Existence and Stability of New Nanoreactors: Highly Swollen

Variation of C0 can be achieved by several means.10 They include the modification of ... Thus, the potential applications of swollen hexagonal phases ...
0 downloads 0 Views 299KB Size
Download PDF
4362

Langmuir 2005, 21, 4362-4369

Existence and Stability of New Nanoreactors: Highly Swollen Hexagonal Liquid Crystals Eduardo Pena dos Santos,†,£ Miriam Sanae Tokumoto,† Geetarani Surendran,†,§ Hynd Remita,§ Claudie Bourgaux,| Philippe Dieudonne´,‡ Eric Prouzet,† and Laurence Ramos*,‡ Institut Europe´ en des Membranes (UMR CNRS 5635), Route de Mende, 34293 Montpellier, France, Laboratoire des Colloı¨des, Verres et Nanomate´ riaux (UMR 5587/ CNRS/Universite´ de Montpellier 2), cc 26, Universite´ Montpellier II, 34095 Montpellier, France, Laboratoire de Chimie Physique (UMR CNRS 8000), Universite´ Paris XI, 91405 Orsay, France, LURE, Centre Universitaire Paris-Sud, BP 34, 91898 Orsay Cedex, France, and Instituto de Quimica, UNESP, Rua Professor Francisco Degni, s/n CEP 14800-900 Araraquara-SP, Brasil Received November 26, 2004. In Final Form: March 1, 2005 We report the preparation of direct hexagonal liquid crystals, constituted of oil-swollen cylinders arranged on a triangular lattice in water. The volume ratio of oil over water, F, can be as large as 3.8. From the lattice parameter measured by small-angle X-ray scattering, we show that all the oil is indeed incorporated into the cylinders, thus allowing the diameter of the cylinders to be controlled over one decade range, provided that the ionic strength of the aqueous medium and F are varied concomitantly. These hexagonal swollen liquid crystals (SLCs) have been first reported with sodium dodecyl sulfate as anionic surfactant, cyclohexane as solvent, 1-pentanol as co-surfactant, and sodium chloride as salt (Ramos, L.; Fabre, P. Langmuir 1997, 13, 13). The stability of these liquid crystals is investigated when the pH of the aqueous medium or the chemical nature of the components (salt and surfactant) is changed. We demonstrate that the range of stability is quite extended, rendering swollen hexagonal phases potentially useful for the fabrication of nanomaterials. As illustrations, we finally show that gelation of inorganic particles in the continuous aqueous medium of a SLC and polymerization within the oil-swollen cylinders of a SLC can be conducted without disrupting the hexagonal order of the system.

1. Introduction Surfactant molecules self-assemble in water into a large variety of morphologies, including liquid crystalline phases such as lamellar or hexagonal phases.1-4 In lamellar phases, the surfactants form bilayers that are regularly stacked in water, while in hexagonal phases, infinite tubes are organized on a triangular lattice with surfactants building the walls. In a direct (respectively reverse) hexagonal phase, surfactant (respectively water) tubes are immersed in a continuous water (respectively surfactant) matrix. For binary mixtures, the characteristic lengths of the surfactant aggregates, such as the thickness of the bilayer for a lamellar phase or the radius of the tubes for a direct hexagonal phase, are imposed by the molecular length of the surfactant. The addition of a nonpolar solvent in the aqueous medium, generally called the swelling agent, may overcome this restriction and hence leads to structures that exhibit long-range order with larger lattice parameters. For a lamellar phase, the swelling corresponds to an increase of the interlamellae spacing. Hence, upon swelling, the intermembrane repul* Author to whom correspondence should be addressed. E-mail: [email protected]. † Institut Europe ´ en des Membranes. ‡ Universite ´ Montpellier II. § Universite ´ Paris XI. | LURE, Centre Universitaire Paris-Sud. £ Instituto de Quimica, UNESP. (1) Handbook of Surfactants, 2nd ed.; Porter, M. R., Ed.; Chapman & Hall: London, UK, 1992; pp 227. (2) Langevin, D. Annu. Rev. Phys. Chem. 1992, 43, 341. (3) Myers, D. Surfactant Science & Technology; VCH Publishers: New York, 1992. (4) Tschierske, C. Curr. Opin. Colloid Interface Sci. 2002, 7, 69.

sions, which stabilize the lamellar phase, decrease but the shape and composition of the membranes themselves can still be preserved. Consequently, as long as the interaction forces are strong enough over a long range, as is the case for the steric interactions of Helfrich type,5 swelling a lamellar phase may be easily achieved. Several examples have indeed demonstrated the swelling of lamellar phases with oil.6-8 By contrast, for an oil-in-water direct hexagonal phase, the radius of the nonpolar cylinders increases continuously upon swelling. However, since a lyotropic hexagonal phase can only be stabilized if the radius of its tubes is very close to the inverse of the spontaneous curvature, C0, of the surfactant film at the oil-water interface,9 swelling the tubes of a hexagonal phase through incorporation of oil inside is not straightforward but requires, a priori, C0 to be appropriately tuned with. Variation of C0 can be achieved by several means.10 They include the modification of physical parameters, such as the solvent ionic strength with ionic surfactants, the temperature, or the addition of a co-surfactant that inserts among the main surfactant within the cylinders walls and modifies the main charge density and/or wall stiffness. We note that a rod-sphere transition can be observed upon solubilization of hydrocarbons in the micellar core.11 In reference 12, we have demonstrated that, by tuning (5) Helfrich, W. Z. Naturfosch. 1978, 33A, 305. (6) Larche´, F.; Appell, J.; Porte, G.; Bassereau, P.; Marignan, J. Phys. Rev. Lett. 1986, 56, 1700. (7) Safinya, C. R.; Roux, D.; Smith, G. S.; Sinha, S. K.; Dimon, P.; Clark, N. A.; Bellocq, A.-M. Phys. Rev. Lett. 1986, 57, 2718. (8) Leaver, M. S.; Olsson, U.; Wennerstrom, H.; Strey, R.; Wurz, U. J. Chem. Soc., Faraday Trans. 1995, 91, 4269. (9) Safran, S. A.; Turkevich, L. A.; Pincus, P. J. Phys. 1984, 45, L69. (10) Israelachvili, J. Intermolecular and surface forces; Academic Press: London, 1991.

