988. 21, 128. (75) Luisi, P. L. Angew. Chem., Int. Ed. Engl. 1985, 24, 439. (76) Gratton, E.;Jamcson, D. M.; Hall, R. D. Annu. Reo. Eiophys. Eiophys. Eioeng. 1984, 13, 105. (77) Jameson. D. M.;Gratton, E.; Hall, R. D. Appl. Spectrosc. Reo. 1984, 20, 5 5 . (78) Lakowicz, J. R.; Lackzo, G.; Gryczynski, 1.; Szmacinski, H.; Wiczk, W. J. Photochem. Photobiol. E Eiol. 1988. 2, 295. (79) Bright, F. V.; Betts, T. A.; Litwiler, K. S.CRC Crit. Reo. Anal. Chem. 1990, 21, 389. (80) Lakowicz, J. R.; Cherek, H.; Balter, A. J . Biochem. Eiophys. Methods 1981. 5, 131. (81) Litwiler, K. S.; Huang, J.; Bright, F. V. Anal. Chem. 1990. 62, 471.
and F ( h ) is the normalized steady-state fluorescence intensity at wavelength A. The time course of the solvent reorganization for I,8-ANS in the core region of AOT reversed micelles can be described by the time-varying spectral center of gravity ( u ( t ) ) :
Under our experimental conditions multifrequency data are (82) Easter. J. H.; DeToma, R. P.; Brand, L. Eiophys. J. 1976, I S , 571.
The Journal of Physical Chemistry, Vol. 95, No. 20,1991 7903
Reorganization of Water within Micelles n VI
6
1.200-
.-C VI
-T=O°C - - T = 10°C
L
- R = 0.00 - - R 1.66
-
’ .
-
R-4.15
R
.-. R -
’
’
T*2O0C
v
8.30 16.60
R = 33.20
400
hemi. (nm)
Figure 1. Steady-state emission spectra for IO-$ M 1,8-ANS in AOT reversed micelles (3% AOT (w/v)) as a function of added water. Actual water added (R)is shown in the inset of the figure. Excitation, 380 nm; temperature, 25.0 ‘C.
collected at equally spaced wavelength intervals by using constant band-pass filters across the entire emission spectrum. Thus, the time-dependent emission center of gravity (in IO3 cm-I) is given by7W72 v ( t ) = 1oooo(~z’(A,t)A-’)( D’(A,r))-I
(10)
in order to more easily quantify the kinetics of the time-evolving emission spectra, we adopted an expression developed for the study of ultrafast solvation in neat l i q ~ i d s , ’ the ~ , ~solvation ~ correlation function ( S ( t ) ) :
- v(-)ll
s(t) = ( [ v ( t )- v(-)l/[v(O)
(11)
Here v(O), v(-), and v ( t ) are the fluorescence emission maxima or centers of gravity (in cm-I) a t time zero, infinity, and t , respectively. By substitution of the terms calculated in eq 10 into eq 1 I , S(t) is easily calculated. From this we then have a simple means of examining the relaxation of the environment directly around the solute on a microscopic leve1.73-74
Results and Discussion Steady-State Fluorescence. Figure 1 shows the steady-state emission spectra for I,8-ANS dissolved in AOT micelles as a function of added water. Practically no fluorescence was observed with I,I-ANS in pure heptane, indicating that the presence of AOT reverse micelles is required to produce the intense emission.” The general features of Figure 1 are very straightforward. In “water-free” AOT micelles, 1 ,I-ANS displays intense fluorescence which appears as a broad structureless band with a maximum intensity a t 453 nm. As shown in Figure I , the fluorescence is quenched drastically by the addition of water with a continuous red shift in the emission maxima (Amax). 1 &AN& like all the anilinonaphthalenesulfonic acids, has unique and well-studied photo physic^^^,^^
- I 1 8-
snp,ps
Slp..s
h ”np. os
e- lransfer
transfer
sct,*s
snp.9.
