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Phase Behavior of Catanionic Surfactant Mixtures: Sodium Bis(2-ethylhexyl)sulfosuccinate-Didodecyldimethylammonium bromide-Water System Annalisa Caria* and Ali Khan Physical Chemistry 1, Center for Chemistry and Chemical Engineering, P. O. Box 124, University of Lund, S-221 00 Lund, Sweden Received June 12, 1996. In Final Form: September 30, 1996X The phase equilibria of the catanionic surfactant mixtures sodium bis(2-ethylhexyl)sulfosuccinate (AOT)didodecyldimethyl ammonium bromide (DDAB)-water has been studied by combined NMR, polarizing microscopy, and cryo-TEM techniques at 303 K. Equimolar mixtures of AOT and DDAB form a reverse hexagonal liquid crystalline phase in the water-poor part of the phase diagram. This hexagonal phase exists in equilibrium with another reverse hexagonal phase which originates from the binary AOT-water system. Lamellar liquid crystalline phases formed by the binary AOT-water and DDAB-water systems can not solubilize any significant amounts of the other parent surfactant. The bicontinuous-type cubic phase of the aqueous AOT system is found to swell substantially with water by the addition of small amounts of DDAB. A L3 solution phase, a cubic phase and a viscous liquid phase are also characterized. The former phase is formed for the AOT-rich area with small amounts of DDAB, and the latter two phases exist for the DDAB-rich area containing small amounts of both AOT and water. Stable polydispersed vesicles are detected in the very dilute part (>98% water) of the phase diagram for both the catanionic pairs (equimolar) as well as the catanionic pairs containing an excess of one of the two parent surfactants. The formation and the stability of the different phases are discussed in terms of surfactant molecular packing constraints and electrostatic effects.
Introduction Catanionic surfactants are prepared by mixing equimolar amounts of two oppositely charged surfactants from which the inorganic counterions are removed.1 When the mixing ratios of the two surfactants are different from equimolarity, the system is termed a catanionic surfactant mixture.2 At an early stage, it was observed that the aqueous mixtures of catanionic systems exhibit many unusual properties which are different from those of their parent surfactants. Dilute solutions exhibit strong synergistic behavior. The synergism is manifested as high surface activity, enhanced adsorption, and a much lower critical micelle concentration (cmc).2,3 Most surfactants used in industrial applications consist of a mixture of surfactants, since mixed systems perform better than the individual components.4-6 A large number of studies have been reported on the mixed micellization and synergistic behavior of the mixed surfactant systems. At high dilution, the uncharged catanionic surfactants often give a precipitate which redissolves on adding an excess of one parent ionic surfactant;7 the resulting solution gives rise to mixed micelles of different shapes and sizes.8 The precipitate finds its use in the quantitative analysis of surfactants.9 * E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, December 1, 1996. (1) Jokela, P.; Jo¨nsson, B.; Khan, A. J. Phys. Chem. 1987, 91, 3291. (2) Khan, A.; Marques, E. In Catanionic Surfactants in Specialist Surfactants; Robb, I. D., Ed.; Blackie Academic and Professional, an Imprint of Chapman & Hall: London, in press. (3) Lucassen-Reynders, E. H. Kolloid Z. Z. Polym. 1972, 250, 356. (4) Lasic, D. D. Liposomes: from Physics to Applications; Elsevier: Amsterdam, 1993. (5) Malmsten, M.; Lindman, B. Langmuir 1989, 5, 1105. (6) Ba¨ckstro¨m, K.; Lindman, B.; Engstro¨m, S. Langmuir 1988, 4, 872. (7) Marques, E.; Khan, A.; Miguel, M. G.; Lindman, B. J. Phys. Chem. 1993, 97, 4729. (8) Malliaris, A.; Binana-Limbele, W.; Zana, R. J. Colloid Interface Sci. 1986, 110, 114.
S0743-7463(96)00581-1 CCC: $12.00
Another novel property of the catanionic surfactant mixtures, discovered recently,10 is that they form thermodynamically stable vesicles with water at an extremely low surfactant concentration . These vesicles are superior to synthetic vesicles which are unstable and ultimately transform into lamellar bilayers.4 Moreover, the size and charge of catanionic vesicles can easily be controlled by adjusting the desired mixing ratios of the two surfactants. Stable vesicles are of potential use as microreactors and agents for controlled drug release.4,11 However, there are limited studies of the catanionic systems for the concentrated regions. Like the zwitterionic lecithin systems,12 the uncharged pseudo-doublechain catanionic surfactant systems form a concentrated lamellar liquid crystalline phase.1,13 The stabilities of lamellar phases formed by uncharged surfactant systems are explained on the basis of the hydration force, a short-range exponentially decaying repulsive force,14,15 and the catanionic systems are found to be good models to investigate the nature of the hydration force.1 Recently, we have shown that the phase equilibrium of the pseudo-three-alkyl-chain surfactant system sodium dodecyl sulfate (SDS)-didodecyldimethylammonium bromide (DDAB)-2H2O is very complex and rich in polymorphism.7 One interesting finding is that the system forms only normal- and bicontinuous-type aggregates. At present, no report is available on the self-assembly process of pseudo-four-alkyl-chain catanionic surfactant mixtures. The microstructures formed in the system compared to (9) Cropton, R. W. G.; Joy, A. S. Analyst 1963, 88, 516. (10) Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasdzinski, J. A. N. Science 1980, 245, 1371. (11) Ostro, M. J. Liposomes: from Biophysics to Therapy; Marcel Dekker: New York, 1987. (12) Sadaghiani, A.; Khan, A.; Lindman, B. J. Colloid Interface Sci. 1989, 132, 352. (13) Jokela, P.; Jo¨nsson, B.; Eichmu¨ller, B.; Fontell, K. Langmuir 1988, 4, 187. (14) Jo¨nsson, B.; Wennestro¨m, H. Chem. Scr. 1985, 25, 117. (15) Jo¨nsson, B.; Wennestro¨m, H. J. Chem. Soc., Faraday Trans. 2 1983, 79, 19.
