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Lyotropic Structures in a Thermotropic Liquid Crystal Guillaume Toquer, Gre´goire Porte, Maurizio Nobili, Jacqueline Appell, and Christophe Blanc* Laboratoire des Colloı¨des, Verres et Nanomate´ riaux, UMR 5587, CNRS-UniVersite´ Montpellier II, Place Euge` ne Bataillon, 34095 Montpellier, France ReceiVed July 12, 2006. In Final Form: December 28, 2006 We have investigated the nature of didodecyldimethylammonium bromide (DDAB)/water aggregates dispersed in 4-n-pentyl-4′-cyanobiphenyl thermotropic liquid crystal (5CB). The structure of this microemulsion has been probed by small-angle neutron and X-ray scattering experiments far above the nematic-to-isotropic phase transition temperature of the solvent. Our data show that the stability of this system is controlled by strong attractive van der Waals interactions between spherical inverted micelles. These interactions also explain why other swollen mesophases in related cosurfactant/ DDAB/water/5CB phase diagrams are not observed. When approaching the isotropic-to-nematic phase transition, scattering experiments additionally confirm the predominance of an increasing attractive interaction due to the 5CB paranematic fluctuations.
Introduction Colloids in liquid crystal (LC) are a fascinating topic that has attracted many studies in the past decade. Many recent works have dealt with micrometer particles, either solid particles or droplets of liquid embedded in a thermotropic LC, either in a nematic or in a smectic phase. Such colloids exhibit a long-range order that results from the competition between the LC anchoring energy on the particles surface and the LC elastic energy.1-3 When the size of the particles is smaller than a critical size (about 1 µm), the preferred anchoring is not fulfilled everywhere on the surface, and the spectacular long-range order of the inclusions2 is usually not observed. At a much smaller scale, several groups have tried to disperse a small amount of nanometric solid particles in an LC. Dispersions of ferroelectric4 or ferromagnetic particles5,6 in a nematic phase have been mainly investigated. Doped mesophases can exhibit very different properties from those of pure ones. For example, ferroelectric nanoparticles dispersed in a nematic phase (only 0.3% by volume) do not modify the temperature of the phase transition and the alignment but greatly enhance the LC dielectric anisotropy.4 It is also tempting to use the LC spatial order to organize the particles. In this way, Mitov et al.7 showed that, when nanoparticles (1.5% by weight) are dispersed in a cholesteric LC, a long-range order is obtained due to the LC helical configuration. In both of the above approaches, the anchoring/ elasticity competition is irrelevant to explain the stability of the mixtures and their properties. For such nanometric inclusions, the stability of the dispersion is mainly controlled by the thermal fluctuations. The nematic orientational fluctuations are also very important at this scale, which is of the order of magnitude of the * Corresponding author. E-mail:
[email protected]; telephone number: 33 (0) 4 67 14 38 54; fax number: 33 (0) 4 67 14 46 37. (1) Poulin, P.; Stark, H.; Lubensky, T. C.; Weitz, D. A. Science 1997, 275, 1770. (2) Loudet, J. C.; P. Barois, P.; Poulin, P. Nature 2000, 407, 611. (3) Smalyukh, I. I.; Lavrentovich, O. D.; Kuzmin, A. N.; Kachynski, A. V.; Prassad, P. N. Phys. ReV. Lett. 2005, 95, 157801. (4) Reznikov, Y.; Reshetnyak, V.; Glushchenko, A.; West, J. Appl. Phys. Lett. 2003, 82, 1917. (5) Walton, D.; Shibli, S. M.; Vega, M. L.; Oliveira, E. A. J. Magn. Magn. Mater. 2004, 292, 310. (6) Da Cruz, C.; Sandre, O.; Cabuil, V. J. Phys. Chem. B 2005, 109, 14292. (7) Mitov, M.; Portet, C.; Bourgerette, C.; Snoeck, E.; Verelst, M. Nat. Mater. 2002, 1, 229.
