Counterion Effects on Aggregate Size and Shape ... - ACS Publications

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Articles Counterion Effects on Aggregate Size and Shape in Dilute Binary Solutions of Fluorinated Ammonium Carboxylate Surfactants O. Regev,* M. S. Leaver,*,† R. Zhou,† and S. Puntambekar†,‡ Chemical Engineering Department, The Ilse Katz Center for Meso and Nanoscale Science and Technology, Ben-Gurion University of the Negev, 84105 Beer-Sheva, Israel, and Centre for Materials Science, Department of Physics, Astronomy and Mathematics, University of Central Lancashire, Preston, PR1 2HE, Lancashire, United Kingdom Received August 24, 2000. In Final Form: May 17, 2001

The dilute liquid phase of a range of substituted ammonium perfluorodecanoate surfactants in water have been investigated using cryo-transmission electron microscopy, nuclear magnetic resonance, and optical techniques. As the counterion became more “hydrophobic”, at a fixed surfactant volume fraction, the aggregates formed in the liquid phase possessed decreasing interfacial curvature. This manifests itself as a structural transition in the aggregates from micelles to vesicles then on to flat bilayers. The aggregation form in the dilute region was found to influence the structure of the lyotropic liquid crystalline phases formed at higher concentrations. The experimental observations made in this contribution are discussed within a simple framework of enhanced counterion binding.

1. Introduction Surfactant aggregates formed at low concentrations are labile and dynamic objects that can adopt a variety of sizes and shapes. These can encompass small spherical micelles, anisotropic (e.g., long flexible rodlike) micelles, or vesicles.1 The aggregate structure which minimizes the free energy of the system at any point in the isotropic phase is prescribed by a delicate balance of forces: both intraaggregate (chain packing and surface interactions of the headgroups) and interaggregate (interactions between the aggregates which can be electrostatic or entropic).1,2 Alteration in external variables such as temperature and/or concentration influences this balance of aggregate interactions, controlling the supramolecular structure, with a concomitant effect on the macroscopic behavior of the sample. While the processes that drive the formation of micelles is clear, the control over the size and shape of these aggregates is complex. Israelachvili and Ninham3-5 have shown that it is possible to predict an optimal aggregate shape, but only when interaggregate interactions are negligible. It will be determined by molecular parameters of the surfactant: the hydrophobic chain volume (v), * Corresponding Authors email: [email protected]; [email protected]. † University of Central Lancashire. ‡ Currently based at Unilever Research, Port Sunlight, Wirral, Merseyside. (1) Wennerstro¨m, H.; Evans, F. The Colloidal Domain; VCH: New York, 1994. (2) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1985. (3) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. Chem. Soc. Faraday Trans. 2 1976, 72, 1525. (4) Ninham, B. W.; Evans, D. F. Discuss. Faraday Soc. 1986, 81, 1. (5) Mitchell, D. J.; Ninham, B. W. J. Chem, Soc. Faraday Trans. 2 1981, 77, 601.

headgroup area (A), and hydrophobic chain length (l). The optimal shape in the low surfactant concentration isotropic phase is then defined by the surfactant parameter, Ns ()v/Al). Ns for well-defined geometrical structures such as spheres, rods, and bilayers is 1/3, 1/2, and 1, respectively, though intermediate values of Ns are possible for other geometries, such as disk-shaped micelles.6 Therefore, Ns can be thought of as a measure of the curvature of the aggregates. The surfactant parameter, while phenomenological, has had considerable success in the prediction of critical micellar concentrations as a function of salt and temperature driven ion binding, as well as chain length.7 Fluorocarbon surfactant self-assembly into either isotropic solutions or lyotropic liquid crystalline phases is qualitatively the same as that of their hydrocarbon analogues. However, there are differences which stem from the fluorination of the hydrophobic surfactant tails.8 The fluorination of the surfactant tail has two main consequences on self-assembly: first, there is an increase in Ns as result of the larger molecular volume of fluorocarbon tails relative to their hydrocarbon analogues, while A and l are essentially unaltered. Therefore, there is a preference for low curvature monomeric aggregates. Second, the enhanced rigidity of the chain makes it easier for the molecules to pack into the core of flat aggregates rather than ones possessing higher interfacial curvature. In both ionic hydro- and fluorocarbon surfactant systems the structure of the aggregates in the liquid phase is dependent on the type of counterion and its binding to the aggregate. It has been established that the curvature of the aggregates in the liquid phase can be decreased by (6) Marques, E.; Regev, O.; Khan, A.; Lindman, B.; Miguel, M. J. Phys. Chem. B 1998, 102, 6746. (7) Parsegian, V.; Rand, R. P. Proc. Natl. Acad. Sci. U.S.A. 1991, 95, 4779. (8) Hoffmann, H.; Wu¨rtz, J. J. Mol. Liq. 1997, 72, 191.

