Langmuir 2000, 16, 2489-2494
2489
Lamellar Structures in Aqueous Solutions of a Dimeric Surfactant A. Knaebel,* R. Oda,† E. Mendes, and S. J. Candau Laboratoire de Dynamique des Fluides Complexes, UMR No. 7506, Universite´ Louis Pasteur, Centre National de la Recherche Scientifique, 4 rue Blaise Pascal, 67070 Strasbourg Cedex, France Received July 13, 1999. In Final Form: November 3, 1999 The effect of salt (KBr) additions on the behavior of the dimeric surfactant 12-2-12 (ethanediyl-1,2bis(dodecyldimethylammonium bromide)) has been investigated using small-angle neutron scattering and electron microscopy. In the range of surfactant volume fraction 0.08-0.25 only wormlike micelles or lamellar structures are obtained depending on the temperature and the salt content. Two different bilayer structures are observed: one, metastable, which obeys the dilution law up to a maximum swelling corresponding to an intermicellar distance of ∼220 Å; the other at equilibrium with a constant swelling corresponding to an interlamellar distance of ∼70 Å. These results confirm the propensity of the gemini surfactants with a short spacer between the polar heads to form collapsed bilayers structured in stacks.
Introduction The phase behavior of aqueous solutions of ionic surfactants is strongly dependent on the geometrical parameters of the surface-active molecule and on the ionic strength of the medium. The latter can be tuned by varying the surfactant and salt concentration and/or the lipophilicity of the counterion and of the co-ion.1 This is generally taken into account through the surfactant packing parameter p ) ν/al, where ν is the effective hydrophobic chain volume, a the area per polar head, and l the surfactant alkyl chain length. For binary aqueous solutions of single-chain surfactants such as sodium dodecyl sulfate (SDS) or cetyltrimethylammonium bromide (CTAB), p is close to 0.4, favoring spherical micelles at high dilution. With increasing surfactant concentration, the micelles grow in length until various lyotropic liquid crystal phases appear (hexagonal, cubic, lamellar). For double-chain ionic surfactants such as didodecyldimethylammonium bromide (DDAB),2,3 or for a singlechain ionic surfactant with a strongly hydrophobic counterion, such as cetyltrimethylammonium 3-hydroxy2-naphthalenecarboxylate (CTAHNC),4-6 the packing parameter p is around 1, allowing for the formation of bilayers at high dilution. In that case, the lyotropic liquid crystal phases present in the phase diagram are of the bilayer type, that is, vesicular or lamellar. Another class of surfactants is that of dimeric (gemini) surfactants in which two quaternary ammonium moieties (CmH2m+1(CH3)2N+Br-) are linked at the level of the * To whom correspondence should be addressed. † Present address: IECB, ENSCPB, Avenue Pey Berland, 33402 Talence Cedex, France. (1) Israelachvili, J.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (2) Dubois, M.; Zemb, T. Langmuir 1991, 7, 1352. (3) Dubois, M.; Zemb, T.; Belloni, L.; Delville, A.; Levitz, P.; Setton, R. J. Chem. Phys. 1992, 96, 2278. (4) Hassan, P. A.; Narayanan, J.; Menon, S. V. G.; Salkar, R. A.; Samant, S. D.; Manohar, C. Colloids Surf., A 1996, 117, 89. (5) Salkar, R. A.; Hassan, P. A.; Samant, S. D.; Valaulikar, B. S.; Kumar, V. V.; Kern, F.; Candau, S. J.; Manohar, C. Chem. Commun. 1996, 1223. (6) Hassan, P. A.; Valaulikar, B. S.; Manohar, C.; Kern, F.; Bourdieu, L.; Candau, S. J. Langmuir 1996, 12, 4350.