© 1996 American Chemical Society
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pseudo-double- and pseudo-triple-chain catanionic mixtures may contribute to a better undestanding of the selfassociation processes of the oppositely charged mixed surfactant systems. Surfactant self-association as well as its phase stability in a solvent (water) depends on a delicate balance of forces acting between surfactant-surfactant, surfactant-solvent, and solvent-solvent molecules. The long-range electrostatic repulsive force16,17 and surfactant molecular geometry18 are shown to play a dominant role in the selfassembly process of ionic surfactant systems. The geometric considerations are based on the critical packing parameter (CPP) of a surfactant given by CPP ) v/al, where, v is the volume of the hydrophobic tail(s), a is the cross-sectional area per headgroup, and l is the length of the hydrophobic tail(s).19 The geometrical properties of a surfactant molecule dictate whether and how a surfactant is able to form a certain aggregate structure; an efficient balance between the optimal area per headgroup, a, and the necessity of packing effectively the hydrophobic tails (of volume v and length l) is required in order to form stable aggregates. From a microscopic point of view, the local curvature of the aggregate is modulated to meet this requirement. In addition to the CPP description, there is another model which is centered on the spontaneous mean curvature of the surfactant film,20 that is, the curvature that a surfactant film would spontaneously form when it is unconstrained. Within this scheme the more stable type of aggregate will be the one having a mean local curvature equal to the spontaneous mean curvature (H0). The relations between the critical packing parameter and the optimal aggregate structure are the following: CPP < 1 (positive spontaneous curvature, H0) implies a tendency toward the formation of normal micellar structures; CPP ≈ 1 (H0 ) 0) gives a lamellar phase, while CPP > 1 (negative H0) favors reverse structures (reverse micelles, reverse hexagonal). CPP values can be changed by varying parameters such as temperature or salt content. For ionic surfactants, the effective headgroup area, a, is largely determined by the electrostatic interactions; consequently, a depends on both surfactant concentration and salt content. However, catanionic surfactant mixtures behave in a more complex way with water. For the mixtures, there will be charge neutralization due to the strong electrostatic attractive force operating between the oppositely charged headgroups and formation of salt. Thus, the effective headgroup area, a*, in catanionic mixtures will not be just a weighted average of the area of the two parent surfactants, but the value will be somewhat smaller depending on the molar ratio of the two surfactants. Due to a very close packing of the headgroups of the catanionic pair, the effective volume, v*, will also be larger compared to the weighted average volumes of the single-parent surfactants. The combined effects will lead to an increased effective CPP value, which depends on the total surfactant concentration (due to sceening effects of counterions) as well as on the mixing ratio of the two surfactants (degree of neutralization). In this report, we present the phase behavior for the pseudo-quarternary-tail catanionic surfactant system (16) Jo¨nsson, B.; Wennestro¨m, H. J. Phys. Chem. 1987, 91, 338. (17) Jo¨nsson, B.; Wennestro¨m, H. J. Colloid Interface Sci. 1981, 80, 482. (18) Israelachvili, J. Intermolecular and surface forces, 2nd ed.; Academic Press: San Diego, CA, 1991. (19) Tanford, C. The hydrophobic effect, 2nd ed.; Wiley: New York, 1980. (20) Helfrich, W. Z. Naturforsch. 1973, 28C, 693.
