Investigation of mixed fluorinated and triblock copolymer liquid crystals: Imprint for mesostructured bimodal silica

Journal of Colloid and Interface Science 446 (2015) 170–176

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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Regular Article

Investigation of mixed fluorinated and triblock copolymer liquid crystals: Imprint for mesostructured bimodal silica Karine Assaker, Issam Naboulsi, Marie-José Stébé, Mélanie Emo, Jean-Luc Blin ⇑ Université de Lorraine/CNRS, SRSMC, UMR7565, F-54506 Vandoeuvre-lès-Nancy cedex, France

g r a p h i c a l a b s t r a c t

P123 

P123 

+

F

H1 –rich liquid crystal

H1H –rich liquid crystal

TMOS addition +hydrothermal treatment + surfactant removal

8.5 nm 2.7 nm

+ Small mesopores mono-modal network

Bimodal mesoporous materials

a r t i c l e

Large mesopores mono-modal network

i n f o

Article history: Received 10 December 2014 Accepted 14 January 2015 Available online 23 January 2015 Keywords: Surfactant mixture Mesoporous materials Liquid crystals Bimodality Silica

a b s t r a c t Due to the difference in «mutual phobicity» between fluorocarbon and hydrocarbon chains, mixtures of fluorinated and hydrogenated surfactants are excellent candidates to design bimodal systems having two types of mesopores. In literature, only a few papers deal with these bimodal systems. Here hexagonal liquid crystal mixtures of the polyoxyethylene fluoroalkyl ether [RF8(EO)9] and the Pluronic [P123] have been used to template this kind of mesostructure through the liquid crystal mechanism, which is barely considered. After the detailed investigation of the RF8(EO)9/P123/water liquid crystal domain, materials have been synthesized and characterized by small angle X-ray scattering, transmission electron microscopy and nitrogen adsorption–desorption analysis. Our results show that this system provides two separate pore sizes in the materials over the mesoporous range. The ratio between the small mesopores and the large ones depends on the proportion between the porogens in the mixture. Nonetheless, we also outline that a minimum quantity of silica is required to recover the two hexagonal networks. Ó 2015 Elsevier Inc. All rights reserved.

1. Introduction Surfactants have applications in many domains such as food, cosmetics, and component in formulation for firefighting foams. They are also used as templates for the design of mesoporous materials, mainly silica. Indeed, these materials are commonly synthesized through the cooperative templating mechanism (CTM) in the presence of micelles [1,2]. Surfactants are often used in mixed systems to obtain some desired performances [3–5], in particular, ⇑ Corresponding author. Fax: +33 3 83 68 43 44. E-mail address: [email protected] (J.-L. Blin). http://dx.doi.org/10.1016/j.jcis.2015.01.030 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.

mixtures of hydrogenated and fluorinated surfactants are useful in many practical applications. In fact, fluorinated surfactants have chemical and physical properties which are different from hydrogenated ones. For example, as a consequence of the high hydrophobicity, fluorinated surfactants can decrease the surface tension of water up to 15–20 mN m1 [6]. The substitution of hydrogen atoms by fluorine ones enhances the chemical and thermal stability of the surfactant. Indeed, the energy of the C–F bond is 552 kJ mol1 instead of 338 kJ mol1 for the C–H bond [6]. In addition, due to the «mutual phobicity» between the fluorocarbon and hydrocarbon chains, non-ideal net repulsive interactions can occur [7–11] and it has been observed that, depending on the associated hydrogenated

