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Time-Resolved Fluorescence Quenching Study of Aqueous Solutions of Perfluorinated Surfactants with the Use of Protiated Luminophore and Quencher† Ewa Szajdzinska-Pietek* and Marian Wolszczak Institute of Applied Radiation Chemistry, Technical University of Lodz, Wroblewskiego 15, 93-590 Lodz, Poland Received July 22, 1999. In Final Form: October 13, 1999 Application of fluorescence methods to study micellar solutions of perfluorinated surfactants is very scarce because good probes and quenchers could not be found; polycyclic aromatic compounds, the most popular fluorescent probes, are difficult to solubilize into fluorinated surfactant micelles. We have found that the incompatibility of protiated probes with perfluorinated chains of surfactant host may be counterbalanced by positive electrostatic interactions. We present the use of a cationic derivative of pyrene,1-pyrenebutyltrimethylammonium bromide, as a luminophore and cationic quenchers, 4-trimethylammonium-2,2,6,6-tetramethyl-piperidine-1-oxyl iodide (nitroxide radical known as CAT1) and 1,1′-dimethyl-4,4′bipyridinium dichloride (methyl viologen), to examine aqueous solutions of two anionic fluorinated surfactants, ammonium perfluorooctanoate (APFO) and tetraethylammonium perfluorooctyl sulfonate (TEAPFOS). On the basis of the Infelta-Tachiya model, we have determined from time-resolved fluorescence quenching data the micellar aggregation numbers, the rate constants of intramicellar quenching, and the rate constants of the quencher exit from the aggregates. The results indicate formation of ellipsoidal micelles in APFO solution, with the aggregation numbers increasing versus surfactant concentration (from 33 at 0.06 M APFO to 109 at 0.5 M APFO), and threadlike micelles in TEAPFOS solution, which consist of interconnected spherical units each containing at least 52 surfactant molecules.
* To whom correspondence should be addressed. E-mail:
[email protected]. † This work was presented in part at 4th Liquid Matter Conference, Granada, Spain, 3-7 July, 1999.
obtained using various luminophore/quencher pairs. The method has been successfully applied to many hydrocarbon surfactant systems, and pyrene and its derivatives are the most pupular fluorescent probes used in these studies.3,5-7 Recently, there is an increasing interest in the studies of physicochemical properties of micellar solutions containing perfluorinated surfactants, as they have numerous applications in industrial and biomedical fields.8,9 It appears, however, that application of fluorescence methods to these systems is very scarce because good probes and quenchers could not be found. It has been reported that polycyclic aromatic compounds, such as pyrene, are difficult to solubilize into fluorinated surfactant micelles,10,11 and our experience fully supports this conclusion. After prolonged stirring and heating of pyrene with aqueous micellar solutions of ammonium perfluorooctanoate or carboxylic derivative of perfluoropolyether we observed practically no absorption from the probe; as a matter of fact it was less than that in neat water saturated with pyrene. Attempts to increase the probe solubility by sonication were also unsuccessful. It is noteworthy that Muto et al.12 did use pyrene to determine the aggregation number for lithium perfluorooctyl sulfonate, but Take’uchi and Moroi point out that the obtained value, N ) 6 at 20 mM surfactant concentration, seems too small.13
(1) (a) Kalyanasundaram, K. In Photochemistry in Organized & Constrained Media; Ramamurthy, V., Ed.; VCH Publishers: New York, 1991; Chapter 2. (b) Bohne, C.; Redmond, R. W.; Scaiano, J. C. Ibid., Chapter 3. (2) Almgren, M. Adv. Colloid Interface Sci. 1992, 41, 9-32 and references therein. (3) Gehlen, M. H.; De Schryver, F. C. Chem. Rev. 1993, 93, 199-221 and references therein. (4) Malliaris, A. Prog. Colloid Polym. Sci. 1987, 73, 161-166. (5) Szajdzinska-Pietek, E.; Wolszczak, M. Chem. Phys. Lett. 1997, 270, 527-532. (6) Alargova, R. G.; Kochijashky I. I.; Sierra, M. L.; Zana, R. Langmuir 1998, 14, 5412-5418.
