Nonionic Fluorinated−Hydrogenated Surfactants for the Design of

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Nonionic Fluorinated-Hydrogenated Surfactants for the Design of Mesoporous Silica Materials F. Michaux, J. L. Blin, and M. J. Ste´be´* Equipe Physico-chimie des Colloı¨des, UMR SRSMC N° 7565 UniVersite´ Henri Poincare´ - Nancy 1/CNRS, Faculte´ des Sciences, BP 239, F-54506 VandoeuVre-les-Nancy cedex ReceiVed: April 23, 2008; ReVised Manuscript ReceiVed: June 10, 2008

We have investigated the influence of the ratio between the volume of the hydrophilic head (VA) and the volume of the hydrophobic part (VB) of the surfactant on the mesopore ordering. To understand the difference of behavior we have performed a complete study dealing with fluorinated [RFm(EO)n] and hydrogenated [RHm(EO)n] surfactants. Their mixtures have also been taken into account. Here only the phase diagrams and the structural parameters of the liquid crystal phases of the mixed systems are reported. We have shown that the mutual or partial miscibility of the fluorinated and the hydrogenated surfactants depends on the number of oxyethylene units of each surfactant. To follow, various systems were used for the preparation of silica mesoporous materials Via a cooperative templating mechanism (CTM). Results clearly reveal that VA/VB ratios in the range between 0.95 and 1.78 lead to the formation of well-ordered mesostructures. Wormhole-like structures are obtained for higher or lower values. Moreover, results show that from the VA/VB point of view, polyoxyethylene fluoroalkyl ether surfactants behave like their hydrogenated analogues. 1. Introduction Nonionic surfactants are used in various domains as emulsifiers, wetting agents, detergents, or solubilizers. In addition micellar solutions of hydrogenated1–5 or fluorinated6–10 surfactants as well as of their mixtures11–15 can be used to prepare mesoporous materials through a cooperative templating mechanism (CTM).16–18 In such a mechanism, the interactions between the surfactant and the inorganic precursor are responsible for the mesoporous materials formation. 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. For example, hexagonal molecular sieves have been obtained with C8F17C2H4(OC2H4)9OH6 [RF8(EO)9], the surfactant concentration in aqueous solution being in a wide range of around 5-25 wt %. In contrast, in the same conditions of synthesissi.e., pH, surfactant/silica molar ratio, hydrothermal temperature and duration - only wormhole-like structures are recovered with C7F15C2H4(OC2H4)8OH19 [RF7(EO)8]. The difference in behavior has been ascribed to the position of the lower consolute boundary and we have evidenced that 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 materials20 is. Indeed, 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 mesostructures 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. Zhao et al.21 have reported the synthesis of silica mesostructures by using the triblock copolymer P85 (EO26PO39EO20) and P65 (E20PO30EO20) as * Corresponding author. Telephone: +33-3-83-68-43-43. Fax: +33-383-68-43-22. E-mail: [email protected].

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. Nevertheless, for some surfactant-based systems such as that of C6F13C2H4(OC2H4)5OH [R6F(EO)5],20 even if the lower consolute curve is shifted toward high temperature, no ordered mesostructure can be recovered. This reflects the fact the pore ordering is also affected by another feature of the surfactant. In a study concerning the improvement of mesoporous silica materials templated from semifluorinated nonionic surfactants with addition of organic additives, Xiao et al.22 consider the variation of the ratio between the volume of the hydrophilic headgroup (VA) and the hydrophobic part (VB) of the surfactant in order to explain the transition from a wormhole-like to a hexagonal mesostructure when trimethylbenzene (TMB) is added. This concept was first introduced by Stucky et al.23 for a series of hydrogenated surfactants. In a paper dealing with the structural design of mesoporous silica by micelle-packing control using blends of amphiphiles [CmH2m+1(OCH2CH2)nOH), m ) 12-18, n ) 2-100], these authors have established a correlation between the final structure and the VA/VB ratio. According to this paper, with a fixed surfactant concentration of 4 wt %, VA/VB ratios between 0.5 and 1.0 result in the formation of a lamellar mesostructure, values of 1.0-1.7 yield 2-D hexagonal mesostructures, whereas 3-D hexagonal and cubic mesostructures can be obtained in the range of 1.2-2.0. Thus, concerning the CmH2m+1(OCH2CH2)nOH series, for a given value of m, when the number of oxyethylene units is increased the transition from lamellar to hexagonal and then from hexagonal to cubic structure of the materials occurs. However, looking at the study reported by Esquena et al.11 it appears that this scale can not be systemati-

10.1021/jp8035378 CCC: $40.75  2008 American Chemical Society Published on Web 08/29/2008

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Figure 1. Composition phase diagram (wt %) of the RF8 (EO)9/RH18-1(EO)10/water (A), the RF6 (EO)11/RH18-1(EO)10/water (B), the RF7 (EO)8/RH18-1(EO)10/ H water (C), and the RF6 (EO)5/R18-1 (EO)10/water (D) systems.