(12)
h vn. os
As shown in eq 12, it is known that after excitation the relatively nonplanar ANS ground state (S,,,,)can undergo a potentially facile intramolecular electron transfer to a charge-transfer state (Snp,= Sa,-).Further, electron transfer to the charge-transfer state is facilitated by an increase in the dielectric relaxation (which is related to the solvent polarity) of the solvent resulting in a bathochromic shift of the emission6j Once in this charge-transfer state, ANS can be quenched via another nonradiative electrontransfer process. Therefore, if the viscosity of the solvent decreases, the quantum yield should also decrease significantly. In addition, because both electron transfers are facilitated in polar solvents one finds that ANS is red-shifted significantly and very weakly fluorescent in purc water.
-
450
500
550
hemi. ( nm) Figure 2. Steady-state emission spectra for IO-’ M I,8-ANS in AOT reversed micelles (3% AOT (w/v)) as a function of temperature. Actual temperatures are shown in the inset of the figure. Excitation, 380 nm; added water content, R = 0.
Because the water pool in the AOT reverse micelle is growing as water is increased, two very different processes could explain the spectral shift and intensity effects seen in Figure 1: (1) a ground-state population distribution of 1,8-ANS in an ensemble of environments with slightly different physicochemical properties (i.e., polarities and viscosities) or (2) an excited-state reorganization process around the 1,8-ANS occurring on the time scale of the excited-state lifetime. Both these types of processes have been reported for A N S emission in a myriad of different environment~.~~J’”~J~ In the first scenario, I,8-ANS would be preferentially located at different, noninteracting domains within the AOT micelle. Following absorption, each “form” of 1 &ANS will be promoted to its own excited state and decay back to its own ground state with a decay time associated with the particular polarity of the domain. That is, there is little interconversion in or during the excited-state lifetime and the lifetime(s) one sees for the fluorescence emission does not change with time and results only from the different physicochemical properties of the domain surrounding a particular 1,8-ANS probe. As the droplet size changes with water content, the 1.8-ANS will tend to associate in a more bulk-water-like domain. Thus, one would observe more fluorescence emitted from I$-ANS in the center of water core and a red-shifted emission spectrum. This scenario is often referred to as ground-state heterogeneity process.77 In the second scenario, I,8-ANS is located initially in far fewer domains, but these domains undergo dipolar solvent reorganization70*7’about the I,8-ANS on a time scale similar to the excited-state lifetime. Upon excitation the dipole moment of I&ANS is increased significantly; however, the Franck-Condon principle states that no nuclei can move during the absorption process. Following absorption one has an excited-state I,8-ANS and its increased dipole moment, but the solvent cage is in the ground-state orientation. Immediately after excitation ceases, the excited-state 1 &ANS begins to emit photons (Le., fluoresce) and simultaneously the solvent begins to reorient to minimize the free energy of the system (i.e., equilibrate with the new dipole moment). As this process proceeds, the apparent polarity near the probe will tend to increase with time and the emission spectrum will be red-shifted in response to this reorganization of the solvent cage. When more water is incorporated into the AOT micelle, the polarity of water pool continually increases and the interactions between the water and the 1 &ANS will become more efficient. Thus, we would anticipate observing emission from the unrelaxed (blue) and relaxed states (red) at low and high concentrations of water, respectively. Even in the absence of any added water (an R value of 1 is probably maintained at this level), temperature too has a pronounced affect on the steady-state emission spectra as illustrated in Figure 2. The fluorescence quantum yield increases strongly with decreasing temperature and the position of the fluorescence band shifts toward the blue. At low temperature, the spectrum
7904 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991
I
04
Zhang and Bright TABLE I: Typical Impulse Response Functions Recovered from Frequency-Domain Measurements for 10-5 M I,I-ANS in 3% (w/v) AOT Reversed Micelles ( R = 1.66), 25 O C wavelength, normal- T~~ = 7.99 ns T~ = 3.50 ns ij= 0.74 ns nm ized FI fflb ff2 a3 42 1 0.197 0.202 0.264 0.534 440 0.632 0.350 0.261 0.389 459 0.957 0.502 0.243 0.255 480 0.951 0.616 0.241 0.143 5 00 0.722 0.737 0.245 0.018 5 20 0.472 0.709 0.214 -0.077 540 0.279 0.682 0.190 -0.127 560 0.157 0.646 0.184 -0.169 x2' = 4.0 a Using a global analysis scheme in which the apparent lifetimes are linked throughout the fit. blall+ la21,+ la3!= 1.00. 'The value of reduced x 2 was calculated based on variances i n phase and modulation of 0.04 and 0.000016, respectively.