K. Assaker et al. / Journal of Colloid and Interface Science 446 (2015) 170–176

and fluorinated compounds, 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 [4,12–18]. In regards to the synthesis of mesoporous materials, we can thus take benefit of these systems to design bimodal mesostructures on the condition that the two surfactants have hydrophobic chains with different sizes [19–24]. For example Antionetti et al. have reported the synthesis from mixed micellar solutions of immiscible fluorinated and hydrogenated surfactants of mesoporous silica monoliths with bimodal pore size distribution via the nanocasting process [20]. The authors have shown that the bimodality is favored when the proportion of the fluorinated surfactant is increased. These bimodal materials are, for example, of interest for applications in catalysis. Indeed, it was reported that a hierarchical combination of mesopores reduces transport limitations, resulting in higher activities and better controlled selectivity [25]. Another way to synthesize the mesoporous materials consists in using the direct liquid crystal templating (LCT) pathway [26– 32]. In that case the inorganic precursors grow around the liquid crystal. After the polymerization and the condensation, the template can be removed, leaving a mesoporous material whose structure, pore size and symmetry are determined by the liquid crystal scaffold. In addition, the high surfactant concentration templating method often leads to monolithic materials rather than powders, which are associated with mesostructured silica prepared from micellar solution [33]. For example, El-Safty et al. have reported the synthesis in strong acid conditions of nanometer-sized silica monolith by using lyotropic liquid crystal mesophases of polyoxyethylene alkyl ether, Triton, Tween or triblock copolymer type surfactants as a structure-directing agent [30,31]. Using nonionic fluorinated surfactant belonging to the polyoxyethylene fluoroalkyl ether family, we have synthesized by this mechanism mesoporous materials with a hexagonal channel array. We have correlated the structural parameters of the mesoporous materials to the ones of the liquid crystal phase used as the fingerprint for their synthesis. Results show that, at concentrations higher than 55 wt.%, the pore diameter of the obtained mesoporous materials fits with the hydrophobic diameter of the cylinders of the hexagonal liquid crystal phase. By contrast, for the lower surfactant contents, the value of the pore diameter is higher and consequently a decrease of the wall thickness is observed [32]. To the best of our knowledge the LCT pathway is barely used to design the bimodal mesoporous silica. In order to take advantage of both the liquid crystal pathway and the behavior of mixed hydrogenated/fluorinated systems, here, mixtures of liquid crystal of a polyoxyethylene fluoroalkyl ether [RF8(EO)9] and a triblock copolymer (Pluronic P123) have been employed as molds to design bimodal mesoporous silica materials. First we have partially investigated the RF8(EO)9/ P123/water ternary phase diagram focusing on the liquid crystal domain.

171

well-closed glass vials to avoid evaporation. They were left at controlled temperature for some hours in order to reach equilibrium. Liquid crystal phase domain was identified by its texture observed with optical microscope equipped with cross polarizers. In order to find the limits of this domain, additional SAXS measurements were also performed. Mesoporous material preparation: Bimodal mesoporous materials have been prepared from the liquid crystal mechanism. 1 g of the surfactant mixture was first dissolved in tetramethoxysilane (TMOS), used as the silica source. The total surfactant concentrations corresponding to a direct hexagonal phase in the RF8(EO)9/ P123/water system was fixed at 55%. The P123/RF8(EO)9 weight ratio was varied from 0/100 to 100/0. The quantity of TMOS was changed from 1 to 2.5 g. Then, in order to form the hexagonal liquid crystal phase, water was added. The pH of the solution was adjusted with chlorhydric acid (HCl) to 1.3. Afterwards, to remove the methanol produced during the hydrolysis of the silica precursor, the mixture was placed under vacuum. Indeed, in a paper dealing with the influence of methanol on the phase behavior of non-ionic fluorinated surfactant [34], we have shown that the released methanol involved a melting of the H1 phase if it is not removed. Then, the obtained samples were heated at 80 °C for 70 h. The final products are recovered after ethanol extraction with a Soxhlet apparatus during 48 h. Characterization: SAXS measurements were carried out using a SAXSess mc2 (Anton Paar) apparatus. It is attached to a ID 3003 laboratory X-ray generator (General Electric), equipped with a sealed X-ray tube (PANalytical, kCu (Ka) = 0.1542 nm, V = 40 kV, I = 50 mA). A multilayer mirror and a block collimator provide a monochromatic primary beam. A translucent beam stop allows the measurement of an attenuated primary beam at q = 0. Mesoporous materials were introduced into a powder cell, whereas liquid crystals were put in a paste cell, before being placed inside an evacuated chamber equipped with a temperature controlled sample holder unit. Acquisition times were typically in the range of 1–5 min for mesoporous materials, and of 20 min to 1 h for liquid crystals. Scattering of X-ray beam is recorded by a CCD detector (Princeton Instruments, 2084  2084 pixels array with 24  24 lm2 pixel size) in the q range 0.3–5 nm1. The detector is placed at 309 mm from the sample holder. N2 adsorption and desorption isotherms were determined on a Micromeritics TRISTAR 3000 sorptometer at 196 °C over a wide relative pressure range from 0.01 to 0.995. The pore diameter and the pore size distribution were determined by the BJH (Barret, Joyner, Halenda) [35] method applied to the adsorption branch of the isotherm. Samples for transmission electron microscopy (TEM) analysis were prepared by crushing some material in ethanol. Afterwards a drop of this slurry was dispersed on a holey carbon coated copper grid. A Philips CM20 microscope, operated at an accelerating voltage of 200 kV, was used to record the images. 3. Results and discussion

