Relation between the Lower Consolute Boundary and the Structure of

Dec 28, 2007 - 7565, Université Henri Poincaré−Nancy 1/CNRS, Faculté des ... Florentin Michaux , Jean-Luc Blin , José Teixeira , and Marie José...
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Relation between the Lower Consolute Boundary and the Structure of Mesoporous Silica Materials F. Michaux, J. L. Blin, and M. J. Ste´be´* Equipe Physico-chimie des Colloı¨des, UMR SRSMC No. 7565, UniVersite´ Henri Poincare´ -Nancy 1/CNRS, Faculte´ des Sciences, BP 239, F-54506 VandoeuVre-les-Nancy cedex ReceiVed September 19, 2007. In Final Form: October 24, 2007 In this study, we have shed some light on the relation between the position of the lower consolute boundary of various nonionic surfactants in water and the structure of the mesoporous silica materials synthesized from these surfactants-based systems. In the first part, the lower consolute boundary was shifted by adding salts. Depending on the features of the phase diagram, we have looked for either a salting out or a salting in effect. Mesoporous materials were prepared from a micellar solution of the investigated surfactants. Results clearly evidenced that the cooperative self-assembly mechanism is not favored if the lower consolute boundary is not shifted toward high temperatures. Moreover, the higher the difference between the phase separation temperature and the temperature at which the silica precursor is added to the surfactant solution, the better the mesopore ordering is. In the second part, this tendency has been confirmed by using a hydrogenated surfactant as additive.

1. Introduction The nonionic surfactants have been widely studied and much physicochemical data, such as micelle formation, intermicellar interaction, structural studies, and phase behavior in various solvents, have been reported in the literature.1-6 One of the main characteristics of the nonionic surfactant-based systems is that a miscibility gap is often encountered in the phase diagrams and it extends over a large area. The curve that determines the gap is called the lower consolute boundary. Below this curve a micellar phase L1 exists, whereas above the lower consolute boundary, a phase separation into two phases is obtained, one is rich in micelles (L′1) and the other one is poor in micelles (L′′1). The minimal temperature at which the appearance of the micellar solution becomes turbid, defined as the cloud point (CP), also labeled the lower consolute temperature. In the case of nonionic polyoxyethylene alkyl ether surfactants [Cm(EO)n], this phenomenon is related to the fact that water around the polyoxyethylene chain is more structured than bulk water1 and it is associated with a strong entropy dominance.7 Indeed, the ethylene headgroups are highly hydrated, so they are characterized by low entropy. When two micelles approach each other, their hydratation spheres are overlapped and some water molecules are freed. As a consequence, the entropy is increased and this phenomenon becomes more important at the CP. On the other hand, to explain the clouding phenomenon in the case of polyethylene oxide solutions Karlstro¨m8,9 has proposed a model where the conformational adaptation to the environment provides the mechanism of changing the free energy of interaction. The CP depends strongly on the hydrophilic-lipophilic balance (HLB) * Corresponding author. E-mail: [email protected]. (1) Sjo¨blom, J.; Stenius, P.; Danielsson, I. In Nonionic Surfactant Physical Chemistry; Schick, M. J., Ed.; Surfactant Science Series Vol. 23; M. Dekker: New York, 1987; pp 369. (2) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; Mc Donald, M. P. J. Chem. Soc. Faraday Trans. 1 1983, 79, 975. (3) Meguro, K.; Takasawa, Y.; Kawahashi, N.; Tabata, Y.; Ueno, M. J. Colloid Interface Sci. 1982, 83, 50. (4) Ravey, J. C. In Microemulsions: Structure and Dynamics; Friberg, S. E., Bothorel, P., Eds.; CRC Press Inc.: Boca Raton, FL, 1988; p 93. (5) Corti, M.; Degiorgio, V. Phys. Res. Lett. 1980, 45, 1045. (6) Kunieda, H.; Shigeta, K.; Ozawa, K. J. Phys. Chem. B 1997, 101, 7952. (7) Kjellander, R. J. Chem. Soc. Faraday Trans. 2 1982, 78, 2025. (8) Karlstro¨m, G.; Carlsson, A.; Bjo¨rn, L. J. Phys. Chem. 1990, 94, 5005. (9) Karlstro¨m, G. J. Phys. Chem. 1985, 89, 4962.

