Fluorescence Quenching Studies of Micellization and Solubilization in

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Langmuir 1997, 13, 4535-4544

4535

Fluorescence Quenching Studies of Micellization and Solubilization in Fluorocarbon-Hydrocarbon Surfactant Mixtures Mats Almgren* and Ke Wang Department of Physical Chemistry, Uppsala University, Uppsala, Sweden

Tsuyoshi Asakawa Department of Chemistry and Chemical Engineering, Faculty of Engineering, Kanazawa University, Kanazawa 920, Japan Received March 3, 1997. In Final Form: May 20, 1997X Time-resolved fluorescence quenching studies of nonionic, anionic, and cationic micelles have been performed to compare two surfactant quenchers, a fluorocarbon surfactant quencher, N-(1,1,2,2tetrahydroperfluorodecanyl)pyridinium chloride (HFDePC) and a hydrocarbon quencher of similar hydrophobicity, C16PC, N-hexadecylpyridinium chloride. The concentration dependence of the apparent aggregation numbers informs on the interaction between the surfactants, which always was repulsive for the fluorocarbon quencher in hydrocarbon micelles, except for the case with a nonionic micelle, where the effectively attractive electrostatic interaction dominated at low ionic strength. The simple theory (Almgren, M.; Hansson, P.; Wang, K. Langmuir 1996, 12, 3855) suggests that the interaction parameter from the slope of the apparent aggregation number versus mole fraction quencher should be the same as the interaction parameter describing the change of the critical micelle concentration with composition according to regular solution theory (Rubingh, D. H. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 1, p 337). The results show that it is not so; not even the sign of the interaction parameter is always the same. The reasons for the difference are discussed. For the weight average aggregation number, obtained by extrapolation to zero quencher concentration, the two quenchers gave results within about 10% for a given surfactant; the values with the fluorocarbon quencher seemed to be systematically lower, as if the aromatic probe strongly avoided micelles containing fluorocarbon quenchers. From solubility studies, pyrene was found to prefer a C16TAC micelle over a HFDePC micelle by a factor of 60. The preference of pyrene for micelles without fluorocarbon surfactants was utilized to show the demixing into fluorocarbon-rich and hydrocarbon-rich micelles in a mixture of lithium perfluorononanoate and lithium dodecyl sulfate and in cetyltrimethylammonium chloride and HFDePC.

Introduction Fluorocarbon surfactants have unique properties that are both useful and of fundamental interest. Of particular interest is the repulsive interaction with normal surfactants, which comes from the segregation observed in liquid hydrocarbon-fluorocarbon mixtures. It was suggested early1-3 that a demixing into separate populations of hydrocarbon- and fluorocarbon-rich surfactant micelles could occur in mixtures of such surfactants. From critical micelle concentration (cmc) measurements and an appropriate theoretical discussion, Shinoda and Nomura3 concluded that demixing occurs in ammonium perfluorononanoate-sodium dodecyl sulfate (SDS) mixtures, whereas sodium perfluorooctanoate mixes with sodium decyl sulfate in all proportions. Numerous attempts have been made to prove the coexistence of two types of micelles; mostly, however, the methods have been indirect and the results not fully conclusive. The NMR self-diffusion measurements by Carlfors and Stilbs4 are among the most convincing studies. Asakawa et al.5 provided a direct demonstration by separating of the two types of micelles in gel filtration. It should also be possible to obtain very convincing evidence from small angle neutron scattering, X

Abstract published in Advance ACS Abstracts, July 15, 1997.

(1) Tiddy, G. J. T.; Wheeler, B. A. J. Colloid Interface Sci. 1974, 47, 59. (2) Murkerjee, P.; Yang, A. Y. S. J. Phys. Chem. 1976, 80, 1388. (3) Shinoda, K.; Nomura, T. J. Phys. Chem. 1980, 84, 365. (4) Carlfors, J.; Stilbs, P. J Phys. Chem. 1984, 88, 4410. (5) Asakawa, T.; Miyagishi, S.; Nishida, M. Langmuir 1987, 3, 821.

S0743-7463(97)00238-2 CCC: $14.00

using index matching.6,7 Hoffmann et al.8 tried other direct methods to discriminate between mixed and demixed systems. By refractive index matching with glycol-water mixtures in light scattering experiments, either the fluorocarbon or the hydrocarbon rich micelles could be made almost invisible, and by labeling the micelles specifically with normal dyes or fluorocarbon-substituted hydrophobic dyes, for the normal and fluorocarbon-rich micelles, respectively, the two types of micelles could be separated in the ultracentrifuge. The characterizations of the mixed and demixed micelles have been hampered by a lack of simple methods to determine the micelle size. Light-scattering techniques are difficult to apply since the fluorocarbon micelles have a refractive index similar to that of water. Small-angle neutron scattering (SANS) is a very useful9,10 but not a freely available method. Successful attempts to utilize time-resolved fluorescence quenching (TRFQ) have been made and given valuable information,11 but there are some problems with the method that require special attention in mixed systems and make it less general useful than for normal micellar systems. The problems derive just from (6) Burkitt, S. J.; Ottewil, R. H.; Hayter, J. B.; Ingram, B. T. Colloid Polym. Sci. 1987, 265, 628. (7) Clapperton, R. M.; Ottewil, R. H.; Ingram, B. T. Langmuir 1994, 10, 51. (8) Haegel, F. H.; Hoffmann, H. Prog. Colloid Polym. Sci. 1988, 76, 132. (9) Burkitt, S. J.; Ottewil, R. H.; Hayter, J. B.; Ingram, B. T. Colloid Polym. Sci. 1987, 265, 619. (10) Hoffmann H.; Kalus, J.; Thurn, H. Colloid Polym. Sci. 1983, 261, 1043. (11) Muto, Y.; Esumi, K; Meguro, K.; Zana, R. J. Colloid Interface Sci. 1987, 120, 162.

