Comparison of Vesicle Formation in Zwitanionic and Catanionic

Sep 11, 2009 - §Lehrstuhl f¨ur Makromolekulare Chemie II, Universit¨at Bayreuth, D-95440 Bayreuth, Germany. Received April 4, 2009. Revised Manuscr...
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Comparison of Vesicle Formation in Zwitanionic and Catanionic Mixtures of Hydrocarbon and Fluorocarbon Surfactants: Phase Behavior and Structural Progression Christian Wolf,† Katharina Bressel,‡ Markus Drechsler,§ and Michael Gradzielski*,‡ †

Lehrstuhl f€ ur Physikalische Chemie I, Universit€ at Bayreuth, D-95440 Bayreuth, Germany, ‡ Stranski-Laboratorium f€ ur Physikalische und Theoretische Chemie, Institut f€ ur Chemie, Strasse des 17. Juni 124, Sekr. TC7, Technische Universit€ at Berlin, D-10623 Berlin, Germany, and § Lehrstuhl f€ ur Makromolekulare Chemie II, Universit€ at Bayreuth, D-95440 Bayreuth, Germany Received April 4, 2009. Revised Manuscript Received July 14, 2009 The phase behavior of zwitanionic and catanionic mixtures of perfluoro and hydrocarbon surfactants has been studied for the case of lithium perfluorooctanoate (LiPFO) as anionic surfactant and tetradecyldimethylamine oxide (TDMAO) as zwitterionic or tetradecyltrimethyl ammonium bromide (TTAB) as cationic surfactant. Samples as a function of the mixing ratio were characterized by means of electric conductivity, light scattering, cryo-TEM, and rheology. Despite the equal chain length of the zwitterionic and cationic surfactant we observe largely different phase behavior in these systems with the formation of precipitates around equimolar mixing for the catanionic system, while no such precipitation is observed for the zwitanionic system. This can be rationalized in terms of the much lower interaction parameter β (from cmc measurements) for the case of the zwitanionic system. Accordingly, in the zwitanionic mixture a larger range of stable unilamellar vesicles is observed, while for the catanionic system larger vesicles are present, with a much stronger tendency for formation of multilamellar vesicles. Another interesting effect is that only for the case of the zwitanionic mixture a substantial increase of viscosity is observed prior to the formation of vesicles, which means that here the transition from spherical micelles to vesicles takes place via strongly anisometric micellar aggregates, which are not observed for the case of the catanionic mixtures. From these results it can be concluded that the structure and stability of vesicles formed in these mixtures are largely controlled by the extent of the electrostatic interactions which allows modification accordingly.

I. Introduction Spontaneous vesicle formation is a phenomenon frequently encountered in mixtures of cationic and anionic surfactants.1-5 For many such systems, unilamellar vesicles are formed,1,2,5,6 provided the chains are not too long as in that case precipitation and formation of crystalline chains may occur,1,7,8 which is driven by electrostatic surfactant ion pairing, thereby liberating their counterions which leads to a substantial gain in entropy. For the case when precipitation is suppressed (for instance by being above the crystallization temperature or due to kinetic hindrance of crystallization and/or precipitation), long-time stable unilamellar vesicles can be observed in catanionic surfactant mixtures.1,2,4-6,9 Much less investigated with respect to vesicle formation are mixtures of zwitterionic and anionic surfactants despite the fact that it is well-known that zwitterionic surfactants exhibit a behavior that is in between that of real nonionic surfactants, *To whom correspondence should be addressed. Tel: þ49 30 314 24934. Fax: þ49 30 31426602. E-mail: [email protected].

(1) Kaler, E. W.; Herrington, K. L.; Murthy, A. K.; Zasadzinski, J. J. Phys. Chem. 1992, 96, 6698. (2) Huang, J. B.; Zhao, G. X. Colloid Polym. Sci. 1995, 273, 156. (3) Safran, S. A.; Pincus, P. A.; Andelman, D.; Mackintosh, F. C. Phys. Rev. A 1991, 43, 1071. (4) Talhout, R; Engberts, B. F. N. Langmuir 1997, 13, 5001. (5) Marques, E. F.; Regev, O.; Khan, A.; Miguel, M. D.; Lindman, B. J. Phys. Chem. B 1998, 102, 6746. (6) Gradzielski, M. J. Phys.: Condens. Matter 2003, 15, R655. (7) Vautrin, C.; Dubois, M.; Zemb, Th.; Schm€olzer, St.; Hoffmann, H.; Gradzielski, M. Colloids Surf. A 2003, 217, 165. (8) Vautrin, C.; Zemb, T.; Schneider, M.; Tanaka, M. J. Phys. Chem. B 2004, 108, 7986. (9) Schm€olzer, St.; Gr€abner, D.; Gradzielski, M.; Narayanan, T. Phys. Rev. Lett. 2002, 88, 258301.

