Langmuir 2006, 22, 9175-9180
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Effect of Temperature and Surfactant Structure on the Microviscosity at the Micelle-Water Interface: Isomeric Alkylbenzenesulfonates Used As Their Own Probe M. Aoudia*,† and M. A. J. Rodgers‡ Department of Chemistry, College of Science, Sultan Qaboos UniVersity, P.O. Box 36, Al-Khodh, Muscat, Sultanate of Oman and Center for Photochemical Science, Department of Chemistry, Bowling Green State UniVersity, Bowling Green, Ohio 43403 ReceiVed May 8, 2006. In Final Form: August 16, 2006 We investigated the effect of temperature and surfactant structure on the microviscosity in aqueous micellar solutions formed by isomeric hexadecylbenzenesulfonates (xφC16, where x ) 4-6 and indicates the position of the benzene ring [φ] along the alkyl chain) by fluorescence polarization and excimer emission spectroscopy. For a given isomer, the degree of polarization (p) was found to decrease with increasing temperature, with no evidence for changes in micellar structure. ηint/τ ratios, where ηint is the microviscosity of the benzene environment in micelles and τ its natural lifetime, were derived from fluorescence polarization measurements and showed a similar temperature behavior to that observed with the degree of polarization, suggesting that a thermal effect is the determinant factor in the variation of ηint. Interestingly, the microviscosity around the benzene ring was found to depend on the isomer structure in the entire range of temperatures investigated (8-60 °C) and is mainly determined by the orientation of the surfactant at the micelle-water interface in which the short alkyl chain is preferentially located at the interface and the long alkyl chain in the micellar core. This micelle conformation was found to prevail in the entire range of temperatures. In contrast to the dependence of p with temperature, excimer to monomer maximum emission ratios (IE/IM) were found to increase with increasing temperature, showing that when IE/IM is high (strong excimer emission), the degree of polarization is low (low microviscosity) and vice versa. Thus, the two independent measurements (IE/IM and p) yield the same information, namely, that the benzene moiety in all xφC16 aqueous micelles resists both translational and rotational diffusion in a similar manner in the entire range of temperatures investigated (∼ 8-60 °C).
Introduction Micellar dispersions have received considerable attention because of their intrinsic interest as microheterogeneous systems and their wide use in many technological areas such as enhanced oil recovery and surfactant-enhanced subsurface remediation.1-3 Among the different techniques aimed at investigating aqueous micellar properties, the fluorescent probe technique has been widely used due to the fact that excited states of fluorescent molecules in general have sufficiently long lifetime to interact with their immediate environment prior to decay. This interaction between fluorescent probe and environment has been largely used to elucidate micellar properties such as size, shape, aggregation number, degree of hydration, and microviscosity.4-8 In particular, much attention was devoted to investigation of the effect of temperature on the microviscosities of micelles from the fluorescence emission properties of extrinsic organic probes. Thus, microviscosity of aqueous micelles differing by the nature of the headgroup, the carbon number of the surfactant alkyl * To whom correspondence should be addressed. Phone: 968 24142374. Fax: 968-24413415. E-mail:
[email protected]. † Sultan Qaboos University. ‡ Bowling Green State University. (1) Lange, K. R. In Surfactants; Lange, K. R., Ed.; CarlHabserVerlag: Muenchen, 1999; p144. (2) Islam, M. R. Energy Sources. 1999, 21 (1-2), 97. (3) Shiao, S. Y.; Chabra, V.; Patist, A.; Free, M.; Huibers, P. D. T.; Gregory, A.; Patel S.; Shah, D. O. AdV. Colloid Interface Sci. 1999, 74, 1. (4) Inoue, Y.; Jiang, P.; Tsukada, E.; Wada, T.; Shimizu, H.; Tai, A.; Ishikawa, M. J. Am. Chem. Soc. 2002, 124 (24), 6942. (5) Benrraou, M.; Bales, B. L.; Zana, R. J. Phys. Chem. B 2003,107 (48), 13432. (6) Dutt, G. B. J. Phys. Chem. B 2002, 106 (29), 7398. (7) Kim, J. H.-H.; Domach, M. M.; Tilton, R. D. Langmuir 2000, 16 (26), 10037. (8) Robson, R. J.; Dennis, E. A. J. Phys. Chem. 1997, 8 (11), 1075-1078.