© 1997 American Chemical Society

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Almgren et al.

the non-ideality of mixing,12,13 which implies that the probes and quenchers used in the measurements distribute in a nonrandom way among the micelles. On the other hand, the nonideal mixing effects on the fluorescence quenching can also be utilized. It was recently shown by Asakawa et al.14 that strong evidence for microscopic demixing is obtained by using an aromatic probe such as pyrene with strong preference for hydrocarbon micelles, together with a fluorocarbon-based quencher. In systems with mixed surfactants, the pyrene fluorescence was quenched by a hydrocarbon-based quencher but not by the fluorocarbon quencher, whereas in both of the pure systems the two quenchers work about equally well and would without doubt continue to do so if only mixed micelles had formed. The studies were done with simple static fluorescence intensity measurements; one aim of the present contribution is to substantiate the interpretation by time-resolved studies of similar systems. A second aim of the paper is to continue the evaluation of the TRFQ method in systems with nonideality,12 in particular by comparing a fluorocarbon surfactant quencher, N-(1,1,2,2-tetrahydroperfluorodecanyl)pyridinium chloride (HFDePC), with a hydrocarbon quencher of similar hydrophobicity, C16PC, hexadecylpyridinium (or cetyl-) chloride. The effect of nonideal mixing on the distribution of surfactants over a set of mixed micelles of equal size was recently investigated.12 A simple mean-field lattice model was used to obtain a surfactant distribution consistent with the pseudophase “regular solution”15 approach usually employed to discuss the distribution of surfactants between the aqueous phase and the micelles. In particular, it was shown that the apparent aggregation number, Napp, from time-resolved fluorescence quenching (TRFQ) measurements, would be linearly dependent on the mole fraction, xq, of a quenching surfactant in the micelles12

Napp ) N0 [1 + (1/2 - R)xq]

(1)

where R is the interaction parameter and N0 ) Ns + Nq the total aggregation number, assumed independent of xq. Index s and q will be used for surfactant and (surfactant) quencher, respectively. In the regular solution approach one uses

cs ) c°s fs xs cq ) c°q fq xq

(2)

where ci is the concentration of free i, c°i the cmc of pure i, and the activity factor, fi, given by

ln fi ) R(1 - xi)2;

i ) s, q

(3)

It was shown that the apparent aggregation number from TRFQ for a nonionic surfactant, using cetylpyridinium chloride as quencher, increased with the quencher concentration, indicating an effectively attractive interaction between the nonionic and ionic surfactant molecules. The electrostatic origin of the interaction was shown by its reduction on addition of salt. The mixed cmc, the sum of cq and cs, was consistently lower than an ideal mixture, so that the sign, but not the value, of the interaction (12) Almgren, M.; Hansson, P.; Wang, K. Langmuir 1996, 12, 3855 (13) Barzykin, A.; Almgren, M. Langmuir 1996, 12, 4672. (14) Asakawa, T.; Hisamatsu, H.; Miyagishi, S. Langmuir 1996, 12, 1204. (15) Rubingh, D. H. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 1, p 337.

parameter was the same from both methods. Qualitatively, the reason for the effect on the distribution is easy to understand. Addition of more than one charged surfactant to a nonionic micelle will be hindered by electrostatic repulsion, and the exchange of an ionic surfactant for a nonionic one in an ionic micelle will be favored by a reduced electrostatic repulsion. This is what is required for the interaction parameter to be negative and favor the mixed micelles more than according to random mixing. It is not fully clear under what minimum assumptions both the regular solution model, eqs 2 and 3, and the composition dependence of the apparent aggregation number according to eq 1, should be valid, but there is no reason to expect that the experimental values of the interaction parameter from both types of measurements should be the same. It is more fruitful to regard both as empirical laws and investigate the ranges of validity and to what extent the interaction parameters from the regular solution model and quenching, respectively, agree. The regular solution approach has been shown to represent a variety of mixed systems fairly well.15,16 A further source of nonideality that must be considered in the TRFQ method is the distribution of the fluorescence probe, often pyrene, between micelles of different composition. Pyrene prefers the hydrocarbon environment strongly, and the subset of micelles selected for study by excitation will be biased toward micelles with few fluorocarbon surfactants. The nonideal distribution of the probe will be given some attention. Experimental Section Chemicals. C12E8 (octaethylene glycol dodecyl ether) was used as received from Nikko Chemicals, Japan. C16TAC (cetyltrimethylammonium chloride) was prepared by ion exchange from the bromide salt (Serva, analytical grade). SDS (sodium dodecyl sulfate) and LiDS (lithium dodecyl sulfate) were used as received from BDH. LiPFN (lithium perfluorononanoate) was prepared by neutralizing perfluorononanoic acid with lithium hydroxide, followed by freeze drying and vacuum drying. Perfluorononanoic acid and perfluoroheptane were purchased from Fluorochem Limited. Lithium hydroxide was purchased from ROTH. NaCl (sodium chloride), LiCl (lithium chloride), ndodecane, and the cationic hydrocarbon quencher C16PC (Ncetylpyridinium chloride), all from Merck, were used as received, whereas C12PC from Aldrich was recrystallized several times from acetone. The synthesis of the cationic fluorocarbon quencher HFDePC (N-(1,1,2,2-tetrahydroperfluorodecanyl)pyridinium chloride) has been described.17 Pyrene (Aldrich) was recrystallized twice from ethanol. Sample Preparation. The samples for fluorescence measurements were prepared by allowing a certain amount of a stock solution of pyrene in ethanol to evaporate in a gentle stream of nitrogen gas. A stock solution of quencher, surfactant, and water was then added to get the desired quencher and surfactant concentrations. The solutions were mixed in a rotational mixer for at least a day. The pyrene concentration was always kept low enough to prevent excimer formation. Solutions with probe only were used in the determination of the natural lifetime τ0. Solutions with probe and quencher were used in the determination of aggregation number. All solutions were prepared in distilled water. Steady-State Fluorescence. Pyrene fluorescence spectra were recorded on a SPEX Fluorolog 1680 using SPEX DM3000 software. Time-Resolved Fluorescence Quenching. The fluorescence decay data were collected with the single photon counting technique, using methods and equipment described earlier.18 (16) Holland, P. M.; Rubingh, D. N. J. Phys. Chem. 1983, 87, 1984. (17) Asakawa, T.; Hisamatsu, H.; Miyagishi, S. Langmuir 1995, 11, 478. (18) Almgren, M.; Hansson, P.; Mukhtar, E.; Stam, J van. Langmuir 1992, 8, 2405.

