Demixing of Fluorinated and Hydrogenated Surfactants into Distinct

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Demixing of Fluorinated and Hydrogenated Surfactants into Distinct Aggregates and into Distinct Regions within Aggregates. A Combined NMR, Fluorescence Quenching, and Cryo-TEM Study M. Kadi,*,† P. Hansson,‡ and M. Almgren† Department of Physical Chemistry, Uppsala University, P.O. Box 532, SE-751 21 Uppsala, Sweden, and Department of Pharmacy, Uppsala Biomedical Centre, P.O. Box 580, SE-75123 Uppsala, Sweden

I. Furo´ Division of Physical Chemistry, Department of Chemistry, Royal Institute of Technology, SE-10044 Stockolm, Sweden Received June 21, 2002. In Final Form: September 11, 2002 The formation of two different kinds of micelles in the cationic surfactant mixture of HFDePC and CTAC as well as the increased mixing with increasing temperature has been investigated. A critical temperature of demixing was appreciated to 42 °C. NMR self-diffusion measurements reveal to us a coexistence of larger fluorocarbon-rich and smaller hydrocarbon-rich micelles, which is also observed using cryo-TEM. We also suggest, from 19F line width data, that the two surfactant species constituting the demixed micelles are “microphase-separated”. The existence of such aggregates has earlier only been speculated upon. From time-resolved fluorescence quenching measurements, we have estimated the fraction of fluorocarbon surfactant in the hydrocarbon-rich micelles to larger than 0.1.

Introduction The nonideal mixing behavior of fluorocarbon and hydrocarbon surfactants has received much attention in recent years. There is a repulsive interaction between the chains that in liquid hydrocarbon-fluorocarbon mixtures leads to segregation and phase separation. The possibility of demixing into two different types of micelles in hydrocarbon-fluorocarbon surfactant mixtures was first investigated by Mukerjee and Yang by electrical conductivity measurements.1 In surfactant mixtures with equally charged headgroups, they concluded that hydrocarbonrich and fluorocarbon-rich micelles coexisted in solution. Mysels was first to theoretically predict the coexistence of two populations of micelles.2 The theory that is most often used to describe mixtures of micelles is a pseudophase separation model where the surfactants are treated either as an ideal mixture3,4 or as a regular solution5,6 where the interaction parameter β describes the nonideality of mixing. Shinoda described the conditions for demixing using this regular solution theory.7 By measuring the miscibility of hydrocarbon-fluorocarbon liquid mixtures, it was concluded that a fluorocarbon chain length of at least eight carbons is necessary to cause phase separation. Many different techniques have been tried in order to experimentally verify this proposed microscopic demixing, for example, cmc measurements by surface tensio† ‡

Uppsala University. Uppsala Biomedical Centre.

(1) Mukerjee, P.; Yang, J. J. Phys. Chem. 1976, 80, 1388. (2) Mysels, K. L. J. Colloid Interface Sci. 1978, 66, 331. (3) Lange, H.; Beck, K. H. Kolloid Z. Z. Polym. 1973, 251, 424. (4) Clint, J. H. J. Chem. Soc., Faraday Trans. 1975, 17, 1327. (5) Holland, P. M.; Rubingh, D. N. J. Phys. Chem. 1983, 87, 1984. (6) Hoffmann, H.; Po¨ssnecker, G. Langmuir 1994, 10, 381. (7) Shinoda, K.; Nomura, T. J. Phys. Chem. 1980, 84, 365.

