J. Phys. Chem. 1996, 100, 11133-11138
11133
Characterization of the Microenvironments in AOT Reverse Micelles Using Multidimensional Spectral Analysis Kerry K. Karukstis,* April A. Frazier, D. Stefan Martula, and Jennifer A. Whiles Department of Chemistry, HarVey Mudd College, Claremont, California 91711 ReceiVed: April 2, 1996; In Final Form: April 25, 1996X
We have used the fluorescence probe Prodan to characterize the structure of reverse micelles formed in the ternary system of surfactant Aerosol OT, sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/heptane/water. Our results demonstrate that Prodan is a novel and powerful probe of the features of reverse micellar systems as a consequence of its solubility and measurable fluorescence intensity in a wide range of solvents of varying polarity. These characteristics govern the distribution of the probe into the microregions of the reverse micelle and yield fluorescence properties simultaneously indicative of multiple locations. We observe four principal microenvironments for Prodan, including an inner “free” water pool, a bound water region, the AOT interface, and the surrounding hydrocarbon solvent phase. As the parameters of surfactant concentration and the molar ratio of water to surfactant are varied, we attribute the observed emission characteristics of Prodan to specific micellar structural features including heterogeneity of the water pool, the variable polarities of the bound and free water regions, the hydrophobicity and permeability of the surfactant interface, and the hydration of Na+ counterions in the bound water region.
Introduction In nonaqueous media certain amphiphilic molecules assemble to form reverse micellessaggregations in which the polar heads of the amphiphiles cluster to form a micellar core and the hydrophobic tails extend into the organic bulk phase. The double-chain surfactant Aerosol OT (sodium bis(2-ethylhexyl) sulfosuccinate, AOT) has been extensively investigated for its ability to form reverse micellar aggregates in nonpolar solvents and to solubilize relatively large amounts of water.1 The structure of the aggregates formed in the ternary system water/ AOT/alkane has been analyzed using a variety of experimental techniques. Light scattering, neutron scattering, viscometric, and ultracentrifugation measurements suggest a spherical shape with a 15-Å radius in the absence of water.2,3 Uniform aggregates with a surfactant aggregation number of 23 have been characterized in heptane.4 As the molar ratio of water to AOT, R ) [water]/[AOT], increases, the hydrodynamic radius of the spherical aqueous micellar core increases monotonically with R.5 Two distinct water domains emerge in the polar core6-8s“bound” water immobilized via hydration of the sodium counterions and ionized sulfosuccinate head groups and “bulk” water in the inner region. Only bound water exists at low R values, with the formation of a pool of “free” water molecules at R values greater than 12.9 A dynamic equilibrium exists between the bound and bulk water molecules.8 The hydrogen-bonded network comprising the bound water layer is estimated to be 3-5 Å thick,6,10,11 while the size, as well as the microviscosity and polarity, of the bulk water pool varies with R.12 As the size of the surfactant-entrapped water pool increases, the microviscosity of the solubilized water decreases as its polarity increases. Addition of water also dramatically increases the average aggregation number of the reverse micelles, with little dependence on the hydrocarbon solvent used or the AOT concentration.13 Evidence for polydispersity increases with increasing micelle size.14 Fluorescence techniques have been used to further characterize the reverse micelle structure.9,12,15-23 Many of these studies employ molecular probes with fluorescence characteristics (e.g., X
Abstract published in AdVance ACS Abstracts, June 1, 1996.