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e.g., such as of CiEj type, and that of cationic surfactants.10 Especially for dodecyl and tetradecyldimethylamine oxide strong synergistic interactions have been observed for their mixtures with anionic surfactants.11-13 Their structural and phase behavior in mixtures with cationic14,15 or anionic surfactants16-19 has been studied in some detail before and also in the presence of cosurfactants.14 However, for mixtures containing an amine oxide and an anionic surfactant the formation of stable unilamellar vesicle dispersions might even be more likely than for the case of catanionic systems because the strong interaction between the oppositely charged head groups, which favors crystallization, is present to a much lesser extent. Therefore, a significantly reduced tendency for precipitation is expected, while there is still a significant synergistic interaction in such mixtures. In general, for vesicle formation an appropriate balance has to be present between a sufficiently strong attractive interaction between the two head groups of the surfactant in order to shift the packing parameter into a region favorable for vesicle formation while being not so strong as to induce crystallization of the surfactant mixture. (10) Rosen, M. J. Langmuir 1991, 7, 885. (11) Kolp, D. G.; Laughlin, R. G.; Krause, F. P.; Zimmerer, R. E. J. Phys. Chem. 1963, 67, 51. (12) Gradzielski, M.; Hoffmann, H. Adv. Colloid Interface Sci. 1992, 42, 149. (13) Gradzielski, M.; Hoffmann, H. Prog. Colloid Polym. Sci. 1993, 93, 167. (14) Valiente, M.; Thunig, C.; Munkert, U.; Lenz, U.; Hoffmann, H. J. Colloid Interface Sci. 1993, 160, 39. (15) Gorski, N.; Gradzielski, M.; Hoffmann, H. Langmuir 1994, 10, 2594. (16) Pilsl, H.; Hoffmann, H.; Hofmann, S.; Kalus, J.; Kencono, A. W.; Lindner, P.; Ulbricht, W. J. Phys. Chem. 1993, 97, 2745. (17) Hoffmann, H.; Hofmann, S.; Illner, J. C. Prog. Colloid Polym. Sci. 1994, 97, 103. (18) Gradzielski, M.; Hoffmann, H. J. Phys. Chem. 1994, 98, 2613. (19) Kakitani, M.; Imae, T.; Furusaka, M. J. Phys. Chem. 1995, 99, 16018.

Published on Web 09/11/2009

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In order to move the packing parameter for a single-chain surfactant toward the formation of bilayers a more bulky alkyl chain is required. This is the case for branched surfactants but also for perfluorinated alkyl chains. Correspondingly, perfluorinated anionic surfactants are known to have a strong propensity of bilayer formation20-22 and can be expected to facilitate the formation of vesicle phases. Previous studies of mixtures of hydrocarbon and perfluoro surfactants have demonstrated the occurrence of synergistic effects in such mixtures.23-26 Due to the known incompatibility of fluorocarbons and normal hydrocarbons an interesting question regarding surfactant mixtures of hydrocarbon and perfluoro surfactants is whether they mix at all. This question has been debated for some time, but thorough investigations, for instance, employing the SANS technique of external contrast variation,27 showed that typically mixed aggregates are formed. Of course, this question arises much less for the case of oppositely charged perfluoro and hydrocarbon surfactants where due to the electrostatic attraction a strongly negative interaction parameter β is to be expected. For instance, for the case of cetyltrimethylammonium bromide (CTAB) and sodium perfluorohexanoate a β parameter of -19.4 has been observed together with the formation of vesicles,28 as in the case of oppositely charged hydrocarbon surfactants. Mixtures of CTAB and sodium perfluorooctanoate are a model case of a vesicleforming catanionic surfactant system; they are stabilized energetically and their bilayers possess a spontaneous curvature.29,30 Similar vesicle formation has been observed in other catanionic hydrocarbon/perfluoro surfactant mixtures.31-33 However, so far the case of zwitanionic hydrocarbon/perfluoro surfactant mixtures has rarely been studied, and one particular case of such vesicle formation has been reported for mixtures of the ammonium salt of perfluoro polyether carboxylate (PFPE) and a n-dodecylbetaine.34,35 The tendency to form amphiphilic bilayers in mixtures of anionic perfluoro and zwitterionic hydrocarbon surfactants has been confirmed by us in previous experiments in which we observed the formation of unilamellar vesicles in mixtures of tetradecyldimethylamine oxide (TDMAO) and lithium perfluorooctanoate (LiPFO), which takes place via the formation of intermediately present disklike micelles. In equimolar mixtures of TDMAO and LiPFO at a total concentration of 50 mM, rather monodisperse vesicles of about 14 nm radius are formed initially which grow substantially in size (up to about 30 nm radius) over (20) Boden, N.; Clements, J.; Jolley, K. W.; Parker, D.; Smith, M. H. J. Chem. Phys. 1990, 93, 9096. (21) Gebel, G.; Ristori, S.; Loppinet, B.; Martini, G. J. Phys. Chem. 1993, 97, 8664. (22) Iijima, H.; Kato, T.; Yoshida, H.; Imai, M. J. Phys. Chem. B 1998, 102, 990. (23) Hoffmann, H.; W€urtz, J. J. Mol. Liq. 1997, 72, 191. (24) Treiner, C.; Chattopadhyay, A. K. J. Colloid Interface Sci. 1984, 98, 447. (25) Muto, Y.; Yoda, K.; Yoshida, N.; Esumi, K.; Meguro, K.; Binana-Limbele, W.; Zana, R. J. Colloid Interface Sci. 1989, 130, 165. (26) Tiddy, G. J. T.; Wheeler, B. A. J. Colloid Interface Sci. 1974, 47, 59. (27) Burkitt, S. J.; Ottewill, R. H.; Hayter, J. B.; Ingram, B. T. Colloid Polym. Sci. 1987, 265, 628. (28) Iampietro, D. J.; Kaler, E. W. Langmuir 1999, 15, 8590. (29) Jung, H. T.; Coldren, B.; Zasadzinski, J. A.; Iampietro, D. J.; Kaler, E. W. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 1353. (30) Jung, H. T.; Lee, S. Y.; Kaler, E. W.; Coldren, B.; Zasadzinski, J. A. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 15318. (31) Pasc-Banu, A.; Stan, R.; Blanzat, M.; Perez, E.; Rico-Lattes, I.; Lattes, A.; Labrot, T.; Oda, R. Colloids Surf. A 2004, 242, 195. (32) Hao, J. C.; Hoffmann, H.; Horbaschek, K. Langmuir 2001, 17, 4151. (33) Regev, O.; Khan, A. J. Colloid Interface Sci. 1996, 182, 95. (34) Martini, G.; Ristori, S.; Rossi, S. J. Phys. Chem. A 1998, 102, 5476. (35) Rossi, S.; Karlsson, G.; Ristori, S.; Martini, G.; Edwards, K. Langmuir 2001, 17, 2340. (36) Weiss, T. M.; Narayanan, T.; Gradzielski, M.; Wolf, C.; Panine, P.; Finet, S.; Helsby, W. I. Phys. Rev. Lett. 2005, 94, 038303.