1
0
-'i
-1 3
-7
4 5
I 10
100 FREQUENCY (MHz)
Figure 3. (Upper panel) Typical multifrequency phase and modulation data for I.8-ANS in AOT reversed micelles. The points denote the
experimental data and traces represent fits to single (---), double (---), and triple (-) exponential decay models. The central and lower panels are the residual plots in phase and modulation, respectively, for (0.B) single-, (A,A) double-, and (0, 0 ) triple-exponentialdecay laws. is more like that in nonpolar solvents. In contrast, as the temperature is increased the spectrum shifts progressively toward longer wavelength. Again, the trends observed here could simply result from (1) changing the distribution of species in the various domains (Le., variation of the equilibrium mole fractions of 1,8ANS in the various domains) or (2) changes in the viscosity and thus extent (rate) of solvent reorganization around the 1,8-ANS. Thus, steady-state experiments alone are incapable of distinguishing between these two extremely different models. Fortunately, time-resolved fluorescence can provide detailed information about this question. Evidence of an Excited-State Process. As we have discussed above, the local environments (polarity, viscosity, etc.) influence strongly the properties of fluorescent probe molecules. To help us understand the details of 1,8-ANS emission in AOT reverse micelles, we first obtained wavelength-dependent multifrequency phase and modulation data over the spectral region from 421 to 560 nm. Figure 3 shows a typical multifrequency data set for 1,8-ANS in AOT in the presence of H 2 0( R = 1.66; rw = 3.32 A) at 480 nm (upper panel). In addition to the experimental data (points), we show also fits to single (-.--), double (---), and triple (-) exponential decay models. The central and lower panels in Figure 3 show the residual plots for (0, m) single, (A,A) double, and (0, 0 ) triple fits for phase and modulation, respectively. Clearly, the data are most accurately fit by a triple-exponential decay model. [Note: This does not necessarily mean that there are three different domains for the 1.8-ANS. As we discuss below, this simply shows that a triple-exponential decay model can be used to fit or parametrize the decay process.] This is in stark contrast to carlicr reports6* on this same system where single-
exponential decay laws were reported. Although we are not completely sure of the reasons for disagreement between our results and those reported previously68we believe it is simply a result of the present instrumentations superior time resolution (=20 ps vs ~ 0 . ns). 5 In effect, the previous authors were probably unable to resolve the individual decay times and thus reported a singleexponential process. Attempts to fit the experimental data to continuous lifetime distribution^^^,^**^ also resulted in poorer fits (higher x2). In addition, fits to models containing four or more decay times were inferior statistically to the triple exponential model. Table I compiles a typical set of recovered apparent lifetimes and preexponential factors for 1,8-ANS in AOT micelle~.~~ From inspection of these results, we notice one very interesting feature. As we progress across the emission contour (toward the red edge of the fluorescence spectrum) we see that one component exhibits an initially positive then increasing negative preexponential factor. This is a clear indication for the presence of an excited-state r e a ~ t i o n . ~ ~ That , ~ ' , is, ~ ~a ,process ~ ~ which occurs subsequent to excitation and results in the longer wavelength emission.70~71*76*77 By using the recovered fluorescence decay parameters we can (by using eq 1 and the data compiled in Table 1) calculate the wavelength-dependent impulse response function ([(A,?)). From this information, we then construct the normalized time-dependent emission (eq 7). Figure 4 shows a typical set of time-resolved emission spectra for I,8-ANS in AOT reversed micelles as a function of time (see insets within each panel for specific time scales) and water concentration. These results show clearly that the emission spectra are continuously evolving (Le., shifting) as a function of and the time scale for the spectral evolution depends critically on the amount of water within the AOT micelle. Further, this process must be accompanied by some form of time-dependent reorientation of the environment surrounding the 1 ,8-ANS.7b72.82 To better illustrate the time scale and effects of water concentration on the time-resolved emission spectra, Figure 5 shows the time-dependent emission centers of gravity (eq IO) at various concentrations of added water. Again, there is clear evidence for nanosecond solvent r e o r g a n i z a t i ~ n ~and ~ ~we ~ -see * ~ qualitatively that the rate of reorganization is faster as we increase the amount of water. Also, as we increase the water content of the reverse micelle, we see a clear decrease in v ( t ) at time zero. This is evidence that there is probably another (at least) faster relaxation process that is simply beyond our present time r e s o l u t i ~ n . ~ ~ ( 83) Alcala, J . R.; Gratton, E.; Prendergast, F. G . Biophys. J . 1987, 51, 591. (84) In the fitting of our multifrequency data we have linked the
fluorescence decay times across the emission spectrum. A exhaustive analysis
of all data sets shows that the decay times and preexponential factors recovered
by this method differ by at most 6% from the values recovered in the individual (unlinked) analysis. In addition, the recovered S(t) traces using both methods are superimpsable. For this reason, we report only on the linked analysis results because the x * is as good or better compared to the unlinked fits and the linked analyses contain far fewer floating parameters.
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7905 C
.~"bols:
-
0-0
L
nnu (ne)
ymbolm: H20 (R)
0
h
v v)
0-0
0.00
.-•
1.11
A-A
1.66
A-A
2.76
0--0
4.15
m-m
B.30
l ; . 0
r"
1.500
.-.
L
5
0.0001 420
460
440
480
500
Emission Wavelength
2
1.500
I
(nm)
.-.
0-0
Vme (ne)
0 1
h
I
22.00, I
VI
E
W
20.00
I
t
"-.-I-.-.-.
19.501 0
1
2
3
!
540
Figure 4. Reconstructed time-resolved emission spectra for 1,8-ANSin AOT reverse micelles as a function of water content ( R ) . (Upper panel) R = 0; (center panel) R = 1.66: (lower panel) R = 8.30. The specific time increments are denoted on the individual panels.
..-
40
of added water are noted on the figure.
Emission Wovelength (nm)
y
30
Figure 6. Recovered solvation correlation functions (S(f))for 1,8-ANS in AOT micelles as a function of added water at 25 OC. Actual amounts
1
520
.F"boh:
5
L
20
Time (ns)
bole: h e (ne) 0-0 0
C
10
4
I
5
Time (ns)
Figure 5. Time-resolved emission center of gravity for I,8-ANS in AOT micelles at 25 O C as a function of water content ( R ) : 0.00 (O), 1.1 1 (a), 1.66 (A),2.76 (A), 4.15 (O), and 8.30 (B).
Solvation of ANS in Reversed Micelles. In an effort to quantify the rate(s) of solvent reorganization, we looked to the solvent correlation functions (S(r);eq 1 I ) . Figure 6 illustrates the S(r) vs time plots for 1,8-ANS in AOT reverse micelles as a function of water content. Nonlinear least-squares analysis of these tracesa6 shows that at low concentrations of water (R 5 3) there are at (85) The recovered u ( t ) traces all indicate that there is at least one additional component that is faster than our 20-ps time resolution. The amplitude of this fast component grows in as we increase the water content of the reverse micelle. From this, one would conclude that it is related more to the type 2 water domain. (86) Leatherbarrow, R. J.; Enzfitter, A. Non-Linear Regression Dota Analysis Program; Elsevier Biosoft: Cambridge, UK, 1987.