10.1021/la047092g CCC: $30.25 © 2005 American Chemical Society Published on Web 04/02/2005

Existence and Stability of New Nanoreactors

Langmuir, Vol. 21, No. 10, 2005 4363

simultaneously the oil content and the ionic strength of the polar medium, highly swollen hexagonal liquid crystals could be obtained such that the diameter of the oil-swollen cylinders can continuously be adjusted over one decade, from 3 to 30 nm. These mesophases comprise five components, water, an inorganic salt (NaCl), a nonpolar solvent (cyclohexane), an anionic surfactant (sodium dodecyl sulfate), and a co-surfactant (1-pentanol). The specificity of these SLCs is that the nonpolar solvent can be added in large excess (the volume ratio of oil over water being as large as 3.8), still keeping the direct hexagonal structure. Other types of swollen hexagonal phases also made of a mixture of five components, oil, water, surfactant, co-surfactant, and salt have been reported in the literature.13-15 However, in these cases, the swollen phases exist over a very narrow range of concentration and temperature, rendering them difficult to handle and likely to destabilize upon weak physical or chemical perturbations. By contrast, the phases described in reference 12 are stable over a large range of temperatures (at least from 15 to 40 °C, and up to higher temperature when the SLCs does not contain salt) and exist for a large range of swelling ratios. Besides their intrinsic interest, structures organized at the mesoscopic level can also be used as structuredirecting systems for the preparation of solid materials. One of the first applications was provided by preparations of metal, chalcogenides, or polymers nanoparticles either in the aqueous inner core of reverse microemulsions or in the organic core of miniemulsions, which led to the preparation of relatively monodisperse nanoparticles.16-32 More recently, mesoporous inorganic or hybrid materials were prepared by using an assembly mechanism involving weak interactions (electrostatic or H-bonding) between organic species and micelles.33-42 Using the aqueous

matrix of a binary mesophase as confining medium was also successful and led to the so-named “true liquid crystal templating” mechanism where binary direct lyotropic hexagonal or cubic phases were used for the elaboration of mesostructured oxides or metals.43-52 In this latter domain, the control of the structure of the mesophase is expected to direct the structure of the final material. Swollen liquid crystals present characteristic sizes that can be adjusted in a broad range, which make them good candidates for the controlled templating of mesostructures. Syntheses similar to those performed in binary systems are in principle realizable with SLCs if we are able, first, to control their structure and, second, to preserve it upon addition of reagents or growth of particles within the hexagonal matrix. We note that the size range theoretically reachable with the surfactant-based swollen hexagonal phases described in this paper can also be attained with hexagonal phases made of copolymer in water.53-55 However, in that case, the tubes are composed almost exclusively of the polymer moiety, precluding a unidimensional confinement inside the tubes. By contrast, in the surfactant-based swollen phases, the tubes comprise a standard nonpolar solvent, rendering possible for instance the confinement of colloids56-59 and the use of the interior of the tubes as nanoreactors for in-situ synthesis. Thus, the potential applications of swollen hexagonal phases with tunable tube size are 2-fold since they can be exploited as both templates for mesoporous materials and nanoreactors for chemical synthesis of anisotropic particles or polymeric materials. In these optics, one of the main objectives is to possess a set of potential nanoreactors based on swollen hexagonal phases media that could be selected as a function of the desired reaction instead of trying to adapt synthesis processes to a defined SLC. However, the use of swollen hexagonal

(11) Hoffmann, H.; Ulbricht, W. J. Colloid Interface Sci. 1989, 129, 388. (12) Ramos, L.; Fabre, P. Langmuir 1997, 13, 682. (13) de Geyer, A. Prog. Colloid Polym. Science 1993, 93, 76. (14) de Geyer, A.; Guillermo, A.; Rodriguez, V.; Molle, B. J. Phys. Chem. B 2000, 104, 6610. (15) Peters, U.; Roux, D.; Sood, A. K. Phys. Rev. Lett. 2001, 86, 3340. (16) Pileni, M.-P. J. Chim. Phys. 1987, 84, 1037. (17) Pileni, M.-P. J. Phys. Chem. 1993, 97, 6961. (18) Eastoe, J.; Stebbing, S.; Dalton, J.; Heenan, R. K. Colloids Surf., A 1996, 119, 123. (19) Pileni, M.-P. Cryst. Res. Technol. 1998, 33, 1155. (20) Calandra, P.; Goffredi, J. M.; Turco Liveri, V. Colloids Surf., A 1999, 160, 9. (21) Castagnola, M. J.; Dutta, P. K. Micro. Meso. Mater. 2000, 34, 61. (22) Calandra, P.; Longo, A.; Turco Liveri, V. Colloid Polym. Sci. 2001, 279, 1112. (23) Pinna, N.; Weiss, K.; Sack-Kongehl, H.; Vogel, W.; Urban, J.; Pileni, M.-P. Langmuir 2001, 17, 7982. (24) Aizpurua, I.; Amalvy, J. I.; de la Cal, J. C.; Barandiaran, M. J. Polymer 2001, 42, 1417. (25) Tiarks, F.; Landfester, K.; Antonietti, M. Langmuir 2001, 17, 5775. (26) Antonietti, M.; Landfester, K. Prog. Polym. Sci. 2002, 27, 689. (27) Asua, J. Prog. Polym. Sci. 2002, 7, 1283. (28) Landfester, K.; Montenegro, R.; Scherf, U.; Gu¨ntner, R.; Asawapirom, U.; Patil, S.; Neher, D.; Kietzke, T. Adv. Mater. 2002, 14, 651. (29) Barre`re, M.; Landfester, K. Polymer 2003, 44, 2833. (30) Ramirez, L. P.; Landfester, K. Macromol. Chem. Phys. 2003, 204, 22. (31) Hota, G.; Jain, S.; Khilar, K. C. Colloids Surf., A 2004, 232, 119. (32) Lu, C.-H.; Wang, H.-C. J. Eur. Ceram. Soc. 2004, 24, 717. (33) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (34) Inagaki, S.; Fukushima, Y.; Kuroda, K. J. Chem. Soc., Chem. Commun. 1993, 680. (35) Beck, J. S.; Vartuli, J. C.; Kennedy, G. J.; Kresge, C. T.; Roth, W. J.; Schramm, S. E. Chem. Mater. 1994, 6, 1816. (36) Beck, J. S.; Vartuli, J. C. Curr. Opin. Solid State Mater. Sci. 1996, 1, 76. (37) Ramsay, J. D. F. Curr. Opin. Colloid Interface Sci. 1996, 1, 208.