(7) Grieser, F.; Drummond, C. J. J. Phys. Chem. 1988, 92, 55805593 and the Supporting Information. (8) Kissa, K. Fluorinated Surfactants. Synthesis, Properties, Applications; Surfactant Sci. Ser. Vol. 50; Marcel Dekker, Inc.: New York, 1994. (9) Krafft, M. P.; Riess, J. G. Biochimie 1998, 80, 489-514. (10) Moroi, Y.; Take’uchi, M.; Yoshida, N.; Yamauchi, A. J Colloid Interface Sci. 1998, 197, 221-229. (11) Sta¨hler, K.; Selb, J.; Barthelemy, P.; Pucci, B.; Candau, F. Langmuir 1998, 14, 4765-4775. (12) Muto, Y.; Esumi, K.; Meguro, K.; Zana, R. J Colloid Interface Sci. 1987, 120, 162-171.
Introduction Photophysical methods employing fluorescence probes are commonly used to study self-assemby of amphiphilic compounds in solutions.1 In particular, from emission intensities and/or from the kinetic behavior of hydrophobic probes embedded in micelles, in the presence of added quenchers, one can deduce micellar aggregation numbers, N.2,3 The approach is based on the assumption that both the luminophore and the quencher molecules are distributed among the aggregates according to Poisson statistics, and the intramicellar quenching process is governed by classical first-order kinetics. Application of the steadystate method is limited to the special case of immobile quencher not exiting the parent micelle during the lifetime of excited state of the probe and so-called static or quasistatic quenching; i.e., the rate constant of the intramicellar process must be much higher than that of sponateneous decay of the excited state (in the absence of the quencher), otherwise underestimated N values are obtained.4-6 The time-resolved fluorescence quenching (TRFQ) method does not require the latter assumptions, and for the systems of spherical micelles, proper aggregation numbers can be
10.1021/la990981x CCC: $19.00 © 2000 American Chemical Society Published on Web 12/01/1999
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Chart 1
Figure 1. Pseudo-first-order rate constants of quenching of the excited PBTMA by CAT1 (9) and MV (b) in water.
In this paper we report that the incompatibility of protiated probes with perfluorinated chains of surfactant host may be counterbalanced by positive electrostatic interactions. We have used a cationic derivative of pyrene, 1-pyrenebutyltrimethylammonium bromide (PBTMA), as a luminophore and cationic quenchers, 4-trimethylammonium-2,2,6,6-tetramethyl-piperidine-1-oxyl iodide (CAT1) and 1,1′-dimethyl-4,4′bipyridinium dichloride (methyl viologen, MV), to determine by TRFQ measurements aggregation numbers of the two anionic fluorinated surfactants, ammonium perfluorooctanoate (APFO) and tetraethylammonium perfluorooctyl sulfonate (TEAPFOS). Chart 1 presents the formula of the compounds used in this study. Experimental Section APFO (purum >98%), TEAPFOS (purum >98%) from Fluka, and PBTMA and CAT1 from Molecular Probes, Eugene, OR, were used as received. MV (98%) from Aldrich was purified by recrystallization.14 Aqueous solutions of surfactants and PBTMA and MV stocks, were prepared in Millipore deionized water. They were mixed together in proper volumes to obtain the desired concentrations of the luminophore and the quencher; the former was ca. 1 × 10-5 M. The stock solution of CAT1 was prepared in ethanol/chloroform mixture (spectroscopic grade). The proper volumes of this stock were picked up into the sample vials, the solvent was evaporated under the stream of argon, and then the quencher was redissolved in the respective surfactant/PBTMA solution. The surfactant free solutions were also prepared in the same way. The samples were deaerated in quartz cuvettes (1 cm width) by careful bubbling with argon (40 min at least), and fluorescence spectra and decay curves were recorded at room temperature. The equipment and experimental details have been described.5,15
Results Kinetics of fluorescence decay of excited PBTMA has been followed in neat water and in surfactant solutions (13) Take’uchi, M.; Moroi, Y. J Colloid Interface Sci. 1998, 197, 230235. (14) Wolszczak, M.; Thomas, J. K. Radiat. Phys. Chem. 1991, 38, 155-164. (15) Szajdzinska-Pietek, E.; Wolszczak, M.; Plonka, A.; Schlick, S. J. Am. Chem. Soc. 1998, 120, 4215-4221.