of the surfactant, and for a given hydrophobic chain length, its value increases with the number of oxyethylene units.2,10 For example, in a paper dealing with polyoxyethylene-type nonionic surfactants, Tiddy and co-workers2 have determined, for the C12(EO)n family, that changing n from 4 to 12 involves a shift of the lower consolute temperature from 4 to 98 °C. The CP is also modified by the presence of additives,11-16 which affects the intermicellar interactions. For instance, it has been shown that the addition of C6H6 in a 1 wt % solution of C9H19C6H4(OCH2CH2)9.2OH lowers the cloud point from 56 °C to below 0 °C, whereas the addition of n-C16H34 raises it to 80 °C.11 A similar effect can be obtained with alcohol, and it is reported17 that the addition of butanol (0.6 mol‚L-1) to the C12(EO)5PO4-water system lowers the CP of about 10 °C, while methanol (1 mol‚L-1) increases the CP of 3 °C. Moreover, it is well-known that the presence of salts strongly modify surface and bulk properties of nonionic surfactant solutions.12,17-22 Therefore, salts affect the critical micellar concentration (cmc) as well as the position of the lower consolute boundary. Depending on the electrolyte, the lower consolute boundary (lcb) can be shifted either toward higher (salting in) or lower (salting out) temperature. As an example, NaF, NaCl, and NaBr decrease the cloud point of C12(EO)10, whereas NaI shifts the temperature position of the boundary in the opposite way.12 The CP data are in peculiar interest for the practical applications of nonionic surfactants. Indeed, these surfactants are commercially available, and they are used as emulsifiers, wetting agents, detergents, or solubilizers, and more recently, surfactant-based systems have been widely used for the (10) Inoue, T.; Ohmura, H.; Murata, D. J. Colloid Interface Sci. 2003, 258, 374. (11) Shinoda, K. In Principles of Solution and Solubility; Lagowski, J. J., Ed.; M. Dekker Inc.: New York, 1978; p 180. (12) Sharma, K. S.; Patil, S. R; Rakshit, A. K. Colloids Surf. A 2003, 219, 67. (13) Schott, H. J. Colloid Interface Sci. 1997, 192, 458. (14) Schott, H.; Royce, A. E.; Han, S. K. J. Colloid Interface Sci. 1984, 98, 196. (15) Valaulikar, B. S.; Manohar, C. J. Colloid Interface Sci. 1985, 108, 403. (16) Attard, G. S.; Fuller, S.; Tiddy, G. J. T. J. Phys. Chem. 2000, 104, 10426. (17) Chai, J.; Mu, J. Colloid J. 2002, 64, 550. (18) Maclay, W. N. J. Colloid Sci. 1956, 11, 272. (19) Kjellander, R.; Florin, E. J. Chem. Soc. Faraday Trans. 1 1981, 77, 2053. (20) Schott, H. J. Colloid Interface Sci. 1997, 189, 117. (21) Weckstro¨m, K.; Zulauf, J. J. Chem. Soc. Faraday Trans. 1 1985, 81, 2947. (22) Weckstro¨m, K.; Papageorgiou, A. C. J. Colloid Interface Sci. 2007, 310, 151.

10.1021/la7029104 CCC: $40.75 © 2008 American Chemical Society Published on Web 12/28/2007

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preparation of mesoporous materials.23-28 These mesostructured compounds display a wide range of pore sizes and symmetries that can readily be tailored by adjusting the physicochemical properties of the surfactant. When the mesoporous material is prepared from a micellar solution, it is today well-accepted that the formation of these materials occurs through the self-assembly mechanism.29-31 In such a mechanism, the interactions between the surfactants and the inorganic precursor are responsible for the formation of mesoporous materials. Moreover, it appears that the characteristics of the recovered materials, such as the structure and the pore diameter, are strongly related to the properties of the surfactant used for their preparation.32-34 As an example, in a paper dealing with the pore size expansion of mesoporous silica prepared from a fluorinated surfactant and by adding fluorocarbons as expanders,32,33 we have shown that the swelling of the hexagonal hybrid mesophase (surfactant-silica) can be related to the oil incorporation in the hexagonal liquid crystal H1. Actually, our investigations have shown that only a very small quantity (≈1 wt %) of oil can be incorporated into the micelles (L1). Thus, in that case we cannot propose that the increase in the mesopore size occurs through a swelling of the micelles, as usually considered to explain this phenomenon.35-38 Even though in the literature many papers deal with the lower consolute temperature of nonionic surfactants, to the best of our knowledge, no detailed investigation about the relation between the location of the lower consolute boundary and the structure of mesoporous materials is reported. In this work, we performed this kind of study. The position of the lower consolute boundary was shifted by adding either salts or a hydrogenated surfactant. 2. Materials and Methods The fluorinated surfactants used, which were provided by DuPont, have an average chemical structure of C8F17C2H4(OC2H4)9OH, C7F15C2H4(OC2H4)8OH, and C6F13C2H4(OC2H4)5OH. They are respectively labeled as RF8(EO)9, RF7(EO)8, and RF6(EO)5. The hydrogenated surfactants C12H25(OC2H4)8 [RH12(EO)8] and C18H35(OC2H4)10 [RH18(EO)10] were respectively provided by Huntsman and purchased from Aldrich. In all cases, the hydrophilic chain moiety exhibited a Gaussian chain length distribution. 2.1. Determination of the Lower Consolute Boundary. The samples were prepared by weighting the required amounts of surfactant and water or salt solutions in well-closed glass vials to avoid evaporation. They were left at controlled temperature for some hours in order to reach equilibrium. The lower consolute boundary was determined visually by noting the temperature at which the turbidity of the surfactant solutions was observed. In order to get this curve (lcb), the cloud point temperature was plotted as a function of the surfactant concentration. (23) Attard, G. S.; Glyde, J. C.; Go¨ltner, C. G. Nature 1995, 378, 366. (24) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (25) Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Science 1995, 269, 1242. (26) Prouzet, E.; Pinnavaia, T. J. Angew. Chem., Int. Ed. Engl. 1997, 36, 516. (27) Blin, J. L.; Le´onard, A.; Su, B. L. Chem. Mater. 2001, 13, 3542. (28) Blin, J. L.; Lesieur, P.; Ste´be´, M. J. Langmuir 2004, 20, 491. (29) Firouzi, A.; Kumar, D.; Bull, L. M.; Besier, T.; Sieger, P.; Huo, Q.; Walker, S. A.; Zasadzinski, J. A.; Glinka, C.; Stucky, G. D. Science 1995, 267, 1138. (30) Lee, Y. S.; Sujardi, D.; Rathman, J. F. Langmuir 1996, 12, 6202. (31) 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. (32) Blin, J. L.; Ste´be´, M. J. J. Phys. Chem. B. 2004, 108, 11399. (33) Blin, J. L.; Ste´be´, M. J. Microporous Mesoporous Mater. 2005, 87, 67. (34) Kim, J. M.; Sakamoto, Y.; Hwang, Y. K.; Kwon, Y. U.; Terasaki, O.; Park, S. E.; Stucky, G. D. J. Phys. Chem. B 2002, 106, 2552. (35) Ulagappan, N.; Rao, C. N. R. Chem. Commun. 1996, 2759. (36) Ottaviani, M. F.; Moscatelli, A.; Desplantier-Giscard, D.; Di Renzo, F.; Kooyman, P.; Galarneau, A. J. Phys. Chem. B 2004, 108, 12123. (37) Jana, S. K.; Nishida, R.; Shindo, K.; Kugita, T.; Namba, S. Microporous Mesoporous Mater. 2004, 68, 133. (38) Boissie`re, C.; Marines, M. A. U.; Tokumoto, M.; Larbot, A.; Prouzet, E. Chem. Mater. 2003, 15, 509.