nematic orientational correlation length8 ξ. Indeed, these fluctuations produce long-range attractive Casimir interactions between the particles, which are expected to become predominant in the vicinity of the isotropic-to-nematic transition.8 A complete experimental study of this phenomenon is, however, still lacking. Besides the inclusion of solid particles, some interest was recently shown for the formation of soft nanostructures in a thermotropic LC as “a new way of constructing useful exotic complex fluids”.9 Such systems might exhibit both a local thermotropic organization due to the LC solvent and a supramolecular surfactant organization. The resulting coupling between these different orders would be strongly temperature dependent. Following this approach, Tanaka et al.9 studied mixtures of water, didodecyldimethylammonium bromide (DDAB) and 4-n-pentyl4′-cyanobiphenyl (5CB), a well-known nematic LC. At a fixed water/surfactant weight ratio, the phase diagram displays a stable microemulsion at high temperatures, above the LC nematic-toisotropic phase transition temperature TNI ≈ 35.3 °C. A twophase region is observed, however, when cooling, and the corresponding phase-separation temperature decreases continuously when the aggregate (water + surfactants) concentration is increased. The phase diagram also reveals that the maximal amount of aggregates in the nematic phase is rather low (less than 2% in weight),10 whereas the isotropic phase accepts a large amount of surfactant (more than 30%). The intriguing behavior of the microemulsion in the isotropic phase raises several issues related to the appearance of the nematic order in a mixture. When cooling, an increase of scattered intensity in light scattering experiments has been observed10,11 by Bellini et al. in a large region above demixing. The authors have claimed that this phenomenon results from increasing attractive interactions between the aggregates due to the LC paranematic fluctuations. The main properties of this system have indeed been retrieved11 by a simple model derived from the Lebwohl-Lasher spin lattice model.12 The surfactant/water aggregates (assumed to be inverted micelles) occupy some sites and thus disturb the nematic (8) Bartolo, D.; Long, D.; Fournier, J. B. Europhys. Lett. 2000, 49, 729. (9) Yamamoto, J.; Tanaka, H. Nature 2001, 409, 321. (10) Bellini, T.; Caggioni, M.; Clark, N. A.; Mantegazza, F.; Maritan, A.; Pelizzola, A. Phys. ReV. Lett. 2003, 91, 8. (11) Caggioni, M.; Giacometti, A.; Bellini, T.; Clark, N. A.; Mantegazza, F.; Maritan, A. J. Chem. Phys. 2005, 122, 214721. (12) Lebwohl, P.; Lasher, G. Phys. ReV. A 1972, 6, 426.
10.1021/la062024a CCC: $37.00 © 2007 American Chemical Society Published on Web 02/23/2007
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correlations. The shape of the phase diagram is correctly rendered by such a model. Although the mean-field treatment of the lattice model does not provide an intermicellar interaction in the paranematic (isotropic) phase, the authors show by a hightemperature expansion of the partition function that an effective interaction is certainly present. These first studies on lyotropic structures in a thermotropic LC raise several questions about the DDAB/water/5CB ternary system as well as general issues about the dispersions of nanoparticles and surfactants in the isotropic phase of an LC. First, the “paranematic” interaction is poorly known, and supplementary experimental data at the aggregates scale is required for developing further theoretical approaches. The existence of such a coupling in the paranematic phase deserves particular attention, since dispersions of particles in an LC are usually first realized in the isotropic phase before cooling. Concerning the specific system studied in refs 9-11, the existence of the paranematic interaction followed from strong hypotheses, such as the independence of the shape of the aggregates with the temperature. Their morphology should therefore be examined closely, especially the temperature dependence. Additionally, the specific detection of the interactions due to the presence of the LC requires the knowledge of all other interactions. Finally, an intriguing fact concerns the absence of other lyotropic structures in 5CB/water/surfactant ternary systems reported in the literature. The polymorphism of lyotropic systems is quite well-known, and phase transition usually occurs easily in ternary systems. The question remains, Can other mesophases easily form in 5CB? To explore these issues, we have closely examined various surfactant/water/5CB systems. Using neutrons and X-ray scattering experiments, we first explored the whole phase diagram of the DDAB/water/5CB system. Our aims were to determine the precise structure of the aggregates, to search for mesophases other than the isotropic L2 microemulsion phase in 5CB, and to examine the dependence of the aggregate interactions on the temperature when approaching the isotropic-to-nematic transition. Two temperature domains were expected. Close to the nematic phase, interactions coming from the paranematic fluctuations are present and can even be predominant. Far from TNI, these interactions should vanish and the stability of the mixtures must be driven by other interactions. In Section II, we explore the possibility of tuning the morphology of the aggregates. We specifically examine the influence of cosurfactant through the evolution of phase diagrams. We then discuss in Section III the origin of the aggregate interactions depending on their size and on temperature. All our findings suggest that the stability of the systems is mainly controlled by two different interactions: the van der Waals forces that are present at any temperature and the paranematic fluctuations that exist close to the temperature of the isotropic-to-nematic phase transition. Experimental Section Materials. As the LC solvent, we used 5CB purchased from Synthon without any further purification. This LC displays an isotropic-to-nematic phase transition at relatively low temperature (TNI ≈ 35.3 °C), which allows one to investigate surfactant/water/ 5CB mixtures in a large range of temperature above TNI. The surfactants, DDAB and dodecyltrimethylammonium bromide (DTAB) from Sigma-Aldrich were used as received. Water was distilled and deionized. For neutron scattering experiments, we also used D2O from Riedel. Components were weighted and mixed. After sonication (Bioblock, 35 kHz), samples were stored in a temperaturecontrolled bath at the required temperature. After a few days, phase diagrams were established by macroscopic observations, scattering techniques, and polarized microscopy.