metry5,8-12 and electrical conductivity,8,12-14 fluorescence spectroscopy,12,15-20 NMR,21-25 gel filtration,26 ultracentrifugation,27 light scattering using index matching,27 and SANS.28 Some groups have obtained results that are contradictive, for example, the results of Mukerjee and Yang1 and Shinoda7 for the sodium dodecanoate-sodium (8) Esumi, K. Colloids Surf. A: Physicochem. Eng. Aspects 1994, 84, 49. (9) Ben Goulam, M.; Moatadid, N.; Graciaa, A.; Marion, C.; Lachaise, J. Langmuir 1996, 12, 5048. (10) Arai, T.; Takasugi, K.; Esumi, K. J. Colloid Interface Sci. 1998, 197, 94. (11) Tamori, K.; Kihara, K.; Esumi, K.; Meguro, K. Colloid Polym. Sci. 1992, 270, 927. (12) Asakawa, T.; Amada, K.; Miyagishi, S. Langmuir 1997, 13, 4569. (13) Asakawa, T.; Shiraishi, T.; Sunazaki, S.; Miyagishi, S. Bull. Chem. Soc. Jpn. 1995, 68, 2503. (14) Tamori, K.; Ishikawa, A.; Kihara, K.; Ishii, Y.; Esumi, K. Colloids Surf. 1992, 67, 1. (15) Almgren, M.; Wang, K.; Asakawa, T. Langmuir 1997, 13, 4535. (16) Muto, Y.; Esumi, K.; Meguro, K.; Zana, R. J. Colloid Interface Sci. 1987, 120, 162. (17) Asakawa, T.; Okamoto, T.; Miyagishi, S. J. Jpn. Chem. Soc. 1997, 46, 777. (18) Asakawa, T.; Saruta, A.; Miyagishi, S. Colloid Polym. Sci. 1997, 275, 958. (19) Asakawa, T.; Hisamatsu, H.; Miyagishi, S. Langmuir 1996, 12, 1204. (20) Asakawa, T.; Miyagishi, S. Langmuir 1999, 15, 3464. (21) Carlfors, J.; Stilbs, P. J. Phys. Chem. 1984, 88, 4410. (22) Asakawa, T.; Imae, T.; Ikeda, S.; Miyagishi, S.; Nishida, M. Langmuir 1991, 7, 262. (23) Clapperton, R. M.; Ottewil, R. H.; Ingram, B. T. Langmuir 1994, 10, 51. (24) Guo, W.; Guzman, E. K.; Heavin, S: D.; Li, Z.; Fung, B. M.; Christian, S. D. Langmuir 1992, 8, 2368. (25) Barthe´le´my, P.; Tomao, V.; Selb, J.; Chaudier, Y.; Pucci, B. Langmuir 2002, 18, 2557. (26) Asakawa, T.; Miyagishi, S.; Nishida, M. Langmuir 1987, 3, 821. (27) Haegel, F. H.; Hoffmann, H. Prog. Colloid Polym. Sci. 1988, 76, 132. (28) Burkitt, S. J.; Ottewill, R. H.; Hayter, J. B.; Ingram, B. T. Colloid Polym. Sci. 1987, 265, 628.

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perfluorooctanoate system. Recently, De Lisi et al.29 performed a thermodynamic study of this system and concluded that there is no demixing region in agreement with the findings of Shinoda.7 Carlfors and Stilbs21 found indications of demixing by NMR self-diffusion measurements on mixtures of sodium decanoate and sodium perfluorooctanoate. In the NMR self-diffusion studies by Asakawa et al.,22 second critical micellar concentrations, assigned to the formation of a second type of micelle with different surfactant composition, were determined by plotting the self-diffusion coefficients vs the reciprocal of the total surfactant concentrations for mixtures of lithium perfluorooctane sulfonate-lithium dodecyl sulfate and sodium perfluorooctanoate-sodium dodecyl sulfate. Because of the very low difference in refractive index between fluorinated compounds and water, conventional light scattering methods are not suitable for studying these types of systems. However, small-angle neutron scattering has been utilized to study anionic hydrocarbon-fluorocarbon surfactant mixtures. In the system studied in ref 28 no demixing into separate populations of micelles was observed, but the possibility of segregation within the mixed micelle was discussed. The effect of temperature on the miscibility or immiscibility of hydrocarbon-fluorocarbon surfactant mixtures has been investigated by a few authors. Asakawa has determined critical demixing temperatures for different fluorocarbon-hydrocarbon surfactant mixtures from fluorescence quenching studies.17 Earlier results from static and time-resolved fluorescence quenching on the cationic surfactant mixture cetyltrimethylammonium chloride-N-(1,1,2,2-tetrahydroperfluorodecanyl)pyridinium chloride (C16TACHFDePC) studied in this paper strongly suggest demixing into two stable populations of micelles.15 A second critical micellar concentration has been determined from surface tension measurements.30 Here we present further evidence for demixing from NMR self-diffusion and time-resolved fluorescence quenching measurements. We also suggest a segregation of fluorocarbon and hydrocarbon surfactants within the demixed micelles from 19F line width data. The increasing miscibility of the micelles with increasing temperature has been investigated. Materials and Methods Chemicals. The cationic fluorocarbon surfactant HFDePC (N-(1,1,2,2-tetrahydroperfluorodecanyl)pyridinium chloride) was a gift from Prof. Asakawa (Kanazawa University, Japan). The synthesis is described elsewhere.30 C16TAC (cetyltrimethylammonium chloride) was prepared from the bromide salt (Serva, analytical grade) by ion exchange. Deuterated water of 99.9% purity was purchased from Aldrich. Pyrene (Aldrich) was recrystallized twice from ethanol. NMR. All samples were prepared in D2O by weighing. Diffusion Measurements. The 1H self-diffusion measurements were performed on a Bruker DMX 200 spectrometer. The DSTEPGSE pulse sequence described by Jerschow and Mu¨ller was chosen to suppress convection artifacts.31 The diffusional decay32 of the headgroup protons of C16TAC and the headgroup protons of HFDePC with increasing gradient strength was recorded. The obtained diffusion coefficients are described by