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DDAB-AOT (sodium bis(2-ethylhexyl)sulfosuccinate)H2O at 303 K over the entire concentration range. 2H NMR and optical and cryo transmission electron microscopy techniques are used to identify single phases and to determine the stability regions of phases. We will show how the electrostatic effect and the molecular packing constraint influence the phase behavior and the stability region of single phases. Experimental Section Materials. DDAB was received from Tokyo-Kasei, and AOT, from Fluka. The chemicals were used without further purification. 2H2O (99.8 atom % 2H) was purchased from Dr. Glaser, AG Basel, Switzerland. Sample Preparation. The samples were prepared by weighing appropriate amounts of the components in 10 mm (i.d.) glass tubes, which were then flame-sealed. The samples in the liquid crystalline regions were mixed by centrifuging back and forth for several times a day, and the processes were repeated for several days until the samples were mixed thoroughly. Samples in the water-poor part of the phase diagram were centrifuged at about 50 °C to facilitate the mixing. The samples in the dilute regions, where one finds isotropic solutions and/or nonviscous fluid mixtures, were mixed by gentle shaking in a bench-type electric shaker for several hours. The samples were then kept standing for 2 weeks at constant temperature (T ) 303 K) for equilibration before the first 2H NMR measurements were carried out. The 2H NMR spectra were recorded again after 3 months, and no significant changes were observed in the spectra. Representative samples were checked periodically during 1 year to assure the reproducibility of the measurements and the stability of the samples. Methods. The samples’ homogeneity was checked through cross polaroids. Examination of the samples against crosspolarizers is also an easy and quick way to identify the isotropic and anisotropic phases and thus have a preliminary knowledge of the phases formed in the system. 2H NMR and polarizing microscopy techniques were used for the identification of the phases and for the determination of the phase boundaries. In addition, the cryo-TEM method was employed to gain microstructural information for the very dilute regions. 2H NMR. 2H atoms possess a quadrupole magnetic moment (spin quantum number I ) 1) which can couple with electric field gradients at the nuclear sites. As a result, for a single nucleus (in an anisotropic phase) within a magnetic field, the excited (Zeeman) level is split into two (in general 2I) levels whose separation (∆) is a function of the orientation of the local environment of the molecule, which provides the electric field gradient and the magnetic field direction. For anisotropic phases, where several sites for the 2H are possible, the theoretical treatment shows that the observed quadrupolar splitting is given by21
∆)
∑|P ϑ
i QiSi|
(1)
where Pi is the fraction of 2H present at site i and (ϑQS)i is the average quadrupole interaction at site i. In a simple two-site model21,22 a further simplification of eq 1 is obtained assuming that only two sites are available for the 2H2O molecules: water molecules bound (Pb) to the surfactant monolayer and free water molecules (Pf). For the latter case the order parameter vanishes and eq 1 becomes
∆ ) |PbϑQbSb|
(2)
ϑQ for 2H in 2H2O is constant (220 kHz).23 The order parameter (Sb) for the bound water molecules is given by the time average (21) Wennestro¨m, H.; Lindman, B.; Lindbolm, G. Chem. Scr. 1974, 6, 97. (22) Wennestro¨m, H.; Persson, N.-O.; Lindman, B. In Colloidal Dispersions and Micellar Behaviour; Mittal, K. L., Ed.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975; Vol. 9, p 253. (23) Glasel, J. A. In Water, a Comprehensive Treatise; Franks, F., Ed.; Plenum Press: New York, 1972; Vol. 1, p 215.
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for the case of an axially symmetric electric field gradient as21
1 Sb ) 〈3 cos2 θMD - 1〉 2
(3)
where θMD is the time-dependent angle between the electric field gradient and the liquid crystal axis (the director). In a lamellar phase the director is perpendicular to the lamellae, while in a hexagonal phase it is parallel to the axes of the cylindrical aggregates. In the hexagonal phase the absolute value of Sb is reduced by half compared with that of the lamellar phase, and the two quadrupolar splitting values for the two liquid crystalline phases, if all other factors are the same, are related by
∆D ) 2∆E
(4)
The reduction in Sb for the hexagonal phase is associated with the rapid diffusion of deuterons around the cylindrical aggregates, causing a further averaging of the expression in eq 1. Thus, for a liquid only a single line is observed in the 2H NMR spectrum, since the isotropic molecular tumbling mediates the quadrupolar interaction to zero. When the deuterons are situated in an anisotropic environment, such as in a lamellar or a hexagonal liquid crystal, the splitting is observable. Using 2H2O instead of 1H2O for the sample preparation allows us to investigate the number and nature of the phases formed for a system by means of the 2H NMR technique. Thus, a splitting of 2H bounded to the amphiphilic surface appears from each anisotropic liquid crystalline phase while a singlet indicates an isotropic phase (micellar solution, cubic phase). For multiphase samples the different phases in equilibrium can be detected even in the absence of macroscopic phase separation, since the observed spectra consist of a superposition of the spectra arising from the different phases. For example, the appearance of a singlet and a splitting in the spectrum is an indication of the coexistence of an isotropic phase with a lamellar or a hexagonal phase; if two splittings are observed, then two anisotropic mesophases are in equilibrium; eventually, two splittings accompanied by a single peak indicate a three-phase equilibriumstwo anisotropic phases and an isotropic one. According to this scheme, one can study the phase behavior by systematically following 2H NMR spectra of samples as a function of composition over the whole concentration range in the phase diagram. Using a large number of samples single phase-boundaries can be accurately located. Also the stability range of two- and three-phase regions can be determined within the desired accuracy. An example of the 2H NMR spectra recorded for the system is shown in Figure 3. The 2H NMR spectra were obtained with a Bruker MSL-100 superconducting spectrometer operating in the Fourier transform mode at a frequency of 15.371 MHz. The samples were kept in a thermostat at 303 K for 1 day before the NMR experiments were carried out. Successively, each sample was quickly transferred to the NMR probe which was preheated at 303 K. The sample was thermally equilibrated before recording the spectra. Usually, a thermal equilibration of 10 min in the probe was sufficient since no differences in the spectra recorded after 10 min and after 1 h were observed. A 90° (8 µs) pulse with a pulse interval of 1 s and a dead time of 400 µs was used. For most of the samples 100-200 pulses were enough to obtain a good quality quadrupole splitting. However, for the more concentrated samples (less than 10 wt % water) up to 1000 pulses were necessary. The quadrupole splitting was measured in hertz as the peak to peak distance in a spectrum (see Figure 3). Optical Microscopy. The optical characteristic feature of an anisotropic liquid crystal is exhibited by its birefringency against cross polaroids. When observed through a polarized light microscope, different liquid crystals can be recognized by following their characteristic microscopic textures. The difference between textures exhibited by a lamellar phase on one side and a hexagonal phase on the other permits their unequivocal identification. Typical lamellar textures are the fine and coarse mosaic and oily streaks, while a hexagonal liquid crystal shows non-geometric textures, like the so-called fanlike texture.24,25 In Figure 4 examples of the textures relative to the hexagonal phase and the (24) Rosevear, F. B. J. Am. Oil Chem. Soc. 1954, 31, 628. (25) Rosevear, F. B. J. Soc. Cosmet. Chem. 1968, 19, 581.