S0022-3654(96)00994-X CCC: $12.00
fluorescence intensity, excitation and emission wavelength maxima, lifetime, polarization, etc.) that reflect their microenvironment. Most probes employed exhibit a particular affinity for one or more of the micelle regions: the water pool, the AOT interface, or the surrounding hydrocarbon solvent phase.13 For example, 1,8-anilinonaphthalenesulfonate (ANS), pyrenesulfonic acid (PSA), rhodamine B, 1-aminonaphthylene-4-sulfonic acid (1-N), 2,6-toluidinonaphthalenesulfonic acid (TNS), auramine O (AO), and pyranine partition into the aqueous micellar core.9,12,15,23 Indoleacetate ions distribute between the AOT interface and the water pools.21 Tryptamine cations,21 amphiphilic pyrene derivatives,22 amphiphilic indole derivatives,17-19 and tryptophan20 localize in the AOT interface. 1,2-Dimethylindole remains predominantly in the hydrocarbon phase,20 while pyrene16 and indole20 molecules partition into both the hydrocarbon continuous phase and the AOT interfacial microphase. Parameters such as probe polarity, hydrophobicity, and overall charge influence the average location of the fluorophore in the micellar system. An ideal fluorescence probe for the water/AOT/alkane system would distribute over all regions of the reverse micelle, displaying fluorescence properties simultaneously indicative of its local environment. Concurrent variations in the properties of the individual micelle domains could be distinguished with a probe capable of partitioning into each region. The neutral, hydrophobic probe 6-propionyl-2-(dimethylamino)naphthalene (Prodan) is a potential candidate for such a molecular reporter as a consequence of its solubility in a variety of media, its lack of electrostatic interaction with the anionic head groups of AOT, and its ability to display appreciable shifts in the wavelength of maximum emission with a variation in solvent.24 This probe also has the advantage of exhibiting measurable fluorescence intensities in both polar and nonpolar media. Isolation of the coincident emission from Prodan in multiple microenvironments requires fluorescence data analysis techniques capable of resolving the emission from a mixture of fluorophores. In this investigation we have used the commercially available software PeakFit (Jandel Scientific) to perform a nonlinear least-squares fitting on a single fluorescence emission spectrum to a sum of overlapping Gaussian curves © 1996 American Chemical Society
11134 J. Phys. Chem., Vol. 100, No. 26, 1996 using an iterative Marquardt-Levenberg fitting algorithm. Because of the sensitivity of the emission of Prodan to its environment, PeakFit analysis of fluorescence data of AOT micellar aggregates with varying R values and AOT concentrations should reveal the site-selective localization of Prodan and permit a further characterization of the structure of AOT reverse micelles. In particular, our results confirm the heterogeneity of the water pool, quantify the variable polarities of the bound and free water regions, observe the existence of the free water pool at R values as low as 8, corroborate the hydrophobic nature of the AOT interface, and reveal a discontinuity of fluorescence parameters at R ∼ 12 that reflects the hydration of the Na+ counterions in the bound water region. We conclude that Prodan is a suitable and sensitive fluorescence probe for the concurrent observation of the distinct microenvironments of AOT reverse micelles. Experimental Section Reverse Micelle Preparation. 6-Propionyl-2-(dimethylamino)naphthalene (Prodan; Molecular Probes, Eugene, OR) was dissolved in dimethyl sulfoxide (DMSO; Aldrich, HPLC grade) to produce a 5 mM stock solution. Spectrophotometric grade n-heptane (Aldrich) and sodium bis(2-ethylhexyl) sulfosuccinate (Sigma, >99% purity) were used without further purification. Samples were prepared by dissolving appropriate amounts of AOT in heptane and adding specific volumes of water to achieve the desired R value. Volume additivity was assumed in calculating AOT concentration and water/AOT molar ratios. Reverse micelle samples were allowed to stand overnight to establish phase equilibrium. Reverse micellar systems with [AOT] varying from 0.060 to 0.300 M and with R ranging from 2 to 40 were examined. Thus, for samples of varying [AOT], detergent/probe ratios ranged from 1.2 × 104 to 6.0 × 104, equivalent to micelle/probe ratios varying from approximately 500 to 2500 (using the surfactant aggregation number of 23 measured for AOT/heptane solutions4). Fluorescence Measurements. Prodan fluorescence emission spectra were obtained using a Perkin-Elmer LS-5 fluorescence spectrophotometer. Fluorescence was induced by varying the excitation wavelength in 10-nm increments between 280 and 360 nm, and emission was monitored from 370 to 530 nm or 390 to 550 nm, depending on solvent. Excitation and emission slit widths were set at 5 nm. In all samples [Prodan] ) 5.0 µM. No fluorescence was observed from heptane solutions of AOT without Prodan. Fluorescence measurements of Prodan emission in aqueous Na+ solutions were obtained for solutions prepared with sodium chloride at concentrations ranging from 0 to 0.400 M. Data Analyses via PeakFit. Individual emission spectra were analyzed using the nonlinear least-squares fitting routine in PeakFit capable of resolving up to eight overlapping curves. A fluorescence emission spectrum at fixed excitation wavelength, R value, and AOT concentration was deconvoluted into a sum of overlapping Gaussian functions with frequency as the independent variable. All fits represent the minimum number of components required to achieve a fit with a minimum r2 of 0.999 and a random scattering of residuals. The reproducibility of the curve fitting was tested by varying the starting positions and amplitudes of the constituent curves. The variation of initialization conditions, while a means of protection against being trapped in a local minimum, does not guarantee the uniqueness of a fit or address the sensitivity of the results to errors in data or the accuracy of a Gaussian model for peak shape. For each final fit of an emission spectrum, the center, amplitude, and width of each Gaussian function were characterized.