as a function of the quencher and the surfactant concentrations. In the absence of the quencher the dacay was exponential with the rate constant k0 equal to (7.4 ( 0.1) × 106 s-1 in water, (5.1 ( 0.2) × 106 s-1 in APFO, and (5.7 ( 0.2) × 106 s-1 in TEAPFOS solutions; the latter two values represent average data for the examined range of surfactant concentrations (0.06-0.7 M APFO and 6-12 mM TEAPFOS). In the surfactant solutions the uncertainty of k0 determination was higher due to difficulties in complete deaeration of the samples, but the values are evidently lower than those obtained in neat water indicating that the probe is bound to the micellar aggregates. In the presence of the quencher the decay was also exponential in water, but in surfactant solutions nonexponential decay curves have been recorded. The results obtained for water are presented in Figure 1; the second-order rate constants of PBTMA fluorescence quenching are determined as (5.26 ( 0.05) × 109 and (4.7 ( 0.1) × 109 M-1 s-1 for CAT1 and MV, respectively. The TRFQ data for surfactant solutions were analyzed in the frames of the model developed by Infelta16 and Tachiya.17 The model assumes that a micelle contains a maximum of one probe, which does not exit the micelle during the lifetime of excited state, and the occupation of micelles by the solutes (probe and quencher) follows Poisson statistics; cf. also ref 3. Using a nonlinear leastsquares procedure the fluorescence decay curves I(t) have been fitted by eq 1
I(t) ) A1 exp{-A2t - A3[1 - exp(-A4t)]}
(1)
where A1 ) I(0) is the emission intensity at zero time (the end of laser pulse), and A2, A3, and A4 are parameters given by eqs 2-4
A 2 ) k0 +
A3 )
k+kqm[Q] (1 + K[M])(k- + kqm)
) k0 + S2[Q] (2)
kqm2[Q] (k- + kqm)2(1/K + [M]) A4 ) kqm + k-
) S3[Q]
(3) (4)
where k0 is the rate constant of fluorescence decay in the (16) Infelta, P. P.; Gra¨tzel, M.; Thomas, J. K. J. Phys. Chem. 1974, 78, 190-195.
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Figure 2. An example of kinetic data fitting for the 0.19 M APFO/PBTMA/1.28 mM MV solution (A2 ) 5.25 × 106 s-1, A3 ) 0.868, A4 ) 6.78 × 107 s-1). (lower panel) Fitting curve (smooth) superimposed on the experimental run (noisy).
absence of the quencher (s-1), k+ is the rate constant of the quencher entry into the micelle (M-1 s-1), k- is the rate constant of the quencher exit from the micelle (s-1), kqm is the pseudo-first-order rate constant of intramicellar fluorescence quenching (s-1), K ) k+/k-, equilibrium constant of the quencher association with the micelle, [Q] is the concentration of the quencher, and [M] is the concentration of micelles. In the case of the so-called immobile quencher, not undergoing intermicellar exchange during the lifetime of the excited state of the luminophore, the parameters A2A4 are given by eqs 5-7, respectively
A2 ) k0
(5)
A3 ) [Q]/[M] ) 〈n〉
(6)
A4 ) kqm
(7)
where 〈n〉 denotes the mean occupancy of micelles by the quencher. In our experiments such behavior was only observed for the PBTMA/MV luminophore/quencher pair in APFO solutions of relatively low concentrations (up to ≈0.2 M); Figure 2 shows an example of the kinetic data fitting. The A2 parameter was independent of the quencher concentration and equal to k0 (within the experimental uncertainty), while the A3 parameter increased linearly vs MV concentration as shown in Figure 3. From the slopes of the straight lines the concentrations of micelles were determined according to eq 6, and the aggregation numbers N were calculated from the relation
Cs - cmc [M] ) N
(8)
where Cs is the surfactant concentration, and cmc denotes the critical micelle concentration, which can be assumed as the concentration of free surfactant in the aqueous phase. Taking cmc ) 0.03 M for APFO,18 our results give N ) 43 and N ) 57 for 0.095 and 0.19 M APFO solutions, (17) Tachiya, M. Chem. Phys. Lett. 1975, 33, 289-292. (18) (a) Kunieda, H.; Shinoda, K. J. Phys. Chem. 1976, 80, 24682470. (b) Guo, W.; Brown, T. A.; Fung, B. M. J. Phys. Chem. 1991, 95, 1829-1836. (c) Ristori, S.; Martini, G. Langmuir 1992, 8, 1937-1942.