Fluorescence Quenching in Surfactant Mixtures

Langmuir, Vol. 13, No. 17, 1997 4537

The decay curves were fitted to the simple Infelta-Tachiya model.19,20

F(t) ) A1 exp{-A2t + A3[exp(-A4t) - 1]}

(4)

A1 ) F(0) A2 ) k0 A3 ) 〈n〉 ) xqN0 A4 ) kq where F(t) is the fluorescence intensity at time t (after the excitation moment), k0 ) 1/τ0, where τ0 is the natural fluorescence lifetime, kq is the first-order quenching rate constant in a micelle with one quencher, 〈n〉 is the average number of quenchers per micelle, and xq the mole fraction of quencher, given by xq ) [quencher]/([surf]total + [quencher] - (cs + cq)). The model assumes that the probes and quenchers are Poisson-distributed over the micelles and that they stay in the same micelles during the observation time period. If the distribution is not Poissonian, an apparent aggregation number, Napp, is obtained from A3, or from

Napp )

F(0) 1 ln xq F∞(0)

(5)

where F∞(0) is amplitude of the final exponential tail of the decay. The use of eq 4 for the decay is actually an inconsistency, when the ideal distribution is assumed to be binominal as here. It was pointed out to us21 that the solution of the problem for a binominal distribution is simple when no migration occurs

F(t) ) F(0) exp(-k0t)[1 - xq{1 - exp(-kqt)}]N0

(6)

Since our fitting programs are built for equations such as eq 4, we still have used it as the starting point. The difference would be small at low quencher concentrations. The mismatch between the binominal and the Poisson distributions is responsible for the term 1/2 in eq 1. Otherwise, the main effect from employing eq 6 instead of eq 4 would be a slightly different value of the quenching constant, kq.

Figure 1. A family of fluorescence decay curves for pyrene in 10 mM SDS-100 mM NaCl, with the quencher (a) C16PC and (b) HFDePC concentration increasing from zero in the uppermost curve to about 3 mol % in the lowest.

Results and Discussion Aggregation Numbers and Interaction Parameters. TRFQ measurements were performed, at 25 °C unless otherwise noted, on micelles formed by cationic (C16TAC), anionic (SDS and LiDS), nonionic (C12E8), and fluorocarbon (LiPFN, lithium perfluorononanoate) surfactants, using both C16PC and HFDePC as quenchers, which for short will be referred to as the HC and the FC quencher, respectively. Typical families of decay curves are displayed in Figure 1. In all cases reported here the quenched decay curves gave well-developed exponential tails, with none or only a weak indication of migration, and fitted very well to the Infelta-Tachiya model. The apparent aggregation numbers thus obtained were plotted against the mole fraction of quencher in the micelles, as shown in Figures 2-5. The slopes of the linear regression lines gave the interaction parameters; all interaction parameters and N0 aggregation numbers are collected in Table 1. C16TAC. The apparent aggregation numbers are shown in Figure 2 for 100 mM C16TAC, without and with added salt. The repulsive interaction with the fluorocarbon (19) Infelta, P. P.; Gra¨tzel, M.; Thomas, J: K. J. Phys. Chem. 1974, 78, 190. Infelta, P. P.; Gra¨tzel, M. J. Chem. Phys. 1983, 78, 5280. (20) Tachiya, M. Chem. Phys. Lett. 1975, 33, 289; J. Chem. Phys. 1982, 76, 340; 1983, 78, 5282. (21) Barzykin, A. Private communication.

Figure 2. Apparent aggregation numbers determined from measurements on solutions of (b) 100 mM C16TAC quenched by C16PC, (×) 100 mM C16TAC, (0) 100 mM C16TAC-100 mM NaCl, (.)100 mM C16TAC-400 mM NaCl quenched by HFDePC by fitting to the model of eq 4. Napp values are given as a function of the mole fraction of quencher C16PC or HFDePC in micelles, XC16PC or HFDePC. The lines represent least-squares fits to the data.

quencher is enlarged when salt is added. The FC quencher gives an N0 value about 14% smaller than the HC quencher; this effect will be discussed below. The value of the quenching constant, kq ) 1.05 × 107 s-1, was almost the same for both quenchers and independent of the quencher concentration. From cmc and surfactant distribution measurements, an R value of -0.2 was reported for C16PC/C16TAC in 0.15

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Almgren et al. Table 1. Aggregation Numbers and Interaction Parameters from Time Resolved Fluorescence Quenching Studies of Several Surfactant Systems, using Both a Hydrocarbon and a Fluorocarbon Surfactant Quenchera surfactant system

Figure 3. Apparent aggregation numbers of 10 mM SDS-100 mM NaCl micelles as a function of mole fraction of quencher (b) C16PC and (O) HFDePC in micelle, and apparent aggregation numbers of 10 mM LiDS-100 mM LiCl micelles as a function of the mole fraction quencher ([) C16PC and (]) HFDePC in the micelle.