2. Materials and methods

3.1. RF8(EO)9/P123/water partial phase diagram

The used fluorinated surfactant, which was provided by DuPont, has an average chemical structure of C8F17C2H4(OC2H4)9OH. It is labeled as RF8(EO)9. The hydrophilic chain moiety exhibits a Gaussian chain length distribution and the hydrophobic part is composed of well defined mixture of fluorinated tails. The selected triblock copolymer is the Pluronic P123, (EO)20(PO)70(EO)20, which was purchased from Aldrich. On the figures, the samples were named as Hx/ Fy where H and F respectively mean hydrogenated and fluorinated surfactants, x and y respectively indicate the weight percentage of the P123 and RF8(EO)9 in the surfactant mixture. Phase diagram: The samples were prepared by weighting the required amounts of fluorinated surfactant, Pluronic and water in

The phase diagram of the RF8(EO)9/P123/water system at 20 °C is reported in Fig. 1. A micellar solution is detected for total surfactant concentration until at least 30 wt.%. A liquid crystal domain, for which only a hexagonal-type structure (H1) is evidenced, appears beyond 40 wt.% of total surfactant. This region (H1) is not only constituted of monophasic domains. It will be described in detail below, thanks to the determination of the structural parameters. Since our goal is to synthesize the bimodal materials through the LCT mechanism, we have focused our investigations on the hexagonal region. For the pure fluorinated system this phase is detected between 50 and 70 wt.% of RF8(EO)9 in water whereas it is located between 40 and 65 wt.% of Pluronic in the P123/water system. SAXS has been

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Water

13.0 12.5

H

F

H1 +H1

F

H1

6.5 6.0

H

H1

0.8

H

Biphasic domain F H H1 + H1

5.5

F R8 (EO)9

P123

P123

Fig. 1. Partial composition (weight fraction) phase diagram of the RF8(EO)9/P123/ water system at 20 °C. L1: direct micellar phase, the polyphasic domain is constituted F of L1, HH 1 and H1.

12.4nm 7.3 4.8 nm H100/F0 H98/F2

4.6 nm

Intensity (a.u.)

H95/F5 H90/F10 H60/F40 H50/F50

H5/F95

6.1nm 3.6 nm

1

H0/F100

3.1 nm

2

q

3

(nm-1 )

Fig. 2. Hexagonal liquid crystal phase: Evolution of the SAXS pattern of the hexagonal phase with the P123/RF8(EO)9 weight ratio for a total surfactant concentration of 55 wt.%.

RF8(EO)9/P123/water

used to analyze the H1 domain in the system. For a total concentration of surfactant of 55 wt.%., Figs. 2 and 3 show the variation of SAXS reflections when changing the proportions of the two surfactants. When 5 wt.% of P123 are incorporated into the RF8(EO)9 hexagonal phase only 3 reflections are observed on the spectrum. They are characteristic of a hexagonal phase having similar features to the pure fluorinated one. Hence the Pluronic and the fluorinated surfactant form a mixed hexagonal phase rich in fluorinated surfactant (HF1). Beyond 5 wt.% of Pluronic, a supplementary reflection at 12.4 nm appears on the SAXS pattern. The intensity of this reflection increases as a function of the P123 content in the surfactant mixture and at the same time, a shoulder which is more and more pronounced appears on the d1 0 0 reflection of the HF1 phase at low q values. Looking at the SAXS pattern of the P123 hexagonal phase, the supplementary reflection and the shoulder can be identified as the (1 0 0) and (1 1 0) reflections of a P123-rich hexagonal F phase (HH 1 ). Therefore the fluorocarbon-rich H1 phase coexists with