10.1021/la001232e CCC: $20.00 © 2001 American Chemical Society Published on Web 07/27/2001

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the introduction of strongly binding counterions or addition of salt.9,10 Wang et al. have recently shown this to be true for salt addition to cationic fluorocarbon surfactants.11 The presence, and identity, of the counterion species associated with ionic surfactants is an important feature of the surfactant’s self-assembly processes. The simplest possible approach to the process of micellization can be reduced to two terms: the removal of hydrophobic surfactant chains from water and the need to minimize the electrostatic repulsions arising among the surfactant headgroups at the resultant micellar surface. For inorganic counterions, considerable work has indicated that for a fixed surfactant headgroup identity the cmc (critical micellization concentration) and micellar aggregation numbers correspond closely to the lyotropic or Hofmeister series for anions.12,13 These studies strongly suggest that the counterions’ effect is dominated by their interaction with water, with those that interact least with water most likely to promote micelle formation.14 It should be noted, however, that for such sequences there are always exceptions to the rule. The behavior of systems containing hydroxide as the counterion is significantly different from that of the corresponding halide ion systems;15 an observation which is associated with the very strong repulsive forces generated by the hydroxide ion in water. For organic counterions the situation is complicated, since they can penetrate into the micellar core, thus altering the effective volume per surfactant molecule, and thereby increasing the surfactant parameter. It was shown16,17 that with increasing size of carboxylate counterion the cmc values decreased and aggregation numbers increased. In addition a possible uptake of hydrophobic portions of the counterion was indicated as the length of the alkyl chains on the counterion increases. Brady et al. have also reported similar results independently in a study of counterion specificity in surfactant aggregation.10 It was found that the phase maps and structure of the liquid crystalline phases in simple perfluorinated carboxylic acid salts (e.g., Cs, Li, ammonium etc) in water are considerably changed if the counterion is replaced by much larger/more hydrophobic tetrabutylammonium (TBA) counterion.18 For the simple counterion systems (Cs+ etc.) the phase behavior is uncommon in comparison with other surfactant systems, in that lyotropic nematic phases are stable and the lamellar phases possess water-filled defects, but is characteristic for many of the simple salts of perfluorinated carboxylates. However, for the tetrabutylammonium counterion the phase maps exhibit lower consulate behavior, a feature only rarely reported for salt free binary ionic systems19 and defect free lamellar phases in which the TBA counterion is playing a key role in the structure of the phase.18 To focus on the effect of counterion identity on the concentrated phase behavior of this class of surfactant, Puntambekar et al.20 have synthesized a (9) Fontell, K.; Lindman, B. J. Phys. Chem. 1983, 87, 3289. (10) Brady, J. E.; Evans, D. F.; Warr, G. G.; Grieser, F.; Ninham, B. W. J. Phys. Chem. 1986, 90, 1853. (11) Wang, K.; Karlsson, G.; Almgren, M.; Asakawa, T. J. Phys. Chem. B 1999, 103, 9237. (12) Anacker, A. W.; Ghose, H. M. J. Phys. Chem. 1963, 67, 1713. (13) Anacker, A. W.; Ghose, H. M. J. Am. Chem. Soc. 1968, 90, 3161. (14) Underwood, A. L.; Anacker, E. W. J. Colloid Interface Sci. 1987, 117, 242. (15) Ninham, B. W.; Evans, D. F.; Wel, G. J. J. Phys. Chem. 1983, 87, 5020. (16) Magid, L. J.; Han, Z.; Warr, G. C.; Cassidy, M. A.; Butler, P. D.; Hamilton, W. A. J. Phys. Chem. B 1997, 101, 7919. (17) Anacker, E. W.; Underwood, A. L. J. Phys. Chem. 1981, 85, 2463. (18) Smith, A. M.; Holmes, M. C.; Pitt, A.; Harrison, W.; Tiddy, G. J. T. Langmuir 1995, 11, 4202. (19) Buckingham, S. A.; Garvey, C. J.; Warr, G. G. J. Phys. Chem 1993, 97, 10236.

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range of surfactants based on the perfluorodecanoate (C10) hydrophobic chain. The counterion has been altered in increasing hydrophobicity order: ammonium (A), tetramethylammonium (TMA), butyltrimethylammonium (BTMA), dimethyldibutyammonium (DMDBA), and tetrabutylammonium (TBA), and the lyotropic liquid crystalline phase behavior has been studied.20 The following experimental study will investigate the counterion effect on the structure of the aggregates within the dilute isotropic liquid phase of the above systems and on the concentrated phase behavior. The lack of long-range order in the isotropic liquid-phase renders the classical scattering and resonance techniques so successful in the structural characterization of the liquid crystalline phases, less precise, and model-dependent. Also, real systems are polydisperse and can have more than one aggregate geometry coexisting within a single phase that will further complicate the interpretation of experimental data. However, the development of the cryotransmission electron microscopy (cryo-TEM) technique,21 a direct visualization method using vitrified samples and electron microscopy, has resolved the variety of supramolecular aggregate forms possible in isotropic liquid phases. This technological advance, which has facilitated direct model-independent visualization of samples without staining or fracture therefore minimizing artifacts, has evidenced all of the previously proven structures for isotropic building blocks and some more obscure examples including the possibility of “lace like” vesicles.22 In this paper the dilute phase behavior of the surfactants as the counterion is changed has been characterized using ocular inspection, light microscopy, and 2H quadrupolar nuclear magnetic resonance (2H NMR). The supramolecular structure of the liquid phases has been visualized using cryo-TEM. Knowledge of the aggregate structure in the isotropic phase facilitates an informed understanding of the phase behavioral changes observed in the phase maps presented. 2. Experimental Section The surfactants are based around the perfluorodecanoate (C10) hydrophobic chain and the counterion has been altered in the sequence: ammonium (A), tetramethylammonium (TMA), butyltrimethylammonium (BTMA), dimethyldibutylammonium (DMDBA), and tetrabutylammonium (TBA). Heavy water, 2H2O that is 99.8% isotopically pure, was obtained from Fluka and used as received. To clearly delineate the effect the counterion is having on the supramolecular structure, the mole fraction of surfactant has been fixed at 0.000 49 for all the cryo-TEM samples. For all surfactants, the resultant solutions, at an appropriate temperature, are monophasic liquids which possess static optical inactivity. 2.1. Sample Preparation. Samples for NMR and phase map studies were prepared by carefully weighing out the required amount of surfactant and 2H2O into a glass tube containing a constriction, which helps in the mixing process. The tube is then centrifuged to get the unmixed components to one end of the tube and immediately flame sealed. The samples were mixed in a single liquid phase by repeatedly centrifuging through the constriction, ensuring the sample remained within the single phase at all times. Once the samples were homogeneous (judged by visual inspection) they were left to equilibrate for several days to ensure complete mixing. Once opened, 5-mm NMR tubes and 0.2-mm path length flat capillaries (for NMR and light microscopy experiments, respectively) were filled and immediately flame sealed with the sample level marked. Prior to (20) Puntambekar, S.; Leaver, M. S., manuscript in preparation. (21) Bellare, J. R.; Davis, H. T.; Scriven, L. E.; Talmon, Y. J. Electron Microsc. Technol. 1988, 10, 87. (22) Edwards, K.; Gustavsson, J.; Almgren, M.; Karlsson, G. J. Colloid Interface Sci. 1993, 161, 299.