TABLE 1: Molar Volume Of The Surfactants

surfactants RF6 (EO)11 RF8 (EO)9 RF7 (EO)8 RF6 (EO)5 H R18-1 (EO)10 RH12(EO)8 RH12(EO)12

molar volume of the surfactant (cm3 · mol-1)

molar volume of the hydrophobic part VB (cm3 · mol-1)

730 626 555 780 774 535 703

253 261 248 253 359 208 208

cally extended to the fluorinated surfactant-based systems. As a matter of fact, these authors have investigated the formation of mesostructured silica using CF3(CF2)7SO2-(C3H7)N-(C2H4O)nH, the value of n was varied from 6 to 20. Referring to the scale established by Stucky et al.,23 lamellar, hexagonal and cubic structures ought to be obtained when the number of oxyethylene units is increased. However, mesoporous silica with a hexagonal channel array are formed only with n ) 10; other values of n lead to disordered structure. In the present paper, we have investigated in detail the effect of the VA/VB ratio on the structure of mesoporous materials prepared from both fluorinated nonionic surfactants and mixed surfactant-based systems. Each surfactant has been selected according to various criteria. First, in regard to the preparation of mesoporous silica it has to form micelles in water. Second,

the VA/VB parameter has been varied by changing either the hydrophilic (VA) or the hydrophobic (VB) volume, or it has been kept constant by modifying the surfactant nature i.e. fluorinated or hydrogenated. To understand the effect of the modification of the VA/VB ratio on the surfactant phase behavior, we have determined the phase diagram of each surfactant system as well as the structural parameters of the liquid crystal phases. Mesoporous materials have been prepared from the various surfactant-based systems in order to continuously vary the VA/ VB ratio. The selected fluorinated surfactants belong to the polyoxyethylene fluoroalkyl ether family [CmF2m+1C2H4(OC2H4)nOH, labeled as RFm(EO)n]. The surfactant blends have been realized by mixing the fluorinated surfactants with a hydrogenated one [RmH(EO)n]. For the sake of clarity, we will H only describe in detail the RFm(EO)n/R18-1 (EO)10/water systems and present the complete characterization of the mesoporous silica materials synthesized from these surfactant-based systems. 2. Materials and Methods The used fluorinated surfactants, which were provided by DuPont, have an average chemical structure of C8F17C2H4(OC2H4)9OH, C7F15C2H4(OC2H4)8OH, C6F13C2H4(OC2H4)5OH, and C6F13C2H4(OC2H4)11OH. They are respectively labeled RF8(EO)9, RF7(EO)8, RF6(EO)5 and RF6(EO)11. RF8(EO)9 and RF7(EO)8 were used as received, whereas RF6 (EO)5 and RF6 (EO)11 have been employed after solvent removal. The hydrogenated sur-

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H Figure 2. Structural parameters of H1 of the RF8 (EO)9/R18-1 (EO)10/ H water (A) and of the RF6 (EO)11/R18-1 (EO)10/water (B) systems as a function of XF. Key: 0, d; O, RH; ∆, S.

H factant C18H35(OC2H4)10 [Brij 97, R18-1 (EO)10] was purchased H from Aldrich. C12H25(OC2H4)8 [R12(EO)8] and C12H25(OC2H4)12 [RH12(EO)12] were provided by Huntsman. In all cases, the hydrophilic chain moiety exhibited a Gaussian chain length distribution. Phase Diagram Determination. The phase diagrams have been established at 20 and 40 °C over the whole range of surfactant/water compositions. The samples were prepared by weighing the required amount of surfactants and water in wellclosed glass vials to avoid evaporation. They were placed in a thermostatic bath for several hours in order to reach the equilibrium. The various phases have been identified by visual observation and with a polarizing light microscope for the liquid crystals. The boundaries lines of the liquid crystal domains were revealed by small angle X-ray scattering (SAXS) experiments. Mesoporous Preparation. In a typical synthesis, a micellar solution of surfactant containing 10 wt % in water was prepared. The weight fraction of RFm(EO)n in the surfactant mixture (XF), was varied from 0 to 1. The pH value of the solution was kept at 7. TMOS was added drop by drop at 40 °C into the micellar solution while stirring. The surfactant/TMOS molar ratio was adjusted to 0.5. The obtained gels were sealed in Teflon autoclaves and heated for 1 day at 80 °C. The surfactant was removed by ethanol extraction using a Soxhlet apparatus for 48 h. The final product was recovered after drying at room temperature. Characterization. SAXS 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