Figure 7. Schematic representation of the two-state model describing water activity in AOT reverse micelles by Zin~li.'~Type 1 water bound to the ionic head groups of AOT surfactant and type 2 water is unbound and located in the center of AOT water pool. Upper and lower illustrations represent low and high degrees of water loading, respectively.
least two rate processes contributing to S ( t ) . From this information we begin to get an understanding of how the solvation process is occurring during the 1,8-ANS excited-state lifetime. As one would expect, the more polar the interior of the AOT micelle becomes the stronger is the coupling between the molecules (water and 1,8-ANS) and the faster the solvent reorganization. Because we are looking at 1,8-ANS in the water pool of a reverse micelle one would anticipate the time scale for the solvation process to be quite different from results in pure water.*' For a reverse micelle, it is well-known that the first amounts of initially added water plays a different role compared to bulk water. Although new models are beginning to emerge,50 Zinsli was the first to describe the water pool of the reversed micelle with a two-state Here (Figure 7) one would have very viscous water close to the polar head groups (type 1) of the reverse micelle in equilibrium with more bulklike water (type 2) in the center of the pool. As the core expands, upon incorporation of water, type 2 water begins to predominate in the reverse micelle. Thus, the first water molecules added to the reversed micelle serve to preferentially solubilize ions and the head groups. Once these regions are suitably solvated the added water begins to swell the micelle and act more like bulk water. Therefore, as we added water to the micelle, we see initially solvation of all ionic species and once these species are well solvated the remaining water begins to be taken into the micelle as bulklike water. This preferential solvation of small ions is further illustrated in Figure 8 as an effect on the fluorescence lifetime by added NaCI. Clearly, since we see an increase in the average lifetime of 1,8-ANS as we add Na+ (87) Jarzeba, W.; Walker, G. C.: Johnson, A. E.; Kahlow, M.A.; Barbara, Chem. 1988, 92, 7039.
P. F. J . f'hys.
Zhang and Bright
7906 The Journal of Physical Chemisrry, Vol. 95, No. 20, 1991
TABLE 111: Temperature-Dependent Solvation Relaxation Rates and Preexponential Factors' for Fits to Experimentally Recovered Solvation Correlation Functionsb
water content ( R ) 1.66
4.15 5
100
10
Frequency (MHz)
Figure 8. Multifrequency phase and modulation data for I,8-ANS in AOT reverse micelles in the presence of H20(R = 8.30; 25 "C), as a
function of added NaCl concentration. Sodium chloride concentrations are 0.0 M ( 0 , O ) . 0.02 M (A,tuo), and 0.04 M (M, 0).Open and filled symbols represent phase angle and demodulation factor, respectively. Solid lines are best fits to triple-exponentialdecay models. TABLE 11: Recovered Solvation Relaxation Rates and Preexponential Factorso for Fits to Experimental Salvation Correlation Functionsb water content (R) k l , ns-' A2 k2. ns-l 0.00 0.64 0.062 0.36 0.920 1.66 0.38 0.198 0.62 1.350 1.521 2.76 0.29 0.286 0.7 1 4.15 1.oo 1.100 8.30 1.oo 2.049
~
A2
k2. ns-I
0.085 0.125 0.160 0.281
0.44 0.52 0.53 0.59
0.600 0.710 1.220 1.822 0.806 0.921
1.00 1.00 1.00 1.00
1.100 1.250
interfacial region F,,ns-I E,, J/mol
R values 1.66 4.15
2350
core region F2,ns-' Ea, J/mol
23.4
16600 35
Uncertainty in recovered parameters is
-1.50
23.5 8.6
1 1 5%.