Counterion Effects on Aggregate Size and Shape experiments being carried out the samples were left overnight to ensure no solvent was lost due to incomplete sealing. 2.2. Visual Characterization. Initial phase characterization of the dilute surfactant samples was carried out by visual inspection of bulk samples. Samples were placed in a Haake water bath (temperature stability (0.1 °C) and periodically checked to observe one or two phase coexistence. The phases were viewed between cross-polarized sheets to establish if the phases were isotropic or birefringent. For transitions where macroscopic phase separation is observed (e.g., L1 + L2 coexistence or the formation of solid crystals in bulk samples), large volume samples are slowly heated and cooled, with the phase transition assigned at the temperature where full phase separation is first observed to occur. Clearly there will be some local microscopic phase separation occurring prior to this, however transitions estimated on the light microscope do not significantly vary from those observed in bulk solution. More accurate determinations of the isotropic to liquid crystalline phase transitions were carried out utilizing light microscopy and 2H NMR, where possible. 2.3. Light Microscopy. The light microscopy was carried out on a Vickers M72 polarizing microscope fitted with a Linkam TH600 hot stage and TMS 92 temperature control unit, which provided a temperature stability of (0.1 °C. Fixed concentration samples were studied in flame sealed glass microslides. Heating and cooling rates were varied during the experiments but 0.5 °C/min was generally used in accurate phase determinations. Liquid crystalline phases were highly birefringence, exhibiting the characteristic Schlieren and oily streak texture for the nematic and lamellar phases, respectively. 2.4. Quadrupolar Nuclear Magnetic Resonance (2H NMR). The quadrupolar splitting of 2H2O was used, in conjunction with the light microscopy, to map the phase representations. The experiments were carried out on a Bruker Avance DPX250 spectrometer working at 38.3 MHz, and controlled using XWIN NMR version 1.1. In all cases spectra were recorded after equilibration at the experimental temperature for 30 min. The temperature dependence of the quadrupolar splitting of the 2H2O was measured during automated runs, in which the sample was heated and cooled in 1 °C steps with an accuracy of (0.1 °C. 2.5. Cryo-Transmission Electron Microscopy. Specimens for cryo- transmission electron microscopy were prepared in the controlled environment vitrification system (CEVS), to ensure correct temperature control (of 30 and 25 °C for the C10A and remaining systems, respectively) and avoid water loss from the solution during sample preparation. The technique and apparatus have been described in detail by Bellare et al.21 In brief, a 5-µL drop of the solution was put on a lacey carbon film, supported by a copper grid (Ted Pella LTD). The drop was then gently blotted with a filter paper, to create a thin liquid film over the grid. Finally the grid was rapidly plunged into liquid ethane at its melting temperature. This sample preparation technique produces vitrified specimens, i.e., the water is supercooled and does not undergo crystallization during fixation. In this way the original fluid microstructure is preserved, and component segregation and rearrangement are prevented. The vitrified specimens were stored under liquid nitrogen and then transferred to a JEOL 1200EXII electron microscope using a Gatan cryoholder and its workstation. Specimens were kept in the microscope and imaged at a temperature of about -170 °C. 100 kV acceleration voltage was used with low electron exposures (low dose mode) to minimize electron beam radiation damage. To improve the phase contrast ∼4 µm underfocusing was used.

3. Results In this section of the paper we will present experimental results that establish the phase equilibria and the supramolecular structure in dilute solution. The phase behavior has been studied for samples above 20 °C and below 15 wt % surfactant with a combination of ocular observation, polarized light microscopy, and NMR. CryoTEM experiments have been carried out on samples at 25 °C (unless stated differently) and at a fixed surfactant mole fraction of 0.000 49. The cryo-TEM directly elucidates the supramolecular structure within the isotropic liquid phase.