Dapp ) pDmic + (1 - p)Dmon

(1)

(29) De Lisi, R.; Inglese, A.; Milioto, S.; Pellerito, A. Langmuir 1997, 13, 192. (30) Asakawa, T.; Hisamatsu, H.; Miyagishi, S. Langmuir 1995, 11, 478. (31) Jerschow, A.; Mu¨ller, N. J. Magn. Reson. 1997, 125, 372. (32) Tanner, J. E. J. Chem. Phys. 1970, 52, 2523.

because of fast exchange between monomeric and micellized surfactant. Dapp is the observed, time-averaged diffusion coefficient, Dmic and Dmon are the diffusion coefficients of the micelles and surfactants in the aqueous phase, and p is the fraction of micellized surfactant.21 At high surfactant concentration Dapp is equal to Dmic, as the monomeric contribution to the observed diffusion coefficient is negligible far above the cmc. The micellar hydrodynamic radius r is related to the measured diffusion coefficients through the Stokes-Einstein equation:

D ) kT/6πηr

(2)

where η is the temperature-dependent viscosity of D2O.33 The obtained hydrodynamic radius values were not corrected for the minor (a few percent) retardation effect imposed by the micelles on each other. 19F Line Width Measurements. 19F line widths of the trifluoromethyl group and of the difluoromethylene group closest to the pyridinium headgroup of HFDePC were determined by recording 19F NMR spectra on two different spectrometers, a Bruker DMX 200 and a Bruker DMX 500 with 19F resonance frequencies 188 and 470 MHz, respectively. Since the magnetic field inhomogeneity (measured by 1H NMR of water protons) was negligible, the line width data were extracted directly from the spectral lines by recording the frequency difference of the half-height points. TRFQ. The samples for the time-resolved fluorescence quenching measurements were prepared by solubilizing pyrene in a stock solution of C16TAC. After at least 3 days of stirring, the cationic fluorocarbon quencher HFDePC was added to the solution. The pyrene concentration was kept low enough to prevent excimer formation. A mode-locked Nd:YAG laser (Spectra Physics model 3800) was used to synchronously pump a cavitydumped dye laser (Spectra Physics model 375,344S) for the excitation. After frequency-doubling the excitation wavelength was 320 nm, and the pyrene emission was measured at 395 nm. This setup has earlier been described in detail.34 Cryo-TEM. The technique has been described elsewhere.35,36 In short, the procedure can be described as follows: A drop of the solution under study was placed onto a copper electron microscopy grid coated with a perforated polymer film. Excess solution was removed by blotting with a filter paper. The preparation of the sample film was done under controlled environment conditions, i.e., in a chamber at a constant temperature of 25°C and with a relative humidity of 98-99% to avoid evaporation of the liquid. Rapid vitrification of the thin film was achieved by plunging the grid into liquid ethane held just above the freezing point. The sample was then transferred to the electron microscope, a Zeiss 902A instrument operating at an accelerating voltage of 80 kV. The temperature was kept low, below -165 °C, during the entire procedure to prevent sample perturbation and formation of ice crystals.