1
-2.50 3.20
Type I
3.30
3.40
3.50
3.60
3 '0
( 1 / T ) X 10-3 (K-1)
0.301
I
(13)
where A, is a preexponential factor and ki is the dipolar solvent reorganization rate. Below R = 3, we required a double-exponential decay model to fit the data well and above R = 3 a simple single-exponential model sufficed. The recovered parameters from fits of S ( t ) to eq 13 are collected in Table 11. Two trends in the recovered parameters (Table 11) are readily apparent. First, the preexponential factor A , decreases as water concentration increases and A2 increases as water increases. Second, both solvation rates ( k , and k,) generally increase with the increasing water content of the AOT reverse micelle. We propose that AI and k l are associated with type 1 water because the rate is moderately slow (higher viscosity in type I water region) and its contribution decreases to effectively zero as we increase the amount of added water. Similarly, the A2 and k2 terms, we propose, correspond in part to reorganization in type 2 water. Here the reorganizational rate is about an order of magnitude faster, more toward the direction of neat water.88 In addition, as we increase the water content within the AOT micelle we see that ~
k l , ns-I
0.56 0.48 0.47 0.41
TABLE IV: Activation Barriers of Solvation of 1,8-ANS in Both Water Regions of AOT Reversed Micelles'
(H20is constant) to the system we conclude that the water within the AOT micelle preferentially solvates the added Na+ and this water is thus unavailable to facilitate the electron-transfer proc e ~ s e s ~that ~ * ~lead ' to a lower quantum yield and red-shifted spectrum (eq 12). Interpretation of Solvation Process in AOT Reversed Micelles. In the previous sections, we demonstrated that there are at least two nanosecond solvent reorganization processes occurring in the AOT-I ,8-ANS system. To investigate this in more detail, we looked to quantify the effects of water content and temperature on the recovered S(t) traces. To this end, we fitE6our S ( t ) data (Figure 6) to a decay model of the form
EA, exp(-kit)
AIcd
2.0 12.2 22.6 37.4 2.1 11.0 25.0 38.5
Parameters were recovered from Enzfitter program (ref 86). bS(r) function analyzed. AI + A2 = 1 .OO. duncertainty in recovered parameters is 110%.
'Parameters were recovered from Enzfitter program (ref 86). bS(r) function analyzed at 25 O C . + A2 = 1.00. duncertainty in recovered parameters is 58%.
S(t) =
temp, O C
~~
(88) It is curious that our fastest recovered relaxation time ( R = 8.30; - 5 0 0 ps) is significantly greater than the relaxation time recovered in neat water (ref 87; = I ps). At present we have no clear cut arguments for the differences in these two relaxation times. However, we speculate here that ( I ) the dielectric properties of the type 2 water compared to pure water differ significantly; (2) I,8-ANS can locate simultaneously in type 1 and 2 water domains; and (3) there is the possibility of the ultrafast process (cf. Figure 5 ) which we cannot recover may contain this picosecond component.
Y
-S __
3.20
3.30
3.40
3.50
3.60
3.70
( 1 / T ) x 10 3 ( K -1)
Figure 9. Arrhenius plots of solvation reactions for I,8-ANS in AOT micelles. (Upper panel) In the presence of water, R = 1.66. (Lower panel) In the presence of water, R = 4.1 5. A single plot is shown in the lower panel because the solvent relaxation processes collapses into a single-exponential rate expression.