Karukstis et al. Results (a) Resolution of Prodan Fluorescence in Water, nHeptane, in AOT/n-Heptane Mixtures, and in Aqueous Na+ Solutions. In preparation for interpreting Prodan emission in AOT/water/n-heptane reverse micelles, we examined the emission of 5 µM Prodan in the individual solvents of water and n-heptane, in solutions of AOT and n-heptane, and in aqueous Na+ solutions. Analysis of fluorescence data acquired in water revealed two distinct fluorescence emission maxima at 428 ( 2 and 519 ( 2 nm with relative amplitudes of 0.066 ( 0.002 to 1, respectively. The minor short-wavelength component has been attributed to solute-solvent complexes formed via hydrogen bonding.25 Prodan emission in heptane was characterized by maxima at 388 ( 1 and 408 ( 1 nm, with relative amplitudes of 0.81 ( 0.01 to 1, respectively. These two emission bands have been attributed to the presence of two closely spaced π f π* transitions in the absorption spectrum of Prodan in various solvents.26 For a 0.200 M AOT solution in heptane, three emission maxima were resolved at 431 ( 2, 414 ( 1, and 384 ( 1 nm with amplitudes in the ratio of 1.00: 0.64:0.087. For aqueous Na+ solutions, no shift in emission λmax was observed for [Na+] up to 0.400 M. (b) Resolution of Prodan Fluorescence in Water/AOT/nHeptane Reverse Micelles. The Prodan fluorescence emission spectrum exhibits increased complexity for Prodan incorporated into AOT reverse micelles. Figure 1 shows characteristic normalized emission spectra at excitation λ ) 350 nm for 5 µM Prodan in (a) heptane, (b) 0.200 M AOT in heptane, (c) AOT/heptane/water reverse micelles with 0.200 M AOT and R ) 2, (d) AOT/heptane/water reverse micelles with 0.200 M AOT and R ) 40, and (e) water. For excitation wavelengths ranging in 10-nm increments from 280 to 360 nm, fluorescence emission spectra were recorded for reverse micellar systems with [AOT] varying from 0.060 to 0.300 M (i.e., detergent/probe ratios of 1.2 × 104-6.0 × 104, respectively) and with R ranging from 2 to 40. We used PeakFit to analyze the emission spectra with excitation λ ) 350 nm as a function of both R value and [AOT]. Figures 2 and 3 are examples of fits obtained for reverse micellar systems of [AOT] ) 0.200 M and R ) 2 and 40, respectively. Table 1 summarizes the maximal emission wavelengths and the amplitude (as a measure of the fluorescence intensity) of each Gaussian component resolved for the 49 emission spectra analyzed. Several key trends in the data are apparent. Component 1. For a given [AOT], as R increases, the fluorescence amplitude of component 1 remains small and fairly constant. The contribution of component 1 to the overall fluorescence declines as [AOT] increases, averaging around 15% ((2%) for the lowest [AOT] of 0.060 M and at 5.5% ((0.9%) at the highest [AOT] of 0.300 M. The λmax of component 1 is generally insensitive to R value or [AOT], averaging at 385.4 ( 0.9 nm. Component 2. Component 2 exhibits a fairly constant λmax at low [AOT] values, but, as [AOT] increased above 0.100 M, the