(38) Sayari, A. Periodic Mesoporous Materials: Synthesis, Characterization and Potential Applications. In Recent Advances and New Horizons in Zeolite Science and Technology; Chon, H., Woo, S. I., Park, S.-E., Eds.; Elsevier: Amsterdam, Lausanne, New York, Oxford, Shannon, Tokyo, 1996; Vol. 102; p 1. (39) Antonelli, D. M.; Ying, J. Y. Curr. Opin. Colloid Interface Sci. 1996, 1, 523. (40) Behrens, P. Angew. Chem., Int. Ed. Engl. 1996, 35, 515. (41) Stein, A.; Melde, B. J.; Schroden, R. C. Adv. Mater. 2000, 12, 1403. (42) Patarin, J.; Lebeau, B.; Zana, R. Curr. Opin. Colloid Interface Sci. 2002, 7, 107. (43) Attard, G. S.; Glyde, J. C.; Go¨ltner, C. G. Nature 1995, 378, 366. (44) Attard, G. S.; Bartlett, P. N.; Coleman, N. R. B.; Elliott, J. M.; Owen, J. R.; Wang, J. H. Science 1997, 278, 838. (45) Attard, G. S.; Go¨ltner, C. G.; Corker, J. M.; Henke, S.; Templer, R. H. Angew. Chem., Int. Ed. Engl. 1997, 36, 1315. (46) Attard, G. S.; Coleman, N. R. B.; Elliott, J. M. The Preparation of Mesoporous Metals from Preformed Surfactant Assemblies. In Mesoporous Molecular Sieves 1998; Bonneviot, L., Beland, F., Danumah, C., Giasson, S., Kaliaguine, S., Eds.; Elsiever Science: Amsterdam, Lausanne, New York, Oxford, Shannon, Singapore, Tokyo, 1998; Vol. 117; p 89. (47) Attard, G. S.; Edgar, M.; Go¨ltner, C. G. Acta Mater. 1998, 46, 751. (48) Go¨ltner, C. G.; Antonietti, M. Adv. Mater. 1997, 9, 431. (49) Go¨ltner, C. G.; Berton, B.; Kra¨mer, E.; Antonietti, M. Adv. Mater. 1999, 11, 395. (50) Feng, P.; Bu, X.; Pine, D. J. Langmuir 2000, 16, 5304. (51) El Safty, S. A.; Evans, J. J. Mater. Chem. 2002, 12, 117. (52) El Safty, S. A.; Hanaoka, T.-A. Chem. Mater. 2004, 16, 384. (53) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H., Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (54) Feng, P.; Bu, X.; Pine, D. J. Langmuir 2000, 16, 5304. (55) Ryan, K. M.; Coleman, N. R. B.; Lyons, D. M.; Hanrahan, J. P.; Spalding, T. R.; Morris, M. A.; Steyler, D. C.; Heenan, R. K.; Holmes, J. D. Langmuir 2002, 18, 4996. (56) Ramos, L.; Fabre, P.; Ober, R. Eur. Phys. J. B 1998, 1, 319. (57) Ramos, L.; Fabre, P.; Fruchter, L. Eur. Phys. J. E 1999, 8, 67. (58) Eiser, E.; Bouchama, F.; Thathagar, M. B.; Rothenberg, G. ChemPhysChem 2003, 4, 526. (59) Bouchama, F.; Thathagar, M. B.; Rothenberg, G.; Turkenburg, D. H.; Eiser, E. Langmuir 2004, 20, 477.

4364

Langmuir, Vol. 21, No. 10, 2005

Pena dos Santos et al.

Table 1. Composition of the SLCs Prepared with SDS with Various Swelling Ratios and Structural Parameters, as Determined thanks to the SAXS Resultsa sample

Cs (mol‚L-1)

cyclohexane (g/mL)

F

pentanol (g/mL)

x

a (nm)

R (nm)

ASDS (nm2)

ab bc cb db ec fc gb hc ib jc kc lc mc nb ob pc qc rc

0 0 0 0 0.05 0.05 0.10 0.09 0.10 0.15 0.20 0.20 0.25 0.30 0.30 0.30 0.40 0.50

0.34/0.42 1.04/1.33 1.10/1.38 1.50/1.92 1.35/1.73 1.53/1.97 1.50/1.92 1.93/2.47 2.33/2.99 2.50/3.20 3.03/3.89 3.60/4.62 3.54/4.54 3.39/4.24 3.90/5.00 4.03/5.17 5.10/6.56 5.95/7.63

0.21 0.66 0.69 0.96 0.86 0.98 0.96 1.24 1.49 1.60 1.95 2.31 2.27 2.12 2.50 2.58 3.28 3.81

0.41/0.52 0.36/0.44 0.24/0.30 0.36/0.44 0.24/0.30 0.36 /0.44 0.26/0.32 0.38/0.47 0.29/0.36 0.41/0.50 0.42/0.52 0.29/0.36 0.43/0.53 0.30/0.37 0.24/0.30 0.43/0.53 0.27/0.34 0.29/0.36

1.68 1.47 0.98 1.47 0.98 1.47 1.07 1.56 1.19 1.68 1.72 1.19 1.76 1.23 0.98 1.76 1.11 1.19

7.9 12.3 11.1 13.7 10.7 16.3 13.3 18.6 19.9 20.3 20.1 23.8 24.4 24.2 29.0 24.7 33.9 40.3

1.0 3.2 2.6 3.9 2.4 5.2 3.7 6.4 7.0 7.2 7.1 9.0 9.3 9.2 11.6 9.4 14.0 17.2

0.77 0.49 0.76 0.58 1.09 0.39 0.67 0.38 0.46 0.43 0.57 0.56 0.47 0.49 0.46 0.55 0.49 0.44

a In a typical preparation, 0.8 g of SDS is added to 2 g of brine with a salt concentration, C , varying between 0 and 0.5 mol‚L-1. After s dissolution (at 35 °C for about 3-5 h), cyclohexane is added and finally 1-pentanol is added dropwise until formation of the SLC. x is the molar ratio of pentanol over surfactant, F is the volume ratio of oil over water, a is the lattice parameter of the triangular lattice measured by SAXS, R is the radius of the cyclohexane cylinders, and ASDS is the surface area per polar head of SDS determined from the composition and structural parameters (see text). b SAXS recorded with a laboratory apparatus. c SAXS recorded on the synchrotron beamline.