Figure 1. Phase diagrams at 303 K for the aqueous systems of (a) DDAB, (b) AOT, and (c) the catanionic surfactant (didodecyldimethylammonium bis(2-ethylhexyl)sulfosuccinate). Phase notations:
lamellar phase are shown. An Axioplan Universal Microscope by Zeiss (Germany) equipped with the differential interference contrast (DIC) and a camera (MC 100) is used to identify textures of the liquid crystalline materials. Cryo-TEM. The preparation of samples for cryo-TEM was performed in the controlled environment vitrification system (CEVS) according to the procedure described previously.26 The system allows us to control the temperature and humidity. The technique consists of spreading a thin liquid drop (∼0.5 µL) on an appropriate grid coated by a holey carbon film. Subsequently the sample is vitrified by plunging it into liquid ethane at its freezing point. The transfer of the specimen to the electron microscope (JEOL 2000 FX equipped with a Gatan cold stage) is done, keeping the samples in liquid nitrogen, at about -170 °C. This temperature is kept also during the transfer of the samples to the microscope. The examination of the samples was performed at -170 °C and using a 100 kV acceleration voltage. The images were recorded on a Kodak SO-163 electron imaging film with a nominal underfocus of 4-6 µm. The film was developed for maximum electron speed.
Results Binary Phase Equilibria. The studied catanionic surfactant may be considered as a pseudo-four-alkyl-chain uncharged surfactant whereas each of the parent surfactants, AOT and DDAB, is a double-alkyl-chain ionic surfactant. The binary phase equilibria of these systems are expected to provide important information on the surfactant self-assembly processes in water. The phase diagrams of both DDAB-water27 and AOT-water28 are reported. We have further studied the dilute regions formed for these systems.29 The phase diagrams for the two systems are presented in Figure 1a and b, respectively. DDAB-Water System. DDAB is a swelling-type surfactant which tends to self-aggregate in water to bilayer structures. The surfactant is sparingly (≈0.1 wt %) soluble in water, forming a slightly bluish solution. Upon increasing the concentration, the samples show flow-birefrigeny and increase in viscosity. Above 2 wt % of surfactant, the samples exhibit static birefringency. On further increasing the surfactant concentration the system forms two different lamellar liquid crystalline phases, D1 and D2. The D1 phase is formed between 4 and 30 wt % DDAB, and D2, between 83 and 90 wt % surfactant. The two lamellar phases coexist within a wide concentration range. (26) Bellare, J. R.; Davis, H. T.; Scriven, L. E.; Talmon, Y. J. Electron Microsc. Tech. 1988, 10, 87. (27) Fontell, K.; Ceglie, A.; Lindman, B.; Ninham, B. W. Acta Chem. Scand. 1986, A40, 247. (28) Rogers, J.; Winsor, P. A. Nature 1967, 216, 477. (29) Regev, O.; Khan, A. Prog. Colloid Polym. Sci. 1994, 97, 298.
Catanionic Surfactant Mixtures
We have reexamined the very dilute part of the DDAB system by the cryo-TEM technique. The micrographs show the presence of single-walled vesicles at concentrations below 0.5 wt % DDAB. At concentrations above 0.5 wt % and below 4 wt % DDAB, the single-walled vesicles are found to coexist with double-walled vesicles as well as with tubules (Figure 2a). At low concentrations, the solution contains mostly double-walled vesicles and the concentration of tubules increases with increased DDAB contents. The observations are in agreement with our results reported previously.29 AOT-Water System. The phase diagram of the system is dominated by a lamellar liquid crystalline phase D (1070 wt % surfactant). A bicontinuous-type cubic phase (73-80 wt % AOT) and a reverse hexagonal liquid crystalline phase (concentration > 82 wt % AOT) are formed on the water-poor part of the phase diagram. On the water-rich side, the D phase is in equilibrium with a solution by a two-phase region, and below 1.3 wt %, the surfactant is soluble with water. Previously,29 we have reported the existence of vesicles in the very dilute solution region. We have reexamined this region by directly imaging the microstructure by the cryo-TEM technique. Large liposomes are detected in this region. The liposomes are highly polydisperse in terms of vesicle size as well as the number of bilayers in a liposome (see Figure 2a). Catanionic Surfactant-Water System. The positively charged surfactant DDAB will interact strongly with negatively charged AOT in water and at equimolar mixtures of the two surfactants, the uncharged catanionic surfactant didodecyldimethylammonium bis(2-ethylhexyl)sulfosuccinate, denoted as A+A-, will be formed while, at the same time, an equimolar amount of the salt NaBr will be released. About 40 samples were prepared to determine the phase behavior for the system A+A--saltwater at 303 K, and the pseudo-binary phase diagram is shown in Figure 1c. At very high water content, with less than 1 wt % of A+A-, a bluish solution is obtained and the cryo-TEM micrographs recorded in this region show the presence of multiwalled vesicles with high polydispersity (Figure 2b). Between 1 and 10 wt % catanionic surfactant a stable precipitate of A+A- is detected. At concentrations above 10 wt % surfactant, the precipitate is redissolved and a liquid crystal in equilibrium with an isotropic liquid is formed at 303 K. The two-phase region, namely, the liquid crystalline phase and the isotropic solution, extends up to 90 wt % A+A-. The samples in this region produce a 2H quadrupole splitting with a central isotropic signal on its NMR spectra similar to the one in Figure 3b, recorded for a two-phase region, hexagonal + isotropic cubic phases. The liquid crystal has microscopic texture similar to that of a hexagonal liquid crystal (Figure 4a). In Figure 4b the texture shown by lamellar phases is included for comparison. The supernatant liquid of a sample is separated from the liquid crystal, and the recorded 1H NMR spectra do not show the presence of any detectable amounts of surfactant in the liquid phase indicating that the hexagonal phase, coexists with almost pure water. Above 90 wt % surfactant, only the single hexagonal liquid crystalline phase is stable. The hexagonal phase produces a single splitting in the 2H NMR spectra similar to that recorded in the single lamellar phase (Figure 3a). However, the splitting values are different between the hexagonal and the lamellar phases. However, the existence region of the single hexagonal phase is limited, and at concentrations above 95 wt % A+A-, the hexagonal phase is found to exist in equilibrium with surfactant
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Figure 2. Cryo-TEM micrographs taken on dilute aqueous solutions of (a) DDAB (2 wt %). Polydispersed multiwalled vesicles are formed. The same kind of polydispersed vesicles are observed with AOT-water mixtures at concentrations between 1.3 and 10 wt % AOT29. (b) Catanionic surfactant (1 wt %), another example of polydispersed multiwalled vesicles (bar ) 200 nm).
