Polar Solvation Dynamics in Nonionic Reverse Micelles and Model

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Langmuir 2000, 16, 10123-10130

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Polar Solvation Dynamics in Nonionic Reverse Micelles and Model Polymer Solutions Debi Pant† and Nancy E. Levinger* Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 Received July 3, 2000. In Final Form: September 18, 2000 The effect of confinement on solvation dynamics has been explored in Brij-30/cyclohexane and Triton X-100/cyclohexane nonionic reverse micelles. Inside the reverse micelles, the polar solvation dynamics become slower and show additional slow relaxation modes not observed for bulk water. The solvation dynamics inside the Triton X-100 reverse micelles is slower than the dynamics inside Brij-30 reverse micelles. The results for solvation dynamics in the reverse micelles contrast solvation dynamics in aqueous tri(ethylene glycol) monoethyl ether solutions comparable to the reverse micellar interiors, which show significantly faster response. Measurements in the nonionic reverse micelles are also compared to previous work on ionic reverse micelles. Results reported here show that the interactions of water with the polyoxyethylene ether, as well as the micellar confinement inside these reverse micelles, effectively immobilize the water in the micellar interiors.

I. Introduction There exist a wide range of surfactants that form reverse micelles in nonpolar solvents. While surfactants containing ionic groups are typically used for reverse micelle formation, nonionic surfactants have recently shown utility in water-in-oil microemulsions and reverse micelles for various applications, for example as a medium for nanoparticles synthesis.1-5 In the work reported here, we investigate reverse micelles formed from Triton X-100 and Brij-30 (see Figure 1). Compared to reverse micelles created from ionic surfactants, there exists substantially less information about nonionic reverse micelles. In a range of studies on Triton X-100 reverse micelles formed in cyclohexane, Schelly and co-workers6-11 found that the reverse micelles are not spherical. They also observed that the reverse micelle size and aggregation number initially increased as the waterto-surfactant ratio, w0, increased from 0 to 1 and then decreased above w0 ) 1, which they attributed to cyclohexane penetration into micelles polar cores. Via spectroscopic studies of solvatochromic dyes solubilized in the interior of Triton X-100 reverse micelles, Qi et al.12 hypothesized the presence of three different water types within the reverse micelles. Using 13C NMR and IR spectroscopy, Kimura et al.13 estimate that two water molecules can hydrate each ethylene oxide moiety on * Corresponding author: email [email protected]. † Current address: Department of Chemical Sciences, Tata Institute of Fundamental Research, Homi Bhabha Road, Coloba, Mumbai 400005 India. (1) Chhabra, V.; Pillai, V.; Mishra, B. K.; Morrone, A.; Shah, D. O. Langmuir 1995, 11, 3307. (2) Haram, S. K.; Mahadeshwar, A. R.; Dixit, S. G. J. Phys. Chem. 1996, 100, 5868. (3) Qi, L. M.; Ma, J. M.; Cheng, H. M.; Zhao, Z. G. J Phys Chem B 1997, 101, 3460. (4) Qi, L.; Ma, J.; Cheng, H.; Zhao, Z. J. Colloid Interface Sci. 1997, 186, 498. (5) Hopwood, J. D.; Mann, S. Chem. Mater. 1997, 9, 1819. (6) Gu, J.; Schelly, Z. A. Langmuir 1997, 13, 4256. (7) Gu, J.; Schelly, Z. A. Langmuir 1997, 13, 4251. (8) Zhu, D.-M.; Feng, K.-I.; Schelly, Z. A. J. Phys. Chem. 1992, 96, 2382. (9) Zhu, D.-M.; Wu, X.; Schelly, Z. A. Langmuir 1992, 8, 1538. (10) Zhu, D.-M.; Wu, X.; Schelly, Z. A. J. Phys. Chem. 1992, 96, 7121. (11) Zhu, D.-M.; Schelly, Z. A. Langmuir 1992, 8, 48. (12) Qi, L.; Ma, J. J. Colloid Interface Sci. 1998, 197, 36.