phases as nanoreactors for different chemical syntheses certainly requires a precise knowledge of their stability, with respect to the sometimes drastic modifications required for, or induced by, chemical reactions. Studying their intrinsic stability and the possible extensions of their domain of stability is thus necessary. This is the issue we want to address in this paper. We therefore studied the stability of swollen hexagonal phases when the pH of the aqueous medium or the chemical nature of the component (salts and surfactant) are changed. We will show that the range of stability is quite extended, rendering the swollen hexagonal phases potentially useful for the fabrication of materials, as demonstrated by the successful synthesis of metals, oxides, and polymers in both phases of the SLC.60,61 We will then illustrate the exploitation of the hexagonal phase for polymerization in a confined medium and the structuration of an inorganic gel. 2. Experimental Section 2.1. Materials and Sample Preparation. The surfactants, sodium dodecyl sulfate (SDS) (Mw(SDS) ) 288.38 g‚mol-1), cetylpyridinium chloride (CpCl) (Mw(CpCl) ) 358.01 g‚mol-1), and cetyltrimethylammonium bromide (CTAB) (Mw(CTAB) ) 364.45 g‚mol-1) were purchased from Acros. Inorganic salts (NaCl, KCl, NaF, and Na2SO4) and cyclohexane and 1-pentanol were purchased from SDS, and Fluka, respectively. The metallic salt (hexachloroplatinic acid H2PtCl6) was purchased from Johnson Matthey. For in-situ polymerization, we used 1,4-diphenylbutadiyne (Aldrich) as monomers and benzoin methyl ether (Fluka) as catalyst. All compounds were used as received. In place of water, an aqueous suspension of zircone sulfate was eventually used, with a molar ratio Zr/SO4)15, whose synthesis has been published elsewhere.62 In a classical preparation of swollen hexagonal phases, the surfactant is first dissolved in the aqueous salted medium, giving a transparent and viscous micellar solution. The addition under stirring of the oil (cyclohexane) into the pristine micellar solution (60) Surendran, G.; Tokumoto, M.; Pena dos Santos, E.; Remita, H.; Ramos, L.; Kooyman, P. J.; Santilli, C. V.; Bourgaux, C.; Dieudonne´, P.; Prouzet, E. Chem. Mater. 2005, 17, 1505. (61) Pena dos Santos, E.; Santilli, C. V.; Pulcinelli, S. H.; Prouzet, E. Chem. Mater. 2004, 74, 213. (62) Chiavacci, L. A.; Santilli, C. V.; Pulcinelli, S. H.; Craievich, A. F. J. Appl. Cryst. 1997, 30, 750.

leads to a white unstable emulsion. A co-surfactant (1-pentanol) is then added to the mixture, which is vortexed for a few minutes, and a perfectly transparent gel is obtained. A slight excess of pentanol gives a lamellar phase, which is easily distinguished by eye from the hexagonal phase, because it is more turbid and less viscous. A further addition of pentanol gives a fluid isotropic phase. Hexagonal phases are extremely sensitive to the quantity of co-surfactant (especially the highly swollen ones). To ease the sample preparation, the exact quantity of pentanol can be adjusted dropwise, the sample being mixed between successive additions of pentanol until a totally transparent gel is formed. Once the correct amount of pentanol had been defined experimentally, further preparation could be performed more quickly with the help of a pristine mixture of pentanol with cyclohexane. All experiments were performed at room temperature. In the following, we define the swelling ratio, F, as the volume ratio of oil over water. We call x the molar ratio of pentanol over surfactant and Y the weight ratio of salted water over surfactant. 2.1.1. SLCs Prepared with an Anionic Surfactant (SDS). The molar ratio of water over surfactant is fixed at 40, which corresponds to Y ) 2.5. The stability of SLCs as a function of the swelling ratio is studied for samples whose composition are given in Table 1 and which are located within the stability area of the phase diagram given in reference 12. The swelling ratio, F, is varied from 0.21 to 3.81 with a salt concentration, Cs, ranging between 0 and 0.5 mol‚L-1. To investigate the stability of SLCs as a function of the nature of the salt, several salts (NaCl, NaF, KCl, Na2SO4, H2PtCl6), pure or mixed, were used in place of NaCl. The different salt mixtures investigated for F ) 2.5 are displayed in Table 2. 2.1.2. SLCs Prepared with Cationic Surfactants. The SLCs containing CTAB and CpCl as surfactants were prepared according to a similar process. For samples prepared at neutral pH, 1.0 g of CTAB or CpCl was dissolved in 2.0 g of salted water (with NaCl or Na2SO4). We note that these quantities correspond to Y ) 2.0 but to a water over surfactant molar ratio of 40, thus equal to that of the SDS-based SLCs. The addition of cyclohexane led to a white emulsion, which was further transformed into a transparent gel as pentanol was added dropwise. For the samples prepared in acidic medium (pH ) -0.6), the swelling ratio was fixed at F ) 2.5. In this case, the salted water was a solution of Na2SO4 at a concentration of 0.15 mol‚L-1, which was acidified by concentrated sulfuric acid (96%). 2.1.3. SLCs Prepared with Precursors. For in-situ polymerization, we used a swollen hexagonal phase with SDS as surfactant and NaCl as salt. 1,4-Diphenylbutadiyne (DPB), the monomer, and benzoin methyl ether (BME), the catalyst, were added into

Existence and Stability of New Nanoreactors

Langmuir, Vol. 21, No. 10, 2005 4365

Table 2. Composition of SLCs Prepared with SDS (G ) 2.5) and Different Salts (NaCl, NaF, KCl, Na2SO4) (Cation Concentration 0.3 mol‚L-1) sample

a (nm)

100% NaClb 100% NaFb 100% KCla 100% Na2SO4b KCl (80%) NaCl (20%)b KCl (60%) NaCl (40%)b KCl (50%) NaCl (50%)b KCl (40%) NaCl (60%)a KCl (20%) NaCl (80%)a NaF (80%) Na2SO4 (20%)b NaF (60%) Na2SO4 (40%)b NaF (50%) Na2SO4 (50%)b NaF (40%) Na2SO4 (60%)b NaF (20%) Na2SO4 (80%)b NaCl (80%) NaF (20%)a NaCl (60%) NaF (40%)a NaCl (40%) NaF (60%)a NaCl (20%) NaF (80%)a NaCl (80%) Na2SO4 (20%)a NaCl (60%) Na2SO4 (40%)b NaCl (40%) Na2SO4 (60%)b NaCl (20%) Na2SO4 (80%)b NaCl (50%) NaF (25%) Na2SO4 (25%)a NaCl (25%) NaF (50%) Na2SO4 (25%)b NaCl (25%) NaF (25%) Na2SO4 (50%)b NaCl (32%) NaF (36%) Na2SO4 (32%)a

28.4 27.6 29.4 26.9 24.5 28.4 28.4 29.4 29.4 25.6 26.9 25.7 25.7 26.6 25.8 28.7 27.4 28.7 25.2 25.1 26.1 25.0 25.9 29.1 32.4 25.4

a SAXS recorded with a laboratory apparatus. b SAXS recorded on the synchrotron beamline.

