Ordered Mesoporous Silica Templated by Nonionic Fluorinated Liquid

J. Phys. Chem. C 2009, 113, 11285–11293

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Ordered Mesoporous Silica Templated by Nonionic Fluorinated Liquid Crystals K. Zimny, 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, France ReceiVed: March 3, 2009; ReVised Manuscript ReceiVed: May 12, 2009

Here, for the first time, liquid crystals of fluorinated surfactants have been used as templates for the preparation of mesostructured silica. We have focused our study on the hexagonal liquid crystal phase of the C8F17C2H4(OC2H4)9OH/water system. The different synthesis parameters such as pH, surfactant/silica precursor ratio, and surfactant concentration have been optimized. The recovered materials have been characterized by SAXS measurements, nitrogen adsorption-desorption, and transmission electron microscopy. Results clearly evidence that the pH and the kind of acid play a crucial role in the synthesis of well-ordered materials. As a matter of fact, by using HCl as acid if the pH value is increased above 2, the regular channel array is lost and the recovered materials exhibit a randomly oriented channel system. Moreover, in the synthesis conditions used, due to the counter effect, HCl should be employed rather than H2SO4. Another parameter that affects the structure of the recovered material is the surfactant/silica ratio. Indeed, the mesopore ordering is detected only for ratios located in the range between 0.119 and 0.175. An increase of the surfactant/silica molar ratio results in materials with a disordered channel array. Finally, the structural parameters of the mesoporous materials have been correlated to those of the liquid crystal phase used as the fingerprint for their synthesis. At high surfactant concentration, the pore diameter matches well with the hydrophobic diameter of the cylinders of the H1 phase. 1. Introduction Due to their applications in various domains such as emulsifiers, wetting agents, detergents, or solubilizers, the phase behavior of nonionic polyoxyethylene alkyl ether surfactants Cm(EO)n has been widely reported.1-6 In aqueous media, the surfactant molecules are able to self-assemble into aggregates with different shapes according to the temperature and the surfactant concentration. Indeed, from their random distribution in the initial solution, these molecules can usually pack together to form spherical, cylindrical micelles and finally liquid crystals while increasing their concentration in water. The liquid crystal domain can be composed of normal hexagonal phase (H1), normal bicontinuous cubic phase (V1), lamellar phase (LR), reversed bicontinuous cubic phase (V2), or reversed hexagonal phase (H2). Combined with the sol-gel chemistry the different surfactant aggregates can be used as building blocks to design mesoporous materials.7-13 Depending on the surfactant concentration, two mechanisms can lead to the formation of the ordered material. The first one is the self-assembly mechanism (CTM); in this case the building blocks are the micelles, so the CTM occurs at low surfactant concentrations.14-16 The second approach to the preparation of ordered mesostructures utilizes the liquid crystal phase and it is labeled as the direct liquid crystal templating (LCT) pathway.17-21 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.22 For example Attard et al.17 have shown that liquid crystal phases * To whom correspondence should be addressed.

formed of octaethylene glycol monododecyl ether (C12EO8) and octaethylene glycol monohexadecyl ether (C16EO8) can be used as templates for the formation of mesoporous silica. El-Safty et al. have reported the synthesis in strong acid conditions of nanometer-sized silica monolith by using lyotropic liquid crystals mesophases of polyoxyethylene alkyl ether, triton, tween, and triblock copolymer-type surfactants as a structuredirecting agent.19,20,23-25 The incorporation of metal nanoparticles into the pore of mesostructured silica was also achieved through the liquid crystal templating approach.26-29 In the case of platinum,26 the authors have evidenced that when the surfactant concentration was less than that required for the formation of a liquid crystal phase, nonstructured material was obtained. Among the nonionic surfactants, the fluorinated ones are of peculiar interest. Indeed, linear fluorocarbon chains are less flexible than the hydrocarbon ones and, thus, present high melting points and as a consequence fluorinated surfactants exhibit a higher thermal stability than their hydrogenated analogues.30 The presence of fluorine atoms also strongly affects the properties of the surfactant and particularly its hydrophobicity and its critical micellar concentration (CMC).31 In the field of mesoporous materials, the use of fluorinated surfactant is more recent and in regards to the synthesis of mesoporous molecular sieves, their main advantage compared to the hydrogenated ones is their high thermal stability. Indeed since the hydrothermal treatment can be performed at higher temperature, the recovered materials exhibit a higher hydrothermal stability.32 Until now the mesoporous materials, prepared by using a fluorinated surfactant, have been synthesized through the self-assembly mechanism.33-38 For example, using nonionic fluorinated surfactant belonging to the polyoxyethylene fluoroalkyl ether family, we have synthesized by this mechanism mesoporous materials with a hexagonal channel array in a wide range of surfactant concentrations (5-20 wt %).33 To take