C16TAC (100 mM) C16TAC (100 mM) 100 mM NaCl C16TAC (100 mM) 400 mM NaCl SDS (10 mM) 100 mM NaCl SDS (10 mM) 100 mM NaCl LiDS 10 mM LiCl 100 mM LiPFN (100 mM) C12E8 (50 mM) C12E8 (50 mM) 200 mM NaCl

quencher N0 C16PC

103

R

quencher

N0

R

1.0

HFDePC

89

4.4

HFDePC

122

6

HFDePC

140

6.5

C16PC

105

2.4

C12PC

97

2.0

C16PC

88

C16PC

37

HFDePC

92 13.6

4.4

HFDePC

80 12.5

4.9

HFDePCb 42b

4.9b

C12PCc

89c -5.3c HFDePC

87 -4.1

C12PCc

91c -0.5c HFDePC

88

2.0

a All measurements at 25 °C unless otherwise indicated. b A turbidity in the solution developed when HFDePC was added. The solutions cleared on heating. The measurements where performed after cooling to 30 °C, but before visible turbidity had returned. c Results from ref 12.

Figure 4. Apparent aggregation numbers of 100 mM LIPFN as a function of the mole fraction quencher (b) C16PC and (O) HFDePC in the micelle.

Figure 5. Apparent aggregation numbers of 50 mM C12E8 without salt as a function of mole fraction of quencher (b) C12PC and ([) HFDePC in micelle, and with 200mM NaCl as a function of mole fraction of quencher (O) C12PC and (]) HFDePC in micelle.

M NaCl.22 As expected, this mixture is close to ideal. The interaction parameter from the fluorescence quenching is somewhat more positive, which may be due to the size polydispersity effects.23,24 (22) Nguyen, C. M.; Rathman, J. F.; Scamehorn, J. F. J. Colloid Interface Sci. 1986, 112, 438.

SDS, LiDS. Figure 3 shows the apparent aggregation numbers in 10 mM surfactant solutions of SDS (LiDS) with 100 mM NaCl (LiCl); the interaction parameters and aggregation numbers are reported in Table 1. A repulsive interaction with the cationic quencher is clearly indicated in all cases, much larger with the fluorocarbon than with the hydrocarbon quencher. It is remarkable that the results indicate strong repulsion already with the HC quencher; an attractive interaction would be anticipated for the combination of a positively charged quencher and an anionic micelle, as was found from cmc measurements.25 The concentrations of the free surfactants are, of course, much smaller outside the anionic-cationic mixed micelles than the ideal shares of the pure cmc values would suggest; both surfactants are attracted stronger to the micelles than in an ideal noncharged mixture. It is also clear, however, that the distribution of the surfactants over the micelles should not be much influenced by this attraction. The reason is that the effect of adding a few cationic surfactants to an anionic micelle is similar to the addition of a few hydrocarbon molecules, as was observed and discussed in connection with studies of effect of additives on the aggregation number of SDS micelles.26 The charged headgroup of the added surfactant will just replace a bound counterion at the micelle surface, with little effect on the electric energy. The main effect of the addition would then be an increase of the hydrophobic volume of the micelle, possibly amplified by an adjustment of the aggregation number to keep the number of anionic surfactants per surface area constant.26 The observed decrease of the apparent aggregation number could be an indication of a nonrandom distribution, due to the fact that the added cationic surfactants (23) Almgren, M.; Lo¨froth, J.-E. J. Chem. Phys. 1982, 86, 2734. Almgren, M.; Alsins, J.; Stam, J. van; Mukhtar, E. Prog. Colloid Polym. Sci. 1988, 76, 68. (24) Warr, C. G.; Grieser, F. J. Chem. Soc., Faraday Trans. 1, 1986, 82, 1825. (25) Holland, P. M.; Rubingh, D. N. In Cationic Surfactants. Physical Chemistry; Surfactant Sci. Ser. vol. 37; Rubingh, D. N., Holland, P. M., Eds.; Dekker: New York, 1991; p 141. (26) Almgren, M.; Swarup, S. J. Phys. Chem. 1983, 87, 876.

Fluorescence Quenching in Surfactant Mixtures

would prefer to enter those micelles that are already somewhat swollen, instead of the smaller ones with only anionic surfactant. This would result in an effectively segregative interaction, but also a polydispersity effect;23,24 both would give an apparent decrease of the aggregation number. In this connection the length of the quencher tail could be important. A series of measurements with 10 mM SDS, 100 mM NaCl, and C12PC as quencher, instead of C16PC, gave only a marginally reduced interaction parameter, Table 1. When the added cationic surfactants have replaced a large part of the “bound” counterions, the change of the electrostatic interactions becomes important, and simultaneously the curvature of the structure built by the closed packed counter charged surfactants becomes so small that a transition to rods or bilayer structures occurs.27 These results show that the theory needs to be developed further when long range electrostatic effects are involved. Such forces are not well described by pairwise interactions, and both the regular solution approach and the lattice model behind eq 1 are strictly based on uncharged components. Roughly speaking, the electrostatic part of the attraction of countercharged surfactants to ionic micelles is mainly an entropic effect from the release of bound counterions. The effect will be about equally large if two countercharged surfactant molecules are both added either to one micelle or to two different micelles and will not affect the distribution of the surfactants over the micelles. Two comprehensive overviews, in the same monograph, have recently discussed the various approaches used in the thermodynamic treatment of mixed micelle formation; however, none of them takes up the distribution of the surfactants over the micelles.28,29 We note again that the aggregation number obtained with the FC quencher is about 10% smaller than that obtained by using the HC quencher. The values are smaller for LiDS than for SDS; this is in accord with many earlier observations. The quenching constants are similar for the two quenchers and somewhat larger for the smaller LiDS micelles (kq ≈ 2.8 and 3.2 × 107 s-1, respectively, in SDS and LiDS). LiPFN. A perfluorinated surfactant was also investigated. The results, reported in Figure 4, indicate very small micelles, in line with the results reported by Muto et al.11 for LiPFOS, lithium perfluorooctyl sulfate. They found that the aggregation numbers depended strongly on surfactant concentration, growing from less than 20 at 0.03 M to about 60 at 0.5 M; the aggregation numbers were close to 40 in 0.1 M solutions. For LiPFN the values were about 10% smaller with the HC quencher than with the FC quencher, but as discussed below there are reasons to regard both values as less reliable than those for the other surfactants. There were some unusual features in our measurements. On the addition of HFDePC to the LiPPFN micelles at room temperature, the solutions became turbid already at low additions, indicating that large structures, probably bilayers, appeared. No such turbidity was observed when CPC was added. Fontell and Lindman27 have mapped the main features of the two-component phase diagrams of perfluorononanoic acid and some of its salts in water. A lamellar phase is the most prominent liquid crystalline phase and appears already in dilute aqueous solutions of the acid or its salts with dimethyl(27) Fontell, K.; Lindman, B. J. Phys. Chem. 1983, 87, 3289. (28) Nishikido, N. In Mixed Surfactant Systems; Surfactant Sci. Ser. vol. 46, Ogino, K., Abe, M., Eds.; Dekker: New York, 1993; p 23. (29) Graciaa, A. P.; Lachaise, J.; Schechter, R. S. In Mixed Surfactant Systems; Surfactant Sci. Ser. vol. 46; Ogino, K., Abe, M., Eds.; Dekker: New York, 1993; p 63.

