13050
J. Phys. Chem. B 2006, 110, 13050-13061
What Can You Learn from a Molecular Probe? New Insights on the Behavior of C343 in Homogeneous Solutions and AOT Reverse Micelles N. Mariano Correa*,† and Nancy E. Levinger* Department of Chemistry, Colorado State UniVersity, Fort Collins, Colorado 80523-1872 ReceiVed: December 12, 2005; In Final Form: May 19, 2006
The behavior of C343, a common molecular probe utilized in solvation dynamics experiments, was studied in homogeneous media and in aqueous and nonaqueous reverse micelles (RMs). In homogeneous media, the Kamlet and Taft solvatochromic comparison method quantified solute-solvent interactions from the absorption and emission bands showing that the solvatochromic behavior of the dye depends not only on the polarity of the medium but also on the hydrogen-bonding properties of the solvent. Specifically, in the ground state the molecule displays a bathochromic shift with the polarity polarizability (π*) and the H-bond acceptor (β) ability of the solvents and a hypsochromic shift with the hydrogen donor ability (R) of the media. The carboxylic acid group causes C343 to display greater sensitivity to the β than to the π* polarity parameter; this sensitivity increases in the excited state, while the dependence on R vanishes. This demonstrates that C343 forms a stable H-bond complex with solvents with high H-bond acceptor ability (high β) and low H-bond donor character (low R). Spectroscopy in nonpolar solvents reveals J-aggregate formation. With information from the Kamlet-Taft analysis, C343 was used to explore RMs composed of water or polar solvents/sodium 1,4bis-2-ethylhexylsulfosuccinate (AOT)/isooctane using absorption, emission, and time-resolved spectroscopies. Sequestered polar solvents included ethylene glycol (EG), formamide (FA), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMA). Dissolved in the AOT RM systems at low concentration, C343 exists as a monomer, and when introduced to the RM samples in its protonated form, C343 remains protonated driving it to reside in the interface rather than the water pool. The solvathochromic behavior of the dye depends the specific polar solvent encapsulated in the RMs, revealing different types of interactions between the solvents and the surfactant. EG and water H-bond with the AOT sulfonate group destroying their bulk H-bonded structures. While water remains well segregated from the nonpolar regions, EG appears to penetrate into the oil side of the interface. In aqueous AOT RMs, C343 interacts with neither the sulfonate group nor the water, perhaps because of intramolecular H-bonding in the dye. DMF and DMA interact primarily through dipoledipole forces, and the strong interactions with AOT sodium counterions destroy their bulk structure. FA also interacts with the Na+ counterions but retains its H-bond network present in bulk solvent. Surprisingly, FA appears to be the only polar solvent other than water forming a “polar-solvent pool” with macroscopic properties similar to the bulk.
I. Introduction Reverse micelles (RMs) are aggregates of surfactants formed in a nonpolar solvent. The polar headgroups of the surfactants point inward and the hydrocarbon chains point toward to the nonpolar medium.1-3 A common surfactant used to form RMs is sodium 1,4-bis(2-ethylhexyl) sulfosuccinate (AOT) (Scheme 1). The RMs formed with this surfactant can solubilize a large quantity of water in a wide range of nonpolar solvents, reaching values of w0)[H2O]/[AOT] as large as 40-60 depending on the nonpolar solvent. 1-3 RMs can be interesting microreactors for heterogeneous chemistry, as well as templates for nanoparticles and models for biological membranes.4 Some polar organic solvents, having high dielectric constants and very low solubility in hydrocarbon solvents, can also be encapsulated in RMs.5 The most common polar solvents used include formamide (FA), dimethylformamide (DMF), dimethylacetamide (DMA), ethylene glycol (EG), propylene glycol (PG), * To whom correspondence should be addressed. E-mail: mcorrea@ exa.unrc.edu.ar (N.M.C.);
[email protected] (N.E.L.). † Permanent address: Departamento de Quı´mica, Universidad Nacional de Rı´o Cuarto, Agencia Postal # 3, C. P. 5800 Rı´o Cuarto, Argentina.
and glycerol (GY).6-26 Studies have shown that these polar solvents are confined to the nanometer-scale core of the RM aggregates where they behave differently from the bulk solvents as a result of the specific interactions and confined geometries.12,17-22 For example, FT-IR11,12,19,20 and 1H NMR19,20 spectroscopy have shown that GY and EG interact with the AOT surfactant polar head through H-bond interactions that maintain the typical spherical RM structure but break the solvent H-bond structure present in the bulk.7,17,19,20 Thus, even at the highest solvent loading ws ) [solubilizate]/[AOT] between 2 and 4, EG and GY show no evidence for bulklike solvent inside the RMs. In contrast, recent studies have shown that FA inside AOT RMs conserves bulklike intersolvent H-bond interactions.26 Moreover, while RMs formed in the absence of water or other polar solvent possess a hydrodynamics radius of ∼1.5 nm,27 the nonaqueous polar solvent containing RMs swell much more rapidly than those containing water, reaching a size similar to w0 ) 10 (aqueous) with ws ≈ 2 (nonaqueous).13,17 Potential applications of these micellar systems require a clear understanding, on the molecular level, of the mechanism and strength of surfactantpolar solvent interactions and the structure of the polar pseudophase, among others.