10.1021/jp9019409 CCC: $40.75  2009 American Chemical Society Published on Web 06/09/2009

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J. Phys. Chem. C, Vol. 113, No. 26, 2009

Figure 1. Evolution of the SAXS pattern as a function of the pH of the solution. (A) HCl is used as acid and materials are prepared at pH 0 (a), 0.3 (b), 1.3 (c), 1.5 (d), 2 (e), and 7 (f). (B) H2SO4 is used as acid and materials are prepared at pH 0 (a), 0.3 (b), 1.3 (c), 2 (d), and 7 (e). (C) TEM micrograph of the sample synthesized at pH 0.3 by using HCl as acid. Samples have been synthesized with 60 wt % of RF8(EO)9. The surfactant/TMOS molar ratio is equal to 0.145. The hydrothermal treatment has been performed during 2 days at 80 °C.

advantage of both the liquid crystal pathway and fluorinated surfactants in this work the liquid crystal phases of polyoxyethylene fluoroalkyl ether surfactants [CmF2m+1C2H4(OC2H4)nOH, labeled as RFm(EO)n] have been employed as molds to design mesoporous silica materials. As the formation of ordered mesostructures depends largely on the synthesis conditions,39,40 in this study we have in particular investigated the effect of various factors such as the pH and the surfactant/silica molar ratio on the structural and textural properties of the recovered materials. For the first time, we have looked for some correlations between the structural parameters of the mesoporous materials and those of the liquid crystal phase used as the fingerprint. 2. Materials and Methods 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 exhibited a Gaussian chain length distribution and the hydrophobic part is composed of a well-defined mixture of fluorinated tails. Mesoporous Preparation. The surfactant was first dissolved in tetramethoxysilane (TMOS), used as the silica source. The surfactant concentration corresponding to a direct hexagonal phase in the RF8(EO)9/water system was located between 53 and 78 wt %.33 The surfactant/silica molar ratio was changed from 0.10 to 0.45. Then in order to form the hexagonal liquid