Figure 1. (A) Lower consolute boundary location of the RF7(EO)8 surfactant in water (a) and in NaI aqueous solutions of the following concentrations (mol‚L-1): (b) 5 × 10-2, (c) 0.1, (d) 0.25, (e) 0.5, (f) 1, (g) 2, and (h) 3. (B) Phase separation temperature of the 10 wt % RF7(EO)8 solution (9) as a function of the NaI concentration and change in CP (O) between the solution without and with salt. 2.2. Mesoporous Preparation. A micellar solution containing a defined weight percent of surfactant in water or in an electrolyte aqueous solution was prepared. The pH value of the solution was kept at 7.0. Tetramethoxysilane (TMOS), used as the silica precursor, was added dropwise into the micellar solution. The temperature at which the silica is added depends on the position of the lcb. The surfactant/silica molar ratio was adjusted to 0.5. The obtained samples were sealed in Teflon autoclaves and heated for 1 day at 80 °C. The final products were recovered after ethanol extraction with a Soxhlet apparatus during 48 h. 2.3. Mesoporous Characterization. X-ray measurements were carried out using a home-built apparatus, equipped with a classical tube (λ ) 1.54 Å). The X-ray beam was focused by means of a curved gold/silica mirror on the detector placed at 527 mm from the sample holder. Nitrogen adsorption-desorption isotherms were obtained at -196 °C over a wide relative pressure range from 0.01 to 0.995 with a volumetric adsorption analyzer TRISTAR 3000 manufactured by Micromeritics. The samples were degassed under vacuum for several hours at 320 °C before nitrogen adsorption measurements. The pore diameter and the pore size distribution were determined by the BJH (Barret, Joyner, Halenda) method.39

3. Results and Discussion 3.1. Addition of Salts. The RF7(EO)8-Based System. The binary F 7(EO)8/water phase diagram (Figure 1A) evidenced that R 7(EO)8 presents a cloud point at 34 °C (Figure 1Aa). The isotropic RF

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Figure 2. RF7(EO)8: (A) SAXS patterns of samples synthesized from NaI aqueous solutions (a) 0 mol‚L-1, (b) 5 × 10-2 mol‚L-1, (c) 0.1 mol‚L-1, (d) 0.25 mol‚L-1, (e) 0.5 mol‚L-1, (f) 1 mol‚L-1, (g) 2 mol‚L-1, and (h) 3 mol‚L-1; the silica precursor has been added to the micellar solution at 20 °C. (B) Adsorption-desorption isotherms and the corresponding BJH pore size distribution (insert) of compounds synthesized from NaI aqueous solutions (a) 0 mol‚L-1, (b) 0.1 mol‚L-1, (c) 0.5 mol‚L-1, and (d) 3 mol‚L-1; the silica precursor has been added to the micellar solution at 20 °C. (C) SAXS patterns of samples synthesized from a 2 mol‚L-1 NaI aqueous solution and by adding the silica precursor to the micellar solution at (a) 20 °C, (b) 30 °C, and (c) 40 °C.