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3.1. Phase Behavior. Figure 1 illustrates the low concentration phase behavior of the C10X surfactants. Note that the phase representations are schematic only and have been mapped by monitoring the temperature dependence of fixed-composition samples. The full phase behavior for all of the surfactants studied is completed and consistent with the low surfactant concentration phase representations published here.20,23 However, since the aim of this publication is to elucidate the structure of the mesogenic units prior to the formation of the lyotropic phases, the phase maps presented concentrate on establishing the thermal and surfactant concentration limits of the single liquid phase and the determination of the first lyotropic phase (or lyotropic containing biphase) observed after this point, namely, surfactant concentrations below 15 wt %. The synthesis of the surfactants and full phase diagrams will be published elsewhere.20 At fixed concentration samples within the isotropic phase (0.000 49 surfactant mole fraction), the C10DMDBA samples were strongly flow birefringent, while the C10BTMA and C10TBA were very weakly flow birefringent. C10A and C10TMA indicated no discernible flow birefringence. In all cases where flow birefringence was observed it was observed to relaxed back to an isotropic texture as soon as the shearing was stopped. It is possible to observe such flow-induced effects from both vesicle and elongated micellar phases. To simplify the presentation of the phase behavior for the surfactant systems, this section is separated into two parts, based on the above shear-induced birefringence observations. 3.1.1. C10A and C10TMA Surfactants. Parts a and b of Figure 1 represent the phase behavior for C10A upon heating and cooling, respectively. From Figure 1a it can be seen that upon cooling for sample conditions above 1 wt % and below 28 °C, C10A samples are observed to separate into a liquid phase that coexists with crystalline surfactant, which indicates that the thermodynamic state of the sample lies below the solubility limit of the surfactant irrespective of the thermal history of the sample. However, the exact location of this boundary differs upon heating and cooling, as supercooling has been observed (see below). The low-temperature dashed lines on the phase maps, Figure 1a,b, represent the point where isolated crystalline surfactant is observed in bulk samples on cooling (Figure 1a) and where solid surfactant crystals are lost upon heating (Figure 1b). The thermodynamically correct location of this boundary is the one determined upon heating from the two-phase region which has been allowed to fully phase separate. Figure 1b has been established in this manner and indicates that the temperature at which the last crystalline surfactant is observed occurs in the temperature range 34-37 °C in the concentration range studied. However, to avoid demixing and equilibration problems, NMR is usually carried out on samples cooled from the single isotropic phase. Figure 1a indicates that in this case the ocular observation of the first crystal formation occurs within the temperature range 25-28 °C in the same concentration interval. This observation of a significant suppression of crystal formation upon cooling has been observed before and indicates a supercooling of the liquid-crystalline phase.24 This supercooling allows the formation of a lamellar phase at 15 wt % C10A, which is not observed upon heating from the two-phase region. This is discussed below, where the NMR spectra of the 15 wt % sample are (23) Puntambeker, S.; Holmes, M. C.; Leaver, M. S. Liq. Cryst. 2000, 27, 743. (24) Boden, N.; Corne, S. A.; Jolley, K. W. J. Phys. Chem. 1987, 91, 4092.

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Figure 1. Dilute phase behavior of (a) C10A upon cooling, (b) C10A upon heating, (c) C10TMA, (d) C10BTMA, (e) C10DMDBA, and (f) C10TBA perfluorinated surfactants in 2H2O mapped by using a combination of light microscopy (solid diamonds), 2H NMR (solid spheres), and direct observation (open squares). L1 and L2 are isotropic normal and reverse micellar, K - crystalline surfactant; N, LR, and LRH are nematic, normal and disrupted lamellar liquid crystalline phases, respectively. A star indicates the concentration and temperature at which the cryo-TEM measurements were carried out (weight percentage for samples containing a surfactant mole fraction of 0.000 49 made up in H2O, see text). Dashed lines indicate phase transitions that were experimentally not possible to unequivocally resolve; see full text for details.

presented. For samples where liquid crystalline phases are formed, the transition temperatures are independent of thermal history within experimental error. As a direct consequence of this solubility problem and in order to avoid phase separation, the cryo-TEM measurements in this system have been conducted at 30 °C. These samples have been cooled from the isotropic phase and no crystal formation has been observed either during or after the experiments. In the following all samples are equilibrated in the isotropic phase and then cooled. Below 10 wt %, no liquid crystalline phases have been observed at any temperature. At 12 wt %, the L1 is replaced by an optically anisotropic

phase which possessed a Schlieren texture when viewed under the polarizing light microscope.25,26 In addition the 2H NMR exhibited a well-resolved doublet, indicating the presence of a nematic phase. At 15 wt % a lamellar phase is formed on cooling from the nematic. This transition is marked by the formation of an optical texture that is still “Schlieren-like”, but where the bright or dark domains are larger and an oily streak texture can be observed to develop upon equilibration on the microscope hot-stage. This type of observation has previously marked the onset (25) Lawson, K. D.; Flautt, T. J. J. Am. Chem. Soc. 1967, 89, 5489. (26) Holmes, M. C.; Boden, N.; Radley, K. Mol. Cryst. Liq. Cryst. 1983, 100, 93.