Zimny et al. crystal phase, water was added. The pH of the solution was adjusted with sulfuric acid (H2SO4) or hydrochloric acid (HCl) to the desired pH value, which varies from 0 to 7. Afterward, to remove the methanol produced during the hydrolysis of the silica precursor, the mixture was placed under vacuum. Indeed, taking into account our previous results dealing with the influence of methanol on the phase behavior of nonionic fluorinated surfactant,41 we know that for the RF8(EO)9/silica molar ratio investigated in this study the released methanol involved a melting of the H1 phase if it is not removed. The obtained samples were sealed in Teflon autoclaves and heated at 80 °C during 2 or 4 days. The final products were recovered after ethanol extraction with a Soxhlet apparatus during 48 h. Characterization. SAXS measurements were carried out with 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. Samples for transmission electron microscopy (TEM) analysis were prepared by crushing some material in ethanol. Afterward 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 make the images. 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 further 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.42 All the analyses have been performed after surfactant removal. 3. Results and Discussion 3.1. Variation of the pH and of the Acid. As mentioned above we have previously determined that the domain of the hexagonal liquid crystal phase goes from 53 to 78 wt % of RF8(EO)9.33 Thus, in order to be in the H1 domain, first the surfactant concentration was fixed to 60 wt %. Attard et al.17 and El-Safty et al.20 have shown that for the C16(EO)10 surfactant, which is the hydrogenated analogue of RF8(EO)9, when the surfactant/silica molar ratio is around 0.14 hexagonal mesoporous materials can be formed through the liquid crystal mechanism. So to begin our study the RF8(EO)9/TMOS molar ratio is fixed to 0.145. Results dealing with the optimization of this parameter will be presented in the second part. The hydrothermal treatment is performed during 2 days at 80 °C. Indeed, we have reported that at this temperature a hexagonal pore ordering can be obtained from the self-assembly mechanism.33 The first parameters that we have optimized are the pH value and the acid. For the compounds prepared in the presence of HCl as acid, in addition to a sharp peak at 5.0 nm, two peaks at 2.8 and 2.5 nm are detected on the SAXS pattern (Figure 1Aa-d) as long as the pH value remains below 2. The presence of these two last peaks is suggestive of a hexagonal organization of the channels. According to the Bragg’s rule, the unit cell dimension (a0), which is the sum of the pore diameter and the thickness of the pore wall, can be deduced and is about 5.8 nm. The hexagonal arrangement is also evidenced by TEM (Figure 1C). If the pH value is further increased, no reflection is detected any longer (Figure 1Ae,f). Thus, the regular channel array is lost and the recovered materials exhibit a randomly oriented channel system. Replacing HCl by H2SO4 the situation is quite different. As a matter of fact, as regards SAXS patterns, we

Ordered Mesoporous Silica

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Figure 2. Evolution of the adsorption-desorption isotherms and of the pore size distribution (insert) as a function of the pH of the solution. (A) HCl is used as acid and materials are prepared at pH 0 (a), 1.5 (b), 2 (c), and 7 (d). (B) H2SO4 is used as acid and materials are prepared at pH 0 (a), 1.3 (b), 2 (c), and 7 (d). Samples have been synthesized with 60 wt % of RF8(EO)9. The surfactant/TMOS molar ratio is equal to 0.145. The hydrothermal treatment has been performed during 2 days at 80 °C.

Figure 3. Variation the of the specific surface area as a function of the pH of the solution; HCl (9) or H2SO4 (O) are used as the acid. Samples have been synthesized with 60 wt % of RF8(EO)9. The surfactant/TMOS molar ratio is equal to 0.145. The hydrothermal treatment has been performed during 2 days at 80 °C.

can see that the hexagonal structure is only detected for a pH value of 1.3 (Figure 1Bc). At pH 0, only one broad peak is observed (Figure 1Ba) and the presence of a single reflection indicates the formation of a disordered structure. In this case, the recovered mesoporous molecular sieves exhibit a wormholelike channel system, analogous to MSU-type materials. At a

Figure 4. Evolution of the SAXS pattern as a function of the time of the hydrothermal treatment at 80 °C: (a) 2, (b) 3, and (c) 5 days.

pH higher than 1.3 the SAXS patterns are characteristic of a random pore arrangement. Moreover, for the same pH value we note that the material synthesized using HCl exhibits a better

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Figure 5. (A) SAXS pattern of the materials synthesized with a surfactant/TMOS molar ratio (R) equal to (a) 0.097, (b) 0.119, (c) 0.145, (d) 0.175, (e) 0.202, (f) 0.228, (g) 0.309, and (h) 0.452. (B) TEM micrograph of the sample synthesized with a surfactant/silica molar ratio ) 0.145. Samples have been synthesized with 60 wt % of RF8(EO)9 at pH 1.3 by using HCl as acid. The hydrothermal treatment has been performed during 4 days at 80 °C.