headgroups by a hydrocarbon spacer (CsH2s).7 The packing parameter and therefore the structure in the high-dilution limit are strongly dependent on both the spacer and hydrophobic chain lengths. As an example, it was found for the series with m ) 12 and 2 e s e 16 (referred to as 12-s-12) that the surfactants aggregate into spherical micelles, cylindrical micelles, or vesicles depending on the value of s. The phase behavior of salt-free aqueous solutions of dissymmetrical gemini surfactants (with two hydrophobic chains of different lengths) and bromide counterion has been recently reported.8 Two remarkable features have been revealed by this study. The first one is an almost systematic tendency for these surfactants to form elongated aggregates: long micelles, ribbons, rectangular stacks of bilayers. The other feature is the strong attraction between bilayers which form multilayer structures with very little water between, whatever the dilution. It was suggested that the conjunction between the two specific features of the gemini surfactants, namely, the directional anisotropy of the molecules in the interface due to their elongated shape along the direction of the two charges, and the strong ionic attraction between bilayers due to the high affinity between the N+ ions and Br- ions, was at the origin of the observed phase behavior. The effect of salt on the phase behavior of 12-2-12 surfactants has also been recently reported.9 It was found that for relatively low surfactant volume fraction (Φ e 0.2), the addition of salt (KBr) induces a transition from an isotropic phase of wormlike micelles to a lamellar phase. At relatively low salt content (Cs = 0.04 M), the latter exhibits a linear swelling upon dilution down to Φ e 0.1. At higher dilutions the interlamellar distance remains constant with a value =220 Å. However, the scattering pattern obtained by small-angle neutron scattering (SANS) experiments revealed in some cases a second peak at (7) (a) Zana, R.; Talmon, Y. Nature 1993, 362, 228. Danino, D.; Talmon, Y.; Zana, R. Langmuir 1995, 11, 1448. (b) Zana, R.; Benrraou, M.; Rueff, P. Langmuir 1991, 7, 1072. (c) Alami, E.; Beinert, G.; Marie, P.; Zana, R. Langmuir 1993, 9, 1465. (d) Alami, E.; Levy, H.; Zana, R.; Skoulios, A. Langmuir 1993, 9, 940. (8) Oda, R.; Huc, I.; Homo, J. C.; Heinrich, B.; Schmutz, M.; Candau, S. J. Langmuir 1999, 15, 2384. (9) Bu¨hler, E.; Mendes, E.; Boltenhagen, P.; Munch, J. P.; Zana, R.; Candau, S. J. Langmuir 1997, 13, 3096.
10.1021/la990937h CCC: $19.00 © 2000 American Chemical Society Published on Web 02/10/2000
2490
Langmuir, Vol. 16, No. 6, 2000
Knaebel et al.
higher scattering vector, indicating a more complex behavior. In this paper we report on a more complete investigation using SANS and electron microscopy of the effect of salt, temperature, and dilution on the behavior of the dimeric 12-2-12 surfactant with special focus on the lamellar domains. We show that depending on the salt content the surfactant self-organizes into two different bilayer structures: one, metastable, which obeys the dilution law with a maximum swelling corresponding to an interlamellar distance of 220 Å; the other with a constant swelling in the range of surfactant volume fraction investigated (Φ e 0.25) corresponding to d ) 70 Å. Materials and Methods The surfactant preparation, purification, and characterization have been reported. Deuterated water was used as a solvent. The samples were prepared by weighing. The density of the surfactant was about 1. The surfactant volume fractions Φ (v/v) have been determined from the concentrations (w/w). SANS. SANS experiments were carried out on a PAXE spectrometer at the Laboratory Le´on Brillouin at Saclay. For the biphasic systems, the two phases of the samples obtained after sedimentation were put into separate scattering cells, having a thickness of 1 mm. The incident wavelength was 10 or 12 Å depending on the set of experiments. For a given wavelength, the range of the amplitude of the transfer vector q was selected by changing the detector distance, to obtain the configuration allowing a good characterization of the scattering peaks. The range is typically 0.01 e q (Å-1) e 0.1 or 0.02 e q e 0.2. The data are put on an absolute scale following the standard procedures. Intensities relative to the incoherent scattering of H2O in a cell with a path of 1 mm are obtained from the measured intensities after subtraction of the solvent and empty cell contributions. The samples were prepared 10 days before the SANS experiments were performed except for one series for which the measurements were carried out, respectively, 2 and 10 days after the preparation of the solutions. Electron Microscopy. Freeze-fracture experiments were performed in an apparatus developed at IGBMC at Strasbourg. Surfactant solutions were sandwiched between two copper specimen holders, which were then frozen with liquid ethane cooled with liquid nitrogen. The frozen sandwich was then fixed to a transport unit under liquid nitrogen and transferred to the fracture replication stage, which was kept at a vacuum of 10-9 mbar at -180 °C; then immediately after fracturing, replication took place by first shadowing with platinum/carbon at 45° and then deposition of carbon at 90°. The replica was then reinforced by collodion before the recuperation from the specimen holders. A drop of a 1% solution of collodion in amyl acetate was deposited on the frozen surface of the replica while it was warmed to room temperature. The solvent then evaporated, leaving collodion to form a thin film over the replica. The replicas were then retrieved and cleaned in water; they were mounted onto grids and soaked in methanol overnight to dissolve the collodion. Observations were made with a Zeiss EM 10CR microscope operated at 100 kV. Images were recorded on Kodak SO 163 films and developed using standard procedures.