chain, the nature of the counterion, and the chemical structure of the surfactant was studied by fluorescence probing, using the viscosity-sensitive monomer-excimer emission of 1,3-dipyrenylpropane (P3P) in the range 15-45 °C.9 The results showed the importance of the packing of the surfactant alkyl chain in determining the micelle microviscosity. In addition, differences observed between anionic micelles on one hand and cationic, nonionic, and zwitterionic micelles on the other was attributed to the difference in solubilization locus of the fluorescent probe within the micelle entity.9 Microviscosities of anionic (sodium dodecyl sulfate) and cationic (hexadecyltrimethylammonium bromide and chloride) micelles were determined from measurement of intramolecular excimer-monomer emission intensities, and correlation with literature data showed that uncertainties in the micellar distribution of the fluorescent probe molecules may make intermolecular dynamics unsuitable for deriving microviscosities of micelles.10 In another study, the effect of temperature on the fluorescence anisotropy decay dynamics of Coumarin153 dye in Triton X-100 and Brij-35 micellar solutions indicated that this effect in Brij-35 is mainly due to the thermal effect on the microviscosity in the micellar phase.11 On the other hand, in the case of Triton X-100, the results indicated that, along with the thermal effect, an additional effect is observed due to the increased size and hydration of the micelle with temperature with the consequence being that the fluorescence anisotropy decay in Triton X-100 is more sensitive to temperature than in Brij-35. Fluorescence depolarization dynamics of different organic fluorescent dye probes (Nile red, cresyl violet, DODCI, rhodamine (9) Zana, R. J. Phys. Chem. B 1999, 103 (43), 9117. (10) Yekta, A. J. Am. Chem. Soc. 1979, 101 (3), 772. (11) Kumbhakar, M.; Mukherjee, T.; Pal, H. Photochem. Photobiol. 2005, 81, 588.
10.1021/la061280k CCC: $33.50 © 2006 American Chemical Society Published on Web 09/20/2006
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B, and rhodamine DPPE) were studied in cationic, anionic, and neutral micelles by picosecond time-resolved single-photoncounting technique.12 In this study, the concept of microviscosity in the micelles was critically discussed in terms of rotational and translational diffusion coefficients and their temperature dependence. Micellization of Pluronic F-68 ((EO)78(PO)30(EO78)) was investigated by fluorescence intensity, polarization, and lifetime of octadecyl rhodamine B as function of temperature.13 From fluorescence anisotropy, a drastic increase in the microviscosity was observed with increasing temperature, suggesting a conformational change of the micelle from a loosely coiled aggregation to a compact structure. Thus, in most studies extrinsic probes have been used even though it is clear that a probe having a relatively considerable molecular size is likely to cause significant perturbation in the region being explored. In a recent study,14 we showed that peptide critical concentrations in buffered aqueous solutions derived from the use of the peptide as its own probe were higher (175 µM) than those derived from the extrinsic probe approach (125 µM) using metalloporphyrin as the extrinsic probe. This difference was attributed to the perturbation introduced by the association of extrinsic molecules (porphyrin) with peptide aggregates. Further complications associated with the use of an extrinsic approach would arise in temperature-dependent investigation of microviscosity in micelles. This is because a change in the temperature may introduce significant changes in micellar properties such as critical micelle concentration, aggregation number, degree of hydration, size, and shape. Consequently, the thermal effect on the microviscosity in the micelle may not be the dominant factor, and a significant increase in the microviscosity with increasing temperature has been reported.13 Furthermore, one also needs to critically consider the effect temperature on the exact locus of solubilization of the extrinsic probe. Temperature changes surely affect the location of the extrinsic probe within the micelle, with the consequence being that the measured microviscosity at different temperatures will be reflecting different probe solubilization sites. Kumbhakar et al.11 showed that Coumarin-153 indeed undergoes a relative migration toward Triton X-100 micellar core to avoid the increased hydration in the micellar palisade as the temperature is increased. As a result, fluorescence probe technique is suitable for micelle systems that do not exhibit significant structural changes with concomitant changes in the locus of solubilization of the probe molecule within the micelle aggregates as the temperature is raised. Such requirements are nearly impossible to fulfill with fluorescent extrinsic probes. For some micellar systems, it is possible to overcome the above disadvantages associated with an extrinsic fluorescent probe using a surfactant molecule having an aromatic (e.g., phenyl) residue. This offers a suitable optically absorbing and fluorescent moiety which thereby serves as an intrinsic probe. In this unique situation, the location of the light-absorbing and -emitting site is fixed, removing the uncertainty on the exact location of the solubilization site as well as the inevitable change in solubilization site with change in temperature. Thus, in the present work, a series of isomeric hexadecylbenzenesulfonates xφC16, where x ) 4-6, which indicates the position of the benzene ring (φ) along the alkyl chain, were used as surfactants. In previous studies,15,16 we clearly demonstrated that the benzene ring is (12) Maiti, N. K.; Krishna, M. M. G.; Britto, P. J.; Periasamy, N. J. Phys. Chem. B 1997, 101 (51), 11051. (13) Nakashima, K.; Anzai, T.; Fujimoto, Y. Langmuir 1994, 10 (3), 685. (14) Aoudia, M.; Rodgers, M. A. J. Langmuir 2005, 21, 10355. (15) Aoudia, M.; Rodgers, M. A. J. Am. Chem. Soc. 1979, 101, 6777. (16) Aoudia, M.; Rodgers, M. A. J.; Wade, H. J. Colloid Interface Sci. 1984, 101 (2), 472.