Toquer et al.
Figure 1. Temperature phase diagram of the water/5CB/DDAB mixture for a given water/(water + surfactant) volume ratio φm ) 0.11. The composition is given by φ, the volume fraction of the aggregates (surfactant + water). The temperature of demixing decreases with the increase in surfactants. Scattering Set-Ups. Small-angle X-ray scattering (SAXS) experiments were carried out at the Cu-KR wavelength (λ ) 1.54 Å) obtained from a rotating anode (Rigaku). The samples, introduced in Lindmann capillaries (diameter 1.5 mm), were kept at fixed temperature via a temperature-controlled bath. The capillaries were sealed in order to maintain the composition. The scattered beam was collected on a two-dimensional Imageplate detector. In the chosen geometry, the scattering vector q spans the range 0.05-0.6 Å-1. Small-angle neutron scattering (SANS) experiments were conducted on the PACE line at LLB (CEA-Saclay, France). The detector consists of 30 concentric rings centered around the neutron beam. The corresponding range of accessible scattering vectors is between 6 × 10-2 and 0.5 Å-1. Samples were studied in 1 mm thick Hellma cells in which the temperature is controlled by circulating fluid. The scattering data were treated according to standard procedure and were put on an absolute scale by using water as the standard. Other Techniques. We also performed dynamic light scattering (DLS) experiments with a 120 mW argon laser. The use of the 514 nm wavelength provides a range of wave-vectors between 3 × 104 and 3 × 105 cm-1. The DLS setup allows one to probe the correlation time from 25 ns to 1 s. Birefringent mesophases were observed under a polarizing optical microscope (Leitz) equipped with an Instec oven (regulation to 0.1K) and a CCD camera.
I. DDAB/Water/5CB Microemulsion Phase Diagram. The DDAB/water/5CB phase diagram was first established by Tanaka9 and amended by Bellini.10,11 The mixture forms a microemulsion for a tiny range of water content and a large range of temperature where the LC is in the isotropic phase (see Figure 1). The microemulsion demixes in the vicinity of TNI under cooling. The temperature of demixing (shown in Figure 1) decreases continuously with the increase of the (surfactant + water) volume fraction φ. The two-phase region consists of a more concentrated microemulsion and an almost pure nematic phase. According to ref 10, the phase-separation is due to “paranematic fluctuation” interactions near TNI and these fluctuations decrease rapidly with temperature. All other interactions become relevant only far from TNI. We therefore probe samples at 60 °C along a dilution line (horizontal dashed arrow in Figure 1). Note that the microemulsion exists only for a small range of water content.9,10,11 The corresponding volume ratio φ ) water/(surfactant + water) is between 0.05 and 0.15 and does not significantly change with dilution and temperature. All the samples used in the scattering experiments have been prepared at φm ) 0.11. Scattering Experiments. Data on the morphology of aggregates (size and shape) at 60 °C were obtained from X-ray and neutron scattering experiments performed on a batch of samples along a dilution line (dashed arrow in Figure 1). Due to the small
Lyotropic Structures in a Thermotropic LC
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Figure 3. (Left) Sketch of the matter distribution inside an inverted micelle. (Right) Corresponding SLD profiles: note that the neutron SLD with “shell” contrast conditions (a) is dominated by the DDAB SLD, whereas the sphere contrast conditions (b) mainly show the D2O core. The X-ray SLD (c), corresponding to both contrast conditions, mainly results from the bromide ions. Table 1. Neutron and X-ray SLDs (1010 cm-2) Used to Compute the Scattering Spectraa 5CB DDA+ (1010
-2
neutron SLD cm ) 1.34 X-ray SLD (1010 cm-2) 8.95 density (at 60 ˚C) 0.98
Figure 2. Neutron (a) and X-ray (b) experimental spectra of the D2O/5CB/DDAB microemulsion at T ) 60 °C for φ ) 0.18 (open circles) and φ ) 0.09 (triangles). In neutron experiments, the “shell” geometry (D2O/H2O core) is represented by the filled circles. The different computed form factors (solid lines) correspond to R1 ) 6.8 Å and ξ ) 5.8 Å (see text).