Experimental Results Partial Phase Diagram. The partial phase diagram of the KBr/12-2-12/D2O system obtained by visual inspection for surfactant volume fractions varying from 0.002 to 0.2 and salt content in the range 10-70 mM KBr at T ) 20 °C and T ) 28 °C is shown in Figure 1. At low salt concentrations (Cs e ∼0.04 M), low temperatures (T e 28 °C), and surfactant volume fraction e∼0.22, one observes an isotropic viscous phase that does not exhibit any birefringence. Intermediate salt contents correspond to a birefringent, apparently monophasic domain: the higher the surfactant concentration, the larger
Figure 1. Partial phase diagrams of [KBr] vs surfactant concentration at (a) T ) 20 °C; (b) T ) 28 °C. At this temperature, the study of the phase diagram was limited to a surfactant volume fraction of 20%. The actual boundary line between the micellar and lamellar phases is likely to be the same as that of (a).
the range of salt concentration over which this domain exists. One does not observe any phase separation over a period of 6 months, or under centrifugation. At higher salt concentrations, the systems separate into an isotropic lower phase of low viscosity and a turbid birefringent upper phase. The phase separation is rather fast (less than 2 days) due to the significant difference of density between 12-2-12 and D2O. SANS experiments performed on the isotropic phase show that the amount of surfactant solubilized in that phase is almost negligible and that this phase is essentially an aqueous KBr solution. In a previous study, it was shown that the tie lines correspond to an equilibrium between a very dilute isotropic dispersion and a birefringent phase.9 Qualitatively, the shape of this phase diagram is quite similar to that reported by Dubois and Zemb for aqueous solutions of DDAB in the presence of KBr at T ) 25 °C.2 However, the domain of existence of the phase of cylindrical micelles is much more extended for the gemini surfactant investigated here. Both the pure birefringent phase and the upper phase of the demixing systems show under the polarizing microscope the same characteristic features of a lamellar phase. Contrary to the DDAB systems, one does not observe here any [LR] phase, which is a dispersion of pure LR
Lamellar Structures in Solutions of a Surfactant
Langmuir, Vol. 16, No. 6, 2000 2491
Figure 2. Partial phase diagrams of [KBr] vs temperature at surfactant concentration (a) Φ ) 0.005 and (b) Φ ) 0.16.
crystallites in an isotropic phase. This phase is generally gel-like, whereas the lamellar systems studied here are very fluid. The phase diagram is sensitive to temperature. The boundary between the isotropic phase and the lamellar one is shifted toward higher salt concentrations with increasing temperature (cf. the comparison between parts a and b of Figure 1). In Figure 2 are represented the diagrams of temperature vs salt concentration for two surfactant concentrations. For the higher surfactant concentration (Φ ) 0.16) one observes at high temperature and high salt concentration a phase separation into an isotropic viscoelastic phase at the top of the tube and a birefringent phase at the bottom. This suggests that the top phase is a concentrated solution of wormlike micelles, whereas the birefringent one is a lamellar phase containing a large amount of water. At lower concentration, i.e., Φ ) 0.05, one obtains the same phase separation at low salt content, whereas at high salt content the system separates into an isotropic nonviscous phase and a birefringent supernatant. In fact, at an ever lower volume fraction (φ ) 0.02) one observes three phases in the tube: nonviscous isotropic at the bottom, birefringent in the middle, and viscoelastic isotropic at the top. Structural Characteristics of the Phases Encountered. Figure 3 shows three characteristic scattering curves obtained by SANS. Figure 3a refers to a viscoelastic isotropic phase. The scattering curve exhibits a broad peak
Figure 3. Characteristic scattering curves: Φ ) 0.16, [KBr] ) 40 mM, T ) 28 °C; Φ ) 0.16, [KBr] ) 40 mM, T ) 20 °C; Φ ) 0.16, [KBr] ) 60 mM, T ) 20 °C.