Relation between Consolute Boundary and Structure

micellar phase L1 is found to be present over a wide range of surfactant compositions. The variation of the lower consolute boundary position with the addition of NaI is shown in Figure 1Ab-g. You can observe that, with the increase of the NaI concentration, the lcb is shifted toward higher temperature (salting in effect) and the surfactant concentration, at which CP appears, is modified. For a 10 wt % surfactant solution, corresponding to the concentration used for the mesoporous materials preparation, a quasilinear relationship is evidenced between the temperature of phase separation and the sodium iodide concentration (Figure 1B). For example, this temperature is shifted from 42.5 to 70 °C, when the NaI concentration is raised from 0 to 3 mol‚L-1. The changes in the lcb location are mainly attributed to the anion. Indeed, cations are known to be smaller and to bind more hydration water than anions. Therefore, the lcb shifts are not caused primarily by competition between the ions for free water, but by a more specific anion effect. Due to its low electronegativity, high polarizability, and weak electrostatic field, I- disrupts the association of water molecules that surround the micelles of surfactant. Thus, I- belongs to the chaotropic family. This kind of anion increases the concentration of single water molecules, which are able to form hydrogen bonds with the ethylene oxide groups of nonionic surfactants. Hence, it salts nonionic surfactants in; i.e., it raises the CP.14,20 After the determination of the lower consolute boundary, mesoporous materials have been prepared by adding the silica precursor to the micellar solutions at 20 °C. The SAXS patterns of the material prepared from the RF7(EO)8/water system or from the RF7(EO)8/water + NaI system, with a NaI concentration lower than 0.1 mol‚L-1, exhibit a single broad reflection at 5.2 nm (Figure 2Aa,b), which indicates the formation of a disordered structure. In this case, the recovered mesoporous molecular sieves exhibit a wormhole-like channel array analogous to the MSU (Michigan State University) compounds. For these kinds of materials, the broad peak that is observed on the SAXS pattern gives an indication of the average pore-to-pore distance in the disordered wormhole framework, which presents a lack of longrange crystallographic order. By contrast, when the concentration of the aqueous electrolyte solution is higher or equal to 0.1 mol‚L-1, in addition to a sharp peak at 4.9 nm, two peaks at 2.8 and 2.4 nm are detected on the SAXS patterns (Figure 2Ac-h). The presence of these two last peaks is suggestive of a hexagonal organization of the channels. Increasing the NaI concentration from 0.1 to 3 mol‚L-1 does not vary in a significative manner the position of the first peak. According to Bragg’s law, the unit cell dimension (a0 ) 2d100/x3), which corresponds to the sum of the pore diameter and the thickness of the pore wall, can be calculated and its value is found to be equal to 5.6 nm. Therefore, we can conclude that, when the silica source is added to the micellar solution at 20 °C, in the investigated range of salt concentrations, the presence of NaI involves a regular channel arrangement. Nevertheless, the ordered structure is lost if the temperature at which the TMOS is incorporated to the 10 wt % RF7(EO)8 solution is too close to the phase separation temperature. As an example, for the materials prepared from a 2 mol‚L-1 sodium iodide aqueous solution, the addition of the TMOS at 20 or 30 °C leads to a hexagonal channel arrangement (Figure 2Ca,b), whereas only wormhole-like structure is obtained when TMOS is added at 40 °C; i.e., at 20 °C below the phase separation (Figure 2Cc). Indeed, while the three reflections, characteristic of the hexagonal structure, are evidenced in Figure 2Ca,b, only (39) Barret, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 37.

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Figure 3. Lower consolute boundary location of the RF6(EO)5 surfactant in (a) water, in (b) 1 mol‚L-1, (c) 2 mol‚L-1, and (d) 3 mol‚L-1 NaI aqueous solutions, and in (e) 3 mol‚L-1 NaSCN aqueous solution.