Scheme 1. Cross-Section of a Swollen Hexagonal Phasea

a R is the radius of the nonpolar cylinders, comprising a c tube of oil of radius R (shaded area) and a shell of surfactant tails of thickness s. Between adjacent cylinders, the thickness of the polar moiety (water plus surfactant heads) is w; a ) 2Rc + w is the lattice parameter.

cyclohexane before the liquid crystal formation. The DPB/ cyclohexane and BME/DPB ratios are 5% (w/w). The preparation of the sample was performed in a standard way, except that the mixture of cyclohexane plus DPB and BME was used in place of pure cyclohexane. For in-situ gelation, we used a swollen hexagonal phase comprising CpCl as surfactant where we replaced the salted water by an aqueous suspension of sulfated zirconium (ratio Zr/SO4 ) 15) at a concentration in zirconium of 3.4 mol‚L-1. In the two cases, the volume ratio of nonpolar medium over aqueous medium is F ) 2.5. 2.2. Characterization of the Hexagonal SLCs. Lyotropic liquid crystals can adopt various geometries clearly identified by small-angle neutron or X-ray scattering. The direct lyotropic hexagonal phases consist in infinite oil cylinders coated by a monolayer of surfactant and co-surfactant arranged in a triangular array in the aqueous medium. A schematic representation is given in Scheme 1. Macroscopically, the hexagonal phase is a completely transparent gel, with an elastic modulus that decreases continuously as the swelling ratio increases.63 Polarized (63) Ramos, L.; Molino, F. Europhys. Lett. 2000, 51, 320.

Figure 1. (a) Phase diagram in the (F, Cs) plane where F is the volume ratio of cyclohexane over water and Cs is the concentration in sodium chloride. The shaded area corresponds to the domain of stability of the SDS-based swollen hexagonal phases as reported by Ramos et al.,12 the letters correspond to the samples prepared (in neutral pH) for this study (see Table 1). The dashed line has equation Cs ) 0.156 F - 0.104. Inset: Typical texture of a hexagonal phase viewed between crossed polarizers. (b) Variation of the lattice parameter, a, as a function of F; the black circles correspond to the samples marked in (a), and the line is a linear fit to the experimental data. Empty symbols correspond to samples prepared with others surfactant, co-surfactant, or salt: (diamond) SDS, pentanol, and H2PtCl6 as salt; (circle) CpCl, pentanol, and NaCl; (square) CpCl, decanol, and NaCl (up triangle) CpCl, pentanol, and Na2SO4; (down triangle) CTAB, pentanol, and Na2SO4. light microscopy and small-angle X-ray scattering (SAXS) are used to characterize the hexagonal phases. The scattering pattern of a hexagonal phase consists of diffraction peaks whose positions are in the ratio 1:x3:2:x7 and correspond, respectively, to the (10), (11), (20), and (21) diffraction planes. The first peak position, q0, allows a direct determination of the hexagonal lattice parameter, a, to be obtained, according to a ) (2/x3)(2π/q0). SAXS experiments were performed using synchrotron radiation and a laboratory apparatus for complementary experiments. Synchrotron experiments were conducted at the D24 beamline of the DCI synchrotron ring at LURE (Orsay, France). A Ge(111) single-crystal curved monochromator provided a beam focused in the horizontal plane. The selected wavelength was 0.149 nm. The incident beam intensity was monitored by an ionization chamber and its size (typically 0.4 × 1.5 mm2) was determined by collimating slits upstream and downstream from the monochromator. To reduce absorption and parasitic scattering, the beam path was kept under vacuum and slits were placed before the sample to suppress parasitic signal. The sample-to-detector distance was adjusted to 2.50 m in order to cover the required scattering vector range. The scattering patterns were recorded with a gas-filled, position-sensitive detector or an image plate. The laboratory experiments were performed using an in-house setup with a copper rotating anode X-ray source (4 kW) equipped with a multilayer focusing “OSMIC” monochromator giving the Cu KR radiation (wavelength λ ) 0.154 nm), a high flux (108 photons/s) and a punctual collimation. The SAXS intensity was collected with a two-dimensional image plate detector. With both setups, samples were put in 1.5 mm thick glass capillaries and measurements were performed in a transmission configuration. Due to the sample high-intensity scattering level, the X-ray diffraction patterns were not corrected for any background scattering. Hexagonal phases are birefringent and exhibit characteristic textures between crossed polarizing windows, such as fan-shaped textures (see inset of Figure 1a), which are observed when the surfactant cylinders are parallel to the walls of the observation cell. Optical microscopy observations were performed with a Leica DMRX polarizing microscope

4366

Langmuir, Vol. 21, No. 10, 2005

Pena dos Santos et al.

3. Results and Discussion 3.1. Stability of Swollen Hexagonal Phases. The phase diagram of the pseudo-quaternary mixtures of SDS, water, NaCl, pentanol, and cyclohexane (with fixed weight ratio of water over SDS, Y ) 2.5) has been studied in reference 12, where it has been shown that direct hexagonal phases can be obtained for a swelling ratio of oil to water (in volume) from 0 up to 3.8 mol‚L-1 and a NaCl concentration, Cs, from 0 to 0.5 mol‚L-1. (For comparison with the parameter very often considered when dealing with liquid crystal templating for synthesis of mesoporous materials, this swelling ratio corresponds to a weight ratio of swelling agentscyclohexanesover surfactant of up to 7.4.) A phase diagram in the plane (F, Cs) is presented in Figure 1a and illustrates that a higher salt concentration is needed to stabilize samples with higher swelling ratios, corresponding to hexagonal phases with larger oil-swollen tubes. From a practical point of view, as Cs and F have to be changed simultaneously, an empirical relationship can be proposed to help to prepare the samples: Cs ) 0.156F - 0.104. This line is plotted in Figure 1a and falls within the domain of stability of swollen hexagonal phases previously determined. The phase diagram can be qualitatively understood by the fact that a higher ionic strength increases the spontaneous radius of curvature of the surfactant monolayer because of a screening of the electrostatic repulsion between the charged polar heads of SDS. This mechanism is, a priori, independent of the nature of the salt in the aqueous medium and should only be controlled by the ionic strength of the medium. Similarly this argument should also hold if the anionic surfactant is replaced by a cationic one. We have therefore tried to stabilize swollen hexagonal phases using different salts and surfactants. We first present the swelling of SDS-based SLCs and then investigate their stability with the pH and as the nature of the salt and of the surfactant is changed. 3.1.1. Swelling of Direct SDS-Based Hexagonal Phases. The starting point of this study is the swollen hexagonal phases comprising SDS as surfactant and NaCl as salt. The locations in the phase diagram previously determined of the samples investigated are indicated by letters in Figure 1a, and their compositions are given in Table 1. The structural parameters of the samples are determined thanks to their SAXS patterns, measured using both synchrotron radiation and the laboratory setup. Two or three Bragg peaks whose positions are in the ratio 1:x3:2 are usually observed for data obtained using the laboratory setup, and up to four peaks whose positions are in the ratio 1:x3:2:x7 are observed when Synchrotron radiation is used. As an illustration, Figure 2 shows SAXS spectra of samples with various swelling ratios, F, and various salt concentrations, Cs, ranging from 0 to 0.5 mol‚L-1. As F increases, the Bragg peaks are found to shift to lower wave-vectors, q, indicating an increase of the characteristic size of the hexagonal arrangement. We find that a varies between 7.9 and 40.3 nm when F varies between 0.21 and 3.81. The values of a are shown in Table 1. The evolution of the lattice parameter a with F is displayed in Figure 1b and follows a linear variation. The best fit to the experimental data gives: a ) (8.5 F + 5.8) nm. From the SAXS data and the sample composition, the structural parameters (see Scheme 1 for a definition of the structural parameters) of the swollen hexagonal phases can be extracted. We call Rc the radius of the nonpolar cylinders and w the thickness of the polar medium (which comprises water and the polar heads of