Figure 3. The best fitting A3 and A4 parameters of fluorescence decay curves for the APFO/PBTMA/MV system versus the quencher concentration. The immobile quencher behavior: b, 0.095 M APFO; 9, 0.190 M APFO. Each point represents the average value obtained for multiple runs (at least four).
Figure 4. An example of kinetic data fitting for the 0.2 M APFO/PBTMA/1.73 mM CAT1 solution (A2 ) 6.09 × 106 s-1, A3 ) 0.431, A4 ) 2.82 × 107 s-1). (lower panel) Fitting curve (smooth) superimposed on the experimental run (noisy).
respectively. The increase of the aggregation number with the surfactant concentration is accompanied by a decrease of the rate constant of intramicellar quenching as indicated by the A4 data included in Figure 3; cf. eq 7. The MV in 0.5 M APFO and in TEAPFOS solutions, as well as CAT1 in all the examined systems behaved as mobile quenchers; an example of the kinetic data fitting in this case is shown in Figure 4. Both the A3 and A2 parameters increased linearly vs [Q], according to eqs 2 and 3, and the A4 parameter was again constant (within
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Figure 5. The best fitting A2, A3, and A4 parameters of fluorescence decay curves for the 0.2 M APFO/PBTMA/CAT1 system versus the quencher concentration. The mobile quencher behavior. Each point represents the average value obtained for multiple runs (at least four).
the error e10%), as illustrated in Figure 5 for the 0.2 M APFO/PBTMA/CAT1 system. To evaluate from such experimental data the rate constants kqm and k-, we have adopted the procedure of successive iterations described by Roelants et al.19 Then the concentration of micelles, and thus the aggregation number, has been determined according to eq 3 assuming that k+[M] . k-. Figure 6 summarizes the results obtained for APFO solutions with the use of CAT1 and MV as quenchers. Both quenchers report the same aggregation numbers (within uncertainty (2) at relatively low surfactant concentrations. The MV data indicate a linear increase of N versus [APFO] up to 0.5 M concentration, but the values obtained from the CAT1 data at [APFO] g 0.5 M are markedly lower. The rate constant of intramicellar quenching decreases, and the rate constant of the quencher exit from micelles increases (neglecting the lowest APFO concentration data), with the growth of the aggregates. It is important to note that the kqm values are significantly higher for MV than for CAT1, the trend opposite to that observed for the quenching rate constants in water; cf. Figure 1. As the critical micellar concentration of TEAPFOS is 1 mM,20 we examined its solutions in concentrations up to 12 mM only. We note that the solutions exhibit a viscoelastic behavior.21 The final results of TRFQ data analysis are summarized in Table 1. The aggregation numbers and the rate constants, kqm and k-, do not depend on [TPFOS] in the examined range, but different values (19) Roelants, E.; Gelade, E.; Smid, J.; De Schryver, F. C. J. Colloid Interface Sci. 1985, 107, 337-344. (20) Hoffman, H. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 988-1001. (21) Watanabe, H.; Osaki, K.; Matsumoto, M.; McNamee C. E.; Nakahara, M.; Yao, M.-L. Rheol. Acta 1998, 37, 470-485.