one broad peak is detected in Figure 2Cc, this reflects the loss of the mesopore ordering. Nitrogen adsorption-desorption isotherms and the corresponding BJH pore size distributions (inset), obtained from an analysis of the adsorption branch of the isotherm, are shown in Figure 2B. Whatever the synthesis conditions, a type IV isotherm, characteristic of mesoporous materials,40 is obtained. When the transition from a disordered to a hexagonal channel array occurs, the pore size distribution becomes narrower and the specific surface area increases. For example, when the concentration of NaI is changed from 0 to 0.1 mol‚L-1, the value of the specific area is raised from 697 to 913 m2/g. The mean pore diameter does not vary and the maximum of the pore size distribution remains centered at 3.9 nm. The RF6(EO)5-Based System. In water, the CP of RF6(EO)5 is situated below 0 °C. This value was not experimentally determined, and from Figure 3a, we can extrapolate the phase separation temperature to be around -20 °C for 10 wt % of surfactant. As I- and SCN- are chaotropic anions, upon addition of NaI or NaSCN, the lower consolute boundary is shifted toward high temperatures (Figure 3b-e). This phenomenon is more pronounced in the presence of NaSCN. Indeed, for this salt at 10 wt % of RF6(EO)5, an electrolyte concentration equal to 3 mol‚L-1 involves an increase of the phase separation temperature up to 45 °C (Figure 3e), whereas at the same surfactant and salt concentrations, the phase separation occurs at 30 °C in the case of NaI (Figure 3d). This observation is in good agreement with the results reported by Schott.20 In a paper dealing with the effect of chaotropic anions on the cloud point of Triton X-100, based on the measurement of the temperature at which the sodium salts of water-structure-breaking (chaotropic) anions increased the CP, this author ranked the salting in effect in the following order: SCN- > I- > [Fe(CN)5NO]2- > ClO4- > BF4-. Concerning the silica materials prepared from the RF6(EO)5 solution at 20 °C, whatever the synthesis conditions, no line is (40) Brunauer, S.; Deming, L. S.; Deming, W. S.; Teller, E. J. Am. Chem. Soc. 1940, 62, 1723.

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Figure 4. RF6(EO)5: SAXS pattern (A) and the nitrogen adsorption-desorption isotherm with the corresponding BJH pore size distribution curve (inset) (B) of samples prepared by adding the silica precursor at 20 °C. The RF6(EO)5 concentration is (a) 20 wt % in water, (b) 20 wt % in a 3 mol‚L-1 NaI aqueous solution, and (c) 10 wt % in a 3 mol‚L-1 NaSCN aqueous solution.

observed on the SAXS pattern (Figure 4Aa), indicating that the recovered compounds exhibit a complete randomly oriented pore structure. Figure 4Ba shows the N2 adsorption-desorption isotherm of this sample. Two distinct capillary condensation steps can be clearly seen at P/P0 values of about 0.50 and 0.75, respectively. The BJH model analysis of this material provides one narrow peak centered at 3.4 nm in the pore size distribution and another broad peak in the region of 5-15 nm, showing a dual mesoporous distribution. The desorption branch also displays two distinct steps. This suggests the formation of two pore systems with different diameters.41,42 In the presence of sodium iodide, the mesoporous materials were synthesized from a 3 mol‚L-1 NaI aqueous solution. In (41) Bagshaw, S. A. Chem. Commun. 1999, 1785. (42) Bagshaw, S. A. J. Mater. Chem. 2001, 11, 831.

order to go as far as possible from the phase separation temperature, the silica precursor was added at 20 °C, and according to the phase diagram, the weight percent of surfactant was varied from 15 to 25 wt %. In this way, the materials are prepared from a micellar solution. Indeed, in a 3 mol‚L-1 salt solution at 15, 20, and 25 wt % of RF6(EO)5, demixion temperatures are respectively 37, 48, and 54 °C. Even if the SAXS analysis evidenced that no hexagonal structure can be recovered by the addition of NaI, the appearance of a reflection line situated at 5.3 nm (Figure 4Ab) shows the formation of a wormhole-like structure. This means that a transition from a randomly oriented pore arrangement to a wormhole-like structure occurs and that the pore ordering has been enhanced. A type IV isotherm with a H2 hysteresis loop is obtained by nitrogen adsorptiondesorption analysis. The pore size distribution (Figure 4Bb, inset)

Relation between Consolute Boundary and Structure

Figure 5. (A) Lower consolute boundary location of the RH12(EO)8 surfactant in (a) water and in (b) 1 mol‚L-1, (c) 2 mol‚L-1, (d) 3 mol‚L-1 NaCl aqueous solutions. (B) Partial temperature-composition phase diagram of the RF8(EO)9 surfactant in a 3 mol‚L-1 NaCl aqueous solution.

is rather broad and it is centered at 5.0 nm. Similar results are obtained when NaSCN is used as salt instead of NaI (Figure 4Ac,Bc). In this case, the addition of NaSCN (3 mol‚L-1) to a surfactant solution at 10 or 20 wt % increased the phase separation temperature respectively to 45 and 64 °C (Figure 3e). The RF8(EO)9- and the RH12(EO)8-Based Systems. For these surfactants the lower consolute boundary is located at high temperatures. Indeed, for the fluorinated surfactant, the CP is higher than 90 °C,28 and for the hydrogenated one, the CP is equal to 71 °C (Figure 5Aa). A shift of the lcb toward lower temperatures is evidenced with the addition of NaCl. For instance, for 10 wt % of RF8(EO)9 in a 3 mol‚L-1 sodium chloride solution the phase separation temperature is decreased to 48 °C (Figure 5B). In a similar way for RH12(EO)8, phase separation temperature at 10 wt % of surfactant is dropped from 75 to 56, 44, and 36 °C, when the sodium chloride concentration is changed from 1, 2, and 3 mol‚L-1 (Figure 5Ab-d). By contrast to I- or SCN-, in the Hofmeister series Cl- is known to be a kosmotropic anion, i.e., a structure-making anion.20 Therefore, it enhances the