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Figure 2. (a) Evolution of the 2H quadrupolar NMR spectra upon cooling from the isotropic liquid phase into the lyotropic liquid crystalline phase of a 15 wt % C10A sample. (b) Quadrupolar splitting versus temperature from a 15 wt % C10A sample that has been cooled from the high-temperature isotropic phase. Note the two-phase coexistence of nematic and isotropic phases (observed at 42 °C) as well as the transition into the lamellar phase at approximately 37 °C. The dotted lines plotted through the data point are an aid to the eye in visualizing the marked change in the behavior of the recorded ∆ as the temperature is decreased through the transition from the nematic to the lamellar phase. Once the lamellar phase is formed there is a temperature below which the recorded ∆ remains more or less constant. This is the super cooled limit of the lamellar phase and the point at which crystal formation should begin to occur. Note that even though the samples were cooled below the thermodynamic solubility limit of the surfactant (determined upon heating the samples), no optical evidence for crystal formation was noted in the NMR tubes after the sample was removed from the spectrometer even though the latter observation is consistent with crystal formation.

of the formation of a disrupted lamellar phase in closely related systems;24 however, we have utilized NMR to detect this transition more accurately. Figure 2a shows an example of the evolution of the recorded 2H spectra for a 15 wt % C10A sample. The isotropic phase is marked by a single line in the NMR spectra and the anisotropic phases by a doublet. A transition into the nematic phase is observed at 41 °C (doublet), with a narrow two-phase region preceding it. There is no marked change in the NMR spectra at the transition from the nematic to the lamellar phase.24 However, Figure 2b indicates that at about 37 °C the slope of the recorded splitting (∆) versus temperature changes, which has previously been shown to mark the onset of the formation of a disrupted LR from a nematic phase.24 Below 26 °C the sample temperature lies below the solubility limit determined for the bulk surfactant samples, see Figure 1a. However, upon ocular investigation of the sample (after removal from the NMR magnet) no crystalline material was present in the sample, indicating that supercooling of the lamellar phase had occurred, an observation that has been recorded in similar surfactant

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systems.24 This observation is also supported by the recorded ∆ remaining more or less constant below 26 °C, indicating an invariant lamellar structure upon further cooling. Figure 1c illustrates the phase behavior of C10TMA, which is slightly different to that of C10A. The phase transitions are shifted to lower temperatures and the formation of crystalline surfactant is not observed to occur above 20 °C. Below 9 wt % surfactant, no liquid crystalline phases are observed and the liquid phase exhibited no flow birefringence. Above 9 wt % the phase behavior observed for C10TMA is consistent with that observed for C10A, but some of the details are slightly less well-resolved. Between the isotropic and lamellar regions the 2H NMR indicates the presence of two biphasic regions that are consistent with a nematic phase coexisting with an isotropic phase (at higher temperatures) and with a lamellar phase (at lower temperatures). However, with the experimental temperature control available it has not been possible to observe a NMR spectrum consistent with a single nematic phase. Since the phase rule precludes the formation of one biphase directly out of another, it is assumed that a narrow nematic phase is stable between these two nematic containing biphases. This uncertainty is presented on the phase representation as a set of dashed double lines, which are less than 1 °C wide, indicating the region of the phase representation in which this single nematic phase is thought to exist. It should be noted that comparisons between the two surfactants in this first section is complicated by the fact that there are hydrogen bonds formed between the ammonium counterions in the C10A system with the carboxylic headgroup of the surfactant heads, which might result in partial charge neutralization at the micellar surface. This will alter the intra-micellar interactions controlling the preferred curvature of this interface, above and beyond the changes induced by change in counterion hydrophobicity. However, since we observe rather similar phase and structural behavior it is assumed that this effect is minor in comparison with the change in counterion identity. 3.1.2. C10BTMA, C10DMDBA and C10TBA Surfactants. Parts d, e, and f of Figure 1 illustrate the phase representations for C10BTMA, C10DMDBA, and C10TBA, respectively. Here, the phase boundaries between the isotropic and anisotropic phases have proven difficult to establish, since bulk two-phase separation does not occur readily. For the C10BTMA system (Figure 1d) at 15 wt %, the 2H NMR spectra indicate the coexistence of an isotropic and anisotropic phase below 78 °C. Below this temperature the sample possessed static birefringence with the polarized light microscope, indicating the presence of oily streaklike features, inferring the presence of a lamellar phase. This observation was confirmed by leaving a sample in the NMR magnet at 25 °C for 12 h and recording the spectra as a function of time. After 12 h the relative intensity of the anisotropic doublet and isotropic singlet had altered slightly, but the biphasic identity remained. This, along with the polarizing microscope observations, suggests a lamellar plus isotropic biphase. For 10 and 7 wt % C10BTMA, it is not possible to resolve a separate doublet from the anisotropic phase in the NMR spectra, probably due to the high water content of the samples. However, the line width at half-height and the baseline width of the spectra undergo a small, but significant, and reproducible increase, upon cooling, at 60 and 40 °C, respectively. This observation, in conjunction with the bulk birefringent optical texture below these tempera-