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and diethylammonium ions. Only for the Li salt was a hexagonal phase observed. It is reasonable to expect the lamellar phase to appear at low concentrations also in HFDePC-LiPFN-water. It is interesting that no indications of a lamellar phase appears with C16PC as the cationic component under similar conditions. On heating, the turbidity disappeared, and the measurements could be performed at 30 °C, without any visible turbidity appearing. An examination with cryoTEM (the results are not shown; for a discussion of the method see ref 30 ) showed rather large aggregates (about 100 nm) to be present also in these solutions (at least in the vitrified specimen). It was impossible to judge how important these precipitates are and if they were of crystalline or liquid crystalline origin; in any case the TRFQ results must in this case be regarded with some caution. The results of the measurements using C16PC as quencher gave another unusual effect in that the kq values increased with the quencher concentration, from 6 × 107 to 7.4 × 107 s-1 over the concentration range measured. The quenching constant extrapolated to zero concentration was close to the constant value found for the FC quencher, 4.7 × 107 and 4.9 × 107 s-1, respectively. The increase of the rate constant indicates that the micelles really decreased in size in this case. C12E8. The nonionic system was studied in an earlier report,12 and the previous results are complemented by measurements with the FC quencher, Figure 5 and Table 1. This is the only case where we obtained attractive interactions with the FC quencher in hydrocarbon micelles. That the attractive interaction from the electrostatics dominates over the repulsion from the hydrocarbon-fluorocarbon interactions is in agreement with the results from cmc measurements on some similar systems.31 In the nonionic system similar N0 values were obtained with the FC and HC quencher. The quenching constant is lower in the nonionic micelles than in the other systems, kq ≈ 6.5 × 106 s-1 for HFDePC. Interaction Parameters from Fluorescence Quenching. With exception for the nonionic system at low salt, all systems with hydrocarbon-fluorocarbon interactions gave large repulsive interaction parameters. In some cases the values measured with the TRFQ method are probably influenced by polydispersity, which also gives a decrease of the apparent aggregation number with quencher concentration, but even so the repulsion is so large (>2RT) that a macroscopic phase separation would have occurred in the corresponding hydrocarbon-fluorocarbon mixtures. In the micellar systems microscopic demixing into coexisting populations of FC-rich and HCrich micelles is expected to occur first at higher mole fractions of the minority component than those used in the quenching studies. The mean field model predicts that phase separation occurs if the interaction parameter exceeds 2.0 (the value is slightly lower in a finite system such as a micelle), whereas a substantially larger value is required according to an exact lattice model, with only nearest neighbor interactions, even in finite systems.13 As remarked above in connection with the comparison of the interaction parameters from the cmc values in the regular solution approach, and the values obtained by fluorescence quenching, the simple models with only nearest neighbors interactions or mean exchange energies do not work well for the long range electrostatic interactions in ionic micelles. It would probably be profitable to (30) Almgren, M.; Edwards, K.; Gustafsson, J. Curr. Opin. Colloid Interface Sci. 1996, 1, 270. (31) Muto, Y.; Asada M.; Takasawa, A.; Esumi, K.; Meguro, K. J. Colloid Interface Sci. 1988, 124, 632.

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Figure 6. A family of fluorescence decay curves for pyrene in water at 5 × 10-7 M, with HFDePC added to concentrations in a range spanning the cmc (0-4 mM).

treat the electrostatics separately and use the exchange interaction parameters only for the local interactions. It is notable that the N0 values determined with FC and HC quenchers in all cases differ in a similar way: about 10% lower values with the FC quencher in HC micelles, and also about 10% lower value with the HC quencher in the FC micelle. We have considered various error sources but not found anything that could explain the findings. An error in the concentration of the quencher solution was ruled out, both from the fact that repeated measurements using freshly prepared solutions gave similar results, and from the close to equal values of the peak absorptivity of CPC and HFDePC in solutions with the same nominal concentration. The only possible explanation seems to be related to the strong effective repulsion between the fluorocarbon and pyrene; that such an effect is important in FC-rich micelles has been observed14 and will be further discussed below. The repulsion leads to a lower occupancy of FC quenchers in the subset of micelles that contains pyrene (in order to prevent excimer formation, the fraction of micelles with pyrene was always kept rather small), and therefore a low apparent aggregation number is obtained. Evidently, such an explanation does not help to explain the results for the fluorocarbon micelle. In that case, however, there are some further peculiarities that indicate a more complex situation. The observed enhancement of the quenching rate constant with the concentration of the HC quencher indicates that in this case the decrease of the aggregation number is not only apparent. Either the micelles containing both probe and quencher change to a smaller size or the reactants are forced together in some other way, when the quencher concentration is increased. Solubilization of Pyrene in Hydrocarbon and Fluorocarbon Micelles. Experiments were designed to quantify the nonideality of the distribution of pyrene between fluorocarbon and hydrocarbon surfactant micelles. The surfactant quencher provides an opportunity to determine the distribution of pyrene between water and micelles in a direct and instructive way. Figures 6 and 7 present families of decay curves for pyrene in water at 5 × 10-7 M, with HFDePC (Figure 6) or C16PC (Figure 7) added to concentrations in a range spanning the cmc (0-4 mM). In the case of the FC quencher, we note a dynamic quenching below the cmc and above the cmc when the micelles have formed a very fast-almost staticquenching, followed by a tail, representing pyrene in water. The decay constant of the tail remained almost constant above the cmc. With C16PC as quencher, a substantial portion of rapid, static quenching is readily observed also