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association of water molecules by hydrogen bonds instead of their association with the surfactant. Thus, it reduces the surfactant hydration and solubility, lowering the CP. For both the fluorinated- and hydrogenated-surfactant-based systems, when the mesoporous materials are prepared from a salt-free micellar solution at 10 wt % and by adding the silica source at 40 °C, three reflections at q ratios 1:x3:2, consistent with a hexagonal symmetry, are observed on the SAXS pattern (Figure 6A). A type IV isotherm is obtained by nitrogen adsorption-desorption analysis, and a H1 type hysteresis loop, in which adsorption and desorption branches are steep, is observed for these samples (Figure 6B). This reflects the homogeneity in pore size. Indeed, an H1 hysteresis loop is commonly encountered from MCM-41-type materials with a very regular arrangement of channels with homogeneous openings. The specific surface areas are respectively 883 and 764 m2/g. The pore diameter distribution is quite narrow and centered at 4.6 and 3.4 nm (Figure 6B, inset). Concerning the materials prepared from a 3 mol‚L-1 NaCl aqueous solution for the fluorinated surfactant as regards the SAXS patterns, it is obvious that the hexagonal structure is maintained when TMOS is added at 40 °C (Figure 7Aa), but only disordered structures are recovered when the silica precursor is incorporated to the surfactant solution at 45 or 50 °C (Figure 7Ab,c). Actually, when the temperature at which TMOS is added to the micellar solution is increased, the secondary reflections disappear and the shape of the nitrogen isotherm is modified. As shown in Figure 7Ba-c, the capillary condensation is spread out over a larger range of relative pressures, meaning that compounds become less homogeneous in pore sizes. This is confirmed by the pore diameter distribution, which evidences the presence of micropores and shows a low dV/dD value in the mesopore range (Figure 7Bb,c, inset). These observations reflect the disorganization of the mesopore network. The same behavior is detected with the hydrogenated surfactant. For example, in the presence of NaCl (1 mol‚L-1), the addition of TMOS at 20 °C, i.e., at 36 °C below the phase separation, leads to well-ordered mesostructure (Figure 7Ad), while at 50 °C, i.e., at 6 °C below the phase separation, only one broad peak is detected in the SAXS pattern (Figure 7Ae). This indicates the formation of MSUtype materials. Moreover, in the latest conditions, the isotherm becomes intermediate between types I and IV (Figure 7Be). According to Dubinin,43 this kind of isotherm is characteristic of supermicroporous materials; i.e., the pore size is located at the limit between the micro- and mesoporous domains. This phenomenon is further confirmed by the analysis of the pore size distribution, the maximum of which is lower than 1.7 nm. It should be also noted that in the absence of salt, when the silica precursor is incorporated at the same temperature (50 °C), i.e., at 25 °C below the phase separation, even if the intensity of the 110 and 200 reflections is weak, a mesostructured compound, with a hexagonal channel array (Figure 7Af) and a homogeneous pore size distribution, is synthesized (Figure 7Bf). From all the results reported above, it appears that the difference between the phase separation temperature and the temperature at which the silica precursor is added to the surfactant solution plays a key role in the mesopore ordering. 3.2. Addition of Hydrogenated Surfactant. Another way to shift the lower consolute boundary consists of adding surfactants. Indeed, the physicochemical properties of the mixtures are generally different from those of homogeneous surfactant solutions. So, in order to confirm the above results, we have mixed the RH18(EO)10 surfactant with the RF7(EO)8 or RF6(EO)5 (43) Dubinin, M. M. In Progress in Surface and Membrane Science 9; Cadenhead, D. A., Ed.; Academic Press: New York, 1975; p 1.

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Figure 6. RF8(EO)9 and RH12(EO)8: SAXS pattern (A) and the nitrogen adsorption-desorption isotherm with the corresponding BJH pore size distribution curve (inset) (B) of samples prepared from the RF8(EO)9/water (a) and RH12(EO)8/water (b) systems. The TMOS was added at 40 °C.