water amount, these aggregates are expected to be inverted micelles. We therefore also prepared samples in which the water core consisted of a D2O/H2O mixture (28.7-71.3% in weight), with the same neutron scattering length as 5CB (“shell” geometry). We assumed a spherical symmetry both for the shape of the micelles and, at least on average, for their interaction potential. The scattered intensity I(q) is then written as
I(q) ) φVP(q)S(q,φ)
(1)
where q is the scattering vector, V is the object volume, φ is the volume fraction of objects, P(q) is the form factor, and S(q,φ) is the structure factor. After subtraction of the cell and solvent scatterings, the normalized scattering spectra I(q)/φ are found to be superimposed up to a volume fraction φ ) 0.3 for both SAXS and SANS experiments (see Figure 2). This provides strong evidence that the scattering pattern reflects in both experiments the form factor of the aggregates, while the structure factor approximates as S(q,φ) ≈ 1 in the considered q range at 60 °C. We first focus on the form factor part. We model the aggregates by a water core of radius R1, which contains the bromide ions of the surrounding surfactants (see Figure 3). Due to electrostatic interactions, bromide ions are assumed to be located near the ammonium polar heads. We confine them in a shell of thickness δ. The surfactant alkyl chains form an outer shell which might be swollen by the 5CB. We then model the local DDAB volume fraction by a Gaussian profile with half-height width ξ. Taking into account the mass conservation, we then numerically compute the form factor of the aggregates written in spherical coordinates13 as
P(q) )
[
4π V
∫0+∞ rF(r)
sin(qr) dr qr
]
2
(2)
-0.32 7.88 0.81
Br1.93 28.4 3.79
H2O + D2O D2O 1.41 9.31 0.98
6.4 9.31 1.09
a DDA+ corresponds to a DDAB surfactant without the associated bromide counterion.
where the local scattering length density (SLD) F(r) is computed from the values of Table 1. The instrumental response function and the polydispersity on the R1 value are taken into account.14 The computed form factor is very sensitive to the R1 value, but a weak polydispersity (less than 10%) changes the form factor slightly. The X-ray-computed spectra are very sensitive to the localization of the bromide ions. A homogeneous dispersion of bromide ions in the water core does not change the neutroncomputed spectra but shifts the X-ray bump to q values higher than what is observed. The localization of the bromide counterions in a thin corona close to the polar heads is thus confirmed. Note also that any reasonable agreement of the computed spectra with the experimental data requires the above hypothesis of surfactant tails largely swollen by the 5CB LC. Best fits of the experimental data, shown in Figure 2, give an inner radius R1 ) 6.8 ( 0.3 Å. The calculated area per polar head of DDAB in this system is thus a ) 66 ( 5 Å2 for a tail length l close to 12 Å (outer shell thickness given nearly by twice the Gaussian half-height width ξ ) 5.8 Å). The polar head area and the surfactant tail size are both consistent with the values 68 Å2 and 13 Å, respectively, found for DDAB microemulsions in alkane solvent.15,16 The water content and thus the size of the water core, however, are much larger in such oils (typically R1 ) 50-100 Å in decane17 and dodecane,18 for example). This latter feature can be explained through the larger swelling of the surfactant tails by 5CB. Due to the steric hindrance of the mesogen, the surfactant alkyl tails are splayed which yields a larger spontaneous curvature of the DDAB monolayer than the one in n-alkane. Note that the small water cores also reported in nonmesogenic aromatic oils19 could be explained by the same mechanism. (13) Gradzielski, M.; Langevin, D. J. Phys. Chem. 1995, 99, 13232. (14) Safran, S. A. Phys. ReV. A 1991, 43, 2903. (15) Dubois, M.; Zemb, T. Langmuir 1991, 7, 1352. (16) Barnes, I. S.; Hyde, S. T.; Ninham, B. W.; Derian, P.; Drifford, M.; Zemb, T. J. Phys. Chem. 1988, 92, 2286. (17) Monduzzi, M.; Caboi, F.; Larche´, F.; Olsson, U. Langmuir 1997, 13, 2184. (18) Skurtweit, R.; Olsson, U. J. Phys. Chem. 1991, 95, 5353. (19) Olla, M.; Monduzzi, M. Langmuir 2000, 16, 6147.