Szajdzinska-Pietek and Wolszczak
Figure 6. Aggregation numbers (A), rate constants of intramicellar quenching (B), and rate constants of the quencher exit from micelles (C) versus surfactant concentration in APFO solution: b, MV data; 9, CAT1 data. In A the straight line, N ) 23.59 + 168.37[APFO], is a least-squares fit to the MV data and the CAT1 data up to [APFO] ) 0.5 M. In B the lines represent an exponential decrease. In C the straight line is a least-squares fit to the CAT1 data in the concentration range [APFO] ) 0.1-0.7 M; in the inset k- values for CAT1 are plotted versus concentration of micelles, and the line represents a second-order polynomial fit. Table 1. Aggregation Numbers (N), Rate Constants of Intramicellar Quenching (kqm), and Rate Constants of the Quencher Exit from Micelles (k-) for TEAPFOS/ PBTMA/MV and TEAPFOS/PBTMA/CAT1 Systems k- × 10-6, s-1
N [TEAPFOS], mM MV CAT1 6 12
50 53
26 30
kqm × 10-7, s-1
MV
CAT1
MV
CAT1
7.70 7.64
13.35 13.25
2.16 2.13
1.79 1.83
have been obtained for MV and CAT1 as quenchers. Similarly as in the case of APFO, for CAT1 the rate constant of the quencher exit from micelles is higher, while the rate constant of intramicellar quenching is lower than that for MV. Discussion Our results clearly indicate that the incompatibility of protiated probes with perfluorinated chains of the surfactant host may be counterbalanced by positive electrostatic interactions. Using the cationic derivative of pyrene as a luminophore and cationic quenchers, it is possible to determine aggregation numbers of anionic perfluorosurfactants from TRFQ data based on the classical model
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developed by Infelta and Tachiya. We note, however, that the obtained N values, except of those for APFO in the range of low surfactant concentrations, depend on the kind of quencher. There are reasons to believe that the data reported by MV as a quencher are more reliable. The MV dication binding to micelles is about twice as strong as that of CAT1 (cf. k- data in Figure 6C and Table 1), and most probably in the case of CAT1 in TEAPFOS solutions and in concentrated APFO solutions (g 0.5 M) the assumption k+[M] . k-, made for evaluation of N values from eq 3, is not fulfilled. Furthermore, higher kqm values for MV in comparison to CAT1 (cf. Figure 6B and Table 1) ensure that the quenching process is accomplished even if the quencher residence time in micelle decreases. The relevant finding of our study is the linear increase of APFO aggregation number with the surfactant concentration, from N ) 33 at 0.06 M APFO up to N ) 109 at 0.5 M APFO; cf. Figure 6A. Such high aggregation numbers for the heptyl chain surfactant suggest that the shape of micelles is ellipsoidal rather than spherical, in accordance with the conclusion of the earlier study by small angle neutron scattering (SANS) technique.22 It is important to note that the aggregation number reported in that study, N ) 43 at 0.12 M APFO concentration, coincides with our data within experimental uncertainty ((2). The growth of micelles was also observed in SANS experiments for sodium perfluorooctanoate.23 In this case N values increased linearly versus the square root of surfactant concentration, from 27 in 0.1 M solution to 51 only in 0.5 M solution, and the results were rationalized assuming the spherical shape of the aggregates. Micellization of perfluorooctanoates is thus markedly affected by the kind of counterion. This is also revealed in the recent NMR study.24 Two cmc points were determined by 19 F and 133Cs chemical shift measurements for cesium perfluorooctanoate solution in D2O, at 0.024 and 0.14 mol kg-1, and the latter one was ascribed to transition from spherical to ellipsoidal micelles. For APFO, however, only one cmc value was detected. Apparently APFO aggregates are ellipsoidal from the onset of micellization; the optically isotropic phase persists up to 55% (w/w) surfactant content (ca. 1.7 M solution), at which transition to the lammellar phase occurs.25 The decrease of the rate constant of intramicellar quenching process with the APFO concentration, cf. Figure 6B, is a natural consequence of the growth of micelles; in bigger aggregates an encounter of the luminophore with the quencher is less probable. The increase of the rate constant of the quencher exit from a host micelle observed for CAT1 above 0.1 M APFO concentration, cf. Figure 6C, is a behavior typical for ionic quenchers in solutions of micelles of the opposite sign.19,26-28 This effect was modeled by Almgren at al.29,30 for spherical aggregates in terms of
decreasing electrostatic potential at the micellar surface with increasing micelle concentration. Our data suggest that in the case of growing ellipsoidal APFO micelles the decrease of surface potential occurs only at higher micelle concentrations, above [M] ≈ 2 mM; in the vicinity of the cmc the potential seems constant or even slightly increasing versus [M]; cf. inset of Figure 6C.31 The observed different behavior of the quenchers in TEAPFOS solutions, as compared to APFO solutions, is not surprising taking into account that the former system consists of threadlike micelles, ca. 3 nm width and a few hundred nanometer length at 10 mM TEAPFOS concentration.32 In the threadlike micelle, spherical or hemispherical units are interconnected (as pearls on the necklace) through a hydrophobic attraction due to tetraethylammonium counterions bound to their surface; hence both MV and CAT1 behave as mobile quenchers. It is reasonable to interpret the aggregation number evaluated from the MV data, N ) 52 ( 2, as a number of surfactant molecules in one unit. Its independence of the surfactant concentration is consistent with the results of the cryogenic transmission electron microscopy (cryo-TEM) studies which indicated that the thread width does not change with increasing [TEAPFOS].32 It is important to bear in mind, however, that the above N value for TEAPFOS may be underestimated, bacause the rate constant of MV exit from micelles, although lower than that of CAT1, is still relatively high and the assumption k+[M] . k- may not be valid. Finally, we have to mention that formation of the nonfluorescent ground-state charge-transfer complex between pyrene and MV was reported for anionic micellar systems,33 and the process also takes place in the systems examined by us.34 Our results indicate that neglecting the probe-quencher association in TRFQ data analysis does not introduce a significant error to aggregation numbers; for APFO solutions of low concentrations the N values obtained with the use of MV and CAT1 are close to each other and coincide with the SANS data.