wavelength resolved at R ) 2 is considerably higher than the values at all other R ratios. For these same higher [AOT] values, the amplitude and percentage contribution of component 2 for R ) 2 is also a significantly higher contribution than at other R values. Component 3. The λmax of this component exhibits a ∼10nm spread, with higher values at low R ratios. For a given R value, the amplitude of the Gaussian peak and the percentage contribution of component 3 generally increase as [AOT]
Microenvironments in AOT Reverse Micelles
J. Phys. Chem., Vol. 100, No. 26, 1996 11135
TABLE 1: Prodan Fluorescence Emission as a Function of [AOT] and R Value.a Emission λmax and Amplitudes of Gaussian Peaks Resolved by PeakFit [AOT]/M
R
C1
C2
C3
C4
0.060
2
384.6 7.64 386.8b 7.10 386.8b 6.88 388.5b 4.51 385.7b 6.48 387.5b 5.14 386.9b 6.44 384.7 7.77 386.3b 7.68 386.6b 6.18 386.3b 5.40 386.3b 6.35 386.0b 6.64 384.4 7.99 384.6 7.26 388.0b 4.97 384.7 8.58 384.6 7.99 387.6b 4.62 387.0b 6.75 384.1 7.30 384.2 6.35 384.8b 8.54 384.8 8.08 384.5 7.60 384.6 7.88 387.3b 5.33 384.6 7.55
401.1 8.55 400.9 9.55 399.8 10.47 397.4 8.06 397.5 7.66 398.7 10.99 400.2 8.11 402.3 8.69 402.2 11.34 400.0 10.94 396.5 6.81 398.5 8.73 397.9 7.70 398.4 7.97 403.8 8.18 401.9 13.01 399.7 8.64 399.0 7.59 395.6 5.74 400.4 8.85 397.2 7.33 412.3 23.96 401.4 7.93 401.4 8.48 399.5 6.27 400.4 8.35 399.1 9.10 398.9 7.08
424.6 18.24 426.0 11.88 425.3 12.59 420.2 11.30 418.5 10.10 426.1 9.44 424.7 11.28 424.4 21.01 426.0 11.47 426.3 13.49 418.7 12.40 421.5 11.16 419.4 10.99 420.2 11.87 423.5 24.81 426.8 12.84 422.5 17.02 421.3 15.08 418.4 13.59 425.9 14.06 419.7 12.62 431.7 29.62 423.2 26.14 424.2 24.05 422.0 22.11 424.8 20.77 425.9 19.12 422.7 16.92
480.7 16.89 498.4 12.46 498.7 12.58 494.5 9.21 488.6 6.42 503.0 9.64 503.4 11.28 474.1 20.17 487.5 13.70 496.6 15.05 486.1 10.09 488.2 8.48 491.1 7.65 489.2 10.25 464.2 26.83 481.9 18.86 486.9 18.37 487.5 16.36 486.0 10.73 501.8 16.40 492.4 11.05 462.7 29.85 480.3 33.79 484.8 29.92 483.0 27.17 492.8 24.66 498.0 23.43 492.3 18.21
8 10 12 20 30 40 0.075
2 8 10 12 20 30 40
0.100
2 8 10 12 20 30 40
0.150
2 8 10 12 20 30 40
C5
[AOT]/M
R
C1
C2
C3
C4
0.200
2
384.3 6.15 385.0 8.37 384.6 7.52 384.8 7.56 384.8 7.74 384.4 7.59 384.7 7.40 384.1 6.09 384.8 6.52 385.3 6.94 384.9 7.97 385.1 8.00 384.9 7.75 385.0 7.19 384.2 5.57 384.9 7.09 385.0 6.33 384.5 5.80 385.4 6.19 385.2 7.41 384.8 6.98
411.2 19.96 405.3 11.26 400.5 5.16 401.8 6.78 401.2 7.35 399.0 6.71 399.3 6.20 412.2 30.71 407.6 1.02 405.8 0.98 405.8 11.13 402.8 6.38 402.5 7.99 401.1 6.47 411.4 29.69 409.1 11.67 407.0 1.10
428.0 40.33 425.7 32.43 422.6 31.31 423.6 28.91 424.5 25.38 423.2 23.05 423.3 21.83 429.9 55.19 422.6 50.32 422.3 42.77 426.0 39.45 424.2 30.64 425.5 28.42 424.5 24.94 428.7 65.24 425.4 46.09 422.5 49.92 422.2 45.83 423.2 39.71 429.1 34.48 424.9 29.78
454.0 40.93 477.5 45.60 479.2 40.85 481.9 39.41 488.7 32.34 490.1 28.04 491.6 26.16 454.1 43.63 475.7 56.77 478.8 51.99 480.1 56.14 484.5 41.24 491.0 38.75 491.4 33.39 451.8 48.91 468.8 65.92 474.3 62.96 475.1 58.59 484.9 48.59 493.5 52.51 490.1 41.14