10.1021/jp0572636 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/14/2006
What Can You Learn from a Molecular Probe SCHEME 1
Frequently fluorescent probe molecules with sensitivity to specific properties of the systems have been used to learn about RMs. We have used the highly fluorescent coumarin dye, C343 (see Scheme 1), as a probe of solvation dynamics in RMs.12,13,27-29 In bulk nonviscous solvents, dipolar solvent relaxation around the fluorophore in the excited state occurs on a time scale much faster than the fluorescence lifetime. Thus, the peak of the fluorescence spectrum, λem,max, does not depend on the excitation wavelength. However, if the dipolar relaxation of the solvent molecules in the excited state is slow enough that the relaxation time is comparable to or longer than the fluorescence lifetime, then λem,max will shift toward lower energy as the excitation wavelength energy decreases. This effect, known as red-edge excitation shift (REES),30,31 directly monitors the microenvironment and dynamics around a fluorophore in motionally restricted media, such as an organized media like RMs.32-34 We already know that the absorption and emission spectroscopy of C343 depends strongly on its environment.13,35-40 The similarities in the structure of C343 to other probes used in solvation dynamics, such as C153 and C102, has led to its use for aqueous environments where the former probes are insoluble.41 Because the steady-state spectra shift with changing solvent dielectric constant, C343 spectral shifts have been used to measure dipolar relaxation.36,40 Additionally, in acidic solution the C343 absorption and emission spectra peak at shorter wavelengths than they do in basic solution, which has been attributed to the deprotonation of the dye (see Scheme 1).27,42 Several sites on the C343 molecule could accept a proton: the carboxylic acid group, the cyclic ester, and cyclic amine moieties; at low pH, the carboxylic acid moiety is the protonation site.27 While the C343 has been used extensively for solvation dynamics,13,27,35-42 several features are still not well understood. For example, if increasing the polarity of the solvent is responsible for a spectral shift to lower energy, then it is not clear why the absorption band of C343 in FA, an extremely polar solvent,43 peaks at shorter wavelength than it does in
J. Phys. Chem. B, Vol. 110, No. 26, 2006 13051 tetrahydrofuran or why the absorption band in DMF peaks at lower energy than it does in the more polar EG. Moreover, the location of the absorption band in water, arguably one of the most polar solvents, does not follow the trend. Thus, polarity alone cannot account for the solvathochromic behavior of C343; specific interactions must also play a role in the spectroscopy. Here, we report results from a systematic study using absorption and emission spectroscopy and time-resolved fluorescence spectroscopy of C343 in homogeneous media, as well as in RMs formed with water or various polar solvents (EG, FA, DMF, or DMA)/AOT/isooctane. The results explain the observed solvatochromism of C343 so that it can be used to gain insight about polar solvents sequestered in RMs and their interactions with the polar head or the counterions of the AOT surfactant. The low solubility of C343 in hydrocarbon solvents leads it to be located in the RM interface or interior avoiding problems of partitioning that other probes suffer18 and making it possible to investigate the effect of the polar solvent addition, ws. The results show that C343 forms a J-aggregate in nonpolar solvents while, it retains its monomeric form in AOT RMs systems. The solvatochromic behavior of C343 can be explained by taking into consideration specific dye-solvent H-bonding interactions. Moreover, we demonstrate that, C343 forms stable H-bond complexes with solvents with high ability as H-bond acceptors (high β) and low ability as H-bond donors (low R). Inside the RMs, the C343 behavior depends on the polar solvent encapsulated, revealing the different types of interactions between the solvents and AOT. II. Materials and Methods Sodium 1,4-bis (2-ethylhexyl) sulfosuccinate (AOT) (Sigma >99% purity) was used as received and was kept under vacuum over P2O5 to minimize H2O absorption. The absence of acidic impurities was confirmed through the 1-methyl-8-oxyquinolinium betaine (QB) absorption bands.44 Isooctane (2,2,4-trimethylpentane, HPLC grade, Aldrich) was used as received. Ultrapure water was Milli-Q filtered, 18.2 MΩ cm resistivity. Formamide (FA), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), and ethylene glycol (EG) all from Aldrich, HPLC grade, were used without further purification. All other solvents were of the highest grade available (spectral of HPLC grade) and used as received. Coumarin 343 (C343, Exciton) was used without further purification. The absorption spectra were measured using a Varian Cary 500 absorption spectrometer at 25 ( 0.1 °C unless otherwise indicated. A Spex ISA Fluorolog apparatus was employed for the fluorescence measurements. Corrected fluorescence spectra were obtained using the correction file provided by the manufacturer. The path length used in the absorption and emission experiments was 1 cm. Because the spectra do not possess a well-defined spectral shape, λmax was measured by taking the midpoint between the two positions of the spectrum where the absorbance is equal to 0.9 × Amax. The uncertainties in λmax are about 0.1 nm. To apply Kamlet-Taft’s solvatochromic comparison method (KTSCM) to the emission frequency and considering that the maximum frequency of an emission spectrum is not necessarily the maximum of the spectrum on a wavelength scale, a correction was performed.45 Thus, the emission spectrum on a wavelength scale was multiplied by the square of the wavelength and replotted (as F(ν) ) (λ2) × F(λ) versus frequency (ν)) to find the maximum. Fluorescence decay data were measured with the timecorrelated single-photon counting technique (TCSPC, IBH Model 5000F) with a subnanosecond pulsed LED emitting at
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Correa and Levinger
372 or 450 nm. Fluctuations in the pulse and intensity were corrected by making an alternate collection of scattering and sample emissions. The quality of the fits was determined by the reduced χ2.46 The stock solutions of AOT in the hydrocarbon solvent were prepared by mass and volumetric dilution. Samples were sonicated to obtain optically clear solutions. The appropriate amount of stock surfactant solution to obtain a 0.10 M surfactant in the micellar media (for all the experiments) was transferred into a volumetric flask. The appropriate amount of polar solvent or water to obtain the desired ws ) [polar solvent]/[AOT] or w0 ) [H2O]/[surfactant] was added using a calibrated microsyringe, and then, isooctane was added to obtain the final volume. C343 was added in excess to the prepared samples; the samples were periodically shaken manually and sonicated for 1-10 min over 24 h, and then they were filtered through 0.45 µm syringe filters (PTFE, Whatman) to remove any excess dye. From the absorption spectra, we estimate that the concentration of dye in the RM samples was around (1-5) × 10-5 M.27 The C343 salt was prepared following a previously described method.28 Briefly, an aqueous solution of C343 was titrated with sodium hydroxide to pH 7; the excess water was evaporated, and the salt was dried under vacuum over P2O5 before use. III. Results and Discussions III.a. Homogeneous Media. C343 in Nonpolar SolVent. Because C343 is practically insoluble in isooctane, the nonpolar solvent used to prepare the AOT RMs,13 but is slightly soluble in cyclohexane, we used cyclohexane for studies of C343 in a nonpolar solvent. These spectral shapes for C343 in cyclohexane have been reported by our group and by others.37,40 Both absorption and emission spectra of C343 in cyclohexane change dramatically with the dye concentration, as shown in Figure 1A and B. In nonpolar environments, dye probes can display coarse vibrational structure.30 Structure in the spectra published for C343 were assumed to come from the monomeric dye.13 Figure 1A clearly shows that structure seen in the C343 spectrum does not result solely from vibrational structure. As the dye concentration increases, the band located near 425 nm increases at the expense of the band located near 405 nm. In the fluorescence spectrum (Figure 1B), a new band emerges near 460 nm as the dye concentration increases. If the features observed in the C343 absorption spectrum result from vibrational structure, then the relative intensity of the spectral features should remain the same as the concentration varies. The ratio of the intensities of the 405 and 425 nm features is shown as a function of C343 concentration in Figure 1C. Clearly the ratio depends on the concentration indicating that different forms of the dye lead to the observed spectra. Also, the C343 absorbance at 425 nm does not obey the Lambert Beer law over the concentration range used. These results suggest that, because of its very low solubility in cyclohexane, C343 forms aggregates in nonpolar solvents. The monomer absorbs at 405 nm and emits at 435 nm, while the aggregate absorption band peaks at 425 nm and emits at 460 nm. Because its absorption spectrum appears at lower energy relative to the monomer, we conclude C343 forms J-aggregates,47,48 that is, aggregates with dye molecules arranged head-to-tail in a slanted stack.47,48 Emission at lower energy relative to the monomer has also been reported for J-aggregates.49-52 While C343 aggregates have not been reported in bulk solution, the dye molecules can aggregate at interfaces. The absorption spectra of C343 attached to ZrO2 nanoparticles showed a dramatic change with dye concentration.53 Experi-
Figure 1. (A) Normalized absorption spectra of C343 in cyclohexane at 1 × 10-7 and 3 × 10-6 M. (B) Normalized emission spectra of C343 in cyclohexane at 1 × 10-7 and 3 × 10-6 M: λexc ) 406 nm. (C) Ratio of absorbance at 406 and 425 nm for C343 in cyclohexane as a function of [C343].