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being solubilized at the micelle-water interface and therefore could reflect changes in the microviscosity at the micelle-water interface (ηint) upon changing the physicochemical conditions of the aqueous surfactant system (temperature, ionic strength, pH, additives). In this work, we amplify our earlier study16 carried out at room temperature by investigating the effect of temperature on the microviscosity in xφC16 micelles from fluorescence polarization and excimer-monomer emission of the surfactant used as its own probe. Experimental Section Materials and Procedures. Chemically and isomerically pure sodium hexadecylbenzenesulfonates were synthesized17 so that the substituted benzene nucleus becomes part of the surfactant molecule as shown, where i + j ) 15. Throughout the text the surfactants are
designed as xφC16, where x is defined as the isomer number and indicates the position of the benzene ring (φ) along the alkyl chain. Excimer-emission spectra and fluorescence polarization measurements were made with a modular spectrofluorimeter fully described elsewhere.16 Aerated solutions were used and made with doubledistilled water at 2 × 10-3 M surfactant, at which concentration all were above the CMC.18
Results and Discussion Fluorescence Polarization. The fluorescence polarization (p) and the molecular anisotropy (r) are defined as
p ) (I| - I⊥)/(I| + I⊥) r ) (I| - I⊥)/(I| + 2 I⊥)
(1)
where I| and I⊥ are the fluorescence intensities observed through polarizers oriented parallel and perpendicular to the plane of polarization of the excitation beam, respectively. These quantities are related by19
r0/r ) [(1/p) - (1/3)]/[(1/p0) - (1/3)] ) 1 + 6Rτ
(2)
where r0 and p0 are the limiting values of r and p when the fluorescent molecules maintain their orientation during excitation and emission. R is the rate of rotation of a sphere under Brownian forces, and τ is the natural lifetime of the excited state. Using the Einstein relationship, R ) (kT/6ηV), where k is the Boltzman constant, T is the absolute temperature, η is the viscosity in the vicinity of the rotating sphere, eq 2 can be rewritten as
r0/r ) [(1/p) - (1/3)]/[(1/p0) - (1/3)] ) 1 + (kT/ηV)
(3)
Thus, eq 3 can be used to derive the microviscosity of the medium surrounding the fluorescent probe given a reliable measurement of p0, V, and τ. In any case, relative measurements of r0/r can be used to obtain relative microviscosity changes under the (17) Doe, P. H.; El-Emary, M.; Wade, W. H. J. Am. Oil. Chem. Soc. 1977, 54, 150. (18) Aoudia, M.; Wade, W. H.; Rodgers, M. A. J. J. Colloid Interface Sci. 1991, 145 (2), 493. (19) Perrin, F. J. Phys. Radium 1926, 7, 930.