508.6 0.50 507.8 1.03 507.0 2.70 507.2 4.56 508.4 0.74
8 10 12 20 30 40 0.250
505.2 4.08 505.9 1.49 506.0 5.71 507.3 6.08 507.2 5.64 507.0 4.64
8 10 12 20 30 40 0.300
504.5 5.82 505.6 5.43 504.9 5.49 506.4 7.89 508.0 1.58 507.7 6.44
2
2 8 10 12 20 30 40
406.6 0.63 406.8 12.31 401.8 6.21
C5
503.4 3.76 503.6 6.63 503.9 6.58 506.0 8.90 506.7 10.76 507.0 12.57 503.7 3.15 502.5 5.35 505.0 6.38 504.9 11.30 506.0 10.03 506.5 12.46 503.4 2.57 503.1 4.51 501.5 8.03 503.4 12.40 507.1 5.40 505.7 14.73
504.1 5.41 504.8 5.56 503.9 8.34 507.1 5.70 507.2 5.06 507.6 9.78
a Emission characteristics presented are the centers (i.e., emission λmax values) in nanometers and amplitudes of Gaussian curves for each component resolved from the overall fluorescence emission spectrum. Emission data correspond to excitation λ ) 350 nm. b An additional emission component at λmax ) 379.6 ( 0.2 nm was resolved with an average amplitude of 5.7 ( 1.1.
increases. For [AOT] < 0.100 M, component 3 is the most significant contributor on a percentage basis to the overall Prodan fluorescence emission spectrum. For all [AOT], the amplitude of component 3 is largest in the R ) 2 micelles and shows a decline with increasing R that is more pronounced for the higher [AOT] values. Component 4. Marked increases (∼30-40 nm) in the emission λmax of component 4 are observed as R increases, particularly at higher [AOT]. On average, the amplitude of this component falls as the surfactant concentration is raised. However, for AOT concentrations at or above 0.150 M, the
amplitude initially rises as R is increased above 2, with a maximum occurring around R ) 8-12, before decreasing with higher AOT/water ratios. Similarly, the maximum percent contribution of component 4 to the overall fluorescence appears around R ) 8-10 for fixed [AOT] values at or above 0.150 M. For [AOT] g 0.100 M, component 4 is the dominant contributor to the overall Prodan emission spectrum. For R values of 2, 8, 10, and 12, as [AOT] increases, the emission λmax of Prodan decreases (e.g., from 480.7 to 451.8 nm for R ) 2; from 498.4 to 468.8 nm for R ) 8; from 498.7 to 474.3 nm for R ) 10; from 494.5 to 475.1 nm for R ) 12).
11136 J. Phys. Chem., Vol. 100, No. 26, 1996
Figure 1. Characteristic normalized emission spectra at excitation λ ) 350 nm for 5 µM Prodan in (a) heptane, (b) 0.200 M AOT in heptane, (c) AOT/heptane/water reverse micelles with 0.200 M AOT and R ) 2, (d) AOT/heptane/water reverse micelles with 0.200 M AOT and R ) 40, and (e) water.
Figure 2. Resolution of the Prodan emission spectrum for reverse micellar systems of [AOT] ) 0.200 M and R ) 2 with excitation λ ) 350 nm. Four overlapping Gaussian curves with centers at 384.3, 411.2, 428.0, and 454.0 nm and amplitudes of 6.15, 19.96, 40.33, and 40.93, respectively, contribute to the overall spectrum. The experimental emission spectrum and the theoretical fit to the experimental curve from the sum of the contributing Gaussian components are superimposed.