ments probing protonated C343 at the water/dichloroethane interfaces interpreted red-shifted emission as being the result of the formation of J-aggregates.38,39 The absence of a clear isosbestic point in the absorption spectrum of C343 suggests that the aggregation process may include higher-order aggregates, beyond a simple monomer-dimer equilibrium. We
What Can You Learn from a Molecular Probe
J. Phys. Chem. B, Vol. 110, No. 26, 2006 13053
TABLE 1: Lifetimes for Coumarin 343 (C343) in Cyclohexane at Different C343 Concentrations λexc ) 372 nm
λexc ) 450 nm
λem ) 435 nm
λem ) 470 nm
λem ) 580 nm
[C343] (M)
τ (ns)
τ (ns)
τ1 (ns)
τ2 (ns)
5 × 10-7 3 × 10-6
2.69 2.69
2.69 2.69
2.50
4.90
observed similar absorption and emission spectra for C343 dissolved in benzene and n-heptane, most likely, indicating that C343 forms aggregates in solvents of low polarity and solvents with low R and β values, where its solubility is not high. We also explored C343 aggregation using time-resolved emission spectroscopy. TCSPC measurements were performed for C343 in cyclohexane at two different concentrations, 5 × 10-7 and 3 × 10-6 M, two different excitation wavelengths, and two different emission wavelengths, as given in Table 1. At low concentration, where the monomer dominates the absorption spectrum, the fluorescence decay fits well to a singleexponential decay with a time constant corresponding to the fluorescence lifetime of the monomer. Probing the higher concentration with excitation into the high energy feature also yields a monoexponential with the same time constant found at lower concentration. In contrast, with excitation at 450 nm and detection at 580 nm, the region where the contribution from the aggregate should be higher, the fluorescence decay exhibits a biexponential decay with 10% of the emission decaying with the monomer lifetime and the remaining 90% decaying with a lifetime, τ2 ) 4.90 ( 0.04 ns, nearly twice the monomer lifetime. These results agree with literature reports of aggregate lifetimes that are longer than the monomer51,54 and support the conclusion that the absorption band appearing at 435 nm corresponds to the J-aggregates of C343 in cyclohexane. C343 Anion. C343 can also be found in an anionic form when the carboxy proton dissociates. The steady-state spectrum of the anionic C343 in water peaks at 423.4 nm, while the fluorescence spectrum peaks near 500 nm. The fluorescence lifetime of the C343 anion in water yields a monoexponential decay with a value of 1.8 ns at λem ) 450 and 550 nm. There are various estimates for the pKa of C343: our latest measurement suggests that it is approximately 6.0, and the literature reports a pKa of 7.30 in a 4:1 dioxane-water mixture.55,56 Thus, the acidity of C343 appears to diminish when the probe is located in a less polar environment. Furthermore, the crystal structure57 and theoretical calculation studies58 performed on C343 shows that the carboxylic acid proton can reside in close proximity to the ester carbonyl interacting through H-bonding. These considerations suggest that intramolecular H-bonding in C343 could account for the observed pKa variations and suggest that the H-bond strengthens when the molecule resides in a nonpolar environment. Kamlet-Taft Analysis of C343. Solvent polarity and solutesolvent interactions are two of the most important factors that control the rates of chemical reaction.59,60 The solvent effects on physical or chemical processes are frequently studied by empirical solvent parameters to determine the predominant interactions. One of the most useful approaches for elucidating and quantifying different solute-solvent interactions is the Kamlet-Taft solvatochromic comparison method (KTSCM).61 According to the KTSCM, absorption and emission band frequencies, ν, can be correlated using
ν ) ν0 + sπ* + aR + bβ
(1)
Figure 2. Normalized absorption and emission spectra of C343 in homogeneous media.
TABLE 2: Frequencies of Absorption and Emission Maximaa (×103 cm-1) for C343 in Different Solvents solvents
νabs max
νems max
π*b
Rb
βb
cyclohexane ethylene glycol methanol dimethylacetamide N,N-dimethylformamide tetrahydrofuran benzene formamide dimethyl sulfoxide water n-heptane acetonitrile ethanol propylene glycol
24.64 23.10 22.73 22.68 22.57 22.97 23.92 23.32 22.67 23.15 24.39 23.04 22.99 23.36
22.98 20.79 20.53 20.32 20.28 20.96 22.72 20.70 20.32 20.40 22.97 21.74 20.43 20.66
0 0.92 0.6 0.85 0.88 0.55 0.55 0.97 1 1.09 -0.08 0.66 0.50 0.92
0 0.9 0.98 0 0 0 0 0.71 0 1.17 0 0.19 0.89 0.90
0 0.52 0.66 0.76 0.69 0.62 0.1 0.48 0.76 0.47 0 0.40 0.75 0.52
a
λexc ) 372 nm. b Values taken from refs 67 and 68.