Triton X-100. Triton X-100 reverse micelles appear sensitive to the nonpolar medium. That is, reverse micelles formed in propylbenzene are significantly smaller than those formed in toluene, most likely because of lower aggregation number and tighter packing of Triton X-100 molecules in propylbenzene or an increased penetration of toluene in the reverse micelle wall. Some aspects of Brij-30 reverse micelles differ from Triton X-100. Electron spin resonance (ESR) spectroscopy14 has shown that Brij-30 aggregates in nonpolar solvents even in the absence of added water; in contrast, no micelles were detected by dynamic light scattering (DLS) in dry Triton X-100/toluene or Triton X-100/ propylbenzene mixtures.15 Using time-resolved fluorescence quenching, Vasilescu et al.16 found that Brij-30 reverse micelles in cyclohexane grow spherically while nonspherical micelles formed in decane and dodecane form with increasing hydration. Using spin and fluorescence probe techniques to study Brij-30 reverse micelles in cyclohexane and decane, Caldararu et al.14 found a nonuniform distribution of water hydrating the PEO chains in the Brij-30 reverse micellar core. Despite interest and some structural characterization in nonionic reverse micelles, there is little known about the dynamical aspects of the micellar interiors. We have applied ultrafast polar solvation dynamics to explore the dynamics of reverse micellar interiors.17-22 Polar solvation dynamics measures the response of a system to an (13) Kimura, N.; Umemura, J.; Hayashi, S. J. Colloid Interface Sci. 1996, 182, 356. (14) Caldararu, H.; Caragheorgheopol, A.; Vasilescu, M.; Dragutan, I.; Lemmetyinen, H. J. Phys. Chem. 1994, 98, 5320. (15) Rodrı´guez, R.; Vargas, S.; Ferna´ndez-Velasco, D. A. J. Colloid Interface Sci. 1998, 197, 21. (16) Vasilescu, M.; Caragheorgheopol, A.; Almgren, M.; Brown, W.; Alsins, J.; Johannsson, R. Langmuir 1995, 11, 2893. (17) Riter, R. E.; Willard, D. M.; Levinger, N. E. J. Phys. Chem. B 1998, 102, 2705. (18) Riter, R. E.; Undiks, E. P.; Levinger, N. E. J. Am. Chem. Soc. 1998, 120, 6062. (19) Riter, R. E.; Undiks, E. P.; Kimmel, J. R.; Levinger, N. E. J. Phys. Chem. B 1998, 102, 7931. (20) Willard, D. M.; Riter, R. E.; Levinger, N. E. J. Am. Chem. Soc. 1998, 120, 4151. (21) Willard, D. M.; Levinger, N. E. J. Phys. Chem. B, in press. (22) Pant, D.; Riter, R. E.; Levinger, N. E. J. Chem. Phys. 1998, 109, 9995.

10.1021/la000932g CCC: $19.00 © 2000 American Chemical Society Published on Web 12/19/2000

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Figure 1. Chemical formulas and structures of Brij-30, Triton X-100, TGE, and Coumarin 343

instantaneous electric perturbation usually induced through a probe molecule. The solvent response function

C(t) )

F(t) - F(∞) F(0) - F(∞)

(1)

characterizes the solvation dynamics, where F(t) is a spectral feature of the probe molecule at various times.23 Solvation dynamics have measured relaxation in a wide range of bulk liquids;23-2524,25 the dynamics of bulk water are especially fast.26,27 Recently, we have used timeresolved fluorescence Stokes shift (TRFSS) methods to probe polar solvation dynamics of water and formamide in a range of reverse micellar environments.17-22 The dynamics of water inside the reverse micelles include significantly slower relaxation modes than bulk water. We have attributed the overall slower dynamics in these reverse micelles to the restricted environment and the interactions of the intramicellar water with ions or ionic components of the surfactant headgroups. In this paper, we report the ultrafast solvation dynamics measurements of water inside the nonionic reverse micelles of Brij-30 and Triton X-100 as a function of hydration. We compare the intramicellar dynamics with solvation dynamics measured in tri(ethylene glycol) monoethyl ether (TGE)/water solutions that mimic the interior of the Brij-30 reverse micelles. These measurements allow us to distinguish whether the observed dynamics are due solely to the interaction of water with the ethylene oxide chains or due to the restricted environment. Additionally, we analyze the results in light of the reverse micellar morphology similar to our results for lecithin reverse micelles in cyclohexane and benzene.20,21 The dynamics of Brij-30 and Triton X-100 reverse micelles with TGE/H2O mixtures are also compared and contrasted to our previous measures of solvation dynamics in other reverse micellar systems. II. Experimental Methods Sample Preparation. Brij-30 (C12E4, tetraethylene glycol monododecyl ether, Aldrich) and tri(ethylene glycol) monoethyl ether (TGE, Aldrich) and Triton X-100 (TX-100, poly(ethylene oxide) tert-octylphenyl ether, Anatrace) were used with no further purification. Representative structures are shown in Figure 1. (23) Maroncelli, M. J. Mol. Liq. 1993, 57, 1. (24) Barbara, P. F.; Jarzeba, W. Ultrafast Photochemical Intramolecular Charge and Excited-State Solvation. In Advances in Photochemistry; Volman, D. H., Hammond, G. S., Gollnick, K., Eds.; John Wiley & Sons: New York, 1990; Vol. 15, p 1. (25) Horng, M. L.; Gardecki, J. A.; Papazyan, A.; Maroncelli, M. J. Phys. Chem. 1995, 99, 17311. (26) Jimenez, R.; Fleming, G. R.; Kumar, P. V.; Maroncelli, M. Nature 1994, 369, 471. (27) Lang, M. J.; Jordanides, X. J.; Song, X.; Fleming, G. R. J. Chem. Phys. 1998, 110, 5884.