Figure 2. Representative SAXS spectra of swollen hexagonal phases with various swelling ratios and salt concentrations: from right to left, samples b (F ) 0.66, Cs ) 0 mol‚L-1), f (F ) 0.98, Cs ) 0.05 mol‚L-1), h (F ) 1.24, Cs ) 0.1 mol‚L-1), l (F ) 2.31, Cs ) 0.2 mol‚L-1), q (F ) 3.28, Cs ) 0.4 mol‚L-1) and r (F ) 3.81, Cs ) 0.5 mol‚L-1). The scattered intensity is normalized by the intensity of the main diffraction peak to ease comparison.

the surfactant and co-surfactant molecules) between two adjacent cylinders: a ) 2Rc + w. The radius of the nonpolar cylinder, which comprises oil and the hydrophobic tails of the surfactant and co-surfactant molecules, can be decomposed into a shell of surfactant tails of thickness s and a core of cyclohexane of radius R, if we assume that the amount of cyclohexane dissolved in the surfactant palisade is negligible. Thus, Rc ) R + s. In the absence of oil: a ) w + 2s. The extrapolation of the experimental values of the lattice parameters toward F ) 0 gives a ) w + 2s ) 5.8 nm. With a numerical value for the surfactant tail length s ) 1 nm, one obtains that the polar medium has a thickness w ) 3.8 nm, in agreement with previous determination.12 The radius, R, of the cyclohexane cylinders can be directly estimated from the lattice parameters knowing w (as the ratio of water over surfactant does not change, w is not expected to change significantly) and s. Numerical values that range between 1.0 and 17.2 nm are reported in Table 1. In addition, an evaluation of the surface area per polar head can be extracted knowing R and the composition of the mixture. The calculation for the surface area per polar head of surfactant is detailed in Appendix 1. We find ASDS + xAol ) 2MW(SDS)/dH2ONAFY(R + s)/R2. In this expression, ASDS and Aol are the surface areas per polar head of SDS and pentanol, respectively, Mw(SDS) ) 288.38 g‚mol-1 is the molar weight of SDS, NA is Avogadro’s number, dH2O ) 1 g‚cm - 3 is the density of water, F is the volume ratio of oil over water, x is the molar ratio of pentanol over surfactant, and Y is the weight ratio of salted water over SDS. In our experiments Y ) 2.5 and x slightly varies from one sample preparation. All numerical values for the molar ratio of pentanol over surfactant are reported in Table 1 and give x ) 1.4 ( 0.3. We take Aol ) 0.1 nm2,12 and calculate for all samples the surface area per SDS head. The numerical values are reported in Table 1 and do not show any clear trend with F. We find ASDS ) (0.56 ( 0.17) nm2. Although rather scattered, the results for ASDS are in excellent agreement with numerical values found in the literature10 and indirectly confirm that the whole amount of oil added in the mixtures is indeed incorporated inside the oil-swollen cylinders. Hence, the structure of the swollen hexagonal phases remains unchanged over the whole range of

Existence and Stability of New Nanoreactors

Figure 3. SAXS spectra of swollen hexagonal phases prepared in neutral pH with SDS as surfactant and F ) 2.5 and with varying salts, NaCl, KCl, NaF, and Na2SO4. In all samples, the cation concentration is 0.3 mol‚L-1.

Figure 4. Image plate recorded using synchrotron radiation of a sample crystallized in the capillary tube into large monocrystalline domains; the surfactant is SDS and F ) 2.5, a mixture of NaF (80%) and Na2SO4 (20%), for a global concentration of 0.3 mol‚L-1 in cation is used.

composition and is well described with the parameters defined in Scheme 1. 3.1.2. Stability with the Nature of the Salt. For phases made with SDS, we have replaced NaCl employed until now12 by two monovalent salts, NaF and KCl, and a divalent one, Na2SO4, without changing the proportions of the different components of the mixture. In Figure 3, we show that the SAXS spectra of the samples with Cs ) 0.3 mol‚L-1 and F ) 2.5 but with the four different salts all superimposed, thus showing that stable phases with unchanged structures are obtained in all cases. (We note that, for monovalent salts, the salt concentration was 0.3 mol‚L-1, while we used 0.15 mol‚L-1 for the divalent salt Na2SO4, thus maintaining a fixed concentration in cation.) This result is more general: we have indeed prepared samples with 22 different mixtures of two or three of the different salts mentioned above, without finding any modification of the structure (data not shown). The various compositions investigated are listed in Table 2 together with the values of the lattice parameters measured by SAXS. We find for all samples the same numerical value within experimental uncertainties for the lattice parameter a ) (27.2 ( 1.9) nm. Moreover, these liquid crystals can organize in large domains when left at rest in the capillary and lead to highly ordered regions as revealed by the SAXS pattern displayed in Figure 4 for a salt composition (NaF, 80%; Na2SO4, 20%). More complex salts can also be used. As an illustration, we show in Figure 5, the SAXS spectra of a SDS-based SLC made with a metallic salt, H2PtCl6 (Cs ) 0.2 mol‚L-1), a well-known platinum

Langmuir, Vol. 21, No. 10, 2005 4367

Figure 5. SAXS spectrum and image plate recorded on the synchrotron beamline of a SLC made with SDS as surfactant and H2PtCl6 as salt. The salt concentration is 0.2 mol‚L-1, and the swelling ratio is F ) 2.