Michaux et al.

H Figure 3. Structural parameters of HH1 of the RF7 (EO)8/R18-1 (EO)10/ H water (A) and of the RF6 (EO)5/R18-1 (EO)10/water (B) systems as a function of R. Key: 0, d; O, RH; ∆, S.

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 outgassed under vacuum for several hours at 320 °C before nitrogen adsorption measurements were taken. The pore diameter and the pore size distribution were determined by the BJH (Barret, Joyner, Halenda) method.24 3. Results and Discussion 3.1. The Phase Behavior of the Surfactant Mixtures. The RF8(EO)9/RH18-1(EO)10/water and the RF6(EO)11/RH18-1(EO)10/water systems exhibit a similar phase diagram. Whatever the proportion of the fluorinated surfactants in the mixture, at 20 °C (Figure 1, parts A and B) for high loading of water a micellar solution H (L1) is observed. For example, for the RF8 (EO)9/R18-1 (EO)10/ water system the micellar domain is unbroken between the limits of the micellar solutions shown in each binary phase diagram; i.e. from 60 wt % of water for the fluorinated system and from 70 wt % of water for the hydrogenated one (Figure 1A). Moreover, for high concentrations of surfactant, whatever the ratio between the hydrogenated and the fluorinated surfactants only one hexagonal liquid crystal phase composed of mixed infinite cylinders packed in a hexagonal array is formed. This observation is in accordance with the existence of real mixed liquid crystals.25,26 Decreasing the number of the oxyethylene groups to 8, a miscibility gap appears. Indeed, for a total concentration of

Mesoporous Silica Materials

Figure 4. RF7(EO)8/RH18-1(EO)10/water system: evolution of the structural parameters of LFR (A) and of LHR (B). Key: 9, d; b, dB; 2, S.

surfactant higher than 18 wt %, a biphasic region is evidenced H for high amounts of R18-1 (EO)10 (Figure 1C). In addition, the liquid crystal domains are no longer continuous and are divided into three parts; two hydrocarbon-rich phases (a hexagonal HH1 and a lamellar LHR), which can accommodate up to 20 wt % of RF7 (EO)8 and one fluorocarbon-rich liquid crystal phase (a lamellar LFR), which can incorporate up to 23 wt % of H R18-1 (EO)10. Finally with the system having 5 EO groups (except the small biphasic domain detected in the fluorocarbon rich part of the diagram, which is due to the lower consolute boundary (lcb) of the fluorinated binary system20) a direct micellar phase L1 is observed (Figure 1D). The presence of the micellar phase is due to the shift of the lcb curve by the addition of the hydrogenated surfactant. As a matter of fact, the cloud point is H increased up to 20 °C when 1 wt % of R18-1 (EO)10 is added to F the R6(EO)5/water solution. The liquid crystal domains deal only with the hydrogenated-rich part of the diagram. The LHR and HH1 can accommodate 30 and 40 wt % of RF6(EO)5 respectively. H In comparison with the RF7 (EO)8/R18-1 (EO)10/water system, for F H that of R6 (EO)5/R18-1(EO)10/water, it should be noted that a higher amount of fluorinated surfactant can be incorporated into the hydrocarbon-rich lamellar and hexagonal phases. This difference in behavior can be related to the length of the fluorocarbon chains. Increasing the number of fluorinated carbon makes the surfactant more oleophobic and consequently the mutual solubility between the hydrogenated and fluorinated compounds is decreased. Generally speaking, for all the systems investigated, at 40 °C the same phase behavior is observed as at 20 °C, but the