whose position characterized by q* varies like φ1/2 in the absence of salt. This behavior is the signature of the presence of entangled wormlike micelles, which is confirmed by CRYO-TEM experiments (not shown here). The scattering curve shown in Figure 3b refers to a birefringent phase that under crossed polarizers exhibits the typical textures of a lamellar phase. One observes a sharp peak and a shoulder on the high-q side. In a previous study this shoulder, which in some cases might emerge as a small peak, was interpreted as a second-order component of the scattering pattern.9 In fact this is the
2492
Langmuir, Vol. 16, No. 6, 2000
Knaebel et al.
Figure 5. Effect of salt concentration on the scattering curves of (LR) samples, Φ ) 0.16, T ) 20 °C. The experiments are performed less than 2 days after the sample preparation.
Figure 4. Electron micrographs: (a) Φ ) 0.16, [KBr] ) 35 mM, T ) 20 °C; (b) Φ ) 0.16, [KBr] ) 65 mM, T ) 20 °C. The bar represents 300 nm.
result of a coincidence as the concentration of the sample investigated was such that the positions of the two maxima of the scattering curves were exactly in a ratio of 2. However, as shown in the following, the first peak is shifted under dilution of the system, whereas the one at higher q is independent of concentration and corresponds to a periodicity of 70 Å. Moreover, this second peak is much broader than that obtained at lower q. Figure 4a shows a freeze-fracture micrograph of such a system. One clearly sees the lamellar structure. In the following we call this type of structure (LR). In Figure 3c is represented a scattering curve typical of other birefringent phases encountered in the phase diagram, still showing under observation between crossed polarizers the textures of lamellar systems. One observes
only the broad peak at a scattering vector that is independent of the surfactant concentration. This means that either the lamellae are perforated as suggested earlier or there is water dispersed in the system outside the lamellae. In fact the latter hypothesis is supported by the cryomicrographs that reveal the presence of interpenetrated mesomorphic and isotropic domains with size in the micrometric range (cf. Figure 4b). In the following this structure is denominated L′R. 3. Stability of the Lamellar Phase. In the course of the study, it was found that the scattering characteristics of the lamellar samples were history dependent. Samples investigated shortly, that is 1 or 2 days, after the dissolution of the surfactant exhibit different structures depending on the salt content. This is illustrated in Figure 5, which shows the scattering curves for a sample with a surfactant volume fraction of 0.16 at temperature T ) 20 °C and with various salt contents. One clearly sees a sharp transition from (LR) to L′R structures upon increasing the salt content, the transition appearing between 50 and 55 mM KBr. It must be noted that the positions of the two peaks are insensitive to the salt concentration within the experimental accuracy. As the first peak disappears (LR phase), the second peak (L′R phase) merges, and its broad spectral width suggests a more grainy structure for this phase. The L′R structure is obtained very rapidly (less than 1 day) and then remains unchanged over time. Centrifugation does not produce any phase separation. On the other hand, for salt contents larger than 65 mM macroscropic phase separation occurs rapidly. In that case, the isotropic phase at the bottom of the tube is an aqueous solution of KBr. The birefringent phase gives scattering curves with a peak corresponding to periodicity ∼70 Å characteristic of L′R. This set of data suggests that the domain of existence of the birefringent phase shown in Figure 1 is divided into two parts, one (LR) and the other L′R. However, the attention must be drawn to the fact that the (LR) system is formed by a coexistence of two lamellar phases with different periodicities. In these systems the reequilibration times are very long as illustrated in Figure 6, which shows two sets of scattering curves obtained, respectively, 2 and 10 days after the surfactant dissolution for a sample with Φ ) 0.25, T ) 20 °C, and various salt contents. Freshly prepared systems show the (LR) structure with predomi-
Lamellar Structures in Solutions of a Surfactant
Langmuir, Vol. 16, No. 6, 2000 2493
Figure 7. Effect of surfactant concentration on the scattering curves of (LR) samples, [KBr] ) 35 mM, T ) 20 °C. The surfactant concentrations are given in the inset. The experiments were performed less than 2 days after the sample preparation.