Component 5. For all [AOT], no emission component in the 500-nm range is resolved when R ) 2. In general, for a fixed [AOT] the amplitude of this component increases with R. Furthermore, for [AOT] g 0.150, the percent contribution of component 5 to the overall fluorescence increases with increasing R. The λmax of component 5 shows a slight decline as [AOT] increased, from an average value of 507.8 ( 0.6 nm at [AOT] ) 0.060 M to 504.0 ( 1.6 nm at [AOT] ) 0.300 M. Discussion Sensitivity of Fluorescence Probes to Microenvironment. In general, the characteristic sensitivity of the emission properties of many fluorophores to their immediate local environment is related to the differences in the electronic distribution, and therefore in the relative polarities, of the ground and excited states of the fluorescing species. An interaction between the solvent and fluorophore molecules affects the energy difference between the ground and excited states of the fluorophore and influences such parameters as emission wavelength and fluorescence quantum yield. In particular, an increase in the dipole moment upon generation of the excited state leads to reorientation of the solvent molecules surrounding the probe molecule. More polar solvents lead to greater stabilization, decreasing the energy of fluorescence emission and thereby shifting the
Karukstis et al.
Figure 3. Resolution of the Prodan emission spectrum for reverse micellar systems of [AOT] ) 0.200 M and R ) 30 with excitation λ ) 350 nm. Five overlapping Gaussian curves with centers at 384.4, 399.0, 423.2, 490.1, and 506.7 nm and amplitudes of 7.59, 6.71, 23.05, 28.04, and 10.76, respectively, contribute to the overall spectrum. The experimental emission spectrum and the theoretical fit to the experimental curve from the sum of the contributing Gaussian components are superimposed.
fluorescence maximum to longer wavelengths. A variation in fluorescence quantum yield is related to the extent of solventfluorophore interaction; increased interaction of a solvent with the excited probe molecule increases the likelihood of nonradiative transitions. The high spectral sensitivity of Prodan to its environment has been attributed27 to behavior consistent with these principles. Prodan Emission in AOT Reverse Micelles. The observed behavior of Prodan fluorescence in water/AOT/n-heptane mixtures is consistent with the premise that Prodan partitions into multiple environments within the reverse micelles. The overall fluorescence spectrum reflects discrete populations of Prodan molecules with surroundings consistent with the four principal regions of the reverse micelle: hydrocarbon continuum, AOT interface, bound water interface, and inner bulk water pool. The characteristics of the distinct components of Prodan emission, particularly emission wavelength maximum and relative intensity, are influenced by numerous factors including the polarity of the particular micelle region, the extent of rotational motion of the probe in that environment, and the variations in micelle structure imposed by changes in surfactant concentration and in the water/surfactant molar ratio. Our data support the following assignments for the resolved Prodan fluorescence components. Components 1 and 2, principally at about 385 and 400 nm, correspond to emission from Prodan in the bulk heptane solvent. Component 3, with λmax ranging from 418 to 432 nm (average 424 nm), coincides with fluorescence emanating from Prodan in the AOT interfacial region. Components 4 and 5 can be attributed to Prodan in two distinct aqueous regions. Fluorescence from Prodan molecules located in the region of “bound” water immobilized via hydration of the AOT sodium counterions and ionized sulfosuccinate head groups can be ascribed to the shorter wavelength component, 4, detected over the range of 452-503 nm and averaging at 484 nm. The longer wavelength component, with an average λmax of 506 nm, is associated with emission from Prodan molecules deep within the “free” water pool. These assignments are consistent with the relative polarities of the microenvironments, the relative viscosities of the bound and free water pools, and the emission observed for Prodan in the pure solvents heptane and water. On the basis of these assignments, we suggest that the following features of the micellar organization may be ascertained from the measured parameters of Prodan fluorescence.