0.2

0.4

0.6

0.8 F

F

d100 F

R8(EO)9

Weight fraction of R 8 (EO)9 Fig. 3. Hexagonal liquid crystal phase: Variation of the position of the d1 0 0 reflection with the weight fraction of RF8(EO)9 in the surfactant mixture for a total F H surfactant concentration of 55 wt.%. s: dF1 0 0 and J dH 1 0 0, where d1 0 0 and d1 0 0 stand for the distance associated to the (1 0 0) line of the fluorinated and hydrogenated systems, respectively.

one hydrocarbon-rich HH 1 hexagonal phase. A similar behavior is noted for the P123-rich part of the diagram. Incorporating the fluorinated surfactant up to 5 wt.%, only one hexagonal phase, characteristic of the P123 hexagonal phase, is observed and the d-spacing slightly decreases. The further addition of RF8(EO)9 gives rise to a second hexagonal network, which corresponds to the fluorinated-rich hexagonal phase (HF1). To sum up, the liquid crystal domain is mainly composed of a biphasic mixture of one fluorocarbon-rich HF1 phase in equilibrium with one hydrocarbon-rich HH 1 hexagonal phase. Each phase can incorporate only a weak fraction of the second surfactant. Assuming that the mixture of surfactant forms a mixed entity, the structural parameters of the two H1 phases have been determined. For a given composition the average molar weight (M) of the mixed entity is

H40/F60 H10/F90

d100

mixed liquid crystals H1

Polyphasic domain

12.0

mixed liquid crystals H1

d100 (nm)

L1

0.5

H

F

0.2

M¼

nF M F þ nH MH nF þ nH

where nF and nH respectively stand for the mole number of fluorinated and hydrogenated surfactant; MF and MH are the corresponding molar weight. The hexagonal phase is composed of infinite cylinders packed in a hexagonal array. In case of direct systems, cylinders are filled by the hydrophobic chains and are covered by both head groups and water. The hexagonal phase is characterizedpby ffiffiffi its typical SAXS profile with the relative peak positions, 1, 3, 2 (Fig. 2). The distance d associated to the first peak is related to the hydrophobic core radius RH by the relation [36]:

VB ¼ V TA þ aV E

pffiffiffi 2 3pRH 2d

where a stands for the number of water molecules per surfactant molecule and VB, VTA, VE respectively stand for the molar volumes of the hydrophobic part of the mixed surfactant, the mixed surfactant and water (VE = 18 cm3/mol). VB and VTA depend on the molar ratio between the two amphiphiles. For example, for a mixture composed of 5 wt.% of P123 and 95 wt.% of RF8(EO)9, these values are VB = 290 and VTA = 665 cm3/mol. The values of VTA and VB for the pure surfactant were calculated from densities and are VTA = 626, VB = 261 cm3/mol for RF8(EO)9 and VTA = 5577 and VB = 4030 cm3/mol for P123. The cross-sectional area S can then be deduced from the following relation [36]:

S¼

2V B N RH

N is the number of Avogadro.