Reverse micelles were prepared from high-purity water (Milli-Q filtered, 18.2 MΩ cm) and cyclohexane (spectrophotometric grade, 99+%, Acros). Before the reverse micellar samples were prepared, the absorption and emission spectra of neat samples of the surfactants and neat TGE were measured in the absence of dye to ensure that the surfactants and TGE neither absorbed nor emitted in the wavelength range of the probe. Neat samples displayed no absorption in the vicinity of the dye and no measurable emission. Reverse micellar samples for steady-state and time-resolved studies were prepared by dissolving the surfactant in cyclohexane to which water was added to make the samples of different w0 values

w0 ) [H2O]/[surfactant]

(2)

The concentration of the surfactants in the solution was 0.1 M. The samples were filtered twice with a 100 nm Teflon filter prior to measurements. Samples yielding microemulsions were determined by visually inspection of the ternary mixtures for phase separation and sample turbidity. The probe molecule for these experiments, Coumarin 343 (C343, Exciton), Figure 1, was used without further purification. The probe molecule was added to the ternary mixtures containing reverse micelles. Samples were periodically shaken manually and sonicated over a 12-h period to ensure that probe molecules were solvated by the reverse micelles; as C343 is highly insoluble in the cyclohexane, it preferentially solvates inside the micelles. The reverse micellar samples were refiltered to remove any unsolubilized excess dye. From the concentration of surfactant and absorbance of the samples, we estimate that there was fewer than 1 C343 molecule per 50 micelles. A range of TGE/H2O mixtures were prepared so that the concentration of water was the same as the concentration of water inside the reverse micelles of different w0, that is, 50, 22, and 11 M TGE/H2O corresponding to w0 ) 1.1, 2.5, and 5.0, respectively. Reverse micellar sizes were measured using dynamic light scattering (DynaPro-MSTC, Protein Solutions). Absorption spectra were recorded with a Cary 2400 UV-Vis-NIR spectrophotometer. Fluorescence spectra were measured with a home-built fluorometer. All experiments were carried out at room temperature, ∼21 °C. The fluorescence upconversion spectrometer used to measure the TRFSS in these experiments has been described in detail previously.17 Briefly, a mode locked Ti:sapphire laser produced output pulses centered at 820 nm, with a duration of 160 fs, at 100 MHz repetition rate, and energies of 6.5 nJ/pulse. Frequencydoubled laser pulses served to excite the sample, while the residual fundamental light was used to gate the sample fluorescence in a nonlinear BBO crystal. The sum frequency of the fluorescence and the gate pulse was detected as a function of the time delay between excitation and gate pulses. The angle between the polarization of the pump and gate pulses was kept at the magic angle to eliminate effects from rotational diffusion. The upconverted signal passed through a monochromator and was detected with a photomultiplier tube. Signals were collected via photon counting interfaced to a computer. Samples were

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Langmuir, Vol. 16, No. 26, 2000 10125

Table 1. Characteristics of Reverse Micelle Size and Coumarin 343 Steady-State Spectroscopy particle size (nm)

solution

absorption peak (nm)

emission peak (nm)

433 434 437

484 486 487

411 415 416

482 486 486

446 448 448 450 428a 456a

489 492 495 497 487a 494a

Brij-30 2.1 (0.7) 2.6 (1.0) 3.2 (0.7)

w0 ) 1.1 w0 ) 2.5 w0 ) 5.0

Triton X-100 33.6 (3.3) 21.7 (4.3) 17.8 (1.9)

w0 ) 1.1 w0 ) 2.5 w0 ) 4.5

TGE/H2O bulk TGE 50 M TGE/H2O 22M TGE/H2O 11 M TGE/H2O bulk H2O, pH > 4 Bulk H2O, pH < 4 a

From ref 18.

circulated with a peristaltic pump fitted with PTFE tubing to ensure the integrity of the samples. Data were collected at nine different wavelengths from 440 to 560 nm at 15-nm intervals. The data were fit to a multiexponential function using an iterative-reconvolution fit program with the cross-correlation of the pump and gate pulses as the instrument response function. From these fits, time-resolved fluorescence spectra were reconstructed. The solvent response function, C(t), was calculated using the peak position of the reconstructed fluorescence spectra fitted to log-normal line shapes. Time-resolved fluorescence anisotropy was measured for the dye in the reverse micellar and bulk solutions to determine where the dye resides in the reverse micellar solutions. These measurements were analyzed via

r(t) )

I|(t) - I⊥(t) I|(t) - 2I⊥(t)

(3)

where I|(t) and I⊥(t) are the time-dependent intensities of the upconverted fluorescence with the polarization of the pump and probe pulses parallel and perpendicular to each other, respectively. The initial anisotropy was ∼0.4 for all samples showing that initially they are completely isotropic.

III. Results and Discussion III.A. Sample Characterization. Before pursuing solvation dynamics measurements, we characterized the systems by a range of methods. Dynamic light scattering was used to estimate the sizes of the Brij-30 and Triton X-100 reverse micelles (Table 1). The apparent hydrodynamic radius for the Triton X-100 w0 ) 1.1 reverse micelles is approximately 10 times larger than Brij-30 or AOT reverse micelles at the same hydration level.17 This size difference has been observed previously and has been attributed to nonspherical morphology of the Triton X-100 reverse micelles.6 As the water content increases, we observe an initial decrease in the Triton X-100 reverse micellar size followed by micelle growth. In contrast, the Brij-30 micelles grow monotonically with increasing water content. These micellar sizes and trends agree with work by Zhu et al.8 and Vasilescu et al.16 III.A.1. Sample Characterization: Steady-State Spectroscopy. The steady-state absorption and emission spectra of C343 in Triton X-100 and Brij-30 reverse micelles are shown in Figures 2 and 3, and data are summarized in Table 1. The steady-state spectroscopy of C343 depends strongly on its environment. It peaks at longer wavelengths in increasingly polar environments;28-30 (28) Reynolds, G. A.; Drexhage, K. H. Opt. Commun. 1975, 13, 222.