Microenvironments in AOT Reverse Micelles Free Water Pool. The data clearly support the previously proposed two-state model for the aqueous microregion of the reverse micelle.6-8 We note that the free water pool is present at as low an R value as 8. An increase in the polarity of this micellar region occurs with increasing amounts of solubilized water, as evidenced by increasing emission λmax for component 5 as R increases for all but the lowest [AOT]. The most marked increase in polarity of this microregion is apparent for the shift in R from 12 to 20. This finding further substantiates the sensitivity of the fluorescence parameters of Prodan to its microenvironment, for previous techniques have detected the existence of the free water pool only at [water]/[AOT] ratios greater than 12.9 The differences in the emission λmax values for Prodan in aqueous solution and in the inner core of the AOT reverse micelle indicate that the properties of the free water pool, even at R values of 40, are not identical to that of a continuous bulk water solvent. Bound Water Layer. The expected reduced polarity of the bound water layer versus the free water pool is consistent with a shorter emission wavelength for Prodan located in this region of immobilized water molecules. From geometrical considerations, the thickness of the bound water shell must decrease with an increase in the inner water pool radius (as R increases5). Furthermore, a redistribution of dissociated Na+ counterions into the free water core is likely with an increase in hydrodynamic radius.4,6,28 The marked increase in the emission λmax of Prodan in the bound water region (component 4) with increasing R parallels this expected decrease in the rigidity and polarity of this perimeter. (Recall that variation in [Na+] was shown to have no effect on the emission λmax of Prodan.) We note, however, that, for R values between 8 and 12, the λmax value plateaus at a rather constant value before further increases with increasing R. This behavior as well as the observed discontinuity in the amplitude of component 4 for R values between 8 and 12 presumably reflects the addition of water molecules to the solvation sphere of the sodium counterions and the ionized AOT head groups. The decrease in λmax as [AOT] increases for R values between 2 and 12 may be evidence for a reduced effective polarity of the water as a consequence of an increased degree of structure for the bound water region. One explanation for such an observation is an increase in the number of water molecules required to solvate each AOT molecule as R increases, as has previously been proposed.7 The increased interaction of water with the sodium counterions reduces the effective separation of charge, and hence polarity, of the water. AOT Interfacial Region. The Prodan emission λmax near 424 nm for component 3 implies a quite hydrophobic nature for the AOT interface (see, for example, λmax in p-dioxane), as previously suggested from the partitioning of various amino acids into the AOT interface.29 Variations in the degree to which surfactant molecules in the interface are accessible to water might be anticipated for changes in R. Increasing the radius of the water pool via an increase in R would decrease the radius of curvature of the interface and increase the effective area of each AOT molecule.30 A more open interfacial structure with enhanced water accessibility would result.22 A shift to longer Prodan emission wavelength as R increases for a fixed [AOT] would be anticipated. No such trend is evident, however, and, in fact, for [AOT] g 0.150 M, a substantial decrease in λmax is observed for increases in R from 2 to 8 (and also from 8 to 10 for [AOT] g 0.200 M). While geometrical constraints would necessitate the more open structure, the fluorescence data suggest that the changing degree of water penetration into the interface may alter the distribution of the probe to a less