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Let us focus now on the structure factor, which approximates as S(q) ≈ 1, even at large volume fractions. First, note that this fact is consistent with the vanishing value of the virial coefficient in the light scattering experiment (equivalent to S(0) ≈ 1) reported in ref 11. Here the phenomenon is observed on a large range of wavevectors, which is rather unusual. The possibility of an attractive force “compensating” the expected hard-core repulsive one, mentioned in ref 11, should respect this highly restrictive constraint. We therefore look for plausible pair potentials between the aggregates and compute the corresponding structure factor. First, the repulsive sterical interaction between micelles is modelized by a hard-sphere potential. The sphere of radius RHS (a first parameter) should contain at least the water core and the surfactant polar heads, which yield an expected value for RHS slightly larger than R1. An attractive van der Waals potential should also be considered despite the small size of the aggregates. The magnitude of the Hamaker constant A is indeed found to be as high as 5 kBT using the Lifshitz theory,20 because of the large refractive index of 5CB (n5CB ≈ 1.6 compared to nH2O ≈ 1.33). The van der Waals potential VVW between two spheres at a centercenter distance d is written as
VVW(s) ) -
2 s-4 A 2 + + ln 6 s-4 s s
[
]
(3)
Figure 4. Computed structure factors for different hard-sphere radii and Hamaker constant values RHS (in Å) and A (in kBT units), respectively, at a large volume fraction (φ ) 0.18).
to explain the experimental structure factor observed at high temperatures, S(q) ≈ 1. In conclusion, the water and DDAB surfactant form inverted micelles in the isotropic phase of 5CB of very small size (water bromide core radius R1 ≈ 7 Å at an intermediate water amount φm ) 0.11). At T ) 60 °C, these micelles interact mainly with a repulsive hard-sphere interaction coupled with a very strong attractive van der Waals interaction.
II. Effect of Adding a Cosurfactant
with s ) (d/r′)2, where r′ ≈ RHS is the van der Waals radius. The Hamaker constant A will be a second parameter in the computation. The paranematic fluctuations of the LC also yield an attractive interaction,10,11 but the latter should be neglected at T ) 60 °C, far above the nematic-to-isotropic phase transition. We can now compute the structure factor given by the overall pairs potential (hard sphere + van der Waals) and check whether a set of parameters (RHS and A) leads to a structure factor practically equal to 1 in the q explored range. The structure factors are calculated with a program provided by Luc Belloni in which the Ornstein-Zernicke integral equation together with the hypernetted chain closure condition is used. The calculated pair correlation function is Fourier transformed to obtain the structure factor.21,22 We then look for the best (RHS, A) pair for which S(q) ) 1 in the considered range of wavevectors. We found that, at large concentrations, a nearly constant value S(q) ≈ 1 can be obtained only for a small range of parameters around the values A ≈ 5.5 kBT and RHS ≈ 10 Å. These results are, of course, also valid for low concentrations where the structure factors naturally tend to 1. At large volume fractions, the computed structure factor is very sensitive to the values of the free parameters RHS and A. A set of typical structure factors for a large volume fraction (φ ) 0.18) around the best values RHS ≈ 10.2 Å and A ≈ 5.5 kBT is shown in Figure 4. Even for the lowest acceptable hardsphere radii (RHS ≈ 8 Å), the single repulsive hard-sphere interaction (A ) 0) gives structure factors much smaller than 1 at small wavevectors. Increasing the Hamaker constant results in an increase in the structure factor. Note that the values found are quite realistic since the hard-sphere radius roughly corresponds to the water core and the aliphatic chains nonwetted by 5CB. The Hamaker constant has the magnitude of the value computed with the Lifshitz theory. van der Waals attractive interactions together with hard-sphere repulsive interactions are therefore sufficient