(hydrated) crystals. From the position in the phase diagram (very low water content) we conclude that the liquid crystalline phase consists of reverse-type hexagonal rods, and the phase is denoted as F2. The phase notations adopted by Ekwall, Fontell, and Mandell30,31 are used in this work. Since more than one reverse hexagonal phase (30) Mandell, L.; Ekwall, P.; Fontell, K. Acta Polytech. Scand. Met. Ser. 1968, 74, 1. (31) Ekwall, P. Adv. Liq. Cryst. 1975, 1, 1.
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Figure 3. Representative 2H NMR spectra recorded for the AOT-DDAB-2H2O system at 303 K. Compositions are given in wt % AOT/DDAB/2H2O. Bar ) 1 kHz: (a) single lamellar phase (68.0/1.0/31.0); (b) two-phase region of hexagonal + cubic phases (74.2/9.2/15.6); (c) two hexagonal liquid crystals in equilibrium (79.0/15.0/5.4); (d) three-phase region, two hexagonal + cubic phases (76.0/13.0/11.0).
is detected in the system, we have reserved the notation F1 for the hexagonal phase formed by single surfactant systems. The precipitate formed at low surfactant content has also been examined under a polarizing microscope. A small amount of the precipitate is separated from the supernatant and washed thoroughly to get rid of the free salt. The precipitate exhibits crystalline texture when viewed under the polarizing microscope (Figure 4c). When the temperature is increased to about 323 K, the precipitate is turned into an anisotropic liquid crystal which exhibits a hexagonal liquid crystalline texture identical to the texture shown in Figure 4a. When the temperature is increased further, the liquid crystal melts, forming an isotropic liquid (below 373 K). Our findings indicate that only a reverse hexagonal liquid crystalline phase exists for the catanionic surfactant-water system.The liquid crystal is formed both by redissolution of the precipitate at a higher surfactant concentration and by transformation of the precipitate at higher temperature with or without the presence of a salt. However, the stability region of the hexagonal phase may be affected if the inorganic counterions Na+ and Br- are removed from the system. Pseudoternary Phase Equilibria. The phase equilibria of the catanionic mixtures of the DDAB-AOTwater system have been studied over the entire concentration range at 303 K, and about 400 samples have been prepared to obtain the phase diagram. Single phases are characterized and their stability ranges are determined by analyzing the 2H NMR spectra of water deuteron and the characteristic microscopic textures of liquid crystalline materials as a function of sample composition. Single phases for the four-component system are shown in Figure 5 as a pseudoternary system in a triangular phase diagram. The data are plotted in weight percent of each component. However, a better understanding of the phase equilibria may be realized by expressing the concentration on a molar basis. Since the difference of the molar mass of DDAB (MW 462.64) and AOT (MW 444.57) is rather small, we have an added advantage for discussing the results in terms of either weight fraction or molar unit of
Figure 4. (a) Micrographs recorded by polarized microscopy for selected samples of the AOT-DDAB-2H2O system at 303 K. Sample composition is in wt % AOT/DDAB/2H2O: (a) “funlike” texture shown by the reverse hexagonal phase F2 (39.0/41.0/ 20.0), magnification ) ×20; (b) mosaic texture exhibited by the lamellar phase (40.0/1.0/59.0), magnification ) ×20; (c) micrograph of the precipitate formed in the dilute region of the DDAB-AOT-water system (2.0/2.1/95.9), magnification ) ×40.
the two surfactants. This section has been organized as follows: First we will describe the dilute (>90 wt % water) region of the mixed system. At water content < 90 wt %, we will discuss the phase behavior under the subtitles AOT-rich Area for the AOT-catanionic-water semitriangle and DDAB-rich Area for the DDAB-catanionicwater semitriangular phase diagram. Dilute Region. In Figure 6 are shown the main features observed for samples containing high amounts of water (>90 wt %). We observed that the vesicles formed for the binary systems DDAB-water and AOT-water can incorporate small amounts of the second surfactant, AOT or DDAB, respectively. The vesicular solution region formed in the mixed system extends along the two binary axes and comprises the equimolar compositions with a maximum of about 1 wt % total surfactant. For higher surfactant contents, along the equimolar line, a precipitate
Catanionic Surfactant Mixtures
Figure 5. Isothermal ternary phase diagram for the system AOT-DDAB-2H2O at 303 K. Phase notations: Xsdilute region, see Figure 6; D1, D2, D3slamellar phases; L2, L3sisotropic, viscous solution phases; I1, I2scubic phases; F1, F2sreverse hexagonal phases.