The variation of the periodicity d ) 2π/q* as a function of the inverse of the surfactant volume fraction is reported in Figure 8b. Within experimental accuracy, d varies linearly with 1/Φ for Φ g 0.08. This shows that the amount of the L′R phase in the system is small enough not to strongly affect the dilution law and that, to a first approximation, the lamellar phase fills the whole volume of the sample. Discussion
Figure 6. Effect of aging on the scattering curves of (LR) samples, Φ ) 0.25, T ) 20 °C: (a) less than 2 days after the sample preparation; (b) 10 days after the sample preparation. The salt contents are given in the insets.
nant contribution of the more swollen phase whatever the salt concentration. Over time, the scattering curves broaden with a decrease of the first peak amplitude. Those at high salt content turn progressively to the shape corresponding to L′R. One can notice in particular that for the sample with 35 mM KBr the collapsed lamellar phase becomes predominant. The location in the phase diagrams of the (LR) and L′R systems for samples aged 10 days is indicated in Figures 1 and 2. The main effect of the temperature is to shift the boundaries between the lamellar and the micellar domains. It is rather difficult to obtain by SANS the actual very long time structure of (LR) as those systems must be maintained in a rather narrow range of temperature, without mechanical perturbations. To obtain information on the space-filling of the surfactant, we have investigated the effect of the dilution on freshly prepared systems. This effect is illustrated in Figure 7, which shows the scattering curves for different surfactant volume fractions with Cs ) 35 mM and T ) 20 °C. One can see that the main peak is shifted toward the low-q range under dilution. On the other hand, the position of the shoulder in the high-q range of the scattering curve remains unchanged. In fact, whatever the salt and temperature conditions and independently of whether the sample was the supernatant of a phase-separated system or directly obtained by dissolution, the L′R peak was always found at the same position (cf. Figure 8a).
The molecular films of two-chain surfactants, either of the gemini type or of the single polar head type, can be considered as intermediates between the structures obtained from a single-chain surfactant associated with a cosurfactant and those formed from phospholipids. In the former case, the fluidity of the film, characterized by moduli of bending rigidity on the order of kBT, allows the lamellar phase to swell virtually up to infinity. For phospholipids these moduli are large compared to kBT, thus limiting the swelling capacity of the lamellae and leading to very long reequilibration times, sometimes prohibitive to determine the equilibrium structure. The results reported above suggest that the films of 12-2-12 are also quite rigid since one observes a limited swelling of the lamellae upon dilution. The same type of behavior and transitions between swollen and collapsed lamellar phases were observed for the bicatenar DDAB surfactant. However, the features of these transitions as well as the general phase diagram of the gemini system are quite different from those of the DDAB as discussed below. The L′r Phase. As mentioned above this phase appears as macroscopically homogeneous and stable with time despite the relatively low viscosity of the system. For systems located in the phase diagram close to the boundary between the L′R phase and the macroscopically biphasic system, one often observes a lumpy appearance. In fact, such systems again became, under centrifugation or even hand shaking, homogeneous. However, on the macroscopic scale, the electron micrographs show clearly the coexistence of an isotropic medium and of a lamellar phase. The isotropic phase can be either brine or a micellar solution. The salt is likely partitioned between the two phases to ensure equality of the osmotic pressure.3 The stability of the system may be explained by the fact that any perturbation (like shaking or a temporary increase of temperature) would tend to
2494
Langmuir, Vol. 16, No. 6, 2000
Knaebel et al.