J. Phys. Chem., Vol. 100, No. 26, 1996 11137 permeable location within the interface. The possibility of variations in micelle aggregation number and in the degree of polydispersity complicate these predictions. The dynamic equilibration of Prodan between micellar regions is also evident in the Prodan fluorescence data. The increase in the amplitude of components 4 and 5 and the concurrent decrease in the amplitude of component 3 suggest a movement of Prodan from the interfacial region to the water domains. From the solubility of Prodan in a range of media, the partitioning of Prodan into the interfacial region is anticipated for all R and increasingly into the bound and/or “free” water regions of the micelles as R increases. Bulk Heptane Continuum. Finally, the fairly constant and low amplitudes of the components attributed to Prodan in the bulk n-heptane solvent (i.e., components 1 and 2) suggest that most Prodan molecules are associated with micelles. A substantial Prodan emission from the hydrocarbon continuum is observed only at low [AOT] and R values, where smaller numbers of micelles exist and fewer micellar domains are present. With higher [AOT] values and higher R ratios, respectively, Prodan molecules may transfer from the heptane region into the additional reverse micelles formed and into the increased number of existing microenvironments. Conclusions The significance of these observations is the demonstration of the ability of Prodan to simultaneously probe all regions of the reverse micelle system. Furthermore, variations in the Prodan emission of each microenvironment as the water/ surfactant ratio or AOT concentration is altered demonstrate the sensitivity of Prodan emission to micelle structure. The partitioning of a fluorescence probe into multiple locations offers a potentially powerful means of characterizing both static and dynamic features of molecular aggregates and macromolecules. Acknowledgment. This research was partially supported by a grant from the National Science Foundation Research Experiences for Undergraduates Program (CHE-9322804). K.K.K. acknowledges the Henry Dreyfus Teacher-Scholar Awards Program for the partial support of this research. References and Notes (1) Ekwall, P.; Mandell, L.; Fontell, K. J. Colloid Interface Sci. 1970, 33, 215. (2) Peri, J. B. J. Colloid Interface Sci. 1969, 29, 6. (3) Kotlarchyk, M.; Huang, J. S.; Chen, S.-H. J. Phys. Chem. 1985, 89, 4382. (4) Wong, M.; Thomas, J. K.; Nowak, J. Am. Chem. Soc. 1977, 99, 4730. (5) Fletcher, P. D. I.; Howe, A. M.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1 1987, 83, 985. (6) Maitra, A. J. Phys. Chem. 1984, 88, 5122. (7) D’Aprano, A.; Lizzio, A.; Turco Liveri, V. J. Phys. Chem. 1987, 91, 4749. (8) D’Aprano, A.; Lizzio, A.; Turco Liveri, V.; Aliotta, F.; Vasi, C.; Migliardo, P. J. Phys. Chem. 1988, 92, 4436. (9) Wong, M.; Thomas, J. K.; Gratzel, M. J. Am. Chem. Soc. 1976, 98, 2391. (10) Eicke, H. F.; Arnold, V. J. Colloid Interface Sci. 1974, 46, 101. (11) Eicke, H. F.; Christen, H. HelV. Chim. Acta 1978, 61, 2258. (12) Hasegawa, M.; Sugimura, T.; Suzaki, Y.; Shindo, Y. J. Phys. Chem. 1994, 98, 2124. (13) Fendler, J. H. Membrane Mimetic Chemistry; Wiley: New York, 1982; Chapter 3. (14) Zulauf, M.; Eicke, H.-F. J. Phys. Chem. 1979, 83, 480. (15) Zinsli, P. E. J. Phys. Chem. 1979, 83, 3223. (16) Backer, C. A.; Whitten, D. G. J. Phys. Chem. 1987, 91, 865. (17) Belletete, M.; Durocher, G. J. Colloid Interface Sci. 1989, 134, 289. (18) Belletete, M.; Lachapelle, M.; Durocher, G. J. Phys. Chem. 1990, 94, 5337.
11138 J. Phys. Chem., Vol. 100, No. 26, 1996 (19) Encinas, M. V.; Lissi, E. A.; Bertolotti, S. G.; Cosa, J. J.; Previtali, C. M. Photochem. Photobiol. 1990, 52, 981. (20) Lissi, E. A.; Encinas, M. V.; Bertolotti, S. G.; Cosa, J. J.; Previtali, C. M. Photochem. Photobiol. 1990, 51, 53. (21) Lissi, E. A.; Engel, D. Langmuir 1992, 8, 452. (22) Borsarelli, C. D.; Cosa, J. J.; Previtali, C. M. Langmuir 1992, 8, 1070. (23) Kondo, H.; Miwa, I.; Sunamoto, J. J. Phys. Chem. 1982, 86, 4826. (24) Weber, G.; Farris, F. J. Biochemistry 1979, 18, 3075. (25) Balter, A.; Nowak, W.; Pawelkiewicz, W.; Kowalczyk, A. Chem. Phys. Lett. 1988, 143, 565.
Karukstis et al. (26) Nowak, W.; Adamczak, P.; Balter, A.; Sygula, A. J. Mol. Struct. 1986, 139, 13. (27) Catalan, J.; Perez, P.; Laynez, J.; Blanco, F. G. J. Fluor. 1991, 1, 215. (28) Eicke, H.-F.; Kubik, R. Faraday Discuss. Chem. Soc. 1983, 76, 18. (29) Leodidis, E. B.; Hatton, T. A. J. Phys. Chem. 1990, 94, 6411. (30) Lang, J.; Jada, A.; Malliaris, A. J. Phys. Chem. 1988, 92, 1946.
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