pore ordering, reflected by the better resolution and the higher intensity of the 110 and 200 secondary reflections (Figure 1, Ac and Bc). From nitrogen adsorption-desorption measurements (Figure 2), we can observe that whatever the acid all the recovered samples exhibit a type IV isotherm, characteristic of mesoporous materials according to the BDDT classification.43 When the pH value of the solution is lower than 2, the adsorption branch of the isotherm is steep and a H1 type hysteresis loop is observed. The pore size distribution is narrow and centered on about 3.3 nm (Figure 2, Aa,b and Ba,b). By contrast for pH values higher than 2 the relative pressure at which the capillary condensation occurs is spread out over a larger domain of relative pressure and the shape of the hysteresis loop is slightly modified. In this case a hysteresis loop similar to a H2 type in which the desorption branch is steep but the adsorption branch is more or less sloping is observed (Figure 2, Ac,d and Bc,d). This change can be related to the absence of pore ordering. Indeed, the H2 type hysteresis loop is often encountered for disordered materials. In addition the pore size distributions become broader (insert of Figure 2, Ac,d and Bc,d). Figure 3 depicts the variation of specific surface area of the materials as a function of the pH of

Zimny et al. the solution. Whatever the acid and the pH value, the specific surface area is rather high (>600 m2/g). However, the lower pH values give the highest surface area, for example, using hydrochloric acid the specific surface area drops from 1130 to 800 m2/g when the pH is varied from 1.5 to 2. Also in the case of silica xerogels, a decrease of specific surface area with increasing pH of the synthesis mixture is a general trend.44 The increase of the pore diameter and the broadening of the pore size distribution upon the increase of pH is in accordance with results published in the literature.45,46 For example, with polyoxyethylene alkyl ether as surfactants, Su et al.45 have evidenced that the mean pore diameter of the mesoporous materials increased from 4.9 to 6.2 nm when the pH of the solution is changed from 2.0 to 10.0. The increase of the pore diameter can be seen as a consequence of a change in the hydration of the surfactant headgroup. As a matter of fact, if the pH value is increased, the hydrogen-bonded water molecules with the oxyethylene units become weaker and consequently the conformation of the hydrophilic chain of the surfactant is modified. Nevertheless, this cannot explain the high value of pore diameter and the broader size distribution observed above pH 2 and this later phenomenon should be rather related to dissolution-reprecipitation processes of silica. Indeed, it was reported by Brinker et al.47 that the dissolution rate of silica increases by 3 orders of magnitude when the pH of the reaction mixture is changed from 3 to 8. Thus, the silica undergoes many rearrangements. The redeposition of the silica could give rise to a pure silica inorganic phase that is not in contact with the surfactant and which is responsible of both the loss of the pore ordering and the larger pore diameters. However, these phenomena cannot explain why using HCl rather than H2SO4 leads to a better channel arrangement at low pH. To answer this question we have to consider the effect of the counterions on the synthesis of mesostructured silica materials. At low pH value, the mesoporous silica organized by nonionic surfactant (S) occurs through an (S0H+) (X-I+) hydrogen bonding pathway. I is the silica precursor and X- is the counterion.48 Even if Xis not present in the final mesoporous material, it plays an important role in the formation of the surfactant mesophase. For example, in a paper dealing with the effect of counterion in acid synthesis of mesoporous materials, Lin et al.49 have demonstrated that the dependence of structure quality and morphology on the counterion concentration can be understood as a consequence of counterion binding. It is also reported48 that the type of acid used determines the time required for silica mesophase precipitation. The relative times followed the sequence HBr ≈ HCl < HI < HNO3 < H2SO4 < H3PO4, which corresponds to the inverse Hofmeister series. So, that is why in the conditions used in this work the divalent sulfate leads to poor hexagonal order. With H2SO4, a longer time is needed before silica precipitation takes place. As a consequence, in the following studies, HCl will used as acid and the pH will be fixed at 1.3. 3.2. Effect of the Surfactant/Silica Ratio. Before performing this study we have taken a look at the influence of the time of the hydrothermal treatment on the structural properties of the materials. As regards Figure 4, we can see that the mesopore ordering is enhanced if the heating time at 80 °C is increased. Indeed, the secondary reflections 110 and 200 observed on the SAXS patterns of the materials recovered after a heating time longer than 2 days exhibit a better resolution and a higher intensity. Thus, to ensure a good mesopore ordering, the hydrothermal treatment has been performed during 4 days at 80 °C. Therefore this part of the study has been led for a



Figure 6. Nitrogen adsorption-desorption isotherms and the pore size distribution (insert) of the materials synthesized with a surfactant/TMOS molar ratio (R) equal to (a) 0.452, (b) 0.202, (c) 0.175, (d) 0.145, (e) 0.119, and (f) 0.097. Samples have been synthesized with 60 wt % of RF8(EO)9 at pH 1.3 by using HCl as acid. The hydrothermal treatment has been performed during 4 days at 80 °C.