this process begins, as expected, to dominate and the relaxation rate steadily increases. Beyond about R = IO the process is so fast that we are no longer able to quantify it accurately. In an effort to understand the energetics of these solvation processes we investigated each using a simple Arrhenius rate law kproccss = F exp(-Ea/(kBT) (14) where kproem is the specific rate recovered from S(t), k B is the Boltzmann constant, Ea is the activation barrier, T i s the absolute temperature, and F is the frequency factor. Table 111 compiles the recovered results of temperature-dependent S(r) experiments in a region where the observed relaxation process is modelled by
Reorganization of Water within Micelles one (R = 4.15) and two (R = 1.66) solvent reorientation rates. As expected, there is a distinct increase in both rates of solvent reorganization with temperature. Figure 9 shows the Arrhenius plots for the solvent reorientation rates and Table 1V collects the recovered frequency factors and activation energies for the R = 1.66 and = 4.15 systems. Interestingly, for the R = 1.66 water sample the activation barriers for the two processes are essentially equal. The main difference is the frequency factors (number of interactions/ns). In apparent contrast, when the micelle is loaded with a significant amount of water (e.g., R = 4.1 5) the activation bamer height decreases 3-fold and the frequency factor too decreases by 3 orders of magnitude. Apparently, even though the frequency factor decreases, it is compensated for by making the energy barrier to relaxation so small that solvent reorientation becomes more facile. Conclusions
In this work, we show that nanosecond solvent reorganization occurs within the interior of AOT reversed micelles. At low concentrations of water, 1,8-ANS solvation is described by at least two rate processes. We attribute these two rate processes to relaxation in t y p 1 and 249water domains. Of course, it is entirely possible that the model proposed by Jain et aLS0could also apply here and there may be three forms of water thus one would expect to observe three solvent reorganization rates.89 However, our present S(r) data does not support three forms of water.g0 This lack of three recovered rates could result from several reasons: ( I ) the I,8-ANS does not locate in the third water region; (2) (89) It is important to note that we are referring to a triple-exponential decay in S(r) and not lifetime. As mentioned earlier, we simply use the recovered lifetimes (e.g., Table I) as a means of parametrizing the decay process. All of our conclusions are based on S(r) which is derived from I(X,t). (90) Detailed analysis of the S(r) traces shows that only a double-exponential decay law models the experimental data well. In the case when single-exponential fits were tried, the agreement between model and data was poor. When triple-exponentialmodels were attempted, the fits were good visually, but one or more of the recovered rates was always negative.
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7907 this relaxation process, which would be predominately in the head region,s0 may be too slow such that the fluorophore emits its photons and then the solvent reorganizes about the 1B-ANS; (3) the polarity of the third region is such that the nanosecond change in local polarity about the I,8-ANS is insignificant; (4) several of the rates may be too similar to resolve; or (5) one or more of the kinetic parameters is beyond our present time resolution. Regardless of the specific reason for the lack of an observed third component (in S ( t ) ) we still show clear evidence for dynamic solvent reorganization in the interior of the AOT micelle. Moreover, we show that as the concentration of water within the core region of the AOT micelle increases the observed solvation processes collapses to a single-exponential decay law, dominated by solvent reorganization in, we propose, type 2 water. Analysis of the activation parameters shows that at low water concentration the barrier heights are essentially equal and the frequency factors influence the rate. In contrast, at higher levels of water loading, the rate is apparently determined by the relatively small height of the activation barrier. Presently, we are expanding on the recent steady-state fluorescence work of Johnston and co-workers9' and investigating the solvation kinetics of this same 1,8-ANS-AOT reversed micelle system in supercritical fluids. Here the question will be how fast is solvent reorganization within the core region of this system compared to a normal liquid and how does fluid density influence the solvation rate?
Acknowledgment. This work was supported in part by a Non-Tenured faculty Grant from 3M, Inc., the National Science Foundation (CHE-8921517), and the Department of Energy (DE-FGO2-9OER14143). We also acknowledge the efforts of Qian Zhao. Registry No. H20,7732-18-5; AOT, 577-1 1-7; I,8ANS, 82-76-8. (91) Yazdi, P.; McFann, G. J.; Fox, M. A.; Johnston, K. P. J. fhys. Chem. 1990, 94, 7224.