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The hydrophobic core radius and the cross sectional area of the pure P123 hexagonal phase are equal to 4.8 ± 0.1 nm and 2.80 ± 0.02 nm2, respectively. Taking into account the values of the different bonds, we can estimate that the length of the extended PPO block is 12 nm. Comparing this value to the hydrophobic core radius, we can conclude that the hydrophobic chains of the P123 hexagonal phase are completely self folded. When up to 5 wt.% of the fluorinated surfactant are incorporated, this corresponds to the upper limit of the HH 1 phase, no significant variation of the hydrophobic core radius is noted, whereas the occupied surface of the surfactant molecule in the interface decreases from 2.8 to 2.5 nm2. Looking at the pure fluorinated hexagonal phase, the hydrophobic core radius is 1.7 nm. Until a presence of 5 wt.% of Pluronic, upper limit of the HF1 phase, no change in the hydrophobic radius value is observed. As the length of an extended chain with 10 carbon atoms is about 1.4 nm, we can therefore assume that, in fluorinated hexagonal phase, the fluorinated chains are completely extended, whereas the PPO blocks are self folded. With the accommodation of the P123 molecules, the cross sectional area increases from 0.5 to 0.56 ± 0.02 nm2. This can be attributed both to the bigger size of the hydrophobic part and to the supplementary oxyethylene unit of the Pluronic. 3.2. Bimodal mesoporous materials from the liquid crystal templating mechanism To prepare the materials, first the quantity of TMOS has been fixed to 2 g. The mesoporous materials synthesized from both the pure fluorinated surfactant or the pure Pluronic exhibit a hexagonal mesopore ordering, characterized by the presence of the (1 0 0), (1 1 0) and (2 0 0) reflections at 5.2, 3.0 and 2.5 nm for RF8(EO)9 and at 10.6, 6.0 and 5.0 nm for P123 (Fig. 4A). For each sample a type IV isotherm (Fig. 5A) is obtained by nitrogen adsorption–desorption analysis. Isotherms have been shifted along the y-axis for clarity reason. The pore size distribution is narrow and centered at 3.0 nm and 8.0 nm (Fig. 5B), when RF8(EO)9 and P123

are respectively used. Looking at the isotherms, it seems that the compounds exhibit some microporosity. The relative pressure range investigated with the Micromeritics TRISTAR 3000 sorptometer does not allow to characterize the micropores. Therefore we have estimated the micropore volume by using the formula developed by Galarneau et al. [37]. As long as the hexagonal structure is maintained, the mesopore volume Vmes, due only to the main channels is given by the relation: Vmes = (D/1.05a0)2 (Vp + 1/qSi), where D, a0, Vp and qSi stand for the pore diameter, the cell parameter, the total pore volume obtained at the end of the pore filling and the density of the amorphous silica. The micropore volume Vs can then be estimated from the following relation: Vs = Vp  Vmes Applying this methodology, the values of the micropore volume are 0.36 and 0.30 cm3/g for materials prepared with the pure RF8(EO)9 and the pure P123, respectively. The presence of the micropores can be relied to the synthesis conditions. Indeed, it is reported that when RF8(EO)9 and P123 are used to synthesize mesoporous silica through the cooperating templating mechanism, micropores are present in the recovered materials if the hydrothermal treatment is performed at low temperatures [37–39]. Adding 10% of P123 to the fluorinated surfactant, a supplementary reflection at 11.2 nm appears on the SAXS pattern of the silica template by RF8(EO)9. By comparison with the pattern of the materials prepared from the pure Pluronic, it can be unambiguously attributed to the mesopore network templated by this copolymer. The cell parameter (a0), which is the sum of the pore diameter and the thickness of the pore pffiffiffi wall, can be calculated according to the Bragg’s law ða0 ¼ 2d1 0 0 = 3Þ. Its value is equal to 6.0 and 12.9 nm for networks induced by the fluorinated and the Pluronic surfactants, respectively. With the increase of the P123 content in the surfactant mixture, the intensity of this reflection also increases. Thus, SAXS analyzes show the coexistence of two hexagonal mesopores networks in the mesoporous silica materials templated by the liquid crystal mixtures. The position of the d1 0 0 reflections suggests that the network templated by RF8(EO)9 is smaller than the one arising from P123. TEM analyzes

B

A 10.6nm

small pores

small pores

6nm 5nm

H100/F0 H94/F6 H90/F10

Intensity (a.u.)

10.6nm 6nm

H80/F20

large pores

large pores

b

100 nm

H60/F40

5nm

11 nm

H50/F50 5.7nm

small pores

small pores

H40/F60

12.6nm

H30/F70

11.2nm 5.2nm

H20/F80

large pores 100 nm

a

large pores

H10/F90 5.2 nm

H5/F95 3nm 2.5nm

H0/F100 0

1

2

3

q (nm-1 ) Fig. 4. Mesoporous materials: A: Evolution of the SAXS pattern as a function of the weight ratio between RF8(EO)9 and P123. B: Representative TEM micrographs of the samples prepared with a P123/RF8(EO)9 weight ratio equal to 60/40 (a) and 50/50 (b). The quantity of TMOS is equal to 2 g.