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tures, is consistent with the presence of an anisotropic phase and isotropic phase in coexistence and hence suggests the formation of the lamellar biphasic region as observed above. At 5 wt % of C10BTMA and below, the NMR spectra consists of a single narrow line, of constant width at half-height and base, for all temperatures studied. This marks the formation of a single L1 phase. The optical observation now is of a single phase with zero static birefringence. However, below 5 wt % surfactant careful light microscopy examination of samples in flat capillaries shows isolated “Maltese cross” textures, indicating the possible presence of vesicles in the liquid phase.27 The C10DMDBA system, Figure 1e, presents similar phase behavior to that of the C10BTMA. Above 5 wt % surfactant a L1 + LR biphasic region exists. Above this concentration all the samples in this region possess static birefringence, with optical textures that consist of either lamellar-like oily streaks or compacted Maltese-crosses. At 15 and 10 wt % C10DMDBA the transition into the two-phase region is marked by the formation of either a well-resolved doublet plus singlet spectra (15 wt % C10DMDBA) or a singlet which is broader than that observed in the low concentration isotropic phase (10 wt % C10DMDBA), marking the onset of the biphasic region as outlined for C10BTMA above. For 7 and 5 wt % C10DMDBA the NMR shows a very small alteration in the line width at half-height and so only polarized light microscopy observations have determined the transition point. The transition is determined at the point where the birefringent texture occupies the whole field of view in the polarized light microscope. As in C10BTMA, the isotropic phase, either above the two-phase region or at low C10DMDBA concentrations, can exhibit bulk isotropic textures with isolated Maltese crosses. A demixing from a single L1 phase to L1 and L2 nonbirefringent phases is observed at the upper right corner of the phase map. Finally Figure 1f illustrates the phase behavior of C10TBA. The figure is dominated by the separation of the low-temperature phase(s) into two non- birefringent liquids, as has been observed for the C10DMDBA system. This occurs for samples above 1 wt % and at decreasing temperatures from roughly 50 °C. The lower temperature phase behavior is particularly difficult to resolve for this counterion. Below 10 wt % C10TBA the static phase is optically inactive, both by eye and under the microscope. The corresponding 2H NMR spectra are narrow single lines with constant line widths for all temperatures. Thus it would appear that below this C10TBA concentration the samples are in the L1 phase. At 15 wt % C10TBA the sample is weakly static birefringent and the 2H NMR spectrum shows a well-resolved doublet possessing a splitting of 28 Hz. This indicates the presence of a lamellar phase. However, at 14 wt % C10TBA, while the weak static birefringence remains, the 2H NMR spectrum consists of a single broad line only. Previously we have interpreted this feature as the presence of a biphase in which a lamellar coexists with a liquid phase, although here it is a lamellar phase in which the splitting is very small, since the optical texture is that of a single lamellar phase. Between 10 and 14 wt % C10TBA, the static birefringence becomes harder to observe and the width at half-height of the 2H NMR spectrum narrows. Therefore, it has become impossible to delineate the exact transition point between the single phases and the region of uncertainty has been shaded. This region may consist of a two-phase lamellar plus liquid region or a continuation of either two single phases until an abrupt second-order phase transition occurs. (27) Hoffmann, H.; Thunig, C.; Schmiedel, P.; Munkert, U. Langmuir 1994, 10, 3972.

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The phase separation into two liquid phases is a common feature to the C10DMDBA and the C10TBA surfactant phase maps as well as a limited number of other binary surfactant systems.19 This behavior has been observed for other binary fluorinated counterion systems that possess a TBA counterion surfactants and has yet to be fully explained. 18 However, Buckingham et al. have attributed a similar observation in a surfactant system containing tetrabutylammonium headgroups to the entropic cost of ordering water around the TBA entities driving a demixing of the system.19 3.2. Aggregate Structure in the Dilute Liquid Phase. To understand the effect that the counterion is having on the phase behavior cryo-TEM has been used to study the counterion-dependent structure of the aggregates at a fixed molar fraction of surfactant, where the phase maps indicate a single L1 phase. The samples for the cryo-TEM measurements were made by using water (H2O), while the phase maps were mapped by using 2H2O. While the isotropic substitution of H2O by 2H2O has been observed to have significant effects upon the phase behavior of related systems by Edwards et al.28 it is not expected to have a dramatic effect on the observed phase behavior at such low surfactant concentrations. The transition temperatures recorded for sample made up in H2O, as opposed to those recorded in 2H2O, did not differ significantly on the light microscope. The points marked by a star on the phase maps (Figure 1) therefore correspond to the surfactant weight percentage for samples containing a surfactant mole fraction of 0.000 49 made up in H2O. 3.2.1. Cryo-TEM. For C10A, a mixture of elongated and ∼3 nm in diameter spheroidal micelles are imaged (Figure 3(A).29 This result is in line with measurements from Burkitt et al. who found that ammonium perfluorononanoate formed short rodlike micelles.30 The elongated micelles are rigid and extend over a few micrometers. This behavior is similar to that reported by Wang et al. for cationic fluorinated surfactant.11 Changing the counterion from A to TMA results in the electron micrographs now indicating that the elongated micelles (with similar size as in the C10A system) are more flexible, since they follow the equithickness lines of the vitrified film (Figure 3(B). Elongated micelles coexist with some small spheroidal micelles. Cryo-TEM results for C10A (Figure 3A) indicate rigid elongated fluorocarbon micelles. These are different from those formed by typical flexible hydrocarbon surfactants, which are aligned along the film edge or equithickness lines (Figure 1a in Danino et al.31). It is argued that the imaged rigidity of the elongated micelles is a true effect of fluorocarbon stiffness reported earlier,32 in contrast to Wang et al.11 who suggested that the micelle rigidity resulted from alignment during the blotting process. Micelle rigidity as an artifact is induced by shear during the blotting action when the sample is prepared for cryo-TEM imaging.21 However, in this latter case, the shear-induced rigid micelles are all oriented in the same direction, which is not the case in Figure 3A,B.33 It is therefore proposed that the rigidity of the rods is a true effect due to the fluorination of the surfactants. (28) Edwards, P. J. B.; Jolley, K. W.; Smith, M. H.; Thomsen, S. J.; Boden, N. Langmuir 1997, 13, 2665. (29) The fully extended chain length of a decanoate is 1.26 nm. Therefore, the diameter of a micelle is 2.5 nm, which is consistent with the TEM observation noted above. (30) Burkitt, S. J.; Ottewill, R. H.; Hayter, J. B.; Ingram, B. T. Colloid Polym. Sci. 1987, 265, 619. (31) Danino, D.; Talmon, Y.; Levy, H.; Beinert, G.; Zana, R. Science 1995, 269, 1420. (32) Regev, O.; Khan, A. J. Colloid Interface Sci. 1996, 182, 95. (33) Talmon, Y., private communication, 2001.