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the course of 10 min36,37 to become even larger and more polydisperse for still longer times. However, this investigation concentrated on the dynamics of the formation process. In addition, it was exclusively dedicated to the study of equimolar mixtures of zwitterionic and anionic surfactants, while the structural and phase behavior as a function of the mixing ratio of the surfactants was not addressed. Accordingly, we were interested in the more general behavior of zwitanionic surfactant mixtures with TMDAO as zwitterionic surfactant that has been studied in much detail previosly38 and LiPFO as anionic surfactant. In the work presented here, we performed a study of the phase behavior of such systems as a function of the mixing ratio of zwitterionic and anionic surfactant. Spontaneous formation of vesicles is frequently observed for mixtures that are highly asymmetric with respect to the length of the alkyl chains of the both surfactants6,28,29,39 or a contained cosurfactant.40 Accordingly, we decided to study a C14-hydrocarbon surfactant together with a C8-fluorocarbon surfactant for which a strong propensity for vesicle formation is expected. For a thorough comparison to a corresponding cationic/anionic system we also investigated the system with tetradecyltrimethylammonium bromide (TTAB) as cationic surfactant, which by its molecular structure closely resembles TDMAO; i.e., one CH3 group is substituted by an oxygen atom. Our aim is then to point out the particular differences observed between a purely catanionic and a zwitanionic surfactant mixture of hydrocarbon and perfluoro surfactant, which apart from the charge of the headgroup most closely resemble each other. In that context, we were mainly interested in the extent of vesicle formation in such mixtures, i.e., the vesicle range in the phase diagram, but in addition from the variation of the mixing ratio we also expect to be able to tune vesicle size and stability as a function of the bilayer composition, i.e., the ratio between the anionic and the zwitterionic surfactant. This is particularly interesting with respect to the impact in the presence of a zwitterionic compared with a cationic surfactant paired with an anionic surfactant in their ability to form stable vesicle systems. This study then enables us to learn how the electrostatic interaction, in otherwise very similar systems, affects the ability to form vesicles and their stability, as electrostatics is known to be of key importance for very small unilamellar vesicles.41

II. Materials and Methods II.1. Materials. Tetradecyldimethylamine oxide (TDMAO) was a gift by Clariant (Germany) and purified by recrystallization twice from acetone. Tetradecyltrimethylammonium bromide (TTAB) was purchased from Aldrich (99%) and purified by crystallization from ether/ethanol. Lithium perfluorooctanoate (LiPFO) was obtained from Fluka and recrystallized from ether prior to use. Perfluorooctanoic acid (C7F15COOH, PFOA) was purchased from Lancaster and used as supplied. Tetradecyltrimethylammonium hydroxide (TTAOH) was prepared starting from TTAB by passing a 200 mM TTAB solution through a strongly basic ion-exchange resin (ion exchanger III, Merck) that had been charged with 1 M NaOH solution. The absence of bromide in the eluate was verified by testing it with AgNO3 solution. The concentration of the TTAOH solution was (37) Weiss, T.; Narayanan, T.; Gradzielski, M. Langmuir 2008, 24, 3759. (38) Hoffmann, H.; Oetter, G.; Schwandner, B. Prog. Coll. Polym. Sci. 1987, 73, 95. (39) Yatcilla, M. T.; Herrington, K. L.; Brasher, L. L.; Kaler, E. W.; Chiruvolu, S. C. J. Phys. Chem. 1996, 100, 5874. (40) Gradzielski, M.; M€uller, M.; Bergmeier, M.; Hoffmann, H.; Hoinkis, E. J. Phys. Chem. B 1999, 103, 1416. (41) Oberdisse, J.; Porte, G. Phys. Rev. E 1997, 56, 1965.

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determined by titration with 50 mM HCl solution. All surfactant solutions were prepared using doubly distilled water. Samples were prepared by mixing appropriate amounts of the corresponding stock solutions of the pure surfactants (typically 50 or 100 mM). For the determination of the phase behavior the samples were shaken strongly (Vortex mixing) for homogenization and placed in a thermostated bath, and Vortex mixing was repeated daily until no further changes with time were observed. In the case of nonhomogeneous samples heating was done for homogenization. The phase behavior was typically recorded one week after the last heating of samples. II.2. Methods. II.2.1. Electrical Conductivity. The electrical conductivity was measured by means of an Automatic Precission Bridge B 905 (Wayne Kerr Instruments Ltd.) of the Wayne Kerr type. For that purpose, a platinated electrode was used and all measurements were done at 25.0 ( 0.1 C. II.2.2. Dynamic Light Scattering (DLS). Dynamic light scattering experiments were performed with a Zetasizer 3000 (Malvern) using laser light of wavelength of 633 nm and measurement under an angle of 90. Measurements were done at 25.0 ( 0.2 C and using PMMA cuvettes of 1  1 cm thickness. Some further measurements were done using an ALV/CGS-3 goniometer DLS instrument with a He-Ne laser of 633 nm wavelength. Here the samples were contained in cylindrical glass cells and thermostated at 25 ( 0.1 C. II.2.3. Turbidity. The extinction of the samples was measured using a single-beam spectrophotometer of the type Spectronic 601 (Milton Roy Co.) at 25.0 ( 0.2 C.