MicroViscosity at the Micelle-Water Interface
Figure 1. Variation of the degree of polarization with temperature at 2 × 10-3 M surfactant concentration in water: λexc ) 337 nm, λem ) 380 nm, heating curve (b), and cooling curve (O).
influence of changing conditions such us temperature and surfactant molecular structure. The degree of polarization (p) was measured for 4φC16, 5φC16, and 6φC16 isomers at different temperatures (Figure 1). As seen from this Figure, p was found to decrease with increasing temperature in the range 8-60 °C. This partial polarization of the fluorescence emitted from the surfactant intrinsic probe (benzene moiety) reports on the viscous resistance opposing molecular rotational diffusion of the probe. In previous studies,15,16 we showed evidence for the solubilization of the benzene ring at the micelle-water interface in xφC16 micellar systems. Therefore, the effect shown in Figure 1 is probably reflecting a decrease in the microviscosity at the micelle surface (ηint) with temperature, suggesting that a thermal effect is the dominant factor on the change of ηint. In contrast, a number of studies based on extrinsic probe fluorescence properties reported increases of the microviscosity in micelles with increasing temperature.13 This effect was attributed to temperature-induced changes in micellar properties such as size, shape, and degree of hydration, with the consequence being that the fluorescence properties of the probe may change for other reasons as well due to a change in the probe environment microviscosity. For instance, it is well documented that higher temperatures usually result in a small decrease in aggregation number for ionic surfactants, presumably because of the increase in the cross-sectional area a0 occupied by the hydrophilic group at the micelle-solution interface due to thermal agitation,20 thereby giving more freedom to the benzene ring to rotate. Consequently, the degree of polarization can decrease with increasing temperature as a result of micellar conformational change from a somehow compact structure to a more loosely one. Our results displayed in Figure 1 clearly show no abrupt change in the variation of the degree of polarization with temperature, suggesting that no micelle structural alteration is occurring in this range of temperatures. Furthermore, measurements of the degree of polarization by first heating the surfactant system and then cooling the same surfactant system back to lower temperatures (Figure 1) reflect a reversible heatingcooling process. This observed lack of hysteresis also might suggest that xφC16 micelles are not significantly perturbed by temperature change, as expected with ionic surfactants. However, one should emphasize that even in the absence of micelle structural (20) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; John Wiley and Sons: New York, 1989.
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Figure 2. Variation of (ηint/τ) with temperature at 2 × 10-3 M surfactant concentration in water: λexc ) 337 nm, λem ) 380 nm, heating curve (b), and cooling curve (O).
alterations, use of an extrinsic probe to monitor the effect of temperature on the microviscosity can be misleading due to the uncertainty in the exact locus of solubilization of the fluorescent probe in the micelles as the temperature is varied. Such a change in the probe locus of solubilization with temperature was reported for Coumarin-153 in Triton X-100 micelles in which relative migration of the extrinsic probe from Triton X-100 palisade layer toward the micellar core occurred in order to avoid the increased hydration in the micellar palisade as the temperature is increased.11 As a result, the measured microviscosity of Coumarin-153 at different temperatures is in fact reflecting different probe solubilization sites within Triton ×100 micelles. On the other hand, in the intrinsic probe approach used in our investigation, the location of the fluorescent probe, i.e., the surfactant benzene moiety, is fixed at the micelle-solution interface and remains unchanged as the temperature is increased. In all xφC16 micelles, the fluorescence polarization change is therefore associated with a true change in the viscosity surrounding the benzene ring at the micelle-water interface. As suggested by eq 3, one can also derive the microviscosity of the medium around the probe molecule if reliable measurements of po, V, and τ were obtained. The limiting polarization (p0), a function of exciting wavelength, has been measured in glycerol at - 40 °C and was found to be 0.33 at 337 nm.15 The volume was calculated assuming that the probe rotates about the bond joining the phenyl group to the alkyl chain and estimated the volume of the rotor to be identical with the volume of an equivalent rotating sphere of radius 1.40 Å (carbon-carbon length in benzene). In addition, the natural lifetime τ was assumed to be independent of the surfactant structure for the xφC16 series investigated in this study, and the ratios ηint/τ were derived at different temperatures and reported in Figure 2. Clearly, the shapes of the