where π* is the solvent polarity/polarizability parameter, R is the H-bond donation ability of the solvent, and β is the H-bond acceptance or electron-pair donation ability to form a coordinated bond. The coefficients s, a, and b measure the relative sensitivity of ν to the indicated solvent property.62 While the KTSCM has been applied to other coumarin dyes,63-66 C343 has not been evaluated. Typical absorption and emission spectra of C343 in homogeneous media are shown in Figure 2. The absorption and emission maxima as well as the solvent parameters, π*, R, and β, used62-67,68 are given in Table 2. Because we have determined that C343 aggregates in nonpolar solvents such as cyclohexane, benzene, and n-heptane, the absorption and emission maxima reported correspond to the C343 monomer. The C343 spectra were used to obtain the KTSCM coefficients s, a, and b, given in Table 3. The confidence level of the regression is 99.5% (t-test). In the correlation of the emission frequency, the R parameter was not statistically significant so it was not included. The results obtained from the C343 absorption spectra reflect interactions of the dye in its ground electronic state. Here, the C343 molecule displays a bathochromic shift with β and π* and a hyspochromic shift with the R parameter. While they share essential structural features with C343, the KTSCM performed on related chromophores Coumarin 153 (C153) and Coumarin 102 (C102) yielded bathochromic shifts for π* and R and little or no correlation with β.65,66 For C343, the b/s ratio is ∼3.03 demonstrating that C343 is three times more sensitive to the H-bond acceptor ability of the solvent than it is to its polarity. This challenges the general interpretation of C343’s solvato-
13054 J. Phys. Chem. B, Vol. 110, No. 26, 2006
Correa and Levinger
TABLE 3: Correlations of C343 Absorption Maxima and Emission Frequency (×103 cm-1) According to Equation 1
νabs νem a
intercept
s
a
b
corrn coeff
na
SDa
24.29 ( 0.06 23.03 ( 0.08
-0.53 ( 0.09 -1.10 ( 0.20
0.21 ( 0.10
-1.61 ( 0.20 -2.78 ( 0.25
0.98 0.99
14 14
0.15 0.13
n is the number of solvents, and SD is the standard deviation.
chromic behavior, which is that it primarily reflects the polarity of the medium.13,27,35,40,41 Instead, the KTSCM parameters (Table 3) reveal that in its ground state C343 is largely sensitive to specific interactions. It is clear from its structure and comparison to the KTSCM for related dyes C153 and C102 that C343 gains its sensitivity to the β parameter through H-bond donation through its carboxylic group. While C343 shares many essential features with C153 and C102, the KTSCM shows it responds to solvents significantly differently from them. C102 and C153 can accept but not donate H-bonds, hence the minuscule correlation with β.64,65 However, the carboxylic group on C343 provides an additional H-bonding site compared to C153 and C102 so we might expect comparatively more stabilization of the ground state. Instead, the hypsochromic shift of a observed for C343 contrasts the bathochromic shifts seen for C153 and C102.65,66 Furthermore, the correlation of C343 with polarity, seen through s, is two to three times smaller than the corresponding parameter for C102 and C153, suggesting that C343 interacts with its surroundings significantly through its carboxylic group rather than the features it shares with C102 and C153. The KTSCM parameters from emission (Table 3) show that in its excited state C343 interacts with its environment differently than in its ground state. The β and π* parameters display a bathochromic shift and the molecule is no longer sensitive to the R parameter. Similar to the results for C102 and C153, the C343 excited state is almost twice as sensitive to the π* and β parameters than it is in its ground state. Upon excitation, the C343 polarity increases significantly most likely because of an increase in the dipole moment.58 The H-bond to the solvent also becomes more facile. These factors combine to overwhelm its capacity as a H-bond acceptor causing the excited-state molecule to lose its sensitivity to the KTSCM R parameter. In contrast, emission of C153 and C102 show substantial increases in sensitivity to both π* and R for the excited state. The fact that C343 is more sensitive to the H-bond acceptor ability of the solvent (β) upon excitation suggests that the intramolecular H-bond between the carboxylic and the ester group (see Scheme 1) weakens in the excited state. In a series of ab initio calculations, Cave et al.58 showed that in the ground-state C343 forms an intramolecular H-bond between the hydrogen of the carboxyl group and the nearby carbonyl group (Scheme 1), while its excited state can exist without the intramolecular H-bond. Fluorescence Lifetimes. The fluorescence decays of C343 in homogeneous media provide additional information about the molecule’s interactions with its environment. Time constants for fluorescence decays monitored both near the emission band maxima and also at a significantly longer wavelength, λem ) 550 nm, are given in Table 4, along with the absorption and emission maxima in the different solvents. All the decays are monoexponential when measured near the wavelength of maximum emission. Previous reports suggest that the C343 emission lifetime varies little in solvents such as ethanol, acetonitrile, and water, and the emission decay traces fit well to a single-exponential decay.38,69 Although the lifetimes are similar in different media, they do increase in highly polar solvents and good H-bond donor solvents such as water and
TABLE 4: Wavelengths of the Absorption and Emission Maximaa and Emission Lifetimesa for Coumarin 343 in Different Solvents λemmax solvents cyclohexane ethylene glycol methanol dimethylacetamide N,N-dimethylformamide tetrahydrofuran benzene formamide dimethyl sulfoxide water n-heptane acetonitrile ethanol propylene glycol a
λabs max λems max (nm) (nm) 405.8 432.9 437.5 440.9 443.0 435.5 418.1 428.8 441.1 430.5 407.5 436.5 434.9 428.0
435.0 481.0 487.0 492.1 493.1 477.2 440.2 482.8 492.1 490.2 434.3 460.1 489.7 484.1
τ (ns)
λem ) 550nm τ1 (ns)
τ2 (ns)
4.09 4.30 4.04 4.01 3.70
4.09 4.20 1.10 (-) 4.02 1.10 (-) 4.02 1.02 (-) 3.70
3.80 3.39 4.60
1.80(-) 3.80 1.50 (-) 3.30 4.61
3.30 4.01 4.10
1.10 (-) 3.25 4.10 4.05
λexc ) 372 nm.