J. Phys. Chem. B, Vol. 112, No. 38, 2008 11953 liquid crystal domains are progressively reduced as the temperature increases. At 40 °C, for the RF7(EO)8/RH18-1(EO)10/water systemsdue to the presence of the cloud point curve of the fluorinated systemsthe micellar region splits into two phases at high levels of RF7 (EO)8. But, since the addition of the hydrogenated surfactant increases the demixion temperature, as H soon as 1 wt % of R18-1 (EO)10 is added, the domain becomes monophasic. The results depicted above show that the miscibility of fluorinated and hydrogenated surfactants strongly depends on the size of the hydrophilic parts of both surfactants. If the number of oxyethylene units is almost the same, as for the RF8(EO)9/RH18-1(EO)10/water system, the mixing between the two surfactants is good and all proportions of mixed phases are formed. Decreasing or increasing the number of the (EO) groups of one of the two surfactants leads to a decrease in miscibility. This observation is in accordance with results obtained by others groups. Indeed, it is reported that the mutual oleophoby of fluorinated surfactants is attenuated when the hydrophilic chains are symmetrical and in this case an ideal mixing behavior is noted.25 3.2. Structural Parameters. The structural parameters of the hexagonal and the lamellar liquid crystal phases have been determined by SAXS experiments. To perform this study, the samples have been prepared in two different manners. If the hydrogenated and the fluorinated surfactants are completely miscible, the water amount has been kept constant and the weight fraction of RmF (EO)n in the surfactant mixture, noted XF, has been varied from 0 to 1. By contrast, if one fluorocarbonrich or one hydrocarbon-rich liquid crystal phase is obtained, the structural parameters have been evaluated for different water/ H R18-1 (EO)10 and/or water/RmF (EO)n ratios and the effect of the second surfactant addition has been observed. The results are reported in order to study the influence of the water content in the liquid crystal phases. Assuming that the mixture of surfactants forms a mixed entity, we can study these systems as binary surfactant/water systems. Hexagonal Phase (H1). The hexagonal phase can be described as a stack of infinite cylinders packed in a hexagonal array. Concerning direct systems, the center of the cylinders is made by the surfactant hydrophobic tails and the polar heads in contact with water located around the cylinders. The hexagonal H1 phases are characterized by their typical SAXS pattern with the relative peak positions 1,
3, 2. The distance d associated to the first peak is related to the hydrophobic radius RH by the following relation27

√3πRH2 VB ) VTA + RVE 2d2 where R corresponds to the number of water molecules per surfactant molecule and VB, VS, VW stand respectively for the molar volumes of the hydrophobic part of the mixed surfactant, of the surfactant, and of water (VW ) 18 cm3/mol). The values of VB, VS of the pure surfactants are given in Table 1. The cross sectional area S can be deduced from the following relation

S)

2VB NRH

where N is Avogadro’s number. H For XF ) 0, i.e. for the R18-1 (EO)10 water system, the values of d, RH, and S are respectively equal to 6.4 nm, 2.0 nm, and 0.6 nm2. These results agree with those reported in the literature.28–30

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H Figure 5. Mesoporous materials: evolution of the SAXS pattern as a function of XF. Materials are prepared from (A) the RF8 (EO)9/R18-1 (EO)10/ F H water system, XF ) 0 (a), 0.25 (b), 0.50 (c), 0.75 (d), 1 (e); (B) the R6 (EO)11/R18-1(EO)10/water system, XF ) 0 (a), 0.25 (b), 0.50 (c), 0.75 (d), 1 H H (e); (C) the RF7 (EO)8/R18-1 (EO)10/water system, XF ) 0 (a), 0.50 (b), 0.80 (c), 0.90 (d), 1 (e); (D) the RF6 (EO)5/R18-1 (EO)10/water system; XF ) 0 (a), 0.50 (b), 0.80 (c), 0.90 (d), 1 (e).