Phase Diagram. At 60 °C, the DDAB/water/5CB phase diagram exhibits a single one-phase region (corresponding to the above microemulsion) in the 5CB-rich corner. The water amount is quite small (φ between 0.05 and 0.15). Adding more water leads to a demixing and the appearance of a water-rich isotropic phase. It is known that the water solubility capacity of a microemulsion can be increased by adding a cosurfactant. For example, this synergistic effect is obtained in various DDAB/ water/alkane oils when adding DTAB.23-26 These references show that the spontaneous curvature of the monolayer changes, and thereby the size and shape of the aggregates are modified. By analogy, we have investigated the effect of adding DTAB in our system in order to increase the size of inverted micelles and eventually to give rise to new mesophases. Figure 5 shows the DDAB/DTAB/water/5CB quaternary phase diagram at T ) 60 °C. In the 5CB-rich corner (LC volume fraction above 0.5), the microemulsion domain remains the only onephase region. Its boundaries are supported by 5CB dilution lines. The slices of the quaternary diagram shape at constant 5CB fraction correspond to the DDAB/DTAB/water ternary phase diagrams (see Figure 5b). When increasing the DTAB volume fraction, the microemulsion domain (see Figure 5) initially slightly expands but disappears at high amounts of DTAB (for a DTAB/ DDAB weight ratio wr around 0.6). Taking an area per polar head a ≈ 66 Å2 for both DDAB and DTAB molecules, we obtain a range of maximal water core radius between 7.6 Å (wr ) 0) and 12.8 Å (wr ) 0.18). Above wr ≈ 0.18, the maximal radius does not increase any more, and a change of behavior is observed. Two different kinds of phase-separation are indeed observed when the maximal water amount is reached. For small DTAB amounts (less than 15% of the total surfactant weight), a waterrich isotropic phase (W) is observed. For higher DTAB content, a birefringent phase is found. Both phases are not located in the slice but at the bottom of the quaternary phase diagram (Figure 5). They therefore contain a very small amount of 5CB. Under
(20) Israelachvili, J. Intermolecular and Surface Forces; Academic Press: London, 1985. (21) Belloni, L. J. Phys.: Condens. Matter 2002, 14, 9323. (22) Klein, R. Interacting colloidal suspensions. In Scattering Methods Applied to Soft Condensed Matter; Linder, P., Zemb, T., Eds.; North Holland: Amsterdam, 2002; p 391.
(23) Bumajdad, A.; Eastoe, J.; Griffiths, P.; Steytler, D. C.; Heenan, R. K.; Lu, J. R.; Timmins, P. Langmuir 1999, 15, 5271. (24) Lusvardi, K. M.; Full, A. P.; Kaler, E. M. Langmuir 1995, 11, 487. (25) Lusvardi, K. M.; Full, A. P.; Kaler, E. W. Langmuir 1995, 11, 4728. (26) Proverbio, Z. E.; Schulz, P. C.; Puig, J. E. Colloid Polym. Sci. 2002, 280, 1045.
Lyotropic Structures in a Thermotropic LC
Figure 5. (a) Schematic DDAB/DTAB/water/5CB quaternary phase diagram at T ) 60 °C. The microemulsion domain (L2) is the only one-phase region in the 5CB-rich corner. (b) Ternary phase diagram corresponding to a slice at a (surfactant + water) volume fraction φ ) 0.18. Point A refers to the same composition as point A in Figure 1. The arrow corresponds to a continuous increase in the size of the micelles, corresponding to a computed water core radius going from 6.8 to 12.8 Å (see section on Temperature Dependence in text).