Figure 6. Phase diagram for the dilute region of the DDABAOT-water system at 303 K: gray area, vesicle phase; diagonal lines, precipitate region; L1, narrow solution phase formed by AOT with water, enlarged for visualization purposes.
of catanionic surfactant crystals is found to separate from the aqueous solution, as described in a previous section; such a precipitate is also produced in all the regions marked with black diagonal lines in Figure 6. The narrow solution phase formed by AOT with water, indicated as L1 and enlarged for visualization purposes in Figure 6, solubilizes very tiny quantities of DDAB, as it can be foreseen given the extremely low solubility of DDAB in water. Phase Equilibria of AOT-rich Area. For the AOT-rich area (Figure 5), there is one new isotropic solution phase formed inside the triangle, in addition to single phases identified for the binary AOT-water and catanionicwater systems. There is a limited solubility of DDAB within the stability range of the AOT lamellar phase. However, the solubility of DDAB in the AOT lamellar phase is relatively higher in the dilute region (e.g., 1 molecule of DDAB per 10 molecules of AOT at 90 wt % water) compared to that in the water-poor part of the lamellar phase (e.g., 1 molecule of DDAB per 80 molecules of AOT at 25 wt % water). These experimental findings can be rationalized in terms of dominant attractive electrostatic interactions between the oppositely charged headgroups for the dilute region and relatively larger CPP effects for the concentrated region. The cubic liquid crystalline phase formed by the binary AOT system is isotropic and stiff and consists of a bicontinuous-type microstructure.32 When DDAB is added to this phase, the phase keeps its stability within a few percent of DDAB (5 wt %), but the stability range of the (32) Fontell, K. In Colloidal Dispersions and Micellar Behaviour ; Mittal, K. L., Ed.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975; Vol. 9, p 270.
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phase extends, just above the lamellar phase, to high water contents (42 wt %), showing a swelling behavior. This phase has a bicontinuous-type structure as in the binary system.33 On further dilution of the cubic phase, the formation of a new isotropic solution phase results. The stability region of the phase is small, between 45 and 52 wt % AOT and 43 and 50 wt % water. The stability range of the phase with respect to DDAB concentration is extremely narrow (4-6 wt % DDAB); there are 8-14 molecules of AOT per DDAB molecule within the stability region of the phase. The solution is not very viscous and often shows flow-birefringency, indicating the presence of large domains dispersed in water. The phase is denoted as L3 to stress that its microstructure is identical to that of the L3 phases detected in several surfactant systems.34 Even though the L3 phase does not exist for the AOTH2O binary system, it is not surprising that the L3 phase exists in the mixed system. AOT is known to have a tendency to form such a phase upon addition of a salt.32,35 It appears that there are analogies between addition of a salt and an oppositely charged surfactant to the AOTwater system. However, on the addition of a salt a very high swelling of the L3 phase is observed compared to the relatively limited stability of the phase with DDAB. Coexistence of Two Hexagonal Liquid Crystalline Phases. The water-poor ( 35 °C while at low temperature (T < 35 °C) (hydrated) surfactant crystals are formed. Multiphase Regions. The homogeneous single phases which we have discussed above also exist in equilibrium through the appropriate multiphase regions. In an usual three-component system one can determine tie lines indicating the direction and the extent of the phase separation occurring in a two-phase sample as well as the three coexisting phases in a three-phase sample. The existence of the appropriate number of two- and threephase heterogeneous regions is a necessary condition for the correctness of the phase diagram for a three-component system. Since our four-component system is plotted as a pseudoternary system in a triangular phase diagram, the tie-lines do not necessarily lie on the surface of the triangle; phases can have extensions and shapes which can accurately be described by a three-dimensional phase diagram in a four-phase pyramid, with the four apices characterizing the compositions of the four components.38,39 The multiphase regions characterized by 2H NMR, ocular inspections, and polarizing microscopy can still be approximately relevant for the pseudoternary phase diagram; however, no attempt has been made to determine the stability ranges of the heterogeneous regions. For the DDAB-rich semitriangle the dilute lamellar phase, D1, is in equilibrium with mixed surfactant crystals (C) that form along the equimolar mixtures and also with the isotropic solution, L1, at the water corner through a two-phase region. The concentrated lamellar phase, D2, is, however, in equilibrium with the cubic phase, I2. A three-phase region, D1 + D2 + L1 is, as expected, also formed. The I2 phase is in equilibrium with the D2 phase and, at higher AOT content, with the F2 phase. A threephase region consisting of F2 + I2 + C is also detected. For the AOT-rich side, both F2 and L3 single phases coexist with the L1 phase and the three single phases are in equilibrium by a three-phase triangle. Other multiphase regions which we have detected are: L3 + I1, I1 + D, and L3 + I1 + D. The two hexagonal phases exist in equilibrium between themselves and also with I1. The F2 phase coexists with the I1 and L3 phases by two- and three-phase regions. No equilibrium is, however, shown between any of the F phases and the lamellar phase. Comparison of Phase Behavior with Other Representative Systems. At this stage it is appropriate to make a comparison of the phase behavior of our system with that of the limited number of catanionic surfactant systems reported in the literature. Such a comparison
(36) Wennerstro¨m, H. Langmuir 1990, 6, 834. (37) Wennerstro¨m, H. In Physics of Amphiphilic Layers; Meunier, J., Langevin, D., Boccara, N., Eds.; Springer-Verlag: Berlin, Heidelberg, 1987; Vol. 21, p 171.