variation of the degree of polarization (p) and the ratio ηint/τ are related in a similar manner. In the absence of micelle structural changes, this behavior is expected since polarization data reports on the viscous resistance opposing molecular rotational diffusion of the benzene moiety at the micelle-water interface. Therefore, the higher the degree of polarization p, the higher the ratio ηint/τ should be. To the best of our knowledge, this is the first instance where direct measurements of temperature-induced microviscosity changes in micelles are reported. More specifically and assuming that the fluorescence lifetime τ is independent of isomer structure, our results show a direct determination of the microfluidity in the micelle-solvent interface (ηint) from the variation of the degree
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of polarization of the fluorescence emitted by the intrinsic fluorescent probe (benzene moiety) with temperature. As the temperature is increased, the ratio ηint/τ decreases, suggesting again that the effect of temperature on the microviscosity in the vicinity of the fixed locus of solubilization (micelle-water interface) of the benzene moiety in xφC16 micelles is indeed largely dominated by thermal effects. Clearly, use of intrinsic fluorescent has a significant advantage over the extrinsic probe approach. The viscosity in micelles determined by various extrinsic fluorescent probes depends on the probe and method used.21,22 In particular, use of an extrinsic probe to monitor microviscosity changes in micelles with temperature is seriously limited by the fact that fluorescence properties may not always give a true value of the viscosity surrounding a fixed site of the micelle but corresponds to the microviscosity felt by the probe. As a result, use of an extrinsic probe for viscosity determination is more suitable for estimation of the relative variation in the microfluidity than for the purpose of obtaining the absolute viscosity. At this juncture, it is interesting to turn our attention to the variation of the degree of polarization p and the ratio ηint/τ with isomer structure. At room temperature (25.0 ( 0.2 °C), the microviscosity in the vicinity of the fluorescent residue in aqueous xφC16 micelles was found to vary considerably with isomer structure.16 More specifically, it was shown that the degree of polarization depends only slightly on the surfactant long alkyl chain and more strongly on the surfactant short alkyl chain. This was taken as an indication that the p-sulfonate group, along with the benzene moiety, in all cases lies at the periphery of the micelles at the water interface and that the long alkyl chain is extended toward the micellar core and largely determines the micellar radius. In this conformation the long alkyl chain has little influence on the benzene moiety and will have little retarding effect upon its rotation. On the other hand, the short alkyl chain is most likely located close to the interfacial region where it contributes to the microenvironment of the benzene moiety and can therefore have a retarding effect on its rotation. This isomer effect observed at room temperature was further investigated at different temperatures. Thus, variation of the degree of polarization p with isomer structure was measured in the range 8-60 °C (Figure 3). The ratios ηint/τ were derived for each isomer, and the results are displayed in Figure 4. Interestingly, both figures show similar variation of p and ηint/τ at all temperatures, indicating that the degree of polarization p and the ratio ηint/τ decrease with isomer number. Why the microviscosity in the vicinity of the fluorescent residue in micellar aggregates should vary in such manner with isomer number in the entire range of temperature investigated (8-60 °C) is not readily obvious. One possibility is that at a fixed temperature the microviscosity at the xφC16-water interface is mainly determined by the configuration of the two alkyl chains in the micelle. Along the series 4φC16-6φC16, the longer chain decreases from 12 to 10 carbon atoms whereas the short alkyl chain increases from 3 to 5 carbon atoms. Thus, as the micelles become smaller in radius, more room has to be found for the increasingly longer short chains. To accommodate this tendency toward crowding, it is somewhat plausible that the surfactant headgroups separate to greater distances, opening up the micellar structure and giving more freedom to the benzene moieties to rotate. Consequently, the degree of polarization p and ηint/τ decrease as show in Figures 3 and 4, respectively. The observed change in the microviscosity at the micelle-water interface with isomer structure appears therefore to be a result (21) Gratzel, M.; Thomas, J. K. J. Am. Chem. Soc. 1973, 95, 6885. (22) Rodgers, M. A. J.; Dasilva, E.; Wheeler, M. E. Chem. Phys. Lett. 1976, 43, 587.
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Figure 3. Variation of the degree of polarization with isomer number at different temperatures for xφ C16 surfactants at 2 × 10-3 M in water: λexc ) 337 nm, λem ) 380 nm.