II.2.4. Cryogenic Transmission Electron Microscopy (Cryo-TEM)42. For the cryo-TEM studies, a drop of the sample

II.2.5. Freeze-Fracture Electron Microscopy (FF-TEM)43. For the freeze-fracture transmission electron microscopy (FFTEM) a small amount of the sample was placed on a 0.1 mm thick copper disk covered with a second copper disk. The sandwich was frozen by plunging it into liquid propane, which was cooled by liquid nitrogen. Fracturing and replication were carried out in a BALZERS BAF 400 freeze-fracture apparatus at a temperature of -150 C. Pt was deposited under an angle of 45. For further stabilization a carbon layer was deposited under an angle of 90. The replicas were afterward examined in a CEM 902 electron microscope (Zeiss, Germany). II.2.6. Rheology. More viscous samples were measured with by means of a RheoStress RS 600 instrument (Thermo Haake), (42) Adrian, M.; Dubochet, J.; Lepault, J.; McDowall, A. W. Nature 1984, 32, 308. (43) Reimer, L. Transmission Electron Microscopy; Springer Verlag: Heidelberg, 1984.

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Figure 1. Macroscopic behavior of mixtures of LiPFO with TTAB (a) or TDMAO (b) at constant total surfactant concentration of 50 mM at 25 C (L1: isotropic micellar phase, PB: permanent birefringence, SB: streaming birefringence, precipitate). for which a cone-plate configuration (C60/1 Ti, O60 mm) was used. For the low viscous samples a RheoStress RS 300 instrument (Thermo Haake) with a double-gap configuration (Searle-type, DG41, ISO 3219) was employed. The samples were always thermostatted at 25 ( 0.2 C. 

was placed on a lacey carbon copper transmission electron microscopy (TEM) grid (200 mesh, Science Services, M€ unchen, Germany) which was hydrophilized by glow discharge prior to usage. The majority of the liquid was removed with blotting paper, leaving a thin film stretched over the grid holes. The specimens were then shock-vitrified by rapid immersion into liquid ethane and cooled to ∼90 K by liquid nitrogen in a temperature-controlled freezing unit (Zeiss Cryobox, Zeiss NTS GmbH, Oberkochen, Germany). The temperature was monitored and kept constant in the chamber during all of the sample preparation steps. After the specimens were frozen, the remaining ethane was removed using blotting paper. The specimen was inserted into a cryo-transfer holder (CT3500, Gatan, M€ unchen, Germany) and transferred to a Zeiss EM922 Omega EFTEM instrument. Examinations were carried out at temperatures around 90 K. The transmission electron microscope was operated at an acceleration voltage of 200 kV. Zero-loss filtered images (ΔE = 0 eV) were taken under reduced dose conditions (100-1000 e/ nm2). All images were registered digitally by a bottom-mounted slow scan CCD camera system (HS 301 F, Proscan, Lagerlechfeld, Germany) combined and processed with a digital imaging processing system (EsiVision, Olympus-SIS, M€ unster, Germany).

III. Results and Discussion III.1. Phase Behavior. As a first step, we studied the phase behavior of systems for a constant total surfactant concentration of 50 mM for mixtures of TDMAO/LiPFO and TTAB/LiPFO as a function of the mixing ratio. This was done by visual inspection of the equilibrated samples with respect to their optical and flow properties. For that purpose, the samples were also observed between crossed polarizers and it was determined if they were isotropic, exhibit permanent (PB), or streaming (SB) birefringence. In addition, in Figure 1 regions of turbid appearance and of higher viscosity are depicted. It should be noted that all these denominations arise from the visual inspection of the equilibrated samples. The corresponding phase descriptions are given in parts a and b of Figure 1. In that context, it should be noted that the mixture TDMAO/LiPFO shows a strong pH variation as a function of the mixing ratio which leads to an increase by about one unit in the intermediate mixing range (see Figure S1 of the Supporting Information). In order to elucidate the pH dependence further, we also performed some additional experiments at pH = 8 in a Tris buffer but observed no significantly different phase behavior. In general, one finds a more pronounced turbidity for the samples at pH = 8, but this is presumably due to the higher ionic strength introduced by the Langmuir 2009, 25(19), 11358–11366

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Figure 2. Photographs taken between crossed polarizers of samples of mixtures of TTAB/LiPFO for a total surfactant concentration of 100, 75, 50, and 25 mM (from left to right) at constant molar ratio xTTAB = 0.45 at 25 C.

buffer which enhances the electrostatic screening. This then leads to a decreased repulsion between the vesicles and thereby to a higher scattering intensity. The phase behavior of the two systems is already markedly different with respect to the fact that only for TTAB precipitation is observed in the equimolar range (as frequently observed for catanionic surfactant systems1,6), while for TDMAO homogeneous solutions are always formed. The extent of nonturbid, micellar phases (L1) reaches about 15 mol % of admixed surfactant for hydrogenated and perfluoro surfactant for the catanionic case, but for the zwitanionic case about 35 mol % of LiPFO can be admixed while still being in the micellar region. The optical appearance of the samples with TDMAO is shown in Figure S2 of the Supporting Information. However, this additional nonturbid range for the TDMAO case is much more viscous than in the case of TTAB, where irrespective of the mixing ratio low viscous solutions are observed. In addition, for the case of TDMAO/LiPFO no permanent birefringence (PB) is observed, as is the case for TTAB/LiPFO mixtures. This is depicted in Figure 2 for TTAB/LiPFO mixtures with a constant molar content of 45% TTAB as a function of total concentration. It is evident that in the range of 25-100 mM all samples are birefringent but the extent of birefringence becomes more pronounced with increasing concentration. It should also be pointed out that the TDMAO samples were long-time stable; i.e., even the turbid samples did not really change in our experimental observation window, which was typically 6 months of observing the samples. Furthermore, it should be noted that the L1-phase in the TDMAO system shows streaming birefringence for a content larger than 15 mol % of LiPFO, and here for slow-moving samples in microslides we also observed birefringent textures in the polarization microscope (see Figure 10). For a comparison, we also studied the salt-free system TTAOH/PFOA but here already the admixture of less than 5 mol % of the respective other surfactant leads to a phase separation and one obtains an upper isotropic and a lower more turbid and birefringent phase which contains vesicles. Apparently, here the stronger electrostatic interaction (as it is not screened by formed salt) does not allow for the formation of single-phase vesicle systems. Polarization microscopy shows that in this range of permanent birefringence (PB) Maltese-crosses are observed (see Figure S5 in the Supporting Information) which are a typical signature of Langmuir 2009, 25(19), 11358–11366