(38) Thalberg, K.; Lindman, B.; Karlstro¨m, G. J. Phys. Chem. 1991, 95, 6004. (39) Jokela, P.; Jo¨nsson, B.; Wennerstro¨m, H. Prog. Colloid Polym. Sci. 1985, 70, 17.
Figure 7. Enlarged view of the two-hexagonal-phase region. Phase notations are as in Figure 5. Table 1. Some Representative Values of the 2H NMR Splittings (∆) Recorded for the Region of the Two Reverse Hexagonal Phases at 303 Ka AOT/wt %
DDAB/wt %
∆1/kHz
85.0 79.0 72.0 70.0 67.0 64.5 60.0 89.0 84.0 79.0 75.0 55.5 50.5
5.0 11.0 18.0 20.0 23.0 25.0 30.0 5.0 9.5 15.0 20.0 35.0 40.0
2.57 1.90 1.37
aThe
1.48 0.62 3.70 2.37 1.50 1.49
∆2/kHz 1.62 0.88 1.21 1.38 1.41 0.53 1.80 0.90 1.44 0.63 0.15
phases F1 F 1 + F2 + I F1 + F2 F2 F 1 + F2 F2 + I F 1 + F2 F1 F 1 + F2 + I F1 + F2 F 1 + F2 + I F2 F2
presence of the isotropic peak is also indicated.
Catanionic Surfactant Mixtures
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for the SDS-DDAB-water system over the entire concentration range.7 We also observed that the present system AOT-DDAB-water is equally complex. One significant difference is that different lamellar phases are found to coexist for SDS-DDAB-water whereas we detected the coexistence of two hexagonal phases in the AOT-DDAB-water system. Though the phase diagrams are complex for both systems, a qualitative understanding of phase equilibria can be rationalized in terms of electrostatic effects combined with surfactant geometric constraints. Figure 8. Pseudo-binary-aqueous-phase diagrams: (a) equimolar ratio of pseudo-four-tail DDAB-AOT at 303 K; (b) equimolar ratio of pseudo-three-tail SDS-DDAB at 313 K; (c) dodecyltrimethylammonium dodecyl sulfate (catanionic), at 313 K, pseudo-two-chain. Phase notations are as in Figure 1.
will enable us to assertain the relative importance of electrostatic and hydrophobic effects and surfactant geometric constraints on the phase equilibria of the system. For two symmetric alkyl chains (C8-C8, octylammonium octanoate,13 and C12-C12, dodecyltrimethylammonium dodecyl sulphate1), the catanionic pair, in the absence of inorganic salts, forms one concentrated lamellar phase with water, and the single lamellar phase is in equilibrium with almost pure water by a large two-phase region. On the other hand, the parent surfactants are separately soluble in water, forming spherical micelles, and at high surfactant concentration, no lamellar phase is found to exist in equilibrium with the micellar solution phase for the systems. The formation of the lamellar phase for the catanionic pair is due to the surfactant molecular geometry constraints. The stability of the lamellar phase for the uncharged system is explained in terms of a shortrange repulsive hydration force.13 The chemical compositions of the polar headgroups do not change the general phase behavior of the catanionic systems. However, the sizes of the headgroup are found to determine the strength of the repulsive force. When inorganic salt is present in the mixed system, the general phase behavior still remains unchanged at equimolar ratios of the two surfactants except that the lamellar phase is found to swell with water. The swelling of the lamellar phase may be due to the long-range electrostatic interaction. However, by increasing the number of alkyl chains in the mixed system, a drastic change in the phase behavior is observed. As shown in Figure 8, adding the singlechain SDS to the double-chain DDAB in water leads to the formation of a concentrated cubic phase followed by a lamellar phase with a very limited stability range. On the other hand, for the pseudo-four-alkyl-chain system DDAB-AOT-H2O, we identified only one concentrated reverse hexagonal phase. One important difference of the phase behavior between the last two systems is that the former one forms aggregates with only average zero curvature whereas aggregates with only reverse curvature are found to be stable for the latter system. At identical sample compositions, the electrostatic interactions are not expected to differ significantly for the two systems, but the relatively large CPP value for the pseudo-four-tail system compared to that of the pseudo-three-tail system lies behind the observed differences in phase behavior. Addition of the ionic parent surfactant sodium octanoate to the binary catanionic octylammonium octanoate (A+A-)-water system causes a dramatic swelling of the lamellar phase. No other new phases are identified. The large swelling of the lamellar phase is rationalized in terms of the long-range electrostatic repulsion compared to the short-range repulsive hydration force operating in the A+A- system. A very complex phase behavior is reported
Discussion An explanation of the phase behavior observed for the catanionic surfactant and the catanionic surfactant mixtures is attempted following the change in CPP when DDAB and AOT are mixed in different proportions. Since the binary DDAB-water system forms only lamellar phases and a lamellar phase is at its maximum stability at CPP ∼ 1, then CPP e 1 for the entire concentration range for the binary system. On the other hand, the phase succession of the binary AOT-water system with increasing surfactant concentration is lamellar f cubic f reverse hexagonal, indicating that CPP ∼ 1 at low concentration and CPP > 1 at higher concentration. The surfactant molecules of the catanionic mixtures will pack in aggregates in a way to have the maximum charge neutralization; two oppositely charged headgroups attract and neutralize each other (only partially apart from the equimolar mixtures), and the surfactant molecules can get very closely packed, except for the steric limitations due to the hydrophobic chains. Consequently the effective a* of the mixed surfactants is not just a weighted average of the two a values of the parent surfactants, but the value is somewhat smaller, depending on the