Results and Discussion NMR. 1H Diffusion Measurements. The hydrodynamic radius determined from the self-diffusion measurements and calculated as given in eq 2 is plotted as a function of temperature for two different total surfactant concentrations and with added salt in Figure 1. The monomer concentrations are low, and the measured diffusion coefficients refer to diffusion of micelles. The larger values observed at low temperatures and high counterion concentrations for the fluorocarbon surfactant show that it spends, on average, more of the time residing in a larger micelle than the hydrocarbon surfactant. This behavior can be explained by considering coexistence of two different (33) Cho, C. H.; Urquidi, J.; Singh, S.; Wilse Robinson, G. J. Phys. Chem. B 1999, 103, 1991. (34) Almgren, M.; Hansson, P.; Mukhtar, E.; Stam, J van. Langmuir 1992, 8, 2405. (35) Bellare, J. R.; Davis, H. T.; Scriven, L. E.; Talmon, Y. J. J. Electron Microsc. Technol. 1988, 10, 87. (36) Almgren, M.; Edwards, K.; Karlsson, G. Colloids Surf., A 2002, 174, 3.

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Figure 1. Hydrodynamic radius of the micelles calculated from diffusion coefficients obtained by 1H PGSE NMR for HFDePC (open symbols) and CTAC (filled symbols) is plotted vs temperature for three different equimolar mixtures. Total surfactant concentration is (a) 20, (b) 5, and (c) 5 wt % in 100 mM NaCl.

types of micelles, one larger rich in fluorinated surfactant and the other smaller and rich in hydrogenated surfactant, which is in agreement with earlier results on this system. As mentioned in the Introduction, fluorescence quenching studies indicate demixing, and from surface tension measurements two different cmc’s have been determined.15,30 At 5 wt %, there is no significant difference in micellar radius, and no conclusions whether there is a segregation or not could be drawn from these results. The small difference actually observed can probably also be explained by different monomer concentrations; the cmc for pure C16TAC is 1.4 mM30 and for pure HFDePC is 2.5 mM,30 which is large enough for the diffusion of free monomers to affect the results.

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The tendency of fluorocarbon surfactants to form cylindrical and hence larger micelles and other types of aggregates with small curvature, such as bilayer structures, is an effect of the larger volume and stiffness of the hydrophobic chain. The dependence of salt and surfactant concentration on the aggregation behavior of HFDePC and other related cationic fluorocarbon surfactants as well as a couple of carboxylates was recently studied using cryo-TEM as the main technique.37 Relatively low concentrations of added salt were enough to cause elongation of the initially small spherical micelles. After addition of only 50 mM NaCl to a 50 mM solution of HFDePC cylindrical micelles were observed. This is interesting because many hydrocarbon cationic surfactants require more strongly bound counterions, such as bromide, to cause a transition into threadlike micelles. Increasing the surfactant concentration also induced micellar elongation much earlier than with cationic hydrocarbon surfactants. Increasing the temperature enhances the mixing process, similar to what is found for liquid hydrocarbonfluorocarbon surfactant mixtures7 and, indeed, for neat hydrocarbon-fluorocarbon mixtures.38,39 Above approximately 315 K (42 °C) only one kind of mixed micelle seems to be present in the solution as shown by Figure 1a,c. Critical demixing temperatures have been determined for different mixtures of anionic fluorocarbon and hydrocarbon surfactants by Asakawa et al.17 LiTS-LiPFN and DEATS-DEAPFN were examined at different temperatures using static fluorescence quenching. The critical temperatures for microscopic phase separation were determined to be 32.1 and 29.6 °C. The temperature above which SPFO-SDS mixtures form only one type of mixed micelle was in another study found to be approximately 40 °C.20 Increasing miscibility was also observed with increasing total surfactant concentration in this mixture. This was explained by formation of larger mixed micelles with a large solubilization capacity.20 Such an enhanced miscibility was not observed in the present study. In the self-diffusion studies of SD-SPFO mixtures by Carlfors and Stilbs, an increasing miscibility was observed when the temperature was raised from 25 to 60 °C.21 Asakawa used a group contribution theory in which molecular structure data and a group interaction parameter are utilized to predict immiscibility in mixed surfactant systems.30 Using this theory, the critical demixing temperature has been calculated for a number of hydrocarbon/fluorocarbon surfactant mixtures. In the system HFDePC-C16PC (cetylpyridinium chloride) the critical temperature of demixing was estimated to 45 °C at a mole fraction of 0.555 of the fluorinated surfactant,30 close to the critical temperature observed by us in HFDePC-C16TAC. In our experiments two different types of micelles were only observed when the difference in micellar radius was large enough to cause a significant difference in the observed diffusion coefficients. Under these circumstances it should be possible to use other methods, for instance, SANS with contrast matching, to settle whether demixing occurs. We plan to perform these kinds of experiments in the near future. Hopefully, they will provide us with additional informtion on, for instance, the composition of the demixed micelles. Refractive index matching was (37) Wang, K.; Karlsson, G.; Almgren, M.; Asakawa, T. J. Phys. Chem. B 1999, 103, 9237. (38) Hildebrand, J. H.; Fischer, B. B.; Benesi, H. A. J. Am. Chem. Soc. 1950, 72, 4348. (39) Campbell, D. N.; Hickman, J. B. J. Am. Chem. Soc. 1953, 75, 2881.