Figure 2. Steady-state absorption (a) and emission spectra (b) of C343 dye in Triton X-100 reverse micelles for different water content.

thus we use it to characterize the milieu that the dye senses. The absorption spectrum of C343 in w0 ) 1.1 Brij30 reverse micelles peaks at slightly longer wavelength than the absorption maximum in bulk water (Table 1). Increasing the water content in the Brij-30 micelles shifts the C343 spectrum toward longer wavelengths. A small shoulder in the absorption spectrum for w0 ) 1.1, observed at ∼420 nm, disappears with added water. The emission spectrum of C343 in Brij-30 reverse micelles peaks at a shorter wavelength compared to the bulk water spectrum. It shifts to longer wavelength with added water but, regardless of water content, never overlaps the bulk water spectrum. The absorption and emission spectra of C343 in Triton X-100 are blue shifted compared to the bulk water (Figure 3). Both absorption and emission spectra shift to longer wavelength with increasing water content but, as with Brij-30, never overlay the bulk water spectrum. The absorption spectrum of C343 in all the Triton X-100 reverse micelles displays a shoulder at ∼450 nm for all hydration levels studied (Figure 3). While the absorption spectra of C343 show significant differences in the two reverse micellar systems, the emission spectra display no substantial differences (Table 1). The steady-state absorption and emission spectra of C343 vary as a function of pH.18 Similarities in the trends for the C343 spectra in Triton X-100 reverse micelles to those we have previously measured suggest that the dye is anionic in these micellar cores. In contrast, the position of the C343 absorption spectrum in the Brij-30 reverse micelles appears to result from the protonated form of the dye. The absorption spectrum of the C343 anion, present at pH > 4.5, peaks near 425 nm, while the neutral, (29) Drexhage, K. H.; Erikson, G. R.; Hawks, G. H.; Reynolds, G. A. Opt. Commun. 1975, 15, 399. (30) Tominaga, K.; Walker, G. C. J. Photochem. Photobiol., A 1995, 87, 192.

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Figure 3. Steady-state absorption (a) and emission spectra (b) of C343 dye in Brij-30 reverse micelles for different water content.

Pant and Levinger

Figure 4. Steady-state absorption (a) and emission spectra (b) of C343 dye for different concentrations of TGE in H2O.

protonated form peaks near 450 nm. The emission spectrum displays a less dramatic but consistent red shift (∼7 nm) between the anionic and neutral forms. Tominaga and Walker30 have observed similar behavior for the absorption and emission spectra of C343 in normal and basic methanol solutions. We conclude that the C343 dye is protonated in the interior polar core of Brij-30, while it is in the anionic form inside Triton X-100 reverse micelles. Others report on anomalous pKa behavior inside Brij30 reverse micelles.31 The pKa of molecular probes in Brij30 reverse micelles differed from corresponding bulk solutions. Further, a decrease in the pKa values was observed inside the Brij-30 reverse micelles with the increase of water content,31 which agrees well with our observation of disappearance of the hump observed in the absorption spectra of the smallest reverse micelles with the increase of hydration. In contrast, the pKa values of probe molecules in Triton X-100 reverse micelles matched bulk solutions from which they were made.31 Clearly, these steady-state spectra indicate that the environments inside the Brij-30 and Triton X-100 reverse micelles differ significantly from bulk water. Similar to our previous studies of AOT reverse micelles,17 the spectra of C343 in Brij-30 and Triton X-100 reverse micelles shift continuously toward the bulk water spectra with the increasing water content but never overlap the spectrum in bulk water. The absorption and the emission spectra of C343 in TGE/H2O mixtures are shown in Figure 4. The C343 absorption spectrum in bulk TGE is significantly red shifted compared to bulk water, while the emission maximum is similar to bulk water. Both the absorption

and emission of C343 are red shifted in the TGE/H2O mixtures compared the spectra in Brij-30 or Triton X-100. The position of the absorption peak indicates that the dye retains its neutral form in solution, similar to the spectrum in methanol.30 On dilution of the samples with water, both absorption and emission maximum shift further toward the spectrum in bulk water (see Table 1). Comparison of the C343 absorption spectra in TGE/ H2O mixtures with C343 in Brij-30 reverse micelles shows that the dye experiences a less polar environment in the micellar core than in the TGE/H2O mixtures. Caldararu et al.14 studied the fluorescence of 1-anilinonaphthalene8-sulfonic acid (ANS) fluorescence in poly(ethylene oxide) (PEO)/water mixtures and in Brij-30 reverse micelles. They found that for the smallest reverse micelle studied, the emission maximum of ANS red shifted compared to ANS in bulk PEO. They attributed this observation to the lower polarity in the micellar solutions compared to the bulk PEO. Correa et al.32 also observed higher polarity in the neat Brij-30 than Brij-30/benzene at w0 ) 0 and assigned this result to the penetration of benzene into the micellar interior core. The continuous red shift in the C343 emission in TGE/H2O we observed with the dilution of TGE is most likely due to the further increase of polarity with the increase of water content in the solution. III.A.2. Sample Characterization: Time-Resolved Fluorescence Anisotropy. An issue of great importance for these measurements is the location of the dye molecule in the reverse micelles. We use time-resolved fluorescence anisotropy to probe the rotation time of the dye in the different solutions studied. These measurements yield information about the local viscosity that the dye senses and provides insight to its location in the micelles. The anisotropy decay curves of dye in different solutions are

(31) Chinelatto, A. M.; Fonseca, M. T. M.; Kiyan, N. Z.; El Seoud, O. A. Ber. Busenges. Phys. Chem. 1990, 94, 882.