Figure 6. SAXS spectra for swollen hexagonal phases with F ) 2.5 and salt (Na2SO4) concentration 0.15 mol‚L-1, in neutral (pH 7) and acidic conditions (pH -0.6); the surfactants used are CTAB (a) and CpCl (b). The data were obtained using Synchrotron radiation (a) and a laboratory apparatus (b).

precursor. Four Bragg peaks whose respective positions are characteristic of a hexagonal phase are observed. Moreover, the lattice parameter is identical to that measured for a phase made with NaCl at equivalent swelling ratio F ≈ 2 (see Figure 1b). Thus, our results suggest that swollen hexagonal phases may be obtained with a large variety of salts, used as pure components or as mixtures. 3.1.3. Stability with the Nature of the Surfactant. We also prepared SLCs by employing single-chain cationic surfactants CpCl and CTAB. The diffraction patterns obtained for SLCs prepared with these surfactants for a swelling ratio F ) 2.5 and using Na2SO4 as salt (Cs ) 0.15 mol‚L-1) are shown in Figure 6. Bragg peaks characteristic of a hexagonal phase are observed; three peaks are detected for the sample (CpCl) probed using the laboratory setup, and four peaks are detected for the sample (CTAB) measured with Synchrotron radiation. Thus, swollen hexagonal phases can equally be obtained with the cationic CpCl and CTAB surfactants. However, to stabilize SLCs prepared with different surfactants, the molar ratio of co-surfactant over surfactant has to be adjusted: it is of the order of 1.4 for SDS, 1 for CpCl, and 0.7 for CTAB. This is the direct consequence of the fact that different surfactant molecules are expected to form films with different spontaneous curvatures. Moreover, we noted that the lattice parameters of samples made with SDS, CpCl, or CTAB were all equal. This is shown in Figure 1b, where we have plotted, together with the F-dependent values of the lattice parameters measured for SLCs prepared with SDS as surfactant and NaCl as salt, the values of a

4368

Langmuir, Vol. 21, No. 10, 2005

measured for samples made of cationic surfactant. The lattice parameter values obtained with CTAB (a ) 26.5 nm for F ) 2.5) and with CpCl (a ) 24.6 nm with Na2SO4 as salt and F ) 2.5 and a ) 14.6 nm with NaCl as salt and F ) 1.28) fall on the curve determined with SDS-based samples. The fact that the lattice parameter are similar suggests that all structural parameters are of the same order of magnitude as for SDS-based SLCs. Finally, it is worth mentioning that hexagonal SLCs can also obtained when replacing cyclohexane by decane and/or 1-pentanol with 1-decanol. 3.1.4. Stability with Acido-Basic Conditions. We explored the stability of the SLCs with pH. Acidic (respectively basic) pH was obtained by addition of hydrochloric acid (N) or concentrated sulfuric acid (respectively sodium hydroxide at 10-4 mol‚L-1 for pH ) 11 and at 10-2 mol‚L-1 for pH ) 9). Stable hexagonal phases with SDS as surfactant and NaCl as salt were obtained in the pH range 1-11 without significant variation of the structure as demonstrated by SAXS (not reported here). However, at pH ) 1, the hexagonal phase structure was found to degrade within a few days (sealed samples of SLCs are usually stable for years). We assign this destructuration to the progressive protonation of the sulfate head of the SDS surfactant, which precludes the long-term stabilization of SDS-based samples at lower pH. The use of cationic surfactants allows one to overcome the problem of stability of SLCs in acidic condition observed with SDS. Figure 6 shows that the spectra obtained for samples prepared with CTAB and CpCl at pH ) -0.6 exactly superpose to the spectra obtained for samples in neutral pH. The remarkable stability suggests that an acidic medium does not modify the hydrophilic interaction of the polar head of CTAB and CpCl. 3.2. Toward the Preparation of Materials Using Swollen Hexagonal Phases. In this section, we illustrate by two examples the robustness of swollen hexagonal phases through their use as structuring agent for inorganic material and as nanoreactors for chemical synthesis. First, the aqueous intercylinder space can be used for the synthesis of materials. The pH-induced gelation of zirconia was taken as an example. We used a swollen hexagonal phase comprising CpCl as surfactant and Na2SO4 as salt, where we replaced the salted aqueous solution with a 3.4 mol‚L-1 suspension of roughly 1 nm sulfated zirconium colloidal particles (ratio Zr/SO4 ) 15). We checked first by SAXS that the presence of the colloids does not alter the structure of the hexagonal mesophase (Figure 7a). Thus, quite surprisingly, although the water compartments are very thin (the spacing between adjacent cylinders is w ≈ 4 nm), they can host the colloidal particles. The gelation of the colloidal suspension was then induced by the addition of a small amount of concentrated sulfuric acid (96%) such that the final acid concentration in the colloidal suspension is 9.5%. Thanks to the stability of SLCs prepared with CpCl in very acidic medium, the addition of sulfuric acid does not disrupt the hexagonal order. Upon addition of acid, however, the samples became immediately more turbid and much more rigid. The gelation mechanism was followed by SAXS. Results are shown in Figure 7a. Remarkably, upon gelation, the structuration of the sample is maintained: two orders of diffraction are observed before and after gelation. This shows that a continuous and nanostructured zirconium oxohydroxide gel forms. However, the characteristic size of the gel shifts toward lower value as gelation proceeds. The lattice parameter a is found equal to 25.6 nm before gelation, equal to 20.3 nm for a fresh gel (about 10 min old), and to 17.7 nm for a 10-day-old gel. We noted also

Pena dos Santos et al.