Figure 3. Specific electric conductivity κ (0) and extinction (4) at 350 nm of mixtures of TTAB and LiPFO at constant total concentration of 50 mM as a function of the mole fraction x(TTAB) at 25 C. In addition, the linear connection of the conductivity between pure TTAB and pure LiPFO is given as a straight dashed line. Indicated are the micellar phase (L1), the vesicle phase (Lv), and the region of precipitation (P).

large multilamellar vesicles.44,45 In general, permanent birefringence can be associated with the presence of either a lamellar phase or multilamellar vesicles, whereas streaming birefringence (SB) is indicative of unilamellar vesicles or larger anisometric aggregates.44,45 From this it is already apparent that in the TDMAO system a much stronger tendency for the formation of unilamellar vesicles prevails. The range of streaming birefringence extends from xTDMAO = 0.15-0.85 for TDMAO/LiPFO, i.e., comprises the largest part of the phase diagram. All samples of the zwitanionic system TDMAO/LiPFO at 50 mM total concentration are isotropic and only become more turbid for a nearly equimolar mixing ratio where this range of turbidity extends more toward the LiPFO-rich side of the phase diagram (see Figure 3 and Figure S2 of the Supporting Information). Inspections of samples at the equimolar mixing ratio of TDMAO/LiPFO show that they become increasingly turbid and viscous with increasing concentration. Here it is interesting (44) Gradzielski, M. In Soft matter: complex materials on mesoscopic scale; Dhont, J. K. G., Gompper, G., Richter, D., Eds.: Forschungszentrum: J€ulich, 2002; Vol. B8, pp 1-23. (45) Platz, G.; Thunig, C.; P€olike, J.; Kirchhoff, W.; Nickel, D. Coll. Surf. A 1994, 88, 113.

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to note that in the range of 40f5 mM the viscosity becomes largely reduced by dilution and in the end is water-like. It should also be noted that only around 50 mM streaming birefringence is encountered at the first time (it is absent at lower concentrations), while samples above 60 mM show stress birefringence (which is an indication that above this concentration the vesicles are densely packed, thereby rendering them prone to deformations and formation of local lamellar structures), i.e., one goes rather sharply from a situation of individually mobile vesicles to a state where they are densely packed and deformed. In order to have some further insight into the chain-length dependence of the phase behavior in such mixtures we did also some exploratory studies on the corresponding mixtures with dodecyldimethylamine oxide (DDMAO) instead of TDMAO. Photographs of the corresponding samples are given in Figure S4 of the Supporting Information. It is interesting to note that the general appearance of the samples is very similar to that of the case of TDMAO. For the intermediate mixing range, where vesicle formation takes place for TDMAO, they are somewhat less turbid but that is presumably due to the fact that the bilayers with DDMAO are expected to be thinner and therefore the whole aggregates should scatter less. III.2. Structural Characterization. The phase descriptions of Figure 1 were determined by visual inspection of the macroscopic behavior. Of course, this is only a first start with respect to a precise characterization of the phase behavior, which was complimented by measurements of dynamic light scattering, turbidity, electrical conductivity, and electron microscopy. III.2.1. TTAB/LiPFO. As a first step, we analyze further the classical catanionic system TTAB/LiPFO. At constant total concentration of 50 mM the specific electrical conductivity and the extinction of the samples as a function of the mixing ratio are summarized in Figure 3. Upon admixture of the respective other component to the pure surfactant systems one observes a rather steep increase in turbidity for about 10 or 16% admixed TTAB or LiPFO, respectively. The region of vesicle formation is relatively symmetric with respect to the mixing ratio being a bit more extended on the side of the LiPFO. On the side rich in LiPFO, the increase in turbidity is significantly less pronounced but this can be explained by the fact that the refractive index of the perfluoro surfactant is relatively close to that of water (the refractive index increment, dn/dcg, is 0.00714 mL/g for LiPFO in the aggregated state46 and for TTAB it should be close to the 0.1506 mL/g reported for DTAB47). Due to this much lower contrast the formed vesicles are scattering light to a much lesser extent than their similar counterparts on the hydrocarbon rich side. At lower content of admixing one observes, in particular for the admixture of LiPFO to TTAB, first an increase of conductivity compared to the simple linear relation between the conductivity of the two pure surfactants. This can be attributed to the fact that upon the addition of the oppositely charged surfactants LiBr is released, which contributes more strongly to the conductivity than the pure ionic surfactants (Figure 3). Around equimolarity precipitation is observed. Only this is a real phase boundary while the line separating L1 and Lv is only drawn according to the different appearance of the samples, but there is no first-order phase transition taking place. In the precipitate region the samples are still relatively turbid, but this depends on time as the precipitation is a rather slow process and even after about a week is still continuing. At the same points where the turbidity is (46) P€ossnecker, G.; Hoffmann, H. Ber. Bunsenges. Phys. Chem. 1990, 94, 579. (47) Preu, H; Schirmer, C; Tomsic, M; Rogac, M. B.; Jamnik, A.; Belloni, L.; Kunz, W. J. Phys. Chem. B 2003, 107, 13862.