Almgren et al.

Figure 7. A family of fluorescence decay curves for pyrene in water at 5 × 10-7 M, with C16PC added to concentrations in a range spanning the cmc (0-2 mM).

Figure 8. Stern-Vollmer plots (O) and the decay constants of the dynamic part for pyrene in water at 5 × 10-7 M (b) versus HFDePC concentration.

Figure 9. Stern-Vollmer plots (O) and the decay constants of the dynamic part for pyrene in water at 5 × 10-7 M (b) versus C16PC concentration.

below the cmc (and looking carefully, some static quenching is seen also in the FC quenching data). This static quenching shows that an association between pyrene and the surfactant occurs already below the cmc. In order to determine the cmc more precisely, the static fluorescence intensities were measured. The results are shown as Stern-Vollmer plots in Figures 8 and 9. For comparison also the decay constants of the dynamic parts, representing quenching in water, are shown. The cmc values obtained from both the intensities and the decay constants are in good agreement, 0.83 and 0.88 mM for C16PC and 2.46 and 2.54 mM for HFDePC, and in good agreement also with earlier determinations.17

Fluorescence Quenching in Surfactant Mixtures

Langmuir, Vol. 13, No. 17, 1997 4541

We assume that the association prior to the cmc yields a 1:1 complex, with association constants, KAF and KAH for the fluorocarbon and hydrocarbon surfactants, respectively. The dynamic quenching is characterized by secondorder quenching rate constants k2F and k2H, respectively, for HFDePC and C16PC, and the distribution between micelle and water by KDF and KDH, defined by

KDH )

[Py]mic

(7)

[Py]free[C16PC]mic

and correspondingly for KDF. Assuming that the decay in the complex occurs with a decay constant k1 and that the decay constant in aqueous solution is given by k2 ) 1/τ0 + k2H[C16PC]aq, the observed decay below the cmc should follow eq 8

F(t) ) B1 exp(-k1t) + B2 exp(-k2t)

(8)

The ratio of the amplitudes gives the ratio of the amounts of pyrene in the complex and in the free form, provided that the absorptivities are the same in these states. It then follows

B1 [complex] ) ) KAH[C16PC]aq B2 [Py]free

(9)

An attempt to fit the combined lifetimes and fluorescence intensities to a simple model of complete static quenching failed, probably because the contribution to the intensity from the complex was not negligible. Above the cmc, the decay may still be fitted fairly well to a two-exponential model. We have not tried to separate the fast initial quenching in contributions from pyrene present in the 1:1 complex, and pyrene in the micelles, where it is even more rapidly quenched than in the complex. The decay constant of the final part, representing free pyrene in the aqueous solution, does not change appreciably above the cmc, Figures 8 and 9, showing that the free concentration of surfactant remains almost constant. At high surfactant concentration a decrease of the decay constant is actually expected, as the free surfactant concentration decreases above the cmc in ionic systems. The amplitude of this part of the decay decreases rapidly with the surfactant concentration, however, as more and more pyrene is micellized, and the rapid decay could not be followed to high enough surfactant concentrations for a decrease to be clearly observed. The distribution constant of pyrene between micelles and water was estimated from the static fluorescence intensities. Assuming that the intensity observed above the cmc is proportional to the fraction of pyrene remaining in water times the intensity at the cmc, we obtain

Figure 10. Change of the solubility of pyrene in 50 mM C16TAC on addition of HFDePC. Table 2. Second-Order Quenching Rate Constant, 1:1 Association Constant, and Micelle to Water Partitioning Constant, for Pyrene with the Surfactant Quenchers in water at 25 °C surfactant

kQS, mol-1 dm3 s-1

KAS, mol-1 dm3

KDS, mol-1 dm3

C16PC HFDePC

6.3 × 109 5.1 × 109

4700 230

20 × 103 1.3 × 103

(11)

the fluorocarbon surfactant, by factors of 20 and 15, respectively. Solubility of Pyrene in Fluorocarbon and Hydrocarbon Solvents and Surfactant Micelles. The solubilities in various solvents give a straightforward measure of the relative strengths of the interactions. The solubilities were determined essentially as described earlier32 by saturating solvents or micellar solutions with crystalline pyrene. The solubilities, at 25 °C, of pyrene in n-dodecane and n-perfluoroheptane were determined as 0.14 M and 2.0 × 10-4 M, respectively, and the solubilities of pyrene in 50 mM micellar solutions were obtained as 5.7 mM for C16TAC and 0.096 mM for HFDePC. Whereas the solubilities differ by a factor of 700 between a hydrocarbon and a fluorocarbon solvent, and the difference in solubility between the micelles amounts to a factor of 60, the results from Table 2 suggest only a factor of about 15 between the affinities for the fluorocarbon and hydrocarbon surfactant micelles. Assuming a solubility of 6 × 10-7 M for pyrene in water,33 the solubility in the micelles corresponds to a distribution constant of 1.9 × 105 M-1 for pyrene between the C16TAC micellar pseudophase and water and 3.2 × 103 M-1 in the case of HFDePC. The value for C16TAC is an order of magnitude larger than that reported in Table 2 for C16PC, but in good agreement with earlier determinations for C16TAB and some other surfactant micelles.34 The discrepancy for the fluorocarbon micelle is smaller than a factor of 2 in the same direction. It is probable that the reason for the discrepancy is related to the fact that the values in Table 2 refer to surfactant concentrations very close to the cmc, where the gradual micelle formation probably means that the micelles are considerably smaller and less good solubilizers than at higher concentrations; also the concentration of micellized surfactant, taken as the difference from the cmc, is uncertain. The premicellar interaction with pyrene may also have effects not accounted for properly in the model of eq 10. We regard,