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utilized in light scattering experiments by Hoffmann. In glycol-water mixtures either the hydrocarbon or the fluorocarbon micelles could be made invisible by varying the composition of the solvent.27 Molecular Separation of Fluorinated and Hydrogenated Surfactants within the Aggregates. One way of studying the molecular (solvent) environment of molecules by NMR is via the solvent-induced changes in the various chemical shifts.40-42 Depending on the nuclei, these shift effects are of vastly different magnitude: usually, 1H shift effects are small while 19F shift effects are large.43,44 Hence, there are numerous examples, also in surfactant solutions,23,45-52 of exploiting 19F chemical shifts for revealing information about molecular environments. There are two NMR parameters to proceed with. If the molecular environments are unchanged on the time scale of the time-dependent NMR signal (microseconds to seconds, depending on samples) for the involved 19F nuclei, the presence of different environments is simply revealed by spectral broadening and/or splitting. Both effects are linear functions of the strength of the magnetic field employed in the NMR experiments. If the exchange among molecular environments with different chemical shifts is fast on the time scale of the time-dependent NMR signal, one can detect no spectral splitting, and the spectral broadening becomes a quadratic function of the magnetic field strength. This latter can be expressed,47,50,53,54 in the case of two sites experiencing a first-order exchange, as 2 Rex 2 ) (δγB0) x(1 - x)τe

(3)

where δ represents the difference between the two isotropic chemical shifts, x the relative population at one of the sites, and τe the exchange time which yields the residence time τx ) τe/(1 - x) of the spin-bearing species at this particular site. (Note that this expression was misprinted in ref 50 although the results there were obtained by evaluating the correct expression.) This expression remains valid as long as

x 1 . δγFB0; . δγFB0 τe (1 - x)τe

(4)