Concerning the RF8(EO)9/RH18-1(EO)10/water and the RF6(EO)11/ systems, for a given content of water, parts A and B of Figure 2 display the variation of d, RH, and S as a function of XF. From these figures you can see that the hydrophobic radius and the cross-sectional area decrease in a linear manner with the increase of XF. As an example for the RF8 (EO)9/RH18-1(EO)10/water system (Figure 2A), RH and S vary from 2.0 to 1.7 nm and from 0.60 to 0.53 nm2 when XF is changed from 0 to 1. For both systems the linear relationship between these parameters and the weight fraction of fluorinated surfactant is in a good agreement with the formation of a real mixed hexagonal phase. Looking at the two others systems, only one hydrocarbonrich HH1 is detected. Figure 3A and 3B describe the evolution of the structural parameters as a function of R, the number of water molecules per surfactant molecule. We can note an increase in the d-spacing due to the hydratation of the headgroup and water may also form a film surrounding the surfactant. In addition, in spite of the variation of XF, RH and S remain constant in the overall HH1 domain and their values (RH ) 2.0 nm; S ) 0.60 nm2) are close to the ones determined for the hexagonal H phases in the R18-1 (EO)10/water system. The hydrophobic radius provides direct information about the conformation of the hydrophobic chain. By comparing the RH value with the length of an extended chain with the same number of carbon atoms for all the investigated mixed systems, we can deduce that the hydrophobic chains of the fluorinated surfactant in the H1 liquid crystal phase are completely extended. Lamellar Phase (Lr). The lamellar phase can be described as an infinite bilayer of surfactants stacked in a parallel manner. The diffraction pattern exhibits two reflections with the relative peak positions 1, 2. The repetition distance corresponds to the layer spacing, which comprises the water separated by the surfactant bilayer. The cross-sectional area can be calculated from the following formula:27

H R18-1 (EO)10/water

S)

2(VS + RVW) Nd001

where d001 stands for the repetition distance. For both fluorocarbon-rich, LFR (Figure 4A) and hydrocarbonrich, LHR (Figure 4B) liquid crystal phases detected in the

H RF7 (EO)8/R18-1 (EO)10/water system, we can note an increase in the d001 values. The values of the cross-sectional area remain constant in both phases and are equal to 0.50 and 0.55 nm2 respectively for LFR and LHR. Since the value of S is unchanged, the hydrophobic thickness (dB) is inferred by the equation:

dB )

2VB S

dB remains constant to 1.7 and 2.1 nm respectively in the overall LFR and LHR domains. The same trend is observed for the H hydrocarbon-rich lamellar phase of the RF6 (EO)5/R18-1 (EO)10/ 2 water system (dB ) 2.4 nm; S ) 0.52 nm ). It appears that the addition of the fluorinated surfactant does not involve any modification of structural parameters of the pure hydrocarbon lamellar phase. 3.3. Mesoporous Materials. One should be reminded that all the mesoporous materials have been prepared from a 10 wt % micellar solution of a surfactant mixture. The proportion of the fluorinated surfactant in the blend was varied from XF ) 0 to 1. Structural Features. For the compounds obtained from the H RF8 (EO)9/R18-1 (EO)10/water system, whatever the value of XF, in addition to a sharp peak, two other reflections are detected on the SAXS pattern (Figure 5Aa-e). The presence of these two last peaks is suggestive of a hexagonal organization of the channels. The position of the first peak slightly varies from 5.8 to 5.2 nm when the RF8 (EO)9 content in the blend is increased. According to Bragg’s rule, the unit cell dimension (a0)swhich is the sum of the pore diameter and the thickness of the pore wallscan be deduced, and its value varies from 6.7 to 6.0 nm when XF changes from 0 to 1. When the materials are prepared from the RF7(EO)8/RH18-1(EO)10/ water or the RF6 (EO)5/RH18-1(EO)10/water systems the hexagonal structure is maintained until XF ) 0.8 (Figure 5, parts Ca-c and Da-c). If the content of the fluorinated surfactant is further increased, no secondary reflections are detected any longer (Figure 5, parts Cd,e and Dd). Thus, the regular channel array is lost, and the presence of a single reflection indicates the formation of a disordered structure. In this case, the recovered mesoporous molecular sieves exhibit a wormhole-like channel system, analogous to MSU-type materials. The broad peak that

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Figure 6. Evolution of the adsorption-desorption isotherms and of the pore size distribution as a function of XF. Materials are prepared from (A) H H the RF8 (EO)9/R18-1 (EO)10/water system, XF ) 0 (a), 0.25 (b), 0.50 (c), 0.75 (d), 1 (e); (B) the RF6 (EO)11/R18-1 (EO)10/water system, XF ) 0 (a), 0.25 H H (b), 0.50 (c), 0.75 (d), 1 (e); (C) the RF7 (EO)8/R18-1 (EO)10/water system, XF ) 0 (a), 0.50 (b), 0.80 (c), 0.90 (d), 1 (e); (D) the RF6 (EO)5/R18-1 (EO)10/ water system; XF ) 0 (a), 0.50 (b), 0.80 (c), 0.90 (d), 1 (e).