Figure 7. SAXS patterns of samples synthesized with a concentration (wt %) of RF8(EO)9 of (a) 50, (b) 55, (c) 60, (d) 65, (e) 70, (f) 71, and (g) 75%. Samples have been synthesized at pH 1.3 by using HCl as acid. The surfactant/TMOS molar ratio is equal to 0.145. The hydrothermal treatment has been performed during 4 days at 80 °C.

RF8(EO)9 concentration equal to 60 wt %. Samples have been synthesized at pH 1.3 by using HCl as acid and the hydrothermal treatment was performed at 80 °C during 4 days. Among the investigated molar ratios of surfactant/silica (R), the mesopore ordering is detected only for ratios located in the

range between 0.119 and 0.175. Indeed, the 3 reflexions, characteristics of the hexagonal structure, are evidenced on Figure 5Ab,c,d. The pore ordering is also noted by TEM (Figure 5B). The SAXS patterns of the material prepared with a molar ratio comprised between 0.202 and 0.228 exhibit only a single broad reflection (Figure 5Ae,f), which indicates the formation of a disordered structure. If R is further increased no line is observed on the SAXS pattern (Figure 5Ag,h), indicating that the recovered compounds exhibit a complete randomly oriented pore structure. A similar behavior is noted when the RF8(EO)9/ TMOS molar ratio is decreased (Figure 5Aa). Nitrogen adsorption-desorption isotherms and the corresponding BJH pore size distributions (insert of Figure 6), obtained from an analysis of the adsorption branch of the isotherm, are shown in Figure 6. Whatever the surfactant/silica molar ratio, a type IV isotherm, characteristic of mesoporous materials, is obtained and the pore diameter remains almost constant at about 3.3 nm. However, when the transition from a hexagonal to a disordered channel array occurs, the relative pressure at which the capillary condensation takes place is spread out over a larger range of relative pressures and the pore size distribution becomes broader (Figure 6a,b). It should be remembered that the hexagonal liquid crystal phase is composed of infinite cylinders packed in a hexagonal array and in the case of direct systems, cylinders are filled by the hydrophobic chains and are covered by both head groups and water. Thus, we can assume that when the surfactant/ TMOS molar ratio exceeds 0.175 the quantity of silica added is not sufficient to interact with all the cylinders. By contrast, once the R value is lower than 0.119 the formation of a disordered structure results from a precipitation of the excess silica as a nonbulk templated phase. This observation is in good agreement with the results reported concerning the influence

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Figure 8. TEM micrographs of samples synthesized with 70 (A) and 60 wt % of RF8(EO)9 (B). Samples have been synthesized at pH 1.3 by using HCl as acid. The surfactant/TMOS molar ratio is equal to 0.145. The hydrothermal treatment has been performed during 4 days at 80 °C.

Figure 10. Relation between the structural parameters of the mesoporous materials and those of the H1 liquid crystal phase: (9) cell parameter and (2) hydrophobic diameter of H1; (3) material pore diameter and (O) mesopore wall thickness.