molar ratio between the two surfactants. The effective alkyl chain length, l*, will also change due to the difference in chain length between the two surfactants, i.e., lA- < l* < lA+; the cationic DDAB (A+) has 12 straight chain carbons against 6 carbons for AOT. The effective volume, v*, also increases due to the 4 voluminous hydrocarbon chains. Therefore, the effective packing parameter for catanionic mixtures will be larger than the weighted average CPP values of the two parent surfactants for a given mixing ratio. CPP > 1 for an equimolar ratio and for almost all other mixing ratios except for compositions lying very close to the DDAB-water base line and at the water-rich part of the AOT-water base line. In other words, one can say that the hydrocarbon chains are packed very tightly in an aggregate so that the monolayer is subjected to frustration which can be released by changing the monolayer curvature toward the water. This can qualitatively explain the strong tendency towards the formation of structures with negative curvature in the mixed system, in particular the reverse hexagonal phase (maximum negative curvature) for the equimolar mixture and the new L3 phase (with excess AOT), which is not present in the two binary systems. The effective CPP can also explain why the phases whose structures are characterized by negative curvature of the surfactant monolayer are more extended in the phase diagram (cubic and reverse hexagonal liquid crystalline phases). Connected to the change in curvature for the catanionic mixtures is the limited stability shown by lamellar phases that otherwise dominate the two binary systems. The phases can incorporate only very small amounts of the second surfactant. Further addition leads to phase transitions toward phases characterized by negative curvature with respect to water. For example, starting
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from the AOT-water base line, the succession of phases obtained by the addition of DDAB to an AOT-water system is approximately the same as those of the binary AOT-water system with increased AOT contents. Addition of a surfactant leads to aggregate structures with more negative curvature (lamellar f cubic f hexagonal). However the transition of phases takes place with the addition of relatively small amounts of the second surfactant (more effective phase destabilization) in the mixed system compared to the binary AOT-water system. For example, starting with 50% AOT, we need to add another ∼30 wt % AOT to form the cubic phase in the binary system, while only about 5 wt % DDAB is enough to destabilize the lamellar phase and form a single cubic phase in the mixed system. Moreover the passage for the catanionic mixtures encompasses the formation of a L3 phase within a narrow range of DDAB concentrations, and the change in curvature for the L3 phase is only slightly different from the value in the lamellar phase. It is interesting to notice that the curvature effect is more pronounced when AOT is added to the DDAB-water system. This is manifested by the two lamellar phases which can solubilize very small amounts of AOT. This behavior may be associated with the difference in behavior of the two surfactants. The existence of an L3, a swelling cubic, and a hexagonal phase in the AOT-rich semitriangle and the absence of these phases in the DDAB-rich triangle are consequences of the different molecular structure and in-water phase behavior of the two surfactants. However we do have a cubic phase also on the DDAB-rich side. This phase does not swell as much as the cubic phase with excess AOT. From its position in the phase diagram, it could be concluded that the DDAB-rich cubic phase is derived by the destabilization of the concentrated DDAB lamellar phase, while the swollen DDAB lamellar phase does not seem able to yield cubic or L3 phases on adding AOT to the system. Effect of the Electrostatic Interactions on the Swelling. Another important observation on the pseudo-
Caria and Khan
ternary phase diagram is that the single-phase regions extend mainly along or very near to the three binary axes (an exception is the cubic phase in the DDAB-rich side of the phase diagram), while the large central part of the phase diagram is dominated by two- and three-phase regions. When one surfactant is added to the oppositely charged surfactant, the surface of the surfactant aggregates becomes less charged due to the neutralization of the surfactant headgroups. Thus the double-layer repulsive forces responsible for the swelling of the liquid crystalline phases become weaker for the mixed system. The repulsive interactions are expected to be at their minimum for the equimolar mixtures of the two surfactants where the surface of the aggregates is almost uncharged. This is evident by the formation of a very concentrated hexagonal phase which does not swell more than 10 wt % with water, and the hexagonal phase is in equilibrium with almost pure water by a wide two-phase region. Conclusions Mixing oppositely charged surfactants in water leads to a very complex phase behavior, with the formation of various new phases with variable stability ranges. The interplay of both surfactant packing parameter and electrostatic effects can qualitatively explain the phase behavior displayed by the mixed systems. The results indicate that the desired shape of surfactant aggregates as well as the stability region of single phases can be formulated by a suitable blending of the two surfactants. The study is important for both theoretical modeling and microstructure engineering. Acknowledgment. We gratefully acknowledge Oren Regev for taking the cryo-TEM micrographs for us. Eduardo Marques is thanked for critically reading the manuscript. The project is financed by the Swedish Research Council for Engineering Science (TFR). LA960581Z