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which possess a diameter ranging from 20 to 500 nm in diameter (Figure 3C). No spheroidal micelles are detected. For C10DMDBA, the elongated micelles disappear completely and polydispersed vesicles (50-500 nm) are the only objects imaged in the solution (Figure 3(D). Finally, for C10TBA, some of the vesicles are opened and form flat bilayers (Figure 3(E). The vesicles are concentrated near the polymer film, while the flat bilayers are imaged toward the center of the vitrified film. Vesicles hold more volume than flat bilayers of the same diameter and, therefore, are found in the thicker region of the vitrified film (Figure 3(E) (close to the polymer support), while the flat bilayers concentrate closer to the center, where the film is thinner. In the micrographs the vesicles differ from flat bilayers by contrast at the edge.34 In vesicles, the edge appears much darker than the core since more surfactant headgroups scatter the electron beam there. This phenomenon is termed mass thickness contrast. For flat bilayers, as oppose to vesicles, there is no difference in contrast between edge and core (arrows in Figure 3D,E), hence facilitating differentiation between the two closely related supramolecular aggregates, as has been reported earlier for disklike micelles.6,34,35 Line intensity profiles (shown as insets in Figure 3D,E) are plotted for the dashed lines in the respective figures. They show the difference between a steep intensity decrease for a projection of vesicle bilayer (R) in comparison to a shallow one for a projection of a flat bilayer (β). A line intensity profile of vesicles (γ, δ) in Figure 3D is given for reference. 4. Discussion

Figure 3. Cryo-TEM micrographs from the isotropic, constant 0.000 49 mol fraction, phase of (A) C10A, (B) C10TMA, (C) C10BTMA, (D) C10DMDBA, and (E) C10TBA. Arrows in (d) and (e) indicate vesicles and flat bilayer, respectively. Line intensity profiles of vesicles (R, γ, δ) and bilayers (β) are shown in the insets of (D)-(E) (for details, see text). Bar ) 100 nm.

In the C10BTMA system, flexible elongated micelles are imaged coexisting with a majority of polydisperse vesicles,

In this section a brief summary of the commonality of the experimental results presented in section 3 will lead into a framework within which it is possible to discuss and propose an explanation of the observed counterion effects in the liquid phase. The experimental results can be divided into two broad groups. The first group consists of C10BTMA, C10DMDBA, and C10TBA surfactants, where cryo-TEM indicates that the dominant aggregate form is vesicular and the first liquid crystalline phase observed in the phase representations is lamellar (or lamellar containing biphasic regions). The observation of a Maltese cross in the light microscopy of the L1 phase supports the finding of the cryo-TEM experiments. The structural building blocks in each of these phases are zero curvature bilayers, suggesting that the transition into the concentrated phase is driven by a positional correlation of the bilayers.36 The second group consists of C10A and C10TMA surfactant. The cryo-TEM micrographs from this group clearly indicate a dominance of high-curvature micellar aggregates. This is consistent with the presented phase maps which exhibit nematic and disrupted lamellar phases as the first concentrated phases formed from the L1, both of which possess significant interfacial curvature.37 Clearly the role the counterion plays in the self-assembly processes in mixtures of dilute surfactant in water is important. The size, shape, charge density, hydration, and hydrophobicity (effectively the strength of the interaction with water) of the counterion will be key in determining its effect on the aggregation processes. In (34) Regev, O.; Marques, E. F.; Khan, A. Langmuir 1999, 15, 642. (35) Marques, E.; Regev, O.; Khan, A.; Miguel, M.; Lindman, B. Macromolecules 1999, 32, 6626. (36) Regev, O.; Guillemet, F. Langmuir 1999, 15, 4357. (37) Holmes, M. C.; Leaver, M. S. Phase Transitions in Liquids; Tole´dano, P., Figueiredo-Neto, A. M., Ed.; World Scientific Press: 1998; p 29.