Figure 7. SAXS spectra of swollen hexagonal phases with precursors and after (a) gelation and (b) polymerization. In (a), the sample is made with CpCl as surfactant and a colloidal suspension of sulfated zircone as aqueous solution. In (b), the sample is made with SDS as surfactant and NaCl as salt. Monomers of DPB and initiator BME are added to the nonpolar medium (cyclohexane). For the sake of clarity, the two spectra have been shifted in the y direction. (b) The black line corresponds to the spectrum before polymerization, and the gray line corresponds to the spectrum after polymerization. Inset: TEM picture of the polymer synthesized in the tubes of a swollen hexagonal phase. Bar: 100 nm. In (a) and (b), the arrows point the positions of the first two Bragg peaks.

an increase of the intensity toward small q-vector due to the aggregation of the colloidal particles, which is the signature of a gelation process. The reduction of the characteristic size, while maintaining the hexagonal order, is too large (it reaches 30% for old gels) to be only due to a compaction of the aqueous compartment upon gelation and implies a shrinking of the oil-swollen cylinders too. We are currently investigating this behavior in more detail. The organic compartments of swollen hexagonal phases can also be used as a confining medium for polymerization. We used a standard swollen hexagonal phase with SDS as surfactant, NaCl as salt, and F ) 2.5. Prior to the liquid crystal formation, we added into cyclohexane 1,4-diphenylbutadiyne (DPB) and benzoin methyl ether (BME); BME acts as a catalyst for the polymerization of DPB, which occurs upon exposure of the samples to UV radiation. Before exposure to UV radiation, the hexagonal phases containing DPB and BME were transparent. By contrast, after a 24 h UV exposure, they turned yellow but they were still translucent and birefringent, while a sample without DPB and BME remained completely transparent after a similar UV exposure. The change of color (characterized by the appearance of an absorption with a maximum at around 380 nm in the UV-vis absorption spectrum) unambiguously signs the polymerization of DPB (see for instance reference 64), which was further confirmed by infrared spectroscopy. Polymerization was also conducted using γ-radiation and gave similar color changes. We checked by SAXS that the polymerization (using both UV- and γ-radiation) does not alter the structure of the mesophase. A hexagonal structure was still obtained after polymerization, without

Existence and Stability of New Nanoreactors

moreover any variation of the lattice parameter. This is clearly seen in Figure 7b where the spectra obtained both before and after polymerization are found to superimpose exactly. The structure of the as-synthesized material was also investigated. A transmission electron microscopy picture of the polymer (after destabilization of the hexagonal phase by addition of a mixture of water and ethanol and recovery of the polymer in the organic medium) is presented in the inset of Figure 7b. It clearly shows nanofibers, which suggests a direct templating by the liquid crystal. Thus, these experiments show that chemical reactions in confined geometry are possible within the oil-swollen tubes, without disrupting the longrange order of the soft hexagonal matrix. This result certainly opens the way to many applications of synthesis in confined geometry. 4. Conclusion We have investigated the stability of swollen hexagonal phases with respect to modifications of the chemical nature of the components and of the pH and have shown that they can be prepared in a wide range of experimental conditions without changing their structural parameters. This opens the way to the use of these highly swollen liquid crystals as nanoreacting media. In particular, reactions based on a charge matching interaction between precursors and surfactants heads can require the use of cationic or anionic surfactants, depending on the precursor charge, as illustrated by the assembly mechanism of mesostructured materials.65,66 These types of reactions can be performed within these SLCs as we have shown they are equally stable with anionic or cationic surfactants. On the other hand, we have shown that metallic salts could be used in place of inert inorganic salt, rendering possible the synthesis of metallic particles within the hexagonal matrix, as demonstrated in references 60 and 67. Finally, syntheses requiring a strongly acidic medium can also be performed within these hexagonal matrix61 since a marked variation of the pH does not affect their stability. We have taken advantage of the remarkable possibilities offered by swollen hexagonal phases60,61,67 as (64) Carpick, R. W.; Sasaki, D. Y.; Marcus, M. S.; Eriksson, M. A.; Burns, A. R. J. Phys.: Condens. Matter 2004, 16, R679. (65) Monnier, A.; Schu¨th, F.; Huo, Q.; Kumar, D.; Margolese, D. I.; Maxwell, R. S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P. M.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993, 261, 1299. (66) Huo, Q.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schu¨th, F.; Stucky, G. D. Nature 1994, 368, 317. (67) Surendran, G.; Tokumoto, M.; Prouzet, E.; Ramos, L.; Beaunier, P.; Kooyman, P. J.; Bourgaux, C.; Etcheberry, A.; Remita, H. submitted for publication.

Langmuir, Vol. 21, No. 10, 2005 4369

nanoreacting media and have exploited these SLCs to design anisotropic polymeric, inorganic, and metallic materials. We believe however that their remarkably large range of swelling together with their extended domain of stability render them promising as nanoreactors for a larger variety of materials. Appendix 1 We recall the parameters defined in the main text to characterize the composition of the swollen hexagonal phases: F ) Voil/VH2O is the volume ratio of oil over water, x ) Nol/Nsurf is the molar ratio of pentanol over surfactant and Y ) mH2O/msurf is the ratio between the volume of salted water and the mass of surfactant. The total surface area of the surfactant and co-surfactant molecules is equal to Sc ) 2πRch. Here, h is the total length of all cylinders and is related to the volume of oil incorporated in the hexagonal phase: Voil ) πR2h ) FVH2O. Thus, Sc ) 2FVH2O (R + s)/R2. On the other hand, the total surface area of the surfactant and co-surfactant molecules reads as the sum of the total surface area of surfactant molecules and the total surface area of cosurfactant (pentanol) molecules: Sc ) NolAol + NsurfAsurf ) Nsurf(xAol + Asurf), where Nsurf (respectively Nol) is the number of molecules of surfactant (respectively co-surfactant) in the mixtures and Asurf (respectively Aol) is the surface area of a surfactant (respectively co-surfactant) molecule. Hence, equaling the two expressions for Sc one obtains: xAol + Asurf ) 2FVH2O(R + s)/R2 1/Nsurf. With Nsurf ) msurf/MW(surf)NA, where MW(surf) is the molecular weight of surfactant and NA is Avogadro’s number, one gets: xAol + Asurf ) 2MW(surf)/NAF(R + s)/R2 VH2O/msurf. Incorporating the weight ratio of water over surfactant, Y ) mH2O/msurf ) VH2O/dH2Omsurf (with dH2O the density of water), one obtains the relation given in the main text that relates the surfactant and cosurfactant surface area to the radius of the cylinders, R, and the parameters defining the composition of the swollen liquid crystal, F, x, and Y: ASDS + xAol ) 2MW(SDS)/ dH2ONAFY(R + s)/R2. We note that in this expression we identify the volume, mass, and density of the salted water with those of pure water. Acknowledgment. The authors acknowledge the French Ministry of Research and Education for its financial support through the “ACI Nanoscience” funding program, French (COFECUB) and Brazilian (CAPES and FAPESP) organisms for their financial support in the SOL French-Brazilian network, as well as for PhD grants. LA047092G