alcohols. In solvents with the largest β and lower R values, we also observe a biexponential decay of the emission at the longer emission wavelength measured (550 nm) where the shorter time component displays a negative preexponential factor. For example, in solvents such as DMF, FA, THF, and DMSO, the C343 fluorescence first rises and then decays, while in alcohols and water, the fluorescence decays monoexponentially. Coumarins, such as C343, have been used extensively as a probe of solvation dynamics in RMs following the molecule’s shifting fluorescence spectrum as a function of time.4,27-29 Solvation dynamics measurements reveal an increasing component to the emission when measured at the longest wavelengths.70 However, in homogeneous solution, solvation dynamics occurs on an ultrafast time scale, much faster than the rise observed here. The C343 emission spectra (Figure 2) in solvents showing a rising component also display a shoulder in frequency around λ ) 530 nm. The negative preexponential factor indicates that before radiative deexcitation occurs, an excited-state process leads to a new emitting state.30,71,72 The biexponential C343 emission in solvents with high β most likely results from specific solvent-fluorophore interactions that also account for its solvatochromism.30,73 Specific interactions can affect the ground state, the excited state or both, and these interactions may weaken or strengthen following excitation.30 The absence of perturbation in the C343 absorption spectrum suggests specific interactions are less important for the fluorophore in its ground state and that the observed emission shifts result from strong interactions between the excited solute and the solvent occurring on the time scale of the excited-state lifetime. Interactions leading to sensitivity in the absorption spectrum with R and β increase upon excitation where the propensity of C343 to donate a H-bond increases as it forms stable complexes. Thus, the low-energy shoulder observed in the C343 emission spectrum in solvents with high β values reflects the molecules involved in H-bonding; the rising component of the fluorescence transient corresponds to formation of the H-bonded complex and may also reflect the solvent
What Can You Learn from a Molecular Probe
J. Phys. Chem. B, Vol. 110, No. 26, 2006 13055 TABLE 5: Fluorescence Lifetimes (τ) of C343 in AOT Reverse Micellar Media as Function of wsa λem ) 450 nm τ (ns)
water
0.0 0.5 1.0 2.0 4.0 20.0
3.06 3.29 3.25 3.50 3.85 3.99
3.10 3.20 3.22 3.49 3.84 4.00
DMF
0.5 1.0 2.0 4.0
3.21 3.31 3.46 4.01
3.20 3.30 0.88(-) 0.90(-)
3.40 3.90
0.5 1.0 2.0 4.0
3.25 3.30 3.50 3.57
3.20 3.30 0.90(-) 0.90(-)
3.43 3.57
0.5 1.0 1.5 2.2
3.70 1.70 (-) 1.80 (-) 1.80 (-)
0.5 1.0 1.5 2.2
3.25 3.25 3.54 3.90
DMA Figure 3. Normalized absorption and emission spectra of C343 in AOT/isooctane RMs containing no polar solvent (ws ) w0 ) 0). Excitation radiation for emission, λexc, is 380 nm.
relaxation that lowers the excited-state energy.30,74,75 In contrast, intramolecular H-bond interactions dominate between the C343 carboxylic acid and ester group in high R solvents. These interactions strengthen in the excited-state precluding the formation of H-bond complex with the solvent found with the solvents with high β and low R. III.b. C343 in AOT Reverse Micelles. Having characterized the spectroscopy of C343 in homogeneous solution, we used it to probe various AOT RMs. We have previously used C343 as a probe of solvation dynamics in RMs.12,13,27-29 C343 makes an effective probe of the RM interfacial region or interior because of its exceptional insolubility in the alkane phase that leads it to reside quantitatively in the RMs, unlike some other molecular probes that partition between pseudophases of the nonaqueous AOT RMs. Studies17,18,20 have shown that the partition constants can differ depending on if the polar solvent encapsulated in the RMs can donate H-bonds. Partitioning of C343 does not appear to change depending on the system.29 Moreover, as a fluorescent probe, C343 can reveal information about interactions from both its ground and excited states. While its partitioning characteristics make it an effective probe of AOT RMs, certain spectral features observed in micellar media remain unclear. For example, while it clearly partitions into the RMs, our previous studies of RMs sequestering nonaqueous polar solvents have not determined the exact C343 location nor the nature of its environment. We used our detailed spectral studies described above to interpret the environment sensed by C343 in a range of AOT RMs. C343 in AOT/Isooctane, w0 ) 0. Figure 3 shows the absorption and emission band of C343 in AOT/ isooctane RMs containing no polar solvent, similar to our previous report with the absorption spectrum, peaking at 405 nm, while the emission maximizes at 448 nm.13,29 Because the absorption of C343 in basic solution (λmaxabs ) 423.4 nm) maximizes at higher energy than the species in acidic bulk water (λmaxabs ) 456 nm),27,36 we previously assumed that interaction of the AOT polar headgroups with C343 led to its deprotonation leaving the anionic dye solubilized inside the RMs.13,27 While this is possible, it does not explain why the absorption maximum of C343 inside RMs at w0 ) 0 does not match the value of the C343 anionic species, which peaks ∼25 nm red of C343 in w0 ) 0 RMs. A comparison of the spectra in Figure 3 to the results for homogeneous solutions (section III.a) reveals that the environment sensed by C343 in “dry” AOT RMs resembles the monomer spectra recorded in cyclohexane rather than that of
λem ) 520 nm
w
solvents
FA
EG
a
τ1 (ns)
τ2 (ns)
3.71 3.74 3.91 3.90 3.20 3.21 0.75(-) 0.76(-)
3.50 3.94
λexc ) 372 nm.
the C343 anion. The data presented here suggest our previous interpretation was in error and that instead C343 resides embedded in the RM interface in its neutral form. Unlike its behavior in cyclohexane, C343 does not aggregate in the RMs, which can be explained through Poissonian statistics. At surfactant concentrations above the critical micelle concentration (cmc), the occupation number is
n)
[dye] [RM]
(2)
where [RM] represents the concentration of RMs
[RM] )
[Surfactant] - cmc Nagg
(3)
with Nagg, the aggregation number, the number of surfactant molecules in the RMs.76 If on average, n < 1, then fewer than one molecule occupies any given RM, and the environment leads to the complete deaggregation of dye aggregates.51,52,77 Here, [AOT] is 0.1 M, [C343] ) 5 × 10-5 M, the cmc of AOT ) 2 × 10-4 M1, and Nagg at w0 ) 0 is around 20,2,27 which leads to n ) 0.01. Thus, finding more than one dye molecule in any RM would be very unlikely and the aggregation process that C343 undergoes in nonpolar environments does not occur in the RMs. The question of the location of C343 in “dry” AOT RMs, w0 ) 0, remains. The emission decay for w0 ) 0 fits nicely to a single exponential at both emission wavelengths studied, 450 and 520 nm, and we see no evidence for a rising component in the fluorescence discussed above, see Table 5. The data presented here show no evidence of a H-bond interaction between the dye and the surfactant either in the ground or excited state; the C343 emission band shown in Figure 3 has no shoulder in the lower-energy region, and there is no negative preexponential factor in the lifetime decay profile in the red side of the emission spectrum (Table 5). The AOT polar headgroup is a very effective H-bond acceptor,78-81 and our
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Figure 4. (A) Normalized absorption spectra of C343 in dimethyl acetamide/AOT/isooctane as function of ws. (B) Normalized absorption spectra of C343 in ethylene glycol/AOT/isooctane as function of ws. (C) Normalized emission spectra of C343 in water/AOT/isooctane as function of w0. The absorption and emission spectra of the C343 salt at w0 ) 5 is shown for comparison. (D) Normalized emission spectra of C343 in formamide/ AOT/isooctane as function of ws. Excitation radiation for emission spectra, λexc, is 380 nm.