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Figure 4. Hydrodynamic radii obtained by means of dynamic light scattering for mixtures of TTAB and LiPFO at constant total concentration of 50 mM as a function of the mole fraction x(TTAB) at 25 C (the sample at x(TTAB) = 0.5 (P) was measured freshly prepared, i.e., before precipitation visually had set in).

sharply increasing one observes a marked relative decrease of conductivity. Both observations can directly be linked to the formation of vesicles, which due to their size leads a corresponding turbidity and a decrease of conductivity due to the entrapped counterions. Dynamic light scattering (DLS) experiments show that in the Lv region aggregates with hydrodynamic radii of 120-220 nm are present (Figure 4), with a tendency to become smaller with increasing content of TTAB. From the autocorrelation functions it can be concluded that they are rather polydisperse, as the cumulant analysis gives typical polydispersity indices in the range of 0.25-0.4. Therefore, DLS clearly confirms the formation of vesicles in this system. A freeze-fracture electron micrograph (Figure 5) taken from a freshly prepared sample of x(TTAB) = 0.5 of 50 mM (precipitation only sets in relatively slowly; therefore, the aggregates observed here should also be representative of the aggregates found in the neighboring regions of the phase diagram) shows very polydisperse vesicles in this sample (see also Figure S6 in the Supporting Information which contains further freezefracture electron micrographs of this sample). The majority have radii in the range of 150-400 nm (in agreement with the values observed by DLS), but there are also many smaller and some bigger ones visible, with radii up to 4 μm. The larger ones apparently are of multilamellar structure as evidenced by the cross-fractures seen (see arrows in Figure 5). III.2.2. TDMAO/LiPFO. More novel and interesting than the classical catanionic system TTAB/LiPFO is the zwitanionic mixture TDMAO/LiPFO with its lacking region of precipitate formation. Accordingly, it was investigated in further detail by means of conductivity and extinction measurements, as well as by dynamic light scattering. The results of these experiments are summarized in Figure 6 for the case of constant total surfactant concentration of 50 mM. From these experiments, it becomes clearly evident that substantial structural changes take place in the mixtures upon changing the mixing ratio in a similar fashion as for the catanionic case. In the region of rather pure surfactants, i.e., where only small amounts of the other surfactant are admixed, clear solutions are observed that contain only very small aggregates. The situation becomes much different for x(TDMAO) = 0.16-0.66. Here, the mixtures are much more turbid, and by dynamic light scattering aggregates with hydrodynamic radii in the range of 100-160 nm Langmuir 2009, 25(19), 11358–11366

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Figure 5. FF-TEM picture of a equimolar sample of TTAB/LiPFO at 50 mM total concentration at 25 C (arrows indicate a case of crossfracturing).

Figure 6. Specific electric conductivity (0), extinction at 350 nm (4), and hydrodynamic radius rh (O) of mixtures of TDMAO and LiPFO at constant total concentration of 50 mM as a function of the mole fraction x(TDMAO) at 25 C. In addition, the linear connection of the conductivity between pure TDMAO and pure LiPFO is given as dashed straight line.

are detected, i.e., much larger than would be feasible for micellar aggregates. At the same time, the electrical conductivity drops suddenly significantly below the line that is to be expected for simple mixtures. Again, this behavior can straightforwardly be explained by the formation of vesicles in this composition range which, by entrapment of the counterions into the vesicular core, lead to a corresponding reduction of conductivity and, by their size, to the corresponding turbidity of the samples. For a better comparison the hydrodynamic radii observed as a function of the mixing ratio are summarized in Table 1 for the TTAB/LiPFO and TDMAO/LiPFO mixtures. One clearly observes that the hydrodynamic radii become suddenly larger around equimolar mixing ratio where the vesicles are formed. In general, the values for the TTAB/LiPFO system are larger, and they are slightly larger for the LiPFO-rich side of the phase diagram. This interpretation and the sizes observed are confirmed further by cryo-TEM pictures of the corresponding samples as shown in Figure 7 for a 50 mM sample with 1:1 mixing ratio of Langmuir 2009, 25(19), 11358–11366

TDMAO/LiPFO. Here one can see very nicely unilamellar vesicles with a majority of radii in the range of 40-110 nm. Further pictures of this sample are given in the Supporting Information (Figure S7) and confirm the presence of polydisperse spherical vesicles with radii between the extremes of 20 and 200 nm where, however, the majority of the vesicles shows radii in the lower size range. It is interesting to note that despite the relatively large degree of polydispersity visible in this sample one has almost exclusively unilamellar vesicles present. As studied by DLS and extinction measurements (Figure 8) for the case of equimolar composition the variation of the total concentration in the range of 5-80 mM has only a rather small influence on the size of the vesicles formed in the solutions. Only for the highest concentration of 100 mM a substantial increase of hydrodynamic radius and extinction is observed. This indicates a substantial increase of the vesicle size, which should be due to the formation of multilamellar vesicles at this higher concentration. This is presumably caused by the fact that at this concentration of 100 mM the vesicles already reach a volume fraction of more than 0.32 (assuming a bilayer thickness of 3 nm and only having unilamellar vesicles present). Apparently, the corresponding dense packing of the vesicles is avoided by forming larger, multilamellar vesicles, and therefore, the effective volume fraction becomes reduced. Of course, an interesting point is how the transition from the micellar phase (which is present in the range of the two pure surfactants) into a vesicle phase takes place, which is initiated by admixing the other surfactant. Here it is interesting to note that prior to vesicle formation one always observes a pronounced increase in viscosity (see Figure 1) for the case of TDMAO/ LiPFO upon increasing the content of LiPFO, while such a viscosity increase is absent for the mixtures of TTAB/LiPFO. The macroscopic flow behavior was only investigated further for the case of the system TDMAO/LiPFO as only here more viscous samples are encountered in the phase diagram. For these samples, the apparent viscosity was measured as a function of the shear rate (Figure 9). It is interesting to note that for mixtures with x(TDMAO) of 0.8 and 0.7 rather viscous samples are present which exhibit pronounced shear thinning. In this DOI: 10.1021/la901191a