That is the condition for the so-called fast exchange regime. The linear field dependence signifies that in the static case the line broadening is inhomogeneous; i.e., different (40) Emsley, J. W.; Feeney, J.; Sutcliffe, L. H. High-Resolution Nuclear Magnetic Resonance Spectroscopy; Pergamon Press: Oxford, 1965; Vol. 1. (41) Rummens, F. H. A. Van der Waals Forces in NMR Intermolecular Shielding Effects; Springer: Berlin, 1975. (42) Dios, A. C. D.; Jameson, C. J. Annu. Rep. NMR Spectrosc. 1994, 29, 1. (43) Abraham, R. J.; Wileman, D. F. J. Chem. Soc., Perkin Trans. 1973, 1027. (44) Abraham, R. J.; Wileman, D. F. J. Chem. Soc., Perkin Trans. 1973, 1521. (45) Muller, N.; Simsohn, H. J. Phys. Chem. 1971, 75, 942. (46) Guo, W.; Brown, T. A.; Fung, B. M. J. Phys. Chem. 1991, 95, 1829. (47) Guo, W.; Fung, B. M.; O’Rear, E. A. J. Phys. Chem. 1992, 96, 10068. (48) Petit, F.; Iliopoulos, I.; Audebert, R. Polymer 1998, 39, 751. (49) Bossev, D. P.; Matsumoto, M.; Nakahara, M. J. Phys. Chem. B 1999, 103, 8251. (50) Furo´, I.; Iliopoulos, I.; Stilbs, P. J. Phys. Chem. B 2000, 104, 485. (51) Oda, R.; Huc, I.; Danino, D.; Talmon, Y. Langmuir 2000, 16, 9759. (52) Iliopoulos, I.; Furo´, I. Langmuir 2001, 17, 8049. (53) Sandstro¨m. J. Dynamic NMR Spectroscopy; Academic Press: London, 1982. (54) Lian, L. Y.; Roberts, G. C. K. Effects of chemical exchange on NMR spectra. In NMR of Macromolecules; Roberts, G. C. K., Ed., Oxford University Press: Oxford, 1993; pp 153-182.

molecules contribute to the spectrum at distinct NMR frequencies. Hence, this inhomogeneous broadening55 can be refocused by spin-echo experiments. On the other hand, the rapidly fluctuating chemical shift in the second case causes line broadening via fast transverse relaxation,55 and this broadening cannot be refocused spin-echo experiments. In the present case, the data at low temperatures in Figure 2 clearly show a line width that has a stronger than linear dependence on the field strength. There is no other plausible transverse relaxation mechanism55 except molecular exchange among environments with different chemical shifts that can provide such a field dependence. Note that other relaxation mechanisms (e.g., dipole-dipole relaxation) usually produce transverse relaxation rates that, if any, decrease with increasing magnetic field. Hence, their simultaneous presence may mitigate the actual field dependence to slower than quadratic. At high temperatures (Figure 2), the obtained line width is roughly independent of the field strength which indicates that those other relaxation mechanisms dominate. We interpret these data together with those obtained in fluorescence, microscopy, and diffusion experiments in terms of the following model. At low temperature and at high ionic strength, there are two sorts of aggregates present: one dominated by HFDePC and one by CTAC. As we are going to see, at least one of these aggregates must contain a significant amount of the “minority” surfactant as well. As the temperature increases and/or the ionic strength decreases, the two sorts of surfactants achieve a better mixing. As concerning the solution with 5% surfactant content, the 19F line width for difluoromethylenes close to the headgroup remains unchanged and roughly field-independent in the whole investigated temperature range. On the other hand, the line width of trifluoromethyls increases with decreasing temperature and with increasing field. This can be explained by exchange of HFDePC between aggregated and aqueous environments with an exchange time that, at low temperatures, is on the order of microseconds. Such an exchange produces a smaller effect on the line width for CF2 groups close to the micellar surface since the difference between their chemical shifts in the two involved environments is small. On the other hand, the chemical shift difference is larger (by a factor of 2-3) for CF3 groups at the end of the tail.45 Hence, the exchange broadening, proportional to the square of the chemical shift difference as given in eq 1, can turn out to be much larger for CF3 than for CF2 groups. Similar effects where observed in other micellar systems of fluorinated surfactants.47,50 The same exchange process cannot explain the data obtained at higher (20%) surfactant concentrations. There, the CF2 line widths at low temperatures were larger and more field dependent than the CF3 line widths. (This by no means excludes aggregate-aqueous exchange, but the presence of that exchange is masked by another process.) Such a behavior requires an exchange between sites for which the chemical shift difference is larger for CF2 than for CF3 fluorines. To provide an effect of the correct magnitude (recall eq 3), the exchange must proceed on the microseconds time scale. The molecular mechanism that accounts for these observations is illustrated in Figure 3, which depicts an aggregate within which the two constituting surfactant species are “microphase-separated”. Although the existence of such aggregates has been (55) Abragam, A. The Principles of Nuclear Magnetism; Clarendon: Oxford, 1961.