results for high-β solvents show C343 can donate H-bonds, especially in its excited state. While sodium counterions exist ion-paired with the AOT sulfonate groups at w0 ) 0, diminishing their H-bond acceptor ability,26,82 we still expect that if C343 resides near the AOT headgroup, it should donate a H-bond. While the C343 w0 ) 0 absorption band matches the value of the monomer in cyclohexane, its emission peaks approximately 13 nm to the red of its emission in cyclohexane. From the results obtained in homogeneous media (Table 3), we know that the C343 excited state is more sensitive to the polarity than the ground state. Thus, the environment of C343 in AOT RMs is more polar than cyclohexane. It seems that in the “empty” RMs, C343 exists as the neutral monomer deeply buried in the hydrophobic part of the RM interface, with the carboxylic end of the molecule pointing toward the polar head of the surfactant. In this nonpolar environment, the C343 intramolecular H-bond is reinforced, and the carboxylic proton is not available to interact with the polar head of AOT. With a labile proton, C343 can undergo deprotonation to its anionic form. To confirm the form of C343 in the RMs at w0 ) 0, protonated or anionic, we explored the spectroscopy and lifetime of the sodium salt of C343. The C343 anion is not incorporated in RMs at w0 ) 0 requiring a value of w0 ) 5 before it dissolves in the microemulsions. Furthermore, the C343 anion spectroscopy differs from the neutral form. Figure 4C
contrasts the absorption and emission spectra of the C343 anion and the neutral form in AOT RMs. It is clear that neither the absorption nor the emission spectra match. Furthermore, the lifetime of the C343 anion found in water is not close to the lifetime that C343 shows in the RMs (Table 5), which confirms that C343 does not deprotonate at the AOT RM interface. C343 in AOT RMs Containing Water or Other Polar SolVents. With the addition of polar solvents to the AOT RMs, the C343 spectra change. Figure 4 shows representative steady-state spectra of C343 as a function of ws in various RM samples. The steady-state absorption spectra in DMA/AOT/isooctane shown in Figure 4A are representative of non-H-bond-donor solvents DMA and DMF, while the absorption spectra of EG/ AOT/isooctane and the emission spectra of water/AOT/isooctane shown in Figure 4B and C, respectively, are representative for the H-bond-donor solvents. We present spectra in FA/AOT/ isooctane RMs separately in Figure 4D because FA maintains its H-bonding inside the RMs,26 in contrast to the H-bond-donor solvents, such as water and polyols,19,20 whose H-bond networks are destroyed upon encapsulation in the RMs. Although we previously studied C343 in water/AOT/isooctane RMs,13,27,29 we have not investigated the very low values of w0 we present here. In all the RMs studied, we never observe an isosbestic point in either absorption or emission spectra. This suggests that the observed shifts do not result from two C343 species in
What Can You Learn from a Molecular Probe
Figure 5. (A) Variation of the C343 absorption maxima as a function of ws or w0 in the different AOT/isooctane systems studied. The values of the absorption maxima in bulk solvents are shown for comparison. (B) Variation of C343 emission maxima as a function of ws or w0 in the different AOT/isooctane systems studied. The values of the emission maximum for C343 in bulk solvents are shown for comparison. Excitation for emission studies, λexc, is 380 nm.
equilibrium either in the ground or in the excited state, such as protonated and deprotonated, or from a partition between distinctly different microenvironments. The spectroscopy of C343 varies with the specific polar solvent encapsulated in the RMs. Data shown in Figure 5A and B, for the C343 absorption and emission maxima, respectively, as a function of ws reveal some interesting results. In every system studied, the initial introduction of the polar solvent causes a blue shift in the absorption spectrum compared to the value at w0 ) 0. As the ws value increases, the band shifts back to longer wavelength. This shows that the intramolecular H-bond present in C343 at ws ) 0, first breaks shifting the absorption band to the blue, then forms a new, stronger H-bond through interaction with the AOT polar head or with the polar solvent depending on the specific system (see below). The magnitude of the shift depends on the specific polar solvent sequestered. When DMF and DMA are encapsulated in the RMs, the redshift is only ∼1 nm compared to w0 ) 0. In contrast, the red shifts for FA and EG are ∼12 and ∼10 nm, respectively. The shift obtained with FA and EG at ws ) 2.2 is even greater than that obtained with water at w0 ) 20. Figure 5A also shows that λabs max of C343 in FA/AOT/isooctane RMs at ws ) 2.2 is very close to its value in bulk solvent. While the C343 absorption spectra display shifts to higher and lower energy, the emission spectra (Figure 5B) only shift to longer wavelength as ws
J. Phys. Chem. B, Vol. 110, No. 26, 2006 13057 increases. Like the results for absorption spectra, the emission maximum for C343 in FA RMs at ws ) 2.2 matches the value of the emission maxima in FA bulk. This is the only solvent showing this kind of behavior of all the systems we have probed, including the largest water-containing RMs. Various interpretations can explain these results. Because the spectroscopy depends on the nature of the solvents, we discuss each class of solvent separately. The non-H-bond-donor solvents, DMF and DMA, have a large affinity for solvating cations which can disrupt their weakly associated structure,83,84 as demonstrated by the preferential solvation of Na+ by DMF in DMF-water mixtures.85-89 IR, Raman, and NMR studies show that ions dissolved in DMF and DMA induce structure in the liquids, suggesting that the molecules’ oxygen atoms interact with cations, leaving the nitrogen atoms to interact with anions through electrostatic interactions.90-93 This view is bolstered by the resonance structures these molecules display.26,90 In AOT RMs, Na+ counterions destroy the DMF and DMA structure, and it appears that the intermolecular interactions between DMF and DMA inside the RMs are so weak that these interactions are broken inside the polar core of the aggregate.22 Thus intramicellar DMF and DMA should preferentially solvate the Na+ counterions leaving the sulfonate ready to accept a H-bond as reflected by the minimal shifting seen for the C343 absorption spectra. While C343 absorption spectra show little variation with ws, the emission spectra shift to significantly longer wavelength as ws increases. The results from the emission spectra show an increase in the polarity and hydrogen-bond-acceptor ability of the RM interface as ws increases, and even at ws ) 4, the emission band is red-shifted compared to the emission band of C343 in water-containing RMs. Likewise, the emission displays a rising component at long wavelengths (Table 5) for higher ws values. Taken together, these results suggest that the C343 interacts with the “naked” sulfonate group of the AOT molecule, forming an intermolecular H-bond between the dye and the surfactant. Figure 6A schematically depicts the interactions of C343 with DMF inside the RM systems. While it shares structural similarities with DMF and DMA, FA maintains its H-bonding bulk structure inside the RMs.26 In FA/AOT/isooctane RMs, the spectral maxima of the C343 absorption and emission both approach the bulk value (Figure 5), and its emission lifetime includes a rising component, even at the maximum emission wavelength (Table 5). The strong dependence of the spectra on ws indicates that C343 interacts with intramicellar FA rather than the AOT sulfonate group. We believe this interaction arises largely through H-bonding from the resonance structure of FA (Figure 6B) that leaves the FA oxygen negatively charged and yields a stable complex. Because its spectroscopy resembles that in bulk solution, the C343-FA H-bonding interaction must be stronger than the C343-AOT sulfonate interaction that occurs when DMF or DMA are sequestered inside the RMs. It appears that FA sequestered inside the RMs forms a polar pool with properties similar to those found in bulk FA but with a major contribution of the electrostatic resonance form, as shown in Figure 6B.26 The situation differs for H-bond-donor solvents, EG and water, sequestered inside the AOT RMs. EG and water interact similarly with AOT, namely, by solvating the surfactant polar headgroup mainly via formation of H-bonds with the oxygen atoms of the sulfonate group, which breaks the H-bond structure of the solvents.1,19,20 EG interacts with the AOT polar head more strongly than water, penetrating into the oil side of the interface.16,20 EG containing AOT RMs display a more fluid interface and a lower threshold for phase separation relative to
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Figure 6. Schematic representation of C343 in AOT/isooctane reverse micelles containing (A) dimethyl formamide, (B) formamide, (C) ethylene glycol, and (D) water.