Figure 4. Variation of (ηint/τ) with isomer number at different temperatures for xφC16 surfactants at 2 × 10-3 M in water: λexc ) 337 nm, λem ) 380 nm.
of micellar conformational changes from a somehow compact micellar structure to a looser one as the isomer number is increased from 4φC16 to 6φC16. Furthermore, it is remarkable that this parallel temperature-induced behavior of p and ηint/τ with isomer structure prevails in the whole range of temperatures investigated, suggesting that surfactant packing in xφC16 micelle is not influenced by the change in temperature. From these observations it is inferred that at any temperature the configuration of the surfactant molecule in the micelle still reflects a situation in which the orientation of the surfactant molecule at the interface is such as to hold the two hydrophobic chains in two chemically different environments. The short alkyl chain remains preferentially located at the interface and the long alkyl chain in the micellar core. This lack of temperature-induced change in xφC16 structural properties rules out any possibility for a change on the probe (benzene moiety) fluorescent properties for other reasons than a change in the microviscosity of the probe surrounding medium (micelle-water interface) and strongly suggests that a thermal effect is undoubtedly the dominant factor in determining the variation of the microviscosity in xφC16 aqueous micelles.
MicroViscosity at the Micelle-Water Interface
Figure 5. Variation of excimer to monomer maximum emission ratio with temperature for the xφC16 isomers at 2 × 10-3 M in water: λexc ) 337 nm, λem ) 380 nm, heating curve (b), and cooling curve (O).
Excimer Emission. Evidence was also found for excimer emission in xφC16 aqueous micellar systems.16 Two bands were seen with different relative intensities at 290 and 350 nm and attributed to monomer and excimer emission, respectively. Thus, excimer-monomer fluorescence from the surfactant intrinsic probe (benzene moiety) was used to investigate the effect of temperature on the microviscosity in xφC16 micelles. Excimer to monomer maximum emission ratios (IE/IM) were derived and plotted against temperature for xφC16 isomers (Figure 5). As shown in this figure, the ratio IE/IM in all cases increases with increasing temperature. The degree of polarization data so far considered reports on the viscous resistance opposing benzene rotational diffusion at the micelle-water interface. On the other hand, excimer emission is a diffusion-controlled process and depends on the translational diffusion of molecules. Interestingly, the shapes in Figures 1 and 5 are related in an inverse manner in the entire temperature range investigated, i.e., the degree of polarization p decreases with temperature whereas the ratio IE/ IM increases. In other words, when IE/IM is high (strong excimer emission), the degree of polarization is low (low microviscosity) and vice versa. Therefore, at any given temperature, viscous forces appear to be the limiting characteristic for the excimer formation process occurring at the micelle-water interface. When the microviscosity around the aromatic chromophore parts of the surfactant molecules is relatively high, the benzene moieties have to overcome high viscous forces for the excimer formation process to be effective. Consequently, excimer formation is less likely to occur and the observed excimer fluorescence is low. In addition, we measured the variation of the ratio IE/IM with temperature by first heating the aqueous surfactant system from 8 to around 60 °C and then cooling the same surfactant system back to lower temperatures (Figure 5). As it observed with the fluorescence polarization-temperature behavior, the variation of IE/IM with temperature was found to be reversible and no hysteresis occurs during the heating-cooling process. All isomers investigated showed no abrupt change in the variation of the excimer-to-monomer maximum ratio (IE/IM) with temperature (Figure 5), suggesting no evidence for micelle structural alteration. The observed temperature behavior of the excimer-monomer emission ratios IE/IM seems to confirm our previous conclusion based on the temperature effect on the degree of polarization p,
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Figure 6. Variation of excimer to monomer maximum emission ratio with isomer number at different temperatures for the xφC16 isomers at 2 × 10-3 M in water: λexc ) 337 nm, λem ) 380 nm.