(32) Correa, N. M.; Biasutti, M. A.; Silber, J. J. J Colloid Interface Sci. 1996, 184, 570.

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Langmuir, Vol. 16, No. 26, 2000 10127

shown in Figure 5 and the parameters are summarized in Table 2. The anisotropy decay of dye in bulk water fits well to a single exponential decay function with a rotational time of 90 ps. In contrast, the anisotropy decays measured for C343 in the reverse micellar solutions require a biexponential function for an acceptable fit. The rotational times are orders-of-magnitude larger than the bulk water. Additionally, the rotational time of the dye in Triton X-100 reverse micelles is longer than that in the Brij-30 reverse micelles. Dye rotational times that are substantially longer than those in bulk water reflect hindered motion or, if the dye is completely immobilized at the micellar surface, motion of the entire micelle. Multiple time constants for the anisotropy decays can be interpreted in several ways. In our previous studies exploring AOT reverse micelles, we observed anisotropy decays with two time components.17 Because the time scale of the two components matched those for free C343 in water17 and the micelle rotational correlation time,33 the biexponential anisotropy decay was interpreted as evidence for two dye locations, bound to the inner surface of the micelle and free in the intramicellar water pool. Another common interpretation invokes hindered motions of the dye molecule that can be modeled by the wobbling motion of the fluorophore. The amplitude of the wobble component determines how much the fluorophore rotates in a conical volume and the associated time component corresponds to the rate that the fluorophore samples different configurations within the cone. Results for Brij30 reverse micelles could be interpreted using either model. However, the probe is only minimally soluble in bulk water and is much more soluble in the TGE solutions. Thus, it is more likely to find the dye where it can be solvated by the PEO chains. In contrast to the results for Brij-30, the rotational correlation time for the C343 probe molecule in the Triton X-100 reverse micelles reflects a severely

constrained environment where the dye can wobble and relax but cannot rotate freely as in bulk water. Our anisotropy measurements coincide with those of Caldararu et al.,14 who used ESR and time-resolved fluorescence anisotropy to measure the rotation time of the ANS probe inside the Brij-30 reverse micelles. They measured the anisotropy as a function of water content and found a significant increase of the rotational time with the initial increase of water content up to w0 ) 1.1. Further increase of water yielded a decrease in the rotation time. The initial increase in the rotational time was attributed to the increase in aggregation number, whereas a decrease in rotational time on further hydration was assigned to the increase in the mobility of the probe. III.B. Solvation Dynamics Measurements. We have studied the solvation dynamics of Brij-30/cyclohexane reverse micelles, Triton X-100/cyclohexane reverse micelles, and TGE/H2O mixtures for different water concentrations. The time correlation function, C(t), fits well to a multiexponential function, C(t) ) ∑iRi exp(-t/τi). All C(t) parameters are given in Table 3. The overall Stokes shift measured depends strongly on the position of the emission spectrum instantaneously after excitation, or the time-zero spectrum. The time resolution for the experiments described here was not sufficient to resolve the fastest underdamped inertial component; thus reported values for ∆ν reflect only the diffusive relaxation measured. Comparison of values listed in Table 1 with those in Table 3 shows our values for ∆ν miss some fraction of the overall shift. For the micellar interfaces, our ∆ν value is smaller than that for bulk water while the overall Stokes shifts between the steady-state fluorescence and absorption spectra are similar. This suggests that the degree of diffusive relaxation is smaller at the interface than it is in bulk water and may indicate that there is significantly more inertial relaxation in micellar interface than is seen in bulk water. III.B.1. Triton X-100/Cyclohexane Reverse Micelles. The normalized C(t) functions for Triton X-100/ cyclohexane reverse micelles with varying w0 values are shown in Figure 6. For all w0 values studied, C(t) fits well to three exponential decays. The dynamics are slowest for w0 ) 1.1 and get faster with increasing hydration level. The overall fluorescence Stokes shifts get larger with increasing water content. These results suggest that the solvent gains mobility with increasing hydration level. The growing fast response and increasing Stokes shift for the intramicellar solvation dynamics suggest that solvent mobility increases as the micelles grow. This agrees well with our previous work on other micellar systems.17,20-22 Interestingly, Schelly and co-workers6-11 suggest that the Triton X-100 reverse micelle size reaches a maximum at w0 ) 1, due to solubilization of the nonpolar solvent in the micellar interior. Thus, smaller sizes for the reverse micelles with larger w0 values do not equate to a more restricted environment as the interaction of the Triton X-100 PEO chains with water is quite different from their interaction with cyclohexane. Likewise, if up to two water molecules hydrate each ethylene oxide moiety, as suggested by Kimura et al.,13 then all the intramicellar water in our Triton X-100 reverse micelles should be bound to the poly(ethylene oxide) chain and the dynamics should vary little with hydration level. Recently, Bhattacharyya and co-workers34 studied the solvation dynamics of 4-aminophthalimide in water-inoil microemulsions of Triton X-100 in mixed benzene and

(33) Krishnakumar, S.; Somasundaran, P. J. Colloid Interface Sci. 1994, 162, 425.