a polarizing microscope, the birefringent phase shows typical lamellar phase textures (oily streaks, focal conic domains, etc.). The lamellar structure has been confirmed by the presence of Bragg diffraction peaks obtained by X-ray scattering experiments. At the microemulsion boundary, the periodicity of the lamellar phase (LR,5CB) is roughly 30 Å (see Figure 6) and is almost independent of the 5CB volume fraction. The periodicity of the lamellar phase (LR) found in the ternary DDAB/DTAB/water system at similar volume fractions is nearly the same. For an example where wr ) 0.4 and φ ) 0.3, the LR,5CB periodicity is 31.4 Å, and the LR periodicity is 26.3 Å in the ternary DDAB/ DTAB/water system. By considering the quaternary phase diagram shape, the LR,5CB lamellar phase of the two-phase region has a water content at least equal to that of the microemulsion aggregates. By comparing the LR,5CB and LR periodicities, we found that the 5CB content is therefore very low in the LR,5CB phase with a corresponding thickness of at most 5 Å. This low content (typically 10-15%) is also the maximal amount possible to add to a DDAB/DTAB/water lamellar phase. In conclusion, the size increase induced by a cosurfactant is limited. At small amounts, the emulsification breakdown observed when the water amount is larger than φ ) 0.15 is due to the large negative spontaneous curvature. Adding more cosurfactant yields larger micelles, but this increase is restricted by the large van der Waals interactions. Larger micelles are unfavored, and a lamellar phase nearly without 5CB is formed. Such a behavior indicates that the design of lyotropic structures in 5CB other than tiny micelles will require other strategies, such as replacing water by another solvent or at least decreasing the large polarizability variation between the aqueous part and 5CB.
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Figure 6. (a) A birefringent phase is observed in the two-phase region (point B in Figure 5) for a high DTAB content (φ ) 0.3 and DTAB/DDAB weight ratio wr ) 0.4). Typical oily streak patterns of a lamellar phase are observed under a polarizing microscope. (b) At the demixing, the X-ray spectrum of the microemulsion is disturbed by a Bragg peak, confirming the presence of a lamellar structure. The corresponding layer spacing is small (31.4 Å), which indicates a poorly swollen 5CB phase.
Temperature Dependence. Under cooling, two kinds of behaviors are observed depending on the DTAB content. At low DTAB content and for any water amount, a microemulsion formed at 60 °C is stable down to the appearance of the nematic phase (as in Figure 1). X-ray and neutron scattering experiments show that the micelles retain their shapes. The form factor does not significantly change under cooling. For example, Figure 7 shows that the SANS spectra are almost superimposed at large wavevectors (q in the range [0.08-0.4 Å-1]). On the contrary, the structure factor strongly changes. The scattering signal at small angles (shown in Figure 7a) increases under cooling, which reveals the appearance of an additional attractive interaction. This effect is drastically enhanced when approaching the isotropic-to-nematic phase transition temperature TNI. Since a change in the shape of the micelles is not observed and the Hamaker constant is expected to vary slightly and smoothly in the whole temperature range, this interaction is probably not due to increasing van der Waals forces but rather arises from the LC paranematic fluctuations.11 From neutron scattering at low q, the dependence of the structure factor on the temperature has been obtained by dividing all intensities by the intensity at 60 °C, which is considered to be a pure form factor. We obtain then the evolution of the structure factor with the variation of the temperature from T ) 60 °C down to the demixing at 31 °C. The structure factor can be nicely fitted by an OrnsteinZernicke form (Figure 8):
S(q) ) 1 +
χ 1 + q 2ξ2
(4)
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Figure 9. Schematic temperature phase diagram along the black arrow of Figure 5b. For weak amounts of DTAB (wr < 0.18), the microemulsion (L2) extends from T ) 60 °C down to the outset of the 5CB nematic (N) phase. Otherwise, a two-phase region with an LR,5CB lamellar phase appears under cooling from T ) 60 °C far above TNI.
Figure 7. Neutron experimental spectra of the D2O/5CB/DDAB microemulsion at T ) 45 °C (diamond), T ) 35 °C (square), and T ) 30 °C (open circle) for φ ) 0.3 and φ ) 0.15. Complete spectra are on top, and a zoom on the medium q range is on the bottom.