one. The hydrogenated surfactant [RH18(EO)10] has been selected according to various criteria. First, according to the literature it exhibits a lower consolute temperature at 75 °C.44 Second, at pH ) 7, well-ordered mesostructured silica can be synthesized from a 10 wt % micellar solution of RH18(EO)10 in water. The TMOS can be added to the micellar solution either at 20 or 40 °C.45 The overall surfactant concentration in the solution was kept equal to 10 wt %, and the content of RH18(EO)10 in the mixture was varied from 0 to 5%. By this way, we can vary the value of CP for the investigated surfactant solution. The variation of the phase separation temperature for surfactant solutions at 10 wt % as a function of the RH18(EO)10 content is displayed in Figure 8, parts A and B, respectively, for the RF7(EO)8 and the RF6(EO)5 systems. In both cases, an increase of phase separation temperature is noted with the incorporation of the hydrogenated surfactant. For example, when the content of RH18(EO)10 is changed from 0 to 5 wt %, the demixion temperature is varied from 42.5 to 63 °C for the RF7(EO)8 solution (Figure 8A) and from below 0 to 68.5 °C for the RF6(EO)5 one (Figure 8B). After the determination of the lower consolute boundary, each fluorinated-hydrogenated surfactant-based system has been employed for the design of mesoporous materials. As mixtures of hydrogenated and fluorinated surfactants are useful in many practical applications, they have been investigated by different techniques.46-49 It appears (44) Shigeta, K.; Suzuki, M.; Kunieda, H. Progr. Colloid Polym. Sci. 1997, 106, 49. (45) Michaux, F.; Blin, J. L.; Ste´be´, M. J. Langmuir 2007, 23, 2138. (46) Almgren, M.; Wang, K. Langmuir 1997, 13, 4535. (47) Kadi, M.; Hansson, P.; Almgren, M. Langmuir 2004, 20, 3933. (48) Amato, M. E.; Caponetti, E.; Martino, D. C.; Pedone, L. J. Phys. Chem. B 2003, 107, 10048. (49) Ravey, J. C.; Gherbi, A.; Ste´be´, M. J. Prog. Colloid Polym. Sci. 1989, 79, 272.

that in these systems either mixed micelles containing both surfactants in a well-defined proportion or two kinds of micelles enriched in one of the two components can be formed. However, it was observed that the micellization of two surfactants with compatible head groups and different alkyl chains usually exhibits ideal mixing behavior.49,50 As RH18(EO)10 and RF7(EO)8 or RH18(EO)10 and RF6(EO)5 bear ethylene oxide in their head groups, we can assume that in the investigated range of concentrations, both fluorinated-hydrogenated surfactant-based systems lead to the formation of mixed micelles in all proportions. Thus, mesoporous materials are prepared from only one type of micelle. The silica precursor has been added to the fluorinatedhydrogenated mixture at 40 °C. From Figure 9, you can note that once again the pore ordering occurs for the systems that have the higher CP value. For example, while disordered silica is prepared from RF7(EO)8 or RF6(EO)5 (Figure 9a,b), well-ordered mesostructures are recovered if the surfactant solution contains 2 wt % of RH18(EO)10 (Figure 9c,d); i.e., when the phase separation temperature of the corresponding solutions has been increased respectively to 50 and 58 °C. 3.3. Discussion. For each investigated surfactant-based system, mesoporous materials have been prepared from a micellar solution (L1) in order for the self-assembly mechanism to occur.51 In such a mechanism, when the silica is added to the micellar solution, hydrogen-bonding interactions between the oxygen atoms of the oxyethylene groups of the surfactant and OH groups of the hydrolyzed TMOS are formed. To complete the polymerization of the tetramethoxysilane, these rod-like supramolecular (50) Guo, W.; Guzman, E. K.; Heavin, S. D.; Li, Z.; Fung, B. M.; Christian, S. D. Langmuir 1992, 8, 2368. (51) Wan, Y.; Zhao, D. Chem. ReV. 2007, 107, 2821.

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Figure 7. SAXS pattern (A) and the nitrogen adsorption-desorption isotherm with the corresponding BJH pore size distribution curve (inset) (B) of samples prepared from 10 wt % RF8(EO)9 in a 3 mol‚L-1 NaCl aqueous solution, with the silica precursor added at (a) 40 °C, (b) 45 °C, (c) 50 °C and of samples prepared from 10 wt % of RH12(EO)8 in a 1 mol‚L-1 NaCl aqueous solution, with TMOS added at (d) 20 °C, (e) 50 °C, and (f) in water, with TMOS added at 50 °C.

assemblies (template-silica) have to pack together, leading to the formation of a hexagonal hybrid mesophase. After the surfactant removal, mesoporous molecular sieves with a hexagonal channel array are synthesized. However, in this study when the silica precursor is incorporated at 20 °C to a 10 wt % micellar solution of RF7(EO)8, which presents a phase separation temperature at 42.5 °C, only wormhole-like structure are formed. Adding sodium iodide or RH18(EO)10 involves a shift of the CP toward higher temperature, and hexagonal mesostructures with a uniform pore size distribution are recovered in the same synthesis conditions. These observations led us to conclude that the position of the lower consolute boundary is a decisive factor, which has to be taken into account, since it plays a key role in the mesopore ordering. From an overall point of view, by comparing the features of the materials prepared from the various surfactant/water systems and regarding the shift of the lower consolute boundary by the addition of salts or of another nonionic surfactant, our results show that the higher the difference between the phase separation temperature and the temperature at which the silica source is added to the surfactant solution, the better the quality of the mesoporous materials is. This can be explained by the fact that in the domain near the lower consolute boundary critical phenomena occur.52,53 Thus, when the silica precursor is added (52) Corti, M.; Degiorgio, V. Phys. ReV. Lett. 1980, 45, 1045. (53) Triolo, R.; Magid, L. J.; Johnson, J. S.; Child, H. R. J. Phys. Chem. 1982, 86, 3689.