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B

A 3

200 cm /g-STP H100/F0

H100/F0

H80/F20 H60/F40

dV/dD (cm /g-nm )

-1

H80/F20

3

Volume adorbed (cm /g-STP)

H97/F3 H97/F3

H50/F50

3

H60/F40 H50/F50 H40/F60

H40/F60 H30/F70

H20/F80

H30/F70 H20/F80 H10/F90

H0/F100

H0/F100 0.0

0.2

0.4

0.6

0.8

1.0

5

Relative pressure p/p0

10

15

20

Pores diameters (nm)

Fig. 5. Mesoporous materials: Nitrogen adsorption–desorption isotherm (A) with the corresponding pore size distribution (B) as a function of the weight ratio between P123 and RF8(EO)9. The quantity of TMOS is equal to 2 g. For clarity reason the curves have been shifted along the y-axis. The corresponding scale is indicated in the figure.

These interactions lead to the formation of two organic–inorganic mesophases. During the material preparation, the silica source polymerizes both around the two kinds of hexagonal phases. Finally, the treatment at 80 °C completes the assembly and the polymerization of the silica source. After surfactant removal, bimodal mesoporous materials having two ordered mesopore networks are obtained. Whatever the P123/RF8(EO)9 weight ratio, the specific surface area is high and its value progressively decreases from 900 to 650 m2/g with the increase of the P123 content in the surfactant mixture (Fig. 6). Keeping the P123/RF8(EO)9 weight ratio constant to 10/90, the quantity of TMOS has been varied from 1 to 2.5 g, this corresponds to a variation of the surfactant/TMOS molar ratio from 0.06 to 0.15. Fig. 7A shows that when the quantity of silica is lower than 1.6 g, only one capillary condensation step is observed on the nitrogen adsorption–desorption isotherm. The recovered materials have a

1200

Specific surface area (m2/g)

confirm the existence of the two mesopore networks with a hexagonal structure. Indeed, either the honeycomb like arrangement, or the hexagonal stacking of the channels is observed on the TEM images (Fig. 4B). Domains corresponding to small and large mesopores are also clearly observed on the photos depicted in Fig. 4B. For P123/RF8(EO)9 weight ratios comprised between 90/10 and 10/90, the materials templated by the fluorinated/hydrogenated liquid crystal mixtures show isotherms with two inflection points both during the adsorption and the desorption steps (Fig. 5B). The bimodality is further confirmed by the mesopore size distributions, which exhibit two populations of pore diameters, one centered at around 2.7 nm, corresponding to the mesopore templated by the fluorinated-rich hexagonal phase and the second one at 8.0 nm (Fig. 5B) attributed to the pores arising from the Pluronic-rich hexagonal phase. At low P123 content in the surfactant mixture, the smaller pores are predominant and with the increase of the Pluronic weight fraction, the reverse of that trend is observed. Reaching a P123/RF8 (EO)9 weight ratio of 94/6, only a mono-modal pore size distribution is detected. A similar behavior is noted when the weight fraction of P123 is lower than 0.05. These results are in accordance with the evolution of the RF8(EO)9/P123/water partial phase diagram. Indeed, it should be reminded that incorporating the fluorinated surfactant up to 5 wt.%, only one Pluronic-rich liquid crystal phase (HH 1 ) is observed. In a same way adding up to 5 wt.% of P123, only one fluorinated-rich hexagonal phase (HF1) is present in the ternary diagram. Therefore, under these conditions when the silica precursor is added to the surfactant mixture, only one liquid crystal templates the formation of the mesoporous materials. By contrast, for P123/ RF8(EO)9 weight ratios comprised between 90/10 and 10/90 the HF1 and the HH 1 hexagonal liquid crystal phases coexist. When the silica source is added to the surfactant solution having a total concentration equal to 55 wt.%, hydrogen-bonding interactions between the oxygen atoms of the oxyethylene groups of the two types of hexagonal phase and hydrogen atoms of the hydrolyzed TMOS are formed.