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Table 1. Hydrodynamic Radius rh (nm) as a Function of the Mole Fraction of LiPFO in Mixtures of TTAB/LiPFO and TDMAO/LiPFO As Obtained by Dynamic Light Scattering at 25 C xLiPFO TTAB/LiPFO TDMAO/LiPFO

0.0 2.3 4.1

0.1 4.2 2.1

0.2 8.4 5.3

0.3 111 13.1

0.4 145 116

0.5 195 112

0.6 216 99

0.7 141 111

0.8 5.5 155

0.9 3.6 12.9

1.0 2.0 2.0

Figure 9. Viscosity as a function of shear viscosity for various molar mixing ratios of TDMAO/LiPFO (total concentration 50 mM; measured with a double-gap geometry (SEARLE-System), 25 C), x(TDMAO) = 0, 0.9; O, 0.8; 4, 0.7; 3, 0.6; ), 0.5). Solid lines are fits with the Cross-equation.

Figure 7. Cryo-TEM picture of a 1:1 TDMAO/LiPFO mixture of 50 mM total concentration taken 1 day after preparation (the black spots are contamination by ice).

Figure 10. Polarization microscopy picture taken of a sample of TDMAO/LiPFO for a total surfactant concentration of 50 mM and a mol fraction x(TDMAO) = 0.7 at 25 C.

Figure 8. Extinction (at 350 nm) (0) and hydrodynamic radius rh (O) for equimolar mixtures of TDMAO and LiPFO as a function of total concentration at 25 C.

composition range a marked maximum of the zero-shear viscosity is passed while for samples containing more or less TDMAO low viscous samples are present. This maximum in viscosity occurs clearly well before the composition for vesicle formation (for x(TDMAO) > 0.65) is reached and upon entering the vesicle phase low viscous samples are present. This means that this viscosity increase takes place just in the region before the micelle-to-vesicle transition. Such a viscosity increase could be due to the formation of large wormlike micelles, but could alternatively also be caused by the formation of lamellar sheets or disk-like micelles. Polarization microscopy of these viscous samples, slowly flowing in a microslide, has shown the appearance of oily streaks (Figure 10) which are a typical signature of lamellar 11364 DOI: 10.1021/la901191a

phases of planar lamellae (at higher concentrations at this mixing ratio one finds a lamellar phase). Apparently, large planar lamellae are formed here which are responsible for the pronounced increase in viscosity and which at higher shear rates become oriented, thereby leading to shear thinning. For the case of the viscous samples the shear thinning behavior is well-described by the Cross-equation 48 η ¼

η0 -η¥ þ η¥ _ m 1þðτc γÞ

ð1Þ

where η0 and η¥ are the zero-shear viscosity and the high shear rate limit of the viscosity, respectively, τc is the characteristic time of orientation in the shear field, and m describes the cooperativity of the shear thinning process (becoming larger the more cooperative the phenomenon). For x(TDMAO) = 0.8 and (48) Cross, M. M. J. Colloid Sci. 1965, 20, 417.

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1, x1 is the mole fraction of component 1 in the mixed micelles, R1 is the mole fraction of surfactant 1 in the surfactant mixture, and f1 is the activity coefficient given by ln f1 ¼ βð1 -x1 Þ2

ð3Þ

Here, β is the dimensionless interaction parameter (obtained by dividing with kT as energy unit) for this surfactant pair that accounts for the effect of nonideal mixing. Therefore, one can directly calculate the interaction parameter β from the experimentally determined cmc’s according to49,52

β ¼ Figure 11. Critical micellar concentration (cmc) of mixtures of

TDMAO/LiPFO (0) and TTAB/LiPFO (4) at 25 C. The dashed lines are the theoretical curves for an interaction parameter β = -12 or -21 (in units of kT), respectively.

0.7 characteristic times τc of 0.070 and 0.106 s, respectively, are observed, with a coefficient m of 1.96 and 1.56, respectively. This means that the structural relaxation time becomes shorter with increasing proximity to the vesicle phase, indicating the presence of smaller anisometric aggregates when approaching the vesicles phase (which is corroborated by the lower zero-shear viscosity as well). In general, the rheological behavior of the zwitanionic TDMAO/LiPFO system is much different from that of the catanionic TTAB/LiPFO system as the latter, for the same range of total concentration, shows no significant viscosity changes and always remains low viscous. This can be attributed to the fact that only TDMAO/LiPFO lamellar sheets are formed previous to the structural transition to vesicles. III.3. Mixed Micellization: cmc’s. A very important aspect for the understanding of mixed micellar systems is the knowledge of the specific interaction between the two surfactants in solution as it can most easily be expressed in terms of the interaction parameter β of the regular solution theory49. The classical way to determine the interaction parameter β is by measuring the cmc of the corresponding surfactant mixtures. This was done by surface tension measurements, and some of these are summarized in Figure S8 (Supporting Information). The results of such measurements are given in Figure 11, and it is evident that the cmc’s of the mixtures are substantially lower than those of the pure surfactants, which are 0.12 mM for TDMAO,50 3.6 mM for TTAB,51 and 31 mM for LiPFO.50 Such a substantial reduction of the cmc means that the interactions in the surfactant mixtures are strongly synergistic, and according to Figure 11, this effect is much more pronounced for TTAB compared to TDMAO. This is not surprising as TTAB will interact with LiPFO more strongly as a cationic/anionic surfactant mixture while TDMAO forms a zwitterionic/anionic surfactant mixture, in which only attractive ion-dipole interactions are present. According to the regular solution theory the cmc’s of the surfactant mixtures are given by49,52 c1 ¼ x1 f 1 3 cmc1 ¼ R1 3 cmc

ð2Þ

where cmc and cmc1 are the cmc’s of the mixture and component 1, respectively, c1 is the solution-phase concentration of surfactant (49) Rubingh, D. N. In Solution Chemistry Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 1, pp 337-354. (50) P€ossnecker, G. Dissertation Universit€at Bayreuth, 1991. (51) Lianos, P.; Zana, R. J. Colloid Interface Sci. 1982, 88, 594. (52) Rosen, M. J.; Hua, X. Y. J. Colloid Interface Sci. 1982, 86, 164.