to the micellar solution at a temperature situated in this region, the self-assembly mechanism is not favored and silica with a disordered channel array is obtained. This conclusion is supported by the observations made by Zhao and co-workers,54 who have reported the synthesis of silica mesostructures by using the triblock copolymer P85 (EO26PO39EO20) and P65 (E20PO30EO20) as structuring agent and tetraethyloxysilane (TEOS) as silica precursor. P85 and P65 have a CP value of 82 °C in water. The TEOS:P65 (or P85):HCl:H2O molar composition was 1:0.0003: 6:166. These authors claim that ordered mesoporous silicates can only be obtained at a temperature higher than 90 °C. To explain this tendency, they assume that the high concentration of H+ and the ethanol released by the hydrolysis of TEOS have increased the CP of the triblock copolymer to a temperature higher than 100 °C. However, they have not performed a detail study and they do not explain why an ordered structure cannot be obtained at lower temperature. We have also to focus on the peculiar behavior of the RF6(EO)5 surfactant-based system. First, the material prepared from 20 wt % RF6(EO)5 in water exhibits a bimodal pore size distribution. One should be reminded that this system has a CP below 0 °C; thus, when the silica precursor is added at 20 °C to a 10 wt % solution of this surfactant, the material is prepared (54) Yuan, M.; Tang, J.; Yu, C.; Chen, Y.; Tu, B.; Zhao, D. Chem. Lett. 2003, 32, 660.

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aggregates, which have different sizes. Second, even if NaI or NaSCN allows a shift of the lower consolute boundary toward high-temperature (salting in effect), whatever the temperature at which TMOS is added to the aqueous salt solution containing the surfactant, only MSU-type materials can be recovered. This reflects the fact the pore ordering is also affected by another feature of the surfactant, i.e., the ratio between the volume of its hydrophilic head group (VA) and its hydrophobic part (VB). This concept was first introduced by Stucky and co-workers34 for a series of hydrogenated surfactants. For RF6(EO)5, the VA/ VB ratio is equal to 1.05, and it appears that for fluorinated surfactants this value is too low to obtain ordered mesostructure. A detailed study dealing with the effect of the VA/VB ratio on the properties of mesoporous materials prepared from fluorinated nonionic surfactants will be reported elsewhere.

4. Conclusions The RF7(EO)8/water system presents a cloud point at 34 °C, and the addition of NaI shifts the lower consolute boundary toward higher temperatures (salting in effect). While only disordered mesostructured are recovered from the RF7(EO)8/ water system, well-ordered mesostructure are synthesized from an aqueous solution of sodium iodide. However, the pore ordering is lost if the temperature at which the silica precursor is added is near the phase separation temperature. Figure 8. Evolution of the phase separation temperature with the weight percent of RH18(EO)10: (A) RF7(EO)8 solution and (B) RF6(EO)5. The overall surfactant concentration is equal to 10 wt %.

By contrast, for the RF8(EO)9 or the RH12(EO)8 surfactants, NaCl involves a shift of the lower consolute boundary toward lower temperatures (salting out effect). In a 3 mol‚L-1 sodium chloride solution, the phase separation temperatures of the surfactant solutions at 10 wt % are 48 and 36 °C, respectively, for RF8(EO)9 and RH12(EO)8, instead of a temperature higher than 90 °C and 75 °C for the salt-free solutions. As regards the mesoporous materials preparation, an hexagonal structure with uniform pore diameter is obtained from the RF8(EO)9/water or the RH12(EO)8/water systems, but the presence of sodium chloride leads to the loss of the pore ordering. Concerning the RF6(EO)5-based system, which has a CP below 0 °C, a transition from a randomly oriented pore structure to a disordered one occurs upon addition of NaI or NaSCN. For this fluorinated surfactant, the ratio between the volume of the hydrophilic head group and the hydrophobic part is too low to give rise to ordered mesostructures.

Figure 9. SAXS pattern of sample prepared from (a) RF7(EO)8/ water, (b) RF6(EO)5/water, (c) RF7(EO)8-RH18(EO)10, and (d) RF6(EO)5-RH18(EO)10. The loading of the hydrogenated surfactant in the mixture is equal to 2 wt %.

from a biphasic system. As a consequence, during the material preparation, the silica source polymerizes around surfactant

Nevertheless, for all the investigated systems, the selfassembly mechanism is favored if the lower consolute boundary is shifted toward high temperatures and if the phase separation temperature is moved away from the temperature at which the silica source is added to the micellar solution. This tendency is further confirmed by using a hydrogenated surfactant as additive. Acknowledgment. Authors would like to thank DuPont de Nemours Belgium for providing the fluorinated surfactants. LA7029104