The results are collected in Table 2. Note that both complex formation and micelle solubilization are much more favored with the hydrocarbon surfactant than with

(32) Almgren, M.; Alsins, J. Prog. Colloid Interface Sci. 1987, 74, 55. (33) Schwartz, F. P. J. Chem. Eng. Data 1977, 22, 399. (34) Almgren, M.; Grieser, F.; Thomas, J. K. J. Am. Chem. Soc. 1979, 101, 279.

I0 I0 [Py]tot ) ) I Icmc [Py]aq

(

)

KDH I0 1+ ([C PC] - cmc) (10) Icmc 1 + KAHcmc 16 The slope of the linear plots of I0/I vs surfactant concentration obtained above the cmc thus gives the distribution constant:

KDH ) (slope)(1 + KAHcmc)

4542 Langmuir, Vol. 13, No. 17, 1997

Almgren et al.

Figure 11. Fluorescence decay curves from pyrene in LiDS-LiPFN mixed micelles, without quencher and with C16PC or HFDePC at a concentration of 0.25 mM as quencher. The total surfactant concentration was 10 mM, 0.100 M LiCl. The mole fractions of LiPFN in the surfactant mixture were 0, 0.1, 0.2, 0.55, 0.8, and 1, in (a) to (f), respectively. In all panels, the uppermost curve is without quencher, and the lowest with C16PC as quencher. Note that in panel c no decernible quenching was produced by HFDePC and that the two quenchers give very similar effects in panel f.

therefore, the distribution constants from the solubilities as more reliable. Another series of measurements were made in which the solubilities of pyrene in mixtures of C16TAC, 50 mM, and small amounts of HFDePC (up to 5 mM), were determined. The results are reported in Figure 10. The interpretation requires some discussion of the mixed solvent environment presented by the mixed micelles. At saturation, the solubility cs, is given by

µ° ) µ* + RT ln cs

(12)

where µ° is the chemical potential of the crystalline solute and µ* its standard state in the solvent in question, representing a state where the solute is entirely surrounded by solvent molecules. The most straightforward way to handle a solute in a mixture of two solvents, H and F, of composition xF, is to assume that the standard chemical potential is given by

µ* ) (1 - xF)µ*H + xFµ*F ) µ*H + xF(µ*F - µ*H) (13) The solubility in the mixed solvent is then given by

ln

cs(xF) cs(0)

) -xF

µ*F - µ*H RT

(14)

The results in Figure 10 give for ∆µ°transf ) (µ*F - µ*H) a value of 1.6RT. The distribution constants from the solubilities in the pure micelles give also a measure of the transfer free energy (in this case involving C16PC instead of C16TAC; the difference should be small), ∆µ°transf ) 4.1RT, which is substantially larger. Equation 14 has direct relevance for the value of N0 obtained in TRFQ. At very low quencher concentrations, where only micelles with no or one quencher are present, the free energy penalty for pyrene to be in a micelle with one quencher among the N0 surfactant molecules would be ∆µ°transf/(N0RT), or in the case studied, less favorable by about 2-5% than in a random distribution, and give an error in N0 of this magnitude. This is not enough to

explain the differences found; but maybe a combination of this bias and random errors is all there is to it. Equations 13 and 14 also shed some light upon the difference between the solubility ratio for hydrocarbon/ fluorocarbon solvents (600) and the corresponding ratio (70) for the hydrocarbon/fluorocarbon micelles. The fluorocarbon surfactant contains 80% fluorocarbon in the tail, and in addition the pyridinium headgroup is not fluorinated. The solubility ratios, interpreted with the eqs 13 and 14, suggest that the milieu in the fluorocarbon micelle is about 73% fluorocarbon, which appears as a reasonable result. Fluorescence Quenching in Microscopically Demixed Systems. When a fluorocarbon surfactant with at least eight fluorinated carbons is mixed with a hydrocarbon surfactant of comparable hydrophobicity and the same type of headgroup, demixing into HC- and FCrich micelles can occur. Results from static fluorescence quenching reported by Asakawa et al.14 strongly suggest that this happens in the LiDS-LiPFN system. This system in 100 mM LiCl, with pyrene as probe and C16PC or HFDePC as quencher, was studied by TRFQ, with results that further corroborate the interpretation. The series of quenching curves in Figure 11 shows that in the pure systems quenching of pyrene occurs with both quenchers, whereas in a 50:50 mixture only the hydrocarbon quencher appears to be active. The only reasonable explanation is that pyrene is almost completely solubilized in micelles without or with little FC surfactant. The hydrocarbon surfactant quencher is then active, whereas the fluorocarbon surfactant quencher very strongly avoids the hydrocarbon-rich micelles and therefore pyrene, so that no quenching is observed. What is notable and peculiar is the fact that no quenching at all by the FC quencher can be observed in the results for the mixture in panel c. We may assume that quenching would have been observed if 5% of the pyrene molecules were present in micelles with fluorocarbon quenchers. The penality for placing pyrene in pure FC micelles would have to be much higher than the ∆µ°transf of 1.6-4.1RT calculated