RMs encapsulating water.16,20,94 The spectroscopy of C343 in EG/AOT/isooctane RMs shows a pronounced red shift with increasing ws, Figure 5, and a rising component to the emission at longer wavelength, Table 5, even though the bulk solvent shows no rising component (Table 4). The strong H-bonding interaction between EG and the polar head of the AOT breaks the H-bond interaction between the EG molecules yielding EG with higher nucleophilicity and lower electrophilicity when sequestered inside the RMs.95 Thus, C343 can donate H-bonds to the intramicellar EG, as shown in Figure 6, unlike its behavior toward the bulk solvent.
Water is the most common polar solvent sequestered in RMs,1-3 and its behavior differs from the other solvents discussed above. In water-containing RMs, the C343 steadystate absorption and emission spectra shift to lower energy as w0 increases. As we have observed previously,29 even with substantial water loading, w0 ) 20, the C343 spectra maximize at a wavelength removed from those in bulk water suggesting that the dye molecule does not sense a bulk water environment. Interestingly, the red shift of the emission band observed in this system is quite small, significantly smaller than any of the other polar solvents encapsulated (Figure 5B). Moreover, Table
What Can You Learn from a Molecular Probe
Figure 7. Red-edge excitation spectra, REES, ∆λem (λem (exc 440 nm) λem (exc 380 nm)) for C343 in the different AOT/isooctane RM systems studied, as a function of ws or w0.
5 shows that at all w0 values studied here and at both emission wavelengths studied, we measured single-exponential emission decays. Others have measured a rising component to the C343 emission when probed in water/AOT/near-critical propane RMs.96 Our results imply that the intramolecular H-bond found in w0 ) 0 RMs is preserved when water is added to the AOT/ isooctane solution, both for the ground and for the excited-state C343 molecule. This reflects the fact that water interacts through H-bonding interactions with the AOT polar head1,2 but does not penetrate the oil side of the RM interface where C343 resides (Figure 6D). Thus, while water should be a good H-bond donor and acceptor, in the RMs, it neither accepts nor donates H-bonds to C343. This suggests that C343 is located away from the intramicellar water and AOT headgroup, where there are no effective H-bonding sites. The red shift of the C343 absorption and emission bands and the increase in the fluorescence lifetime do point toward the increasing polarity of the interface upon the addition of water.44,51 However, there is no evidence for the formation of H-bonding interactions between the dye and its environment when w0 is small, less than 10. C343 appears to move toward the interior water phase for w0 > 10. Red-Edge Excitation Shift Studies (REES). When solvent relaxation occurs on a time scale comparable to or longer than the fluorescence lifetime, the slow excited-state relaxation leads the peak of the emission spectrum to shift to longer wavelength when excited at a longer wavelength.30,31 This effect, known as the red-edge excitation shift (REES), reflects slow solvent motion or motional restriction of the chromophores.32-34 Our results for C343 suggest that it resides buried in the RM interface and thus prompted us to explore it using REES. We characterized the REES C343 by marking the difference in the emission peak maximum when excited at 440 and 380 nm
∆λem ) λem(excitation ) 440 nm) - λem(excitation ) 380 nm). (4) Figure 7 shows the plots of ∆λem as a function of ws for the RMs studied here. At ws ) w0 ) 0, ∆λem ≈ 13 nm and reflects the motionally constrained environment of C343.30,31 In the RMs with polar solvents, the REES depend on the specific polar solvent. For EG and the non-H-bonding solvents, DMF and DMA, the REES first increases with the addition of solvent, until ws ≈ 1, then decreases. We observe the largest REES for
J. Phys. Chem. B, Vol. 110, No. 26, 2006 13059 C343 in EG/AOT/isooctane RMs at small ws where the interactions between EG and the AOT polar head are the strongest and the C343 molecule senses the most motionally restricted medium. ∆λem drops quickly for EG suggesting that the dye’s environment becomes significantly more fluid. In contrast, the REES for DMF- and DMA-containing RMs leave C343 confined through its H-bonds to the AOT sulfonate group. This behavior differs from our recent results using acridine orange base as molecular probe.20 Acridine orange base senses a less rigid environment in DMF/AOT RMs than it does for EG sequestered inside the RM systems.20 These differences reflect differing dye locations for each probe in the RMs; acridine orange base sits in the less rigid region formed by the tails of the surfactant and the oil pseudophase, while C343 resides in the oil side of the interface but close to the AOT sulfonate group. In AOT RMs containing FA, the C343 REES value is significantly smaller than the value for w0 ) 0, and it decreases as ws increases. This supports our interpretation that FA moves reasonably freely in the intramicellar pool and the C343 molecule finds itself solvated by the FA. The REES value of ∼2 nm shows that the dye does not exist in a rigid or motionally restricted medium. For the water/AOT/isooctane RMs, the REES value is practically the same as the value obtained at w0 ) 0 until a value of w0 ) 2. For larger RMs, the REES value decreases. Satoh et al. have recently reported a REES for water/AOT/ isooctane RMs with w0 ) 30.97 Our REES results indicate that the C343 moves from its hydrophobic location in w0 ) 0 RMs only when there are enough water molecules at the interface to increase the polarity.1,2 Because the REES value for C343 in DMA and DMF containing RMs at