of the L′R phase, the logarithmic correction to the law of dilution is undetectable, which is the signature of a bending rigidity much larger than kBT. Therefore, the contribution of the undulation force can be neglected. The same observation was made for the DDAB lamellar phases that can also swell up to a lamellar distance ∼20 times κ-1, but in the latter case the systems appear to be stable. Here the (LR) phase is metastable and transforms progressively into the equilibrium L′R phase. Another feature of the scattering curves obtained in the (LR) phase is the upturn of scattered intensity at low scattering angle that appears mainly upon decreasing the surfactant concentration (see for instance the 0.12 sample in Figure 7). This upturn is generally attributed to the concentration fluctuations associated with the displacement of the layers. Its amplitude is inversely proportional to B h , the compressibility modulus of the layers at constant surfactant chemical potential.11 However, for rigid films this contribution becomes negligible. Therefore, the lowangle scattering observed here must be associated with structure defects in the lamellar phase. In particular the small amount of the L′R phase present in the system can be dispersed as small domains that give a contribution to the scattering at low angle. For the (LR) phase there is also a maximum swelling occurring for Φ e 8 × 10-2.9 In a previous work it was concluded that the limiting swelling resulted from the formation of a large amount of holes in the lamellae, but at this stage one cannot discard the coexistence of an (LR) phase and an isotropic phase. In fact the optical observations do not allow two phases to be identified. Furthermore, it is very difficult to perform cryoelectron microscopy experiments because the domain of existence of the lamellar phase is very sensitive to temperature. Figure 8. Variation of the interlamellar distances of two lamellar phases as a function of the surfactant concentration for the (LR) sample. The straight line in (b) has been calculated assuming a linear swelling of the bilayer with thickess δ ) 20 Å.
homogenize the system into a (LR) structure. So, if locally the isotropic phase wants to flow down to separate from the L′R, it might locally form LR lamellae that will prevent the separation. The less salty the system, the more efficient the stabilizing effect. Beyond a critical salt content, the (LR) phase cannot form and the system macroscopically phase separates. The observation that the periodicity of the L′R lamellae is independent of surfactant concentration, salt, and temperature suggests that it corresponds to a maximum swelling in the range of concentration investigated. The ratio d/κ-1 is less than 5, which is compatible with balanced van der Waals and electrostatic forces. The (Lr) Phase. The linear swelling behavior of the (LR) phase in the presence of 35 mM salt is observed for surfactant volume fractions larger than ∼0.08.9 At such a volume fraction, the interlamellar distance is about 20 times the Debye-Hu¨ckel length κ-1. One could assume that, under such conditions, the system is stabilized by Helfrich interactions.10 In that case there is a logarithmic deviation in the linear variation of the lamellar periodicity with the surfactant volume fraction. However, within experimental accuracy, which is here limited because of the presence of a small amount (10) Helfrich, W. Z. Naturforsch. 1978, 33a, 305.
Conclusion The results reported here that the addition of salt to aqueous solutions of the 12-2-12 gemini surfactant induces a micellar-lamellar transition. Shortly after the sample preparation, and for volume fraction larger than 0.08, the surfactant self-associates into a swollen lamellar phase filling mostly all the space available. However, this lamellar phase always coexists with a detectable amount of a collapsed lamellar phase. Over time the swollen lamellar phase transforms progressively into the collapsed one, characterized by a maximum swelling with a period of ∼70 Å. Other examples of lamellar systems with maximum swelling have been reported previously. However, they form multilamellar spherical crystallites dispersed in an isotropic medium. In the systems investigated here, one observes instead a mosaic-like distribution of lamellar and isotropic domains that remains stable over months. This observation must be brought together with the results obtained in recent experiments on salt-free solutions of gemini surfactants that have shown a strong trend for these surfactants to form collapsed bilayers structured in ribbons or stacks.8 The results suggest the existence of an anisotropic fluid-like arrangement of headgroups in the membranes, analogous to nematic order in bulk liquid crystal phases. Such an anisotropy is likely at the origin due to the fact that it seems impossible to spherically bend the lamellae of gemini surfactants with a two carbon atom spacer. LA990937H (11) Porte, G.; Marignan, J.; Bassereau, P.; May, R. Europhys. Lett. 1988, 7, 713.