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Figure 3. Schematic picture of the microphase separation indicated by the 19F line width data. Fluorocarbon surfactants, with the headgroups crowded mostly in domain A, provide a fluorocarbon intermolecular environment for their CF2 groups located close to the headgroups. In domain B, this environment is hydrogenated and thereby provides a different 19F chemical shift for those CF2 groups. On the other hand, surfactant mobility and tail flexibility, however limited for fluorocarbon surfactants,53,54 make that the trifluoromethyl groups reside in a mixed fluorocarbon-hydrocarbon environment that changes little if the headgroup is moved from domain A to domain B. Hence, the CF3 chemical shift is less strongly modulated by this or related exchange.

Figure 2. 19F line widths vs temperature obtained at 470 MHz (filled circles) and 188 MHz (open circles) resonance frequencies. (a) and (b) show the data from the difluoromethylene and the trifluoromethyl groups, respectively, for the 5 wt % sample and (c) and (d) the same for the 20 wt % sample.

upon,56-58

speculated to the authors’ knowledge there is, as yet, no convincing experimental evidence for their

existence. Within such an aggregate, the intermolecular environment of CF3 fluorines is, for obvious geometrical reasons, rather insensitive to whether a HFDePC surfactant reside in region A (dominated on the micellar surface by HFDePC) or region B (dominated by CTAC). On the other hand, the intermolecular environment of CF2 groups close to the headgroups changes a lot upon moving an HFDePC molecule between the two regions because the 19F chemical shift depends on whether the hydrophobic intermolecular environment is fluorinated or hydrogenated.23 Hence, an exchange process of HFDePC molecules between the two regions results in exchange broadening of the 19F lines that is larger for the CF2 than for the CF3 groups. This exchange occurs either between two different micelles, of which at least one must consist of microphase-separated surfactants as described above, or between different regions within the same aggregate. It is possible that only the fluorocarbon-rich larger micelles contain segregated surfactants, since in these cylindrical micelles the chains are more closely packed, which leads to an increased repulsion between them.19 We found no other plausible molecular mechanism to explain this unusual finding. Time-Resolved Fluorescence Quenching. Nonexponential fluorescence decay curves for pyrene in a 50: 50% mixture of C16TAC-HFDePC at a total surfactant concentration of 5 wt % are shown in Figure 4 at a low and a high temperature. The upper exponential decay curves in both figures represent pyrene decay in pure C16TAC micelles. At 27 °C the lifetime of the most long-lived fraction of pyrene in the mixture, given by the tail in the nonexponential decay, is approximately the same as the pyrene lifetime in pure C16TAC micelles. This indicates that, even in a 50:50% mixture of C16TAC and the fluorocarbon quencher HFDePC, there must be pyrene solubilized in micelles without fluorocarbon quenchers. The fraction of quencher-free micelles can be determined from the fraction of the intensity in the first channel that stems from pyrene in micelles without quencher, which is close to 0.0001. By assuming a Poissonian distribution of the fluorocarbon quenchers among the hydrocarbon(56) Fromherz, P. Chem. Phys. Lett. 1981, 77, 460. (57) Kamogawa, K.; Tajima, K. J. Phys. Chem. 1993, 97, 9506. (58) Ristori, S.; Magiulli, C.; Appell, J.; Marchionni, G.; Martini, G. J. Phys. Chem. B 1997, 101, 4155.