(22) Burkitt, S. J.; Ottewill, R. H.; Hayter, J. B.; Ingram, B. T. Colloid Polym. Sci. 1987, 265, 619-627. (23) Berr, S. S.; Jones, R. R. M. J. Phys. Chem. 1989, 93, 2555-2558. (24) Iijima, H.; Koyama S.; Fujio. K.; Uzu, Y. Bull. Chem. Soc. Jpn. 1999, 72, 171-177. (25) Tiddy, G. J. T. J Chem. Soc., Faraday Trans. 1 1972, 68, 608612. (26) Dederen, J. C.; Van der Auweraer, M.; De Schryver, F. C. Chem. Phys. Lett. 1979, 68, 451-454; J. Phys. Chem. 1981, 85, 1198-1202. (27) Grieser, F.; Tausch-Treml, R. J. Am. Chem. Soc. 1980, 102, 72587264. (28) Malliaris, A.; Lang, J.; Zana, R. J. Chem. Soc., Faraday Trans. 1 1986, 82, 109-118. (29) Almgren, M.; Gunnarsson, G.; Linse, P. Chem. Phys. Lett. 1982, 85, 451-455. (30) Almgren, M.; Linse, P.; Van der Auweraer, M.; De Schryver, F. C.; Gelade, E.; Croonen, Y. J. Phys. Chem. 1984, 88, 289-295.
Conclusions For the studies of aggregation of ionic perfluorinated surfactants in aqueous solutions by TRFQ technique we recommend the use of oppositely charged ionic derivatives of protiated luminophore and quencher. Of the two quenchers examined by us in anionic micellar systems the methyl viologen dication seems to be a better choice; at a relatively low concentration of isolated aggregates it does not undergo intermicellar exchange, so the data analysis is simplified. The use of monovalent CAT1 may lead to underestimated aggregation numbers. In this case, however, rate constants of the quencher exit from aggregates can be evaluated from TRFQ results (with no assumption concerning aggregation number), and these data are relevant in connection with the use of CAT1 nitroxide as a spin probe in ESR studies. (31) The concentration of micelles was calculated using N values given by the least-squares straight line in Figure 6A and cmc ) 0.03 M. (32) Knoblich, A.; Matsumoto, M.; Murata, K.; Fujiyoshi, Y. Langmuir 1995, 11, 2361-2366. (33) Gehlen, M.; De Schryver, F. C. J. Phys. Chem. 1993, 97, 1124211248. (34) Our complementary experiments for the APFO system revealed that the Stern-Volmer plots obtained from the steady-state measurements have a more pronounced upward curvature than those from timeresolved measurements. Furthermore, with increasing MV concentration a red shift of the absorption spectrum of PBTMA occurs and a new band appears in the visible part of the spectrum.
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The present study confirms formation of ellipsoidal micelles in APFO solution, with the aggregation numbers increasing versus surfactant concentration, and threadlike micelles in TEAPFOS solution, with the aggregation number of spherical units N g 52.
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Acknowledgment. This work has been supported by the Polish Committee of Scientific Research (in part KBN Project 3 T09A 034 11). LA990981X