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  R1 cmc ln x1 3cmc1 3

ð1 -x1 Þ2

ð4Þ

This calculation was then used to fit the whole series of cmc data sets for the various mixing ratios. From this analysis, it was found that the best value for β is -12 ( 1.5 for the TDMAO/LiPFO mixture and -21 ( 1.5 for the TTAB/LiPFO mixture. Obviously, the interaction between the oppositely charged surfactant pair is about twice as intensive as for the zwitterionic/anionic pair. It is interesting to note that the value for the TDMAO/LiPFO pair is similar to that observed for mixtures of alkyl sulfates and alkyl betaines, which was found to be in the range of -11 to -15, depending on the length of the alkyl chains.53 Similarly, mixtures of sodium dodecyl sulfate and dodecyltrimethylammonium bromide show a β value of -25.5.54 Apparently, the values observed for our perfluoro mixtures are rather similar although somewhat less negative than would be expected for identically long hydrocarbon chains. They are shifted by about 2-4 kT, this effect being attributed to the repulsion arising from having the perfluoro and hydrocarbon alkyl chains paired together.

IV. Conclusions In this work, we studied vesicle formation in mixtures of hydrocarbon/perfluoro surfactants with LiPFO as the anionic perfluoro surfactant and cationic TTAB or zwitterionic TDMAO as the hydrocarbon surfactant. The presence of the perfluoro surfactant stabilizes the formation of bilayers, thereby leading to an extended range in the phase diagram where vesicles are found. The chain lengths of the hydrocarbon surfactants are identical, and they differ only with respect to the charge of their head groups (while otherwise the head groups resemble each other closely). Therefore, we compare a classical catanionic surfactant mixture with a zwitanionic (zwitterionic/anionic) one. This comparison was done in the concentration range below 100 mM, concentrating on series of constant total surfactant concentration of 50 mM. The two surfactant pairs are clearly different with respect to their interaction parameter β, which was determined by cmc measurements, being -21 for TTAB/LiPFO and -12 for TDMAO/ LiPFO, thereby evidencing the much weaker interaction present in the zwitanionic case. Their phase behavior differs already largely with respect to the fact that only for the catanionic case precipitation is observed while for the zwitanionic system homogeneous samples are obtained throughout the phase diagram. For salt-free catanionic mixtures no precipitation takes place, but instead a phase (53) Tajima, K.; Nakamura, A.; Tsutsui, T. Bull. Chem. Soc. Jpn. 1979, 52, 2060. (54) Lucassen-Reynders, E. H.; Lucassen, J.; Giles, D. J. Colloid Interface Sci. 1981, 81, 150.

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separation into a vesicle phase and water-rich phase is observed. Apparently, the electrostatic interactions are very important for the aggregation behavior observed in catanionic mixtures. In both mixed systems the formation of vesicle phases is observed for an extended range around equimolar mixing. Electron microscopy and dynamic light scattering show that for the zwitanionic case unilamellar vesicles with typical radii in the range of 70-100 nm are present, while for the catanionic mixtures less well-defined larger and partly multilamellar vesicles are observed. In addition, it is interesting to note that only for the zwitanionic case of TDMAO/LiPFO a pronounced viscosity increase is observed prior to the formation of the more turbid vesicle phase, i.e., at the border between micellar and vesicle phase. From the investigation by means of polarization microscopy it can be concluded that this increase in viscosity is most likely due to the formation of extended lamellar sheets. Apparently, the structural progression with the variation of the mixing ratio strongly depends on the extent of the electrostatic interaction and leads only for the less strongly interacting TDMAO to the formation of large anisometric aggregates. For TTAB no viscous phase is formed, but instead one observes directly a vesicle phase without passing through a region of a large disklike micelles or lamellar sheets. However, instead at about equimolar mixing ratio precipitate formation takes place at room temperature, and for slightly off-equimolar composition stable dispersions of rather large, often multilamellar vesicles are formed, which exhibit

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permanent birefringence. This a pronounced difference compared to the zwitanionic case in which smaller and unilamellar vesicles are observed that only transform to multilamellar ones at high concentrations above 100 mM. In summary, this means that stability, size, and morphology of the vesicles formed in these surfactant mixtures depend pronouncedly on the strength of the electrostatic interaction of their head groups. For the catanionic case, multilamellar and larger vesicles are formed compared to the zwitanionic case, and they also tend to display more precipitation. This finding allows the way for a systematic control of the vesicle properties and stability by controlling the electrostatic interaction of their head groups. Acknowledgment. For the preparation of the FF-TEM pictures at the Universit€at Bayreuth we thank R. Abdel-Rahem. For very substantial help with taking the polarization microscopy pictures we are very grateful to C. Thunig. In addition, we thank C. Abu-Hani for help with the manuscript. For financial support of parts of this work we are grateful to Sfb448 (funded by DFG). Supporting Information Available: Photographs taken of mixtures of TDMAO/LiPFO, polarization microscopic pictures, and additional freeze-fracture and cryo-transmission electron microscopy pictures. This material is available free of charge via the Internet at http://pubs.acs.org

Langmuir 2009, 25(19), 11358–11366