(34) Mandal, D.; Datta, A.; Pal, S. K.; Bhattacharyya, K. J. Phys. Chem. B 1998, 102, 9070.

Figure 5. Time-resolved anisotropy decay curves for C343 dye in bulk water (anionic form of the dye), Brij-30 reverse micelles, and Triton X-100 reverse micelles. Table 2. Time-Resolved Anisotropy Measurements Yielding Rotational Correlation Times for Various Reverse Micelles with w0 ) 1.1 and for Bulk Watera probe ANSb

probe C343 solution

R1

τ1 (ps)

R2

τ2 (ns)

bulk H2O Brij-30 Triton X-100

0.36 0.11 0.06

96 73 13

0.30 0.34

0.82 2.9

a

τ (ns) 0.07 0.96

Error is (10%. b From ref 14.

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Table 3. Solvation Dynamics Parametersa ∆ν (Stokes shift, cm-1)

R1

τ1

w0 ) 1.1 w0 ) 2.5 w0 ) 4.5

694 835 860

0.14 0.07 0.23

1.4 1.1 0.4

w0 ) 1.1 w0 ) 2.5 w0 ) 5.0

851 950 1040

0.38 0.39 0.63

255 280 300 370 965

0.10 0.34 0.22 0.26 0.29

solution

bulk TGE 50 M TGE/H2O 22 M TGE/H2O 11 M TGE/H2O bulk H2O a

R3

τ3

τ0a

Triton X-100 0.39 47 0.41 21 0.57 18

0.47 0.52 0.20

175 114 137

8.9 11.1 1.8

0.62 0.10 0.09

Brij-30 0.24 15 0.16 2 0.07 2

0.38 0.45 0.30

157 112 103

0.26 0.13 0.14 0.18 0.22

TGE/H2O 0.19 24 0.15 7 0.22 7 0.74 19 0.71 0.59

0.71 0.51 0.56

106 84 33

All times are given in picoseconds. Error is (10%.

R2

a

τ2

〈t〉c

t1eb 75 50 30

131 58 38

1.6 0.26 0.14

45 35 25

63 53 31

2.5 0.38 0.62 0.67 0.40

50 40 16 12 0.45

80 44 20 14 0.55

τ0-1 ) ∑iRiτi-1. b t1e, time required for C(t) to reach to 1/e. c 〈τ〉 ) ∑iRiτii.

Figure 6. Normalized solvent response functions, C(t), displaying the dynamic response for C343 in Triton X-100 reverse micelles for different water content. Points are experimental data, and lines are fits to the data. Values for the fits are given in Table 3.

Figure 7. Normalized solvent response functions, C(t), displaying the dynamic response for C343 in Brij-30 reverse micelles for different water content. Points are experimental data and lines are fits to the data. Values for the fits are given in Table 3.

hexane solvent using the picosecond time-resolved Stokes shift. While their limited instrumental resolution could not resolve the ultrafast time component in the solvation dynamics, they observed two longer time components that differed significantly from bulk water dynamics. III.B.2. Brij-30/Cyclohexane Reverse Micelles. Figure 7 shows the solvent response function for Brij-30 reverse micelles in cyclohexane for three different hydration levels. Like Triton X-100, the dynamics inside the Brij-30 reverse micelles get faster and the overall fluorescence Stokes shift increases with increasing water content. However, compared to the Triton X-100 reverse micelles, the overall response is significantly faster, and the amplitude corresponding to the shortest component is larger (Table 3). The amplitude of the shortest time component increases with hydration as the decay time grows shorter. Several aspects of the intramicellar solvation dynamics mirror our previous results for related reverse micellar systems.17-22 For example, compared to the solvation dynamics of bulk water, the dynamics inside the Brij-30 reverse micelles are significantly slower, including a long, subnanosecond decay component in addition to picosecond and subpicosecond components. With the increase in the hydration level of the micelles, the dynamics get faster, but even for the largest reverse micelles studied, w0 ) 5, the overall dynamics is substantially slower than that seen in bulk water. However, in contrast to other reverse micellar systems we have probed, in the Brij-30 reverse micelles at all hydration levels, we observe a significant