Figure 9. Nitrogen adsorption-desorption isotherms (A) and pore size distribution (B) with various RF8(EO)9 concentrations. Samples have been synthesized at pH 1.3 by using HCl as acid. The surfactant/TMOS molar ratio is equal to 0.145. The hydrothermal treatment has been performed during 4 days at 80 °C.

of the surfactant/silica ratio on the preparation of MCM-41 through the self-assembly mechanism.50,51 For instance, Ekloff et al.50 attributed the poor hexagonal long-range order of their particles obtained at surfactant/silica ratio higher than 0.66 to the polymerization of the silica source into solid amorphous silica due to the excess of surfactant. In the present study similar arguments can be taken into account to explain the transition from a well-ordered mesopore ordering to a randomly oriented pore structure when the value of R is varied from 0.119 to 0.097. In this case the hexagonal liquid crystal phase is diluted into an amorphous silica matrix. In general, the high intensity of the reflection lines means high crystallinity for mesoporous material. As a consequence a RF8(EO)9/TMOS ratio of 0.14 was

selected as optimum to investigate the influence of the concentration of surfactant. 3.3. Variation of the RF8(EO)9 Concentration. Looking at the phase diagram of the RF8(EO)9/water system,41 we can see that at 20 °C, the domain of the hexagonal liquid crystal phase goes from 53 to 78 wt % of surfactant. So mesoporous materials have been synthesized in this range of concentration. The pH was fixed to 1.3 by using HCl as the acid. The RF8(EO)9 molar ratio was equal to 0.145 and the hydrothermal treatment has been performed during 4 days at 80 °C. Figure 7 depicts the variation of the SAXS pattern with the surfactant weight percentage in the aqueous solution. While a disordered structure is formed when the materials are prepared with a surfactant weight percent equal to 50 (Figure 7a) and 75 (Figure 7g), the hexagonal pore ordering is recovered for RF8(EO)9 concentration between 55 and 70 wt % (Figure 7b-f). In addition, except for 75 wt % of RF8(EO)9, which corresponds to the limit of the H1 phase, the better resolution and the highest intensity of the second reflection are observed on the SAXS pattern of the material prepared with high concentrations of RF8(EO)9 (>60 wt %). We can conclude that these materials exhibit better pore ordering. The hexagonal arrangement of the channels is further confirmed by TEM micrographs of different samples (Figure 8). Indeed, either the honeycomb-like arrangement (Figure 8a) or the hexagonal stacking of the channels is evidenced by the TEM analysis. The unit cell a0 varies from 5.4 to 6.1 nm with the increase of the water content in the solution. The wall thickness is deduced by subtracting the pore size determined by the BJH method (see below) from the dimension of the unit cell. Its value varies from 2 to 1 nm when



Figure 11. Scheme illustrated the variation of the pore size with the increase of water content.

the RF8(EO)9 weight percent decreases from 70 to 50. The slight increase in the a0 value associated with the decrease in wall thickness, observed when the water content is raised, indicates the enlargement of the pore size with the decrease of the RF8(EO)9 concentration. Figure 9 shows the nitrogen adsorption-desorption isotherms (Figure 9A) and the pore size distribution (Figure 9B) of the material obtained with different RF8(EO)9 concentrations. A IV-type nitrogen isotherm (Figure 9A), characteristic of mesoporous compounds according to the BDDT classification, is obtained. With an increase in the water content, i.e., decreasing the surfactant concentration in the mixture, the relative pressure for which capillary condensation takes place is shifted toward higher values. Since the p/p0 position of the inflection point is related to the pore diameter, it can be inferred that an enlargement of the mean pore diameter occurs when the water content is raised. This increase in pore diameter is further confirmed by the pore size distribution (Figure 9B), whose maximum is shifted from 2.8 to 4.3 nm when the RF8(EO)9 concentration is progressively decreased from 70 to 50 wt % (Figure 9B). Moreover from Figure 9B it also can be noted that for materials prepared with low water content, the pore size distribution is rather narrow and it becomes broader with further addition of water. Materials are prepared through the liquid crystal mechanism so the hexagonal liquid crystal is the building block to synthesize the mesostructured silica. To shed some light on the relation between the properties of the recovered mesoporous material and those of the fingerprint used for its formation we have evaluated the structural parameters of the H1 phase. The distance

d associated with the first peak is related to the hydrophobic radius RH by the relation:52