namely, that the thermal effect is the dominant factor in the variation of the microviscosity at the micelle-water interface. Finally, the excimer to monomer maximum emission ratio IE/IM was also found to be dependent on isomer structure. In the range of temperatures investigated (8-64 °C), the ratio IE/IM increases with increasing isomer number (Figure 6). In other words, as the short alkyl chain increases (and consequently the longer alkyl chain decreases), the ratio IE/IM increases. It is inferred from these results that excimer to monomer maximum emission ratio (IE/IM) and degree of polarization (p) variations with isomer composition are also related in an inverse manner in the whole range of temperatures, suggesting that the two different measurements are clearly yielding the same information. At any given temperature in the range 8-64 °C the benzene moiety appears to resist both translational (IE/IM) and rotational (p) in a similar manner. The suggested conformational change of the micelle from a compact structure to a looser one upon increasing the isomer number that gives more freedom to the benzene ring to rotate can also lead to a situation where the benzene moiety has more freedom to translate within the micelle-water interface. In such a situation, the degree of polarization is expected to decrease and consequently the ratio IE/IM is expected to increase. Indeed, this is observed in both Figures 2 and 6. As a final note, it seems interesting to mention that in all previous studies fluorescence properties of extrinsic probes have been used to monitor the effect of temperature on the microviscosity in micelles,23-29 whereas to the best of our knowledge no investigation has reported on the use of intrinsic probes. The extrinsic approach is certainly misleading due the expected temperature-induced changes in (i) micelle size and shape, (ii) (23) Ranganathan, R.; Vautier-Giongo, C.; Bales, B. L. J. Phys. Chem. B 2003, 107 (3), 10312. (24) Fujiwara, Y.; Taga, Y.; Tomonari, T.; Akimoto, Y.; Aoki, T.; Janimoto, Y. Bull. Chem. Soc. Jpn. 2001, 74 (2), 237. (25) Prananik, R.; Kumar Das, P.; Bagchi, S. Phys. Chem. Chem. Phys. 2000, 2 (19), 4307. (26) Evertsson, H.; Nilsson, S. Carbohydr. Polym. 1998, 35 (3-4), 135. (27) Zana, R.; In, M.; Levy, H.; Duportai, G. Langmuir 1997, 13 (21), 5552. (28) Lin, Y.; Alexandridis, P. J. Phys. Chem. B 2002, 106 (42), 10845. (29) Turro, N. J.; Kuo, P. L. Langmuir. 1986, 2 (4), 438-42.
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micelle degree of hydration, (iii) surfactant packing in the micelle, and particularly (iv) probe solubilization site in micelles. Consequently, fluorescence properties such as fluorescence depolarization and excimer formation may change upon changing temperature for other reasons as well due to a change in the viscosity of the probe microenvironment. In such an event, use of an extrinsic probe to monitor temperature-induced viscosity changes in micelles is certainly inappropriate because of the uncertainty in the exact locus of solubilization of the probe within the micelle entity as well as in the changes in micelle structural properties. The results reported in this study certainly suggest that use of the surfactant as its own probe can overcome the many limitations inherent to the extrinsic approach, presumably because the intrinsic probe approach offers a unique situation in which the location of the intrinsic probe (benzene moiety in this investigation) site is fixed at the micelle-water interface and not affected by temperature changes as long as no temperatureinduced micelle structural alterations are present.
Conclusions The effect of temperature on the degree of polarization (p) and excimer to monomer maximum emission ratio (IE/IM) have been investigated in xφC16 (x ) 3-5) aqueous micellar systems using the surfactant molecule as its own intrinsic fluorescent probe. For a given isomer the degree of polarization p was found to decrease with increasing temperature with no evidence for a temperature-induced micelle structural change. Accordingly, the variation of the microviscosity around the benzene moiety at the micelle-solution interface with temperature is believed to be
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mainly due to thermal effects. This was confirmed from the observed decrease of ratio ηint/τ with temperature, where ηint is the microviscosity of the benzene environment in xφC16 micelles. In the range of temperatures investigated (8-60 °C), the effect of isomer structure on the degree of polarization p and the ratio ηint/τ showed that both quantities decrease as the isomer number increases from 4φC16 to 6φC16. This was attributed to the disposition of the surfactant long alkyl chain and the short alkyl chain in two chemically different environments, namely, the micellar core and the micelle-water interface, respectively. Excimer to monomer maximum emission ratios (IE/IM) were found to increase with increasing temperature, in accord with the corresponding observed decrease in the degree of polarization (p). Thus, excimer emission and fluorescence polarization showed that the benzene moiety of the surfactant molecule in xφC16 micelles resists both translational and rotational diffusion in a similar manner within the range of temperatures investigated (8-60 °C). In addition, at any temperature the ratio IE/IM was found to increase with increasing isomer number, again in accord with the corresponding observed decrease in the degree of polarization. This was attributed to the opening up of the micellar structure as the short alkyl chain increases and the long alkyl chain decreases with the consequence being that micelles change from a somehow compact structure to a looser one upon increasing the isomer number from 4φC16 to 6φC16, giving more translational (high IE/IM) and rotational (low p) freedom to the benzene ring. LA061280K