ultrafast component to the solvation dynamics. In AOT or lecithin reverse micelles, the ultrafast components to the relaxation are not present at low hydration levels, developing only with significant additions of water. In general, we associate a subpicosecond component to the solvent response to bulk water relaxation. III.B.3. TGE/H2O Mixtures. The polar core of these nonionic reverse micelles is actually an aqueous polyether solution. To directly compare and contrast the dynamics inside the reverse micelles, we have measured solvation dynamics of bulk TGE as well as aqueous solutions of TGE corresponding to the concentrations found in the micellar interiors. The solvent response function, C(t), is shown for neat TGE and the aqueous solutions in Figure 8. For bulk TGE, C(t) fits a triple exponential decay. The shortest time components are similar to bulk water, while the longer time components are considerably slower than bulk water (Table 3). For 50 M TGE in H2O, comparable to the intramicellar concentration for w0 ) 1.1, the time components are slightly faster than those obtained in bulk TGE. A further decrease in the TGE concentration yields faster dynamics. The C(t) for the 11 M TGE/H2O solution (comparable to Brij-30 w0 ) 5) fits very well with only two exponential decays. Still the longer time component is significantly slower than the relaxation observed for bulk water. Solvation dynamics measurements have been made on a few polymeric systems. Argaman and Huppert35 studied (35) Argaman, R.; Hupert, D. J. Phys. Chem. 1998, 102, 6215.

Polar Solvation Dynamics in Nonionic Reverse Micelles

Figure 8. Normalized solvent response functions, C(t), displaying the dynamic response for C343 in TGE/H2O mixtures for different concentrations of TGE in water. Points are experimental data and lines are fits to the data. Values for the fits are given in Table 3.

the solvation dynamics of coumarin 153 in ethylene glycol dimethyl ethers as a function of number of repeat units (n ) 2, 3, and 4) and temperature. They found that the solvation dynamics for these small polymers span from ∼50 fs to more than 100 ps. They ascribe the shortest subpicosecond component to the interplay of intra- and intermolecular interactions. All longer components are ascribed to solvent relaxation. In analogy with work by Olender and Nitzan,36 the solvation dynamics component appearing in the 10-20 ps range is attributed to segmental motion of the polymer. Argaman and Huppert also observed a relaxation component occurring with a time constant longer than 100 ps that displayed almost no dependence on the polyether size. They assigned this component to cooperative motion of the polymer matrix. Using molecular dynamics simulations, Olender and Nitzan36 have recently studied solvation and solvation dynamics in a series of ethers of increasing molecular weights. They found that the ultrafast component for all the ethers studied was the same in amplitude and duration as they observed for the simplest system. However, this component only fit well to a Gaussian function for ethyl methyl ether. They attributed this component to inertial free streaming motion of the solvent molecules. They suggested that the ultrafast time component for ethers of higher molecular weights originates from damped solvent vibrations about inherent solvent structures and could not be considered an inertial component of solvation. Using the three-pulse photon echo peak shift method, Fleming and co-workers37 studied the dye IR144 in polyvinylformal and the dye 3,3′-diethylthiatricarbocyanine iodide in poly(methyl methacrylate). An ultrafast component corresponding to a decay of ∼100 fs time scale in the transition frequency correlation function was found in all the cases and was attributed to the inertial response of the solvent. Solvation dynamics in slowly relaxing solvents may also bear resemblance to the polyether dynamics. Castner and co-workers38 have measured the ultrafast solvent relaxation of liquid ethylene glycol using the optical-heterodynedetected Raman-induced Kerr-effect spectroscopy. They found that the ultrafast responses of ethylene glycol and water were similar and attributed to the similarities in the hydrogen bonding properties of these two liquids. The (36) Olender, R.; Nitzan, A. J. Chem. Phys. 1995, 102, 7180. (37) Nagasawa, Y.; Yu, J.-Y.; Fleming, G. R. J. Chem. Phys. 1998, 109, 6175. (38) Chang, Y. J.; Castner, E. W. J. Chem. Phys. 1993, 99, 7289.

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diffusive rotation was significantly slower for ethylene glycol and glycerol triacetate compared to the bulk water. Maroncelli and co-workers25 studied the polar solvation dynamics of ethylene glycol using the time-resolved emission spectroscopy and found three relaxation components: 187 fs, 5 ps, and 32 ps. The ultrafast component they observed for glycol ether is similar to the ultrafast component of bulk water. III.C. Comparisons. The changes in solvation dynamics with hydration can be attributed to increased mobility of the solvent in the micellar interior. In spherical reverse micelles, as the w0 increases, a water pool forms in the micellar core leaving proportionately less water bound to the micellar interface. In analogy to other studies,6 the three solvation time components observed inside the reverse micelles can be assigned to three types of water. The ultrafast time component grows faster and stronger due the formation of a bulklike water pool inside the reverse micelles. The picosecond time components are assigned to water molecules bound to the micellar interface. As the water pool size increases, the relative amount of water bound to the interface decreases resulting in faster dynamics with water content. The subnanosecond time component is assigned to water strongly bound to the poly(ethylene oxide) oxides and/or concomitant water and surfactant headgroup motion. III.C.1. Brij-30/Cyclohexane vs TGE/H2O. In many ways the solvation dynamics of water inside the Brij-30/ cyclohexane reverse micelles resembles the solvation dynamics in the TGE/H2O mixtures. First, the overall solvation dynamics are significantly slower than those in the bulk water. Second, both C(t) functions fit well to multiexponential decays. Third, with increasing water concentration, the dynamics get faster both in the reverse micelles and in the polymer solution. Despite similarities, there are substantial differences between the solvation dynamics of water in Brij-30 reverse micelles and TGE/H2O solution. The most striking difference is that the amplitude corresponding to faster component in Brij-30 reverse micelles is significantly larger than it is in the TGE solutions. In contrast, no significant change in the amplitude is observed for TGE/ H2O mixtures. This suggests that a bulklike water pool forms with increasing hydration in the reverse micelles while water continues to interact with the PEO chains in TGE solutions. Another major difference is that the overall dynamics inside the Brij-30/cyclohexane is slower than that in the TGE/H2O mixtures, largely due to the significantly longer time constant for the dynamics in the reverse micelles. This implies that the restricted environment inside the reverse micelles accounts for a significant amount of immobilization observed for the micellar solvation dynamics. It also shows that the dynamics are not solely due to the polymer-like solution inside the reverse micelles. Brij-30 vs Triton X-100 vs AOT. Brij-30 (C12E4) reverse micelles show both similarities and differences from Triton X-100. Like Triton X-100 reverse micelles,12 NMR, ESR, and NIR spectroscopic studies39 on poly(ethylene oxide) nonylphenyl ether reverse micelles in cyclohexane revealed three types of water present in the polar core, namely, water interacting directly with the ethylene oxide groups of the surfactant, bound water next to the hydrated ethylene oxide groups, and bulklike water. Amararene et al.40 used an ultrasound velocimeter to measure compressibility of Brij-30 reverse micelle solutions as a (39) Kawai, T.; Shindo, N.; Kon-No, K. Colloid Polym. Sci. 1995, 273, 195.