VTA

√3πRH2 VB ) + RVE 2d2

where R stands for the number of water molecules per surfactant molecule and VB, VTA, and VE respectively stand for the molar volumes of the hydrophobic part of the surfactant (VB ) 261 cm3/mol), the surfactant (VTA ) 626 cm3/mol), and water (VE ) 18 cm3/mol). The hydrophobic radius (RH), which provides direct information about the conformation of the hydrophobic chain, increases from 1.45 to 1.60 nm when the surfactant concentration is changed from 50 to 75 wt %. As the length of an extended chain with 10 carbon atoms is about 1.4 nm, we can assume that hydrophobic chains adopt an extended conformation. The values of RH are slightly higher than 1.4 nm; as the surfactant is composed of fluorinated chains with different lengths this can be due to the contribution of the longer tails. In addition we can see from Figure 10 that for surfactant 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. For the lower surfactant contents the value of the pore diameter is higher than two times RH. Moreover we can also note from Figure 10 that the increase of the pore diameter with the water content is accompanied by a decrease of the wall thickness. This phenomenon can be explained according to the scheme

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represented in Figure 11. As indicated above, in the case of direct systems (H1), cylinders of the hexagonal liquid crystal phase are filled by the hydrophobic chains and are covered by both head groups and water. The thickness of the aqueous film is strongly related to the water content (step 1). For example, for a concentration of RF8(EO)9 equal to 70 wt % the aqueous film is rather thin and the cylinders of surfactant are close to each other, whereas they are more isolated for 50% of RF8(EO)9. When the silica precursor is added, its condensation around the surfactant cylinders occurs and hydrogen-bonding interactions between the oxygen atoms of the oxyethylene groups of the surfactant and OH groups of the hydrolyzed TMOS are formed (step 2). Then the polymerization of the tetramethoxysilane is completed during the hydrothermal treatment (step 3). In the case of surfactant-rich systems (Figure 11A), the intercylinder condensation of silica is favored. As a consequence the pore diameter of the obtained mesoporous materials is close to the hydrophobic diameter of the liquid crystal phase, used as the building blocks. By contrast, if the water content is increased (Figure 11B) as the rods of surfactant are more separated, the intercylinder condensation of silica is more difficult and to ensure the formation of the silica framework, the expansion of the cylinders should take place. As a consequence the degree of the mesopore ordering is lower, as shown by the SAXS analysis, which reveals a lower intensity of the 110 and 200 reflections. Moreover, as the expansion of the surfactant cylinders is made without any reorganization of the liquid crystal phase, a larger amount of silica is needed to cover each rod. This involves a decrease of the wall thickness of the prepared mesoporous silica (Figure 10). 4. Conclusions Mesoporous materials have been prepared by using the liquid crystal phase of the C8F17C2H4(OC2H4)9OH/water system as building blocks. During the hydrolysis of the silica precursor methanol is released. As this alcohol can act as a phase breaker it has to be removed during the synthesis. SAXS analysis shows that the mesopore ordering strongly depends on the pH and on the acid used during the preparation. Indeed whatever the acid used, hexagonal mesostructure are recovered at pH 1.3. Nevertheless, a better pore ordering is obtained by using HCl instead of H2SO4. The difference in behavior has been ascribed to the counterion effect. In both cases, above pH 2 disordered structures are obtained and the pore size distribution becomes broader. These phenomena have been related to dissolutionreprecipitation processes of silica. The optimum surfactant/silica ratio is found to be in the range from 0.119 to 0.175. When the surfactant/TMOS molar ratio exceeds 0.175 the quantity of silica added is not sufficient enough to interact with all the cylinders of the H1 phase. By contrast, at low ratios, the hexagonal liquid crystal phase is diluted into an amorphous silica matrix. As a consequence a disordered structure is formed, which results from a precipitation of the excess silica as a nonbulk templated phase. Finally, the structural parameters of the mesoporous materials have been correlated to those of the liquid crystal phase used as the fingerprint for their synthesis. For surfactant 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.

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JP9019409

Ordered Mesoporous Silica Templated by Nonionic Fluorinated Liquid

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