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Figure 4. Nonexponential pyrene decay curves in 50-50% mixtures of HFDePC and CTAC at a total surfactant concentration of 5 wt % at two different temperatures: (a) 25 °C and (b) 60 °C. The upper exponential decay curves in both figures represent pyrene decay in pure CTAC micelles.

rich micelles, the average number of quenchers per micelle required to give a fraction of 0.0001 of quencher free micelles would be 9.2. This corresponds to a mole fraction of about 0.1 of the fluorinated surfactant in the hydrocarbon-rich population of micelles, assuming the aggregation number to be ≈100. Effects of nonideality were not taken into account in this calculation. From solubility studies15 pyrene was found to prefer C16TAC micelles over HFDePC micelles by a factor of 60, and among the C16TAC-rich micelles it strongly avoids those containing one or more quencher surfactants. This avoidance of the quencher for micelles containing pyrene results in less quenching, and the average number of quenchers per micelle is probably larger than 9.2. The mole fraction of fluorinated surfactant in the hydrocarbon-rich population should for this reason be substantially larger than the value of 0.1 calculated above. Pyrene decay curves for mixtures at higher total surfactant concentrations (12 and 20 wt %) with added salt (100 mM NaCl) and with molar fractions of the fluorocarbon surfactant varied between 0.30 and 0.60 were also collected (not shown). The results indicated the same fraction of fluorocarbon surfactant in the hydrocarbonrich population as was found for the 50:50% mixture at 5 wt %. This is expected when demixing into different populations of micelles occurs. Changing the composition of the mixture leads to a change in the number of micelles of the two types, but the composition of the micelles remains the same. This mixture has earlier been studied in detail at different molar ratios using both static and time-resoved fluorescence quenching.15 Those results indicated demixing in a region with a mole fraction of the fluorocarbon surfactant below 0.88. The fraction of quencher free micelles was here determined to 0.001, i.e., a factor of 10

Figure 5. Cryo-TEM micrographs taken from samples with a total surfactant concentration of (a) 5 wt % in water and (b) 5 wt % in 100 mM NaCl. Bar equals 100 nm.

larger than the present result. In that case the total surfactant concentration was much smaller, and it is possible that the fraction of HFDePC present in water cannot be neglected. Our new results seem more reasonable and are in better agreement with calculations. From the group contribution theory Asakawa predicts demixing at mole fractions of the fluorocarbon surfactant between 0.89 and 0.17,30 which agrees well with cmc values determined from surface tension measurements.30 Raising the temperature to 60 °C increased mixing, and quencher free micelles could no longer be observed Figure 4b. This agrees well with the results from the NMR selfdiffusion measurements where two types of micelles, fluorocarbon-rich and hydrocarbon-rich, were conjectured to coexist at temperatures below 42 °C. Note that for the 5 wt % sample no significant difference in micellar radius was observed, and from those measurements alone nothing could be said about a possible segregation. Moreover, an exchange of surfactants between hydrocarbon- and fluorocarbon-rich microdomains within the aggregates was not observed at this low concentration, but the fluorescence quenching results suggest demixing into two different populations of micelles independent of total surfactant concentration. Note also that the NMR line widths are also a function of the exchange kinetics that has been observed to vary a lot even into very slow regimes in mixed fluorocarbon-hydrocarbon surfactant systems.24 Cryo-TEM. The micrograph shown in Figure 5a was taken on an equimolar sample of HFDePC-C16TAC with a total surfactant concentration of 5 wt % in water at 25 °C. Only spherical micelles were observed. In the sample with 5 wt % surfactant in 100 mM NaCl spherical micelles

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were seen together with larger cylindrical micelles (Figure 5b). This was what was expected from the NMR diffusion measurements, where two different sizes of the micelles were determined. Conclusions The main conclusions from the combination of the different techniques in this study is the coexistence of hydrocarbon- and fluorocarbon-rich micelles in the solutions, which is in line with earlier findings on this cationic surfactant mixture. The strongest indication for demixing was obtained from the NMR self-diffusion results, where two different micellar radii have been determined. The only plausible explanation is that the fluorinated and the hydrogenated surfactant resides in micelles that differ in

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size. This is further supported by the fluorescence quenching measurements, where separate solubilization of the fluorescent probe pyrene and the fluorocarbon quencher HFDePC into different kinds of micelles was observed. The 19F line width measurements at high surfactant concentration and with added salt suggest that in addition to demixing into separate micelles there is also a segregation of the surfactants within the demixed micelles. Increasing the temperature led to an increased mixing of the surfactants, and a critical demixing temperature was determined to approximately 42 °C. By visual inspection using cryo-TEM, cylindrical micelles were seen together with spherical micelles. LA020579+