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this particular system the surfactant chain length has been kept constant and the counterion identity varied. In progressing from ammonium to tetrabutylammonium ions the bare size of the counterion has been increased. More specifically the only parameter being altered is the number of carbon atoms in the hydrocarbon chains attached to the substituted ammonium ions, Nc. The surfactants in the first group (C10BTMA, C10DMDBA, and C10TBA) contain significantly more hydrocarbons, 7, 10, and 16 respectively, than the second (C10A and C10TMA), which contain 0 and 4 hydrocarbons, respectively. However, in progressing from A to TBA the “hydrophobic character” of the counterion is also increasing. For any hydrophobic counterion, water molecules must organize locally around that species, generating a highly ordered structure. Therefore there is an entropic cost to the system in having such ions present in the bulk water, i.e., counterions residing as free entities within bulk water. The larger the counterion the larger the entropic cost. In surfactant systems this cost can be reduced by bringing the counterion in closer proximity to the surface of the micelle and the hydrophobic core. Buckingham et al. have noted lower consolute behavior in quaternary ammonium surfactant systems as the size of the headgroup is increased.19 In the following discussion the term “counterion hydrophobicity” is used as a “catch all” to explain the change in micellar curvature observed in this work, while the deconvolution of the precise importance of each of the effects mentioned at the start of this paragraph is beyond the scope of this paper. As the counterion becomes more hydrophobic the predominant structure, imaged by cryo-TEM, is decreasing in interfacial curvature from spheroidal micelles and rodsrods-vesicles and rods-vesicles-bilayer and vesicles, indicating an increase in Ns from 1/2 to 1. In this system an increase in Ns is only possible by a decrease in A, which in turn requires a reduction in the headgroup repulsion (assuming that there is no change in the intermicellar interactions upon change of counterion identity or that they are negligible all together). Such an effect could be achieved by an increased counterion binding to the surface of the aggregate. Counterion identity will have an effect on both fluorocarbon tail net interaction and headgroup repulsion, while the effect on the latter is expected to be more pronounced. A closer association of the counterion with the surface of the micelle is going to result in a more effective screening of the repulsive interaction between the headgroups, resulting in surfactant aggregates with lower curvature. It should be mentioned that increasing hydrophobicity might not be the only reason for a decrease in curvature. It is also associated with the size of the counterion. This might lead to a crowding of the counterions close to the surface of the aggregates, resulting in an enhanced screening of the Coulombic headgroup repulsions. Moreover, if the counterions are very close to the aggregate interface there might be incompatibility between the fluorocarbons of the surfactant tail and the hydrocarbons of the counterion, which should prevent penetration into the aggregate. Measurements on counterion binding values (e.g., by self-diffusion NMR) and repeating the experiments with fluorocarbon-substituted ammonium counterions are expected to shed more light on this point. In the present study the temperature and concentration of the counterion have been kept constant and the net effect on changing from A to TBA is a change in micellar identity. We propose that this can only be driven by an increase in the counterion binding as the counterion identity changes. Further confirmation of this hypothesis

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Figure 4. Schematic summary of the estimated surfactant parameter based upon the supramolecular structures imaged by cryo-TEM at a fixed surfactant mole fraction in the isotropic phase versus the hydrophobicity (Nc) of the counterion (see text).

will take the form of the measurement of counterion binding using NMR self-diffusion, as has been mentioned earlier, or use of a quartz crystal microbalance. The above observations are summarized in Figure 4 where Ns values, estimated by cryo-TEM micrographs, are plotted vs the hydrophobicity, indicating an increase in Ns values with increasing hydrophobicity. For the dilute isotropic solutions Ns has been observed to increase as the surfactant changes from C10A to C10TBA. In surfactant systems it has been accepted that Ns increases as the surfactant concentration is increased.38 Therefore in systems where Ns is close to 1 in the isotropic phase, as in the second group presented above, the first lyotropic liquid crystalline phase observed at higher concentration can only be a lamellar phase, as seen in Figure 1c-e. However, for the first group Ns is closer to a 1/2 in the isotropic phase, since micellar aggregates are the predominant structures observed, and indeed the first concentrated liquid crystalline phase observed possess non zero curvature, either as a nematic phase for the C10A (Figure 1a,b) or a disrupted lamellar phase in C10TMA24,39 (Figure 1c). 5. Conclusion This study has indicated that as the counterion becomes more hydrophobic for a series of fluorocarbon surfactants with a fixed chain length (C10) the imaged aggregate architecture alters from discrete micellar units to vesicle and bilayer fragments. Such an observation is consistent with a decrease in the interfacial curvature of the interface between the surfactant headgroups and water. For dilute surfactant solutions this observation is reflected in an increase in the surfactant parameter (Ns). It is argued in this case that the increase in hydrophobicity of the counterion drives an increased binding to the surface of the surfactant aggregates, thereby facilitating a decrease in the surface area each headgroup occupies at the interface and concomitantly increasing Ns. This change in micellar architecture with counterion identity at low surfactant concentrations, imaged by cryo-TEM, is reflected in the first liquid crystalline phase formed as the concentration of the surfactants is increased. This observation, while not unique, reinforces the importance (38) Jo¨nsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactant and Polymers in Aqueous Solution; John Wiley and Sons: New York, 1998; p 1. (39) Dombrowski, J. P.; Edwards, P. J. B.; Jolley, K. W.; Boden, N. Liq. Cryst. 1995, 18 (1), 51.

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that the aggregate structure in the isotropic liquid phase has on the lyotropic liquid crystalline phase behavior in surfactant water systems. Acknowledgment. S.P. and R.Z. thank the University of Central Lancashire for postgraduate bursaries. ML would like to thank The Academic Study Group on Israel

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& the Middle East for a travel grant to work in Beer Sheva. Grant 8624 of the Israeli Ministry of Science and Art is gratefully acknowledged. O.R. would like to acknowledge The Jolly Friar for excellent fish and chip lunches which inspired much of the discussion presented here. LA001232E