ws ) 4 is lower than it is for the corresponding water RM, C343 most likely exists in a more polar and more H-bond-accepting environment, perhaps interacting through H-bonding interactions with the AOT sulfonate group when DMF and DMA are sequestered inside the RM systems (see Figure 5B). IV. Conclusion We have performed a detailed study of the spectroscopy of coumarin 343, a common molecular probe in bulk homogeneous solution and in reverse micelles encapsulating a range of nonaqueous polar solvents. The solvatochromic behavior of the dye depends not only on the polarity of the media, but on other specific solvent properties. A Kamlet-Taft analysis shows that the C343 absorption spectrum shifts to longer wavelength with solvent polarity-polarizability (π*) and H-bond-acceptor (β) ability and displays a blue-shift with the increasing hydrogendonor ability (R) of the medium. In its ground state, C343 appears almost three times more sensitive to the β parameter than to the π* polarity parameter demonstrating its dependence on specific interactions. In the excited state, the sensitivity to π* and β increases, while it loses all sensitivity to R. C343 forms a stable H-bond complex with solvents with high H-bondacceptor ability (high β) and low H-bond-donor character (low R). Spectroscopy in cyclohexane shows C343 forms J-aggregates in nonpolar solvents. Dissolved in the AOT RM systems at low concentration, C343 exists in its monomeric form. Furthermore, when introduced to the RMs in its protonated form, C343 remains protonated driving it to reside in the interface rather than in the water pool. In RMs, the dye’s solvathochromic behavior depends on the specific polar solvent encapsulated, revealing the different interactions that the solvents have with the AOT. EG and water
13060 J. Phys. Chem. B, Vol. 110, No. 26, 2006 interact with AOT through H-bonding with the sulfonate group destroying their bulk H-bonded structures. While water remains well segregated from the nonpolar regions, EG penetrates into the oil side of the interface and forms H-bonding interactions with the C343 carboxylic acid group. With DMF and DMA, the strong interactions with AOT sodium counterions destroy their bulk structure. FA also interacts with the Na+ counterions but maintains its H-bond network present in bulk solvent. Surprisingly, FA appears to be the only polar solvent other than water that maintains bulk H-bonding structure with macroscopic properties similar to the bulk. Moreover, it is also possible to detect the H-bond interactions between C343 and the AOT sulfonate or the polar solvents depending on the specific system. In aqueous AOT RMs, C343 neither interacts with the sulfonate group nor the water, perhaps because of the intramolecular H-bonding that the dye forms. The work presented here shows the wealth of information available from relatively mundane experiments that make it possible to understand the complex environments present in RMs. We plan to utilize these results to help interpret time-resolved fluorescence Stokes shift measurements using C343 as a probe. Acknowledgment. This material is based on work supported by the National Science Foundation under Grant 0415260. N.M.C. holds a research position at CONICET, and thanks to the Fulbright Commission for the Research Award obtained in support of travel to Colorado State University, Fort Collins, CO. References and Notes (1) Silber, J. J.; Biasutti, M. A.; Abuin, E.; Lissi, E. AdV. Colloid Interface Sci. 1999, 82, 189. (2) De, T. K.; Maitra, A. AdV. Colloid Interface Sci. 1995, 59, 95. (3) Moulik, S. P.; Paul, B. K. AdV. Colloid Interface Sci. 1998, 78, 99. (4) Hazra, P.; Chakrabarty, D.; Sarkar, N. Langmuir 2002, 18, 7872. (5) Martino, A.; Kaler, E. W. J. Phys. Chem. 1990, 94, 1627. (6) Ray, S.; Moulik, S. P. Langmuir 1994, 10, 2511. (7) Fletcher, P. D. I.; Galal, M. F.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1 1984, 80, 3307. (8) Fletcher, P. D. I.; Grice, D. D.; Haswell, S. J. Phys. Chem. Chem. Phys. 2001, 3, 1067. (9) Friberg, S. E.; Liang, P. Colloid Polym. Sci. 1986, 264, 449. (10) Rico, I.; Lattes, A. J. Colloid Interface Sci. 1984, 102, 285. (11) Arcoleo, V.; Aliotta, F.; Goffredi, M.; La Manna, G.; Turco Liveri, V. Mater. Sci. Eng. C 1997, 5, 47. (12) Riter, R. E.; Undiks, E. P.; Kimmel, J. R.; Levinger, N. E. J. Phys. Chem B 1998, 102, 7931 and references therein. (13) Riter, E. R.; Kimmel, J. R.; Undiks, E. P.; Levinger, N. E. J. Phys. Chem. B 1997, 101, 8292. (14) Mathew, C.; Saidi, Z.; Peyrelasse, J.; Boned, C. Phys. ReV. A 1991, 43, 873. (15) Lopez-Cornejo, P.; Costa, S. M. B. Langmuir 1998, 14, 2042. (16) Laia, C. A. T.; Lopez-Cornejo, P.; Costa, S. M. B.; d’Oliveira, J.; Martinho, J. M. G. Langmuir 1998, 14, 3531 (17) Falcone, R. D.; Correa, N. M.; Biasutti, M. A.; Silber, J. J. Langmuir 2000, 16, 3070. (18) Silber, J. J.; Falcone, R. D.; Correa, N. M.; Biasutti, M. A.; Abuin, E.; Lissi, E.; Campodonico, P. Langmuir 2003, 19, 2067. (19) Novaki, L. P.; Correa, N. M.; Silber, J. J.; El Seoud, O. A. Langmuir 2000, 16, 5573. (20) Falcone, R. D.; Correa, N. M.; Biasutti, M. A.; Silber, J. J. J. Colloid Interface Sci. 2006, 296, 356. (21) El Seoud, O. A.; Correa, N. M.; Novaki, L. P. Langmuir 2001, 17, 1847. (22) Shirota, H.; Segawa, H. Langmuir 2004, 20, 329. (23) Raju, B. B.; Costa, S. M. B. Spectrochim. Acta, Part A 2000, 56, 1703. (24) Laia, C. A. T.; Brown, W.; Almgrem, M.; Costa, S. M. B. Langmuir 2000, 16, 8763. (25) Laia, C. A. T.; Costa, S. M. B. Langmuir 2002, 18, 1494. (26) Correa, N. M.; Pires, P. A. R.; Silber, J. J.; El Seoud, O. J. Phys. Chem. B. 2005, 109, 21209. (27) Riter, R. E.; Undiks, E. P.; Levinger, N. E. J. Am. Chem. Soc. 1998, 120, 6062.
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