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function of water concentration. Their results show that the compressibility of water confined to the micellar interior differs from bulk. Comparison of the dynamics inside the Brij-30 and Triton X-100 reverse micelles shows that the average solvation time in Brij-30 is much shorter than that in Triton X-100. This arises because the amplitude of the shortest time component is larger in Brij-30 than it is in Triton X-100 reverse micelles. This suggests that Triton X-100 effectively solvates the water in the micellar interior precluding water pool formation while a bulklike water pool forms in the Brij-30 reverse micelles. Triton X-100 possesses longer PEO chains (∼9.5 repeat units) compared to Brij-30 (4 repeat units) presenting more than twice the possible ethylene oxide binding sites for water on the polymer tail. The differences in the solvation dynamics of water in Brij-30 and Triton X-100 can be also explained due to the differences in the micellar shape and size. Brij30 forms well-defined spherical reverse micelles, while Triton X-100 is hypothesized to form nonspherical disk like reverse micelles.8 Moreover, the diameter of Brij-30 reverse micelles is 10 times smaller than that of Triton X-100 reverse micelles. At least at low hydration levels, Triton X-100 does not form a well-defined water pool to the same extent Brij-30 does. This is consistent with our previous results for lecithin reverse micelles in cyclohexane and benzene.21 In these experiments, tubular lecithin reverse micelles formed in cyclohexane displayed substantially slower relaxation than spherical micelles formed in benzene. In our previous AOT reverse micelles,17 water in very small reverse micelles displayed virtually no motion on the time scale of the experiment. However, as the reverse micelles swelled, the water exhibited increased mobility. On comparison of the dynamics for the same hydration level at w0 ) 5, the dynamics inside the Brij-30 reverse micelles are faster compared to that of the AOT, while inside the Triton X-100 reverse micelles the dynamics are slower than that of the AOT reverse micelles. Because the solvation dynamics results for Brij-30 and Triton X-100 (40) Amararene, A.; Gindre, M.; LeHuerou, J. Y.; Nicot, C.; Urbach, W.; Waks, M. J. Phys. Chem. B 1997, 101, 10751.

Pant and Levinger

reverse micelles diverge in different directions from results for AOT, the strong interactions of water with ions in the AOT reverse micelles cannot account for the differences that we observe in these nonionic reverse micelles. IV. Summary and Conclusions The overall results of this study are as follows: i. The solvation dynamics inside the nonionic reverse micelles are substantially reduced compared to the solvation dynamics of bulk water. ii. The overall solvation dynamics of water inside Triton X-100 reverse micelles are slower than the dynamics inside the Brij-30 or AOT reverse micelles. The dynamics of water inside the Brij-30 reverse micelles are relatively faster than dynamics of AOT reverse micelles at the same hydration level. iii. The measured solvation dynamics are significantly slower for TGE/H2O mixtures compared to those for bulk water. iv. The restricted environment plays a substantial role in immobilizing the intramicellar interior in Triton X-100 reverse micelles. Interestingly, despite perturbation of the micellar interior by the PEO chains, water pool formation still appears possible, specifically in the Brij30 reverse micelles. The solvation dynamics of TGE/H2O mixtures are faster compared to the dynamics in reverse micelles showing that the observed dynamics cannot be attributed solely to interactions with the PEO chains. In conclusion, we have seen that solvation dynamics in the confined interior of nonionic reverse micelles depends strongly on the form of the micelles. These results demonstrate the wide range of differing confined environments that can be found in reverse micelles and show that caution should be exercised when making broad generalizations about these nanoscopic environments based on structural data. Furthermore, these results show that dynamical measurements can reflect differences in intramicellar structure. Acknowledgment. This work was supported by the National Science Foundation. LA000932G