1000 800 600 400 200

RF8(EO)9

0.2

0.4

0.6

0.8

P123

P123 weight fraction

Fig. 6. Mesoporous materials: Variation of the specific surface area as a function of the P123 proportion in the surfactant mixture. The quantity of TMOS is equal to 2 g.

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A

B 2. 3 nm

2.5g TMOS

3

50 cm /g-STP

2g TMOS

Volume adsorbed (cm /g-STP)

1.6g TMOS

6.6 nm

2.5g TMOS

-1

2g TMOS

3

3

1g TMOS

dV/dD (cm /g-nm )

1.2g TMOS

1.6g TMOS

1.2g TMOS 1g TMOS

0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure p/p0

5

10

15

20

Pore diameters (nm)

Fig. 7. Mesoporous materials: Nitrogen adsorption–desorption isotherm (A) with the corresponding pore size distribution (B) as a function of the quantity of TMOS. The P123/RF8(EO)9 weight ratio is fixed to 10/90. For clarity reason the curves have been shifted along the y-axis. The corresponding scale is indicated in the figure.

of TMOS is lower than 1.6 g (surfactant/TMOS molar ratio exceeds 0.1) the quantity of silica added is not sufficient to interact with all the cylinders of the two H1 phases.

11.4 nm

2.5 nm

Intensity (a.u.)

2g TMOS 2.8 nm 2.5 nm

1.6g TMOS 5 nm

1.2g TMOS

1g TMOS

1

2

3

4

5

q (nm-1) Fig. 8. Mesoporous materials: Evolution of the SAXS pattern as a function of the quantity of TMOS. The P123/RF8(EO)9 weight ratio is fixed to 10/90.

single mesopore size distribution (Fig. 7B), corresponding to the fluorinated template, even if the dH 1 0 0 reflection of the mesopore network of the P123 mold is detected on the SAXS pattern for 1 g of TMOS (Fig. 8). However, for these TMOS amounts (<1.6 g), the dH 1 0 0 intensity is very low, but it increases as a function of the TMOS quantity (Fig. 8). SAXS and nitrogen adsorption–desorption analyzes clearly evidence that the two networks appear for 1.6 g of TMOS. A minimum amount is thus required to get the bimodal mesoporous material through the liquid crystal pathway. It should be reminded that the hexagonal liquid crystal phase is composed of infinite cylinders packed in a hexagonal array and in case of direct system, cylinders are filled by the hydrophobic chains and are covered by both head groups and water. Thus, we can assume that when the amount

4. Conclusion The development of hierarchically ordered structures at multiple length scales has attracted much interest over the past few years. Indeed, they are of particular interest for catalysis, for sorption and for the engineering of pore systems [40,41]. Many studies are focused on the synthesis of meso-macro, micro–macro or micro-mesoporous materials [42–48], but only a few of them deal with systems in which bimodal mesopore structures are present [20–24]. In addition, the recovered materials adopt either disordered bimodal mesopore arrangements or ordered mono-modal mesopore with small size template molecular systems embedded in larger entities [49]. The reported studies concerning the bimodal mesoporous silica mainly deal with the cooperative templating mechanism (CTM). That means the use of micelles as building blocks. Our aim in this paper has been to take benefit of the «mutual phobicity» between fluorocarbon and hydrocarbon chains [7–11] to design such materials through the liquid crystal pathway, which is barely considered. But this mechanism is interesting to tune the nature of the inorganic framework. For example it can be used to get ordered mesoporous titania [50]. In this context, the liquid crystal domain of the RF8(EO)9/P123/ water phase diagram has been investigated in detail to better address the synthesis of the bimodal materials via the LCT route. Thanks to the determination of the structural parameters, we have shown that the hexagonal region is mainly composed of a biphasic mixture of one fluorocarbon-rich HF1 hexagonal phase in equilibrium with one hydrocarbon-rich HH 1 hexagonal phase. Each phase can incorporate only a weak fraction of the second surfactant. Adding silF ica to the HH 1 and H1 template mixture, the bimodal mesoporous silica materials (2.7 and 8.0 nm) can be recovered. It should also be noted that both the fluorinated and the P123 surfactants, when used separately only lead to the formation of mono-modal mesoporous

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