Figure 8. (a) Experimental structure factor for several temperatures obtained from neutron scattering experiments (φ ) 0.18, ψ ) 0.11, corresponding to the vertical arrow in Figure 1). The solid lines correspond to fits derived from an Ornstein-Zernicke equation. (b) Corresponding correlation length and inverse osmotic compressibility with the temperature.
where χ is the inverse of an osmotic compressibility, and ξ is the correlation length of the interaction. Note that the correlation length remains small even at the demixing (ξ ) 50 Å). For greater DTAB contents (above wr ) 0.18) and for small water contents, the behavior is similar under cooling. At larger water amounts (along the arrow of Figure 5, which corresponds to a continuous increase in the size of the micelle from A), the collapsed lamellar phase is formed before the outset of the 5CB nematic phase. A three-phase region is then observed at room temperature. The behavior along the arrow of Figure 5 is
schematically summarized in Figure 9. Note finally that other ammonium-terminated cosurfactants, such as hexadecyltrimethylammonium bromide, give rise to similar phenomena when mixed with DDAB, water, and 5CB. No other thermodynamically stable mesophase has been evidenced in the 5CB-rich corner of the related phase diagrams. Finally, we did not succeed in obtaining a stable water-in5CB phase (a swollen lamellar one, for example) other than the L2 phase despite several attempts, but our data explain some aspects of the puzzling behavior of water/surfactant/5CB ternary systems. The microemulsion contains only a small amount of water, which has therefore limited our study to the smallest micelles. Similar low water contents have already been reported for DDAB-based microemulsions in nonmesogenic aromatic oils.19 This can be related to the large negative spontaneous curvature c0 of the surfactant monolayer at the water-5CB interface. This hypothesis is in agreement with the scattering experiments described in Section I, which show that the DDAB area per polar head remains close to the one observed in alkyl solvents, whereas aromatic molecules penetrate the monolayer alkyl part. The emulsification breakdown observed when the water amount is larger than φ ) 0.15 is therefore due to the curvature. The use of a cosurfactant yields an increase in the size of the micelles, but large van der Waals intermicellar interactions limit this phenomenon and favor the formation of a poorly swollen lamellar phase when increasing the size as well as when decreasing the temperature. For the smallest micelles, the overall data, however, confirm that their morphology does not vary with the temperature and clearly reveal the presence of additional attractive interactions due to the LC paranematic fluctuations in the neighborhood of TNI. A further quantitative study coupled with a theoretical analysis will be detailed elsewhere.
Conclusion New interactions are expected to arise in mixtures of surfactants and thermotropic LCs due to the mesogenic properties of the latter. The coupling of the solvent organization with the surfactant self-assembly could give rise to exotic supramolecular assemblies. Up to now, however, very few experimental systems have been designed. We have investigated the behavior of water/DDAB/ DTAB aggregates in 5CB, a standard thermotropic LC. We found a single thermodynamically stable swollen phase at temperatures largely above the 5CB nematic-to-isotropic phase transition temperature. Scattering experiments have shown that this microemulsion is made of nanometer-sized spherical inverted micelles. Besides the hard-sphere repulsive interaction, the aggregates experience large attractive van der Waals interactions.
Lyotropic Structures in a Thermotropic LC
The large value of the Hamaker constant, due to the large contrast of polarizability between water and 5CB, prevents the formation of other LC-swollen phases. When approaching the 5CB isotropicto-nematic phase transition, scattering experiments have revealed the presence of an additional large attractive interaction between the micelles. This confirms the scenario of an increasing paranematic fluctuation interaction responsible for the demixing of the microemulsion around TNI. Scattering techniques are an appropriate tool to probe the properties of this unusual interaction. Note, however, that a systematic investigation of the micelles size dependence on paranematic fluctuation will require the use of an other system (either another kind of LC or another aqueous
Langmuir, Vol. 23, No. 7, 2007 4087
part). For large micelles, the microemulsion indeed becomes unstable far above TNI, where a 5CB-poor lamellar phase forms and van der Waals interactions prevent the formation of any other swollen mesophases. Acknowledgment. The authors gratefully acknowledge fruitful discussions with Martin In and Julian Oberdisse. We thank Luc Belloni for having kindly provided the structure factor program. We acknowledge partial support of this work by the European Network of Excellence “SoftComp”. LA062024A