Fluorescence Quenching in Surfactant Mixtures

Figure 12. Pyrene fluorescence intensity as a function of composition in HFDePC-C16TAC mixed micelles. Diamonds and filled circles represent two separate series of measurements, with the concentration of HFDePC fixed at 9 and 10 mM, respectively. The difference in pure HFDePC is due to the fact that a larger fraction of pyrene is in the aqueous phase when the surfactant concentration is lower.

above, if 95% of the pyrene molecules would be present in micelles with only hydrocarbon surfactants. The system HFDePC-C16TAC was studied in greater detail. In this case the quencher is the only fluorocarbon surfactant. The fluorescence intensity as a function of composition is shown in Figure 12, and representative decay curves are shown in Figure 13. In a micelle solution of 10 mM FC the measured fluorescence stems from pyrene in the aqueous subphase. Addition of C16TAC first reduces the intensity, as more of the pyrene is transferred to the mixed micelles. The intensity then increases slowly with further addition of C16TAC, but is still strongly reduced compared to the value in pure C16TAC micelles. Without C16TAC or with C16TAC concentrations up to the intensity minimum, the decay curves show that almost all intensity stems from pyrene in water, quenched to about the same degree. The increase of the intensity at higher C16TAC concentration is accompanied by an increased lifetime of

Langmuir, Vol. 13, No. 17, 1997 4543

the most long-lived portion of pyrene, monitoring the decreased concentration of unmicellized HFDePC. Already at an addition of 1.6 mM C16TAC to the 10 mM FC solution, a fraction of pyrene seems to be present in micelles without FC quencher. At still higher C16TAC concentrations, from 4 to 16 mM, the decay curves remain almost unchanged. A fraction of about 0.001 of the intensity in the first channelswhich is 1.58 ns widesseems to come from pyrene in micelles without quencher, decaying with the same decay constant as in pure C16TAC. The deactivation leading to this final population of most long-lived probes also occurs in a very similar way at all C16TAC concentrations in this range. Since the deactivation in micelles with many quenchers is extremely quick, we had to check that the true zerotime intensity was captured in the measurements. The use of a better time-resolution, 0.79 ns per channel, gave the same results for samples with high concentrations of C16TAC (but indicated for HFDePC micelles with no or little C16TAC a fraction of pyrene in the aqueous solution that was strongly reduced, by factors of 2-3 compared to the results with low time resolution). Consequently, the fraction of micelles without FC surfactant in the C16TACrich samples should be close to 0.001, as discussed above. The invariance of the decay curves above 4 mM C16TAC is in accord with a demixing occurring close to the composition that corresponds to the minimum intensity, i.e., at a mole fraction of about 0.88 FC in the system. From group contribution calculations Asakawa et al.17 predicted demixing between micelles with FC mole fractions of 0.89 and 0.17 in this system, which agreed with the measured mixed cmc values. The preference of pyrene for the C16TAC-rich micelles is strong, and among those it would prefer the micelles without any FC surfactant. However, with an average of about 15 FC quenchers per micelle in the C16TAC-rich population, the fraction of micelles without quenchers would be only 3 × 10-7, if a random (Poissonian) distribution prevailed. Even though the repulsion between the FC and HC surfactants would increase it substantially, the fraction of quencherfree micelles must be very low in the nonideal mixture,

Figure 13. Pyrene fluorescence decay curves in HFDePC-C16TAC mixed micelles, as a function of composition. The uppermost curve in all panels represents the decay in C16TAC. The concentration of HFDePC was 10 mM, and C16TAC concentration were 0, 1.2, 1.6, 4.0, 10, and 15 mM, respectively, in panels a to f.

4544 Langmuir, Vol. 13, No. 17, 1997

much smaller than the value 0.001 referred to above, which hardly can be explained by the preference of pyrene for the FC-free micelles. With a Poissonian distribution, a fraction of micelles without FC quencher of 10-3, on the other hand, would correspond to an average of 6.9 quenchers per micelle, or a mole fraction in the order of 0.07, and taking the nonideality into account a mole fraction in the order of 0.1 for the FC-poor micelles would probably be required to give the observed results. There is strong evidence, in another system, that the demixing is much closer to complete than what the regular solution approach suggests.35 The present result is a further indication in that direction. Such an increased segregation on demixing is also supported by the coexistence curve calculated for the two-dimensional lattice model (2-D Ising model) with only nearest neighbor interactions, presented in ref 13, Figure 2. The coexistence curve is much flatter in the Ising model than in the mean field model and suggests a much more distinct demixing. The observed behavior with respect to quenching by the FC surfactant quencher strongly supports the proposition that demixing into two populations of micelles occurs in this system. The measurements do not monitor how size and shape of the micelles vary. (35) Asakawa. T.; Amada, K.; Miyagashi, S. Submitted to Langmuir.

Almgren et al.

Conclusions The TRFQ aggregation numbers for a variety of micelles are equal within about 10% when determined with both a hydrocarbon and a fluorocarbon surfactant quencher. There is a tendency to smaller values with the FQ quencher, which at least partly is due to a preference of the probe for micelles free from fluorocarbons. The repulsive interactions between hydrocarbons and fluorocarbons show up in an apparent aggregation number that decreases with the quencher concentration; the interaction parameter determined from this change is not in accord with that obtained from the cmc values of the mixed micelles. The preference of the aromatic probe for micelles without fluorocarbon, and of the fluorocarbon quencher for fluorocarbon surfactant micelles, enables a discrimination between fluorocarbon-rich and hydrocarbon-rich micelles, obtained on microscopic demixing: a HC quencher is effective in the mixture, but not a FC quencher. Acknowledgment. We are indebted to Dr. Jan Alsins for help with the solubility studies. The work was supported by the Swedish National Science Research Council, and by Knut and Alice Wallenberg’s Foundation. Ke Wang is thankful for a scholarship from World Laboratory, ICSC, Lausanne. LA970238+