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TABLE 2: Mesoporous Materialsa RF8 (EO)9

H R18-1 (EO)10 H R18-1 (EO)10

RF6 (EO)11

H R18-1 (EO)10

RF7 (EO)8

H R18-1 (EO)10

RF6 (EO)5

H R18-1 (EO)10

xF

VA/VB

structure

d100 (nm)

SBET (m2/g)

Vp (cm3/g)

φ (nm)

a (nm)

wall thickness (nm)

0 0.25 0.50 0.75 1 0.25 0.50 0.75 1 0.25 0.50 0.75 0.80 0.90 1 0.25 0.50 0.75 0.80 0.90 1

1.16 1.20 1.25 1.31 1.40 1.26 1.41 1.61 1.89 1.89 1.20 1.21 1.22 1.23 1.24 1.11 1.05 0.98 0.96 0.93 0.90

hexagonal hexagonal hexagonal hexagonal hexagonal hexagonal hexagonal wormlike wormlike wormlike hexagonal hexagonal hexagonal wormlike wormlike hexagonal hexagonal hexagonal hexagonal wormlike disordered

5.8 5.6 5.5 5.4 5.2 5.5 5.4 * * 5.6 5.5 5.4 5.3 * * 5.7 5.6 5.5 5.4 * *

1047 858 953 934 985 1048 877 1124 1124 1044 1043 908 967 960 989 1010 925 872 1016 966 740

1.20 0.87 1.29 1.30 1.30 0.92 0.78 0.75 1.09 1.28 1.12 0.92 1.15 1.22 1.22 1.18 0.75 0.86 1.27 1.51 0.33

4.0 4.4 4.3 4.5 4.1 2.5-3.8 2.5-4.2 2.7-3.4 3.2 4.0 4.0 4.0 4.0 4.3 3.9 4.0 4.3 4.1 4.0 3.8 700 m2/g, Table 2). 3.3. Discussion When mesoporous materials are prepared from a micellar solution, in the literature,35 it is now admitted that the interactions between the surfactant and the silica are the driving force for the formation of the materials. In the present study, the mesoporous materials have been synthesized from a surfactant solution containing 10 wt % in water and by varying the weight fraction of RFm(EO)n in the surfactant mixture. The analysis of the results obtained in this study reveals that whatever XF, the H CTM mechanism is not disturbed in the RF8 (EO)9/R18-1 (EO)10/ water system. In contrast, it does not occur for both the RF7(EO)8/ RH18-1(EO)10/water and the RF6 (EO)5/RH18-1(EO)10/water systems as long as XF is higher than 0.8 or for the RF6(EO)11/RH18-1(EO)10/ water if XF > 0.5. In order to explain the difference in behavior between the various surfactant mixtures we have to take into account different parameters. First, we have to look at the phase diagram. Indeed, in a paper dealing with the relation between the lower consolute boundary and the structure of mesoporous silica materials we have proved that the self-assembly mechanism is favored if the lower consolute boundary is 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 ordering35 is. When the synthesis of mesoporous materials is carried out in the RF7(EO)8/RH18-1(EO)10/water system, the formation of disordered structures for XF > 0.8 can unambiguously be related to the presence of the lcb curve. As an example, for a 10 wt % micellar solution of RF7 (EO)8, the demixion temperature is 42.5 °C, i.e., 2.5 °C below the temperature at which TMOS is added. An increase in the phase separation temperature is noted with the incorporation of the hydrogenated surfactant. As a consequence the transition from a wormhole-like to a hexagonal structure occurs. Concerning the RF6(EO)11/water system, the situation is quite different. Even if no lcb curve is detected, no pore ordering is obtained until XF e 0.5. This reflects the fact that the pore ordering is also affected by another parameter of the surfactant. As mentioned

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TABLE 3: Structural Properties of the Materials Prepared with Various Systemsa X RF6 (EO)11 RH12(EO)8 0 0.25 0.50 0.75 1 RF8 (EO)9 RH12(EO)8 0 0.25 0.50 0.75 1 RF7 (EO)8 RH12(EO)8 0 0.25 0.50 0.75 0.80 0.90 1 RF6 (EO)11 RF8 (EO)9 0 0.25 0.50 0.75 1 RF6 (EO)11 RF7 (EO)8 0 0.25 0.50 0.75 1 RF6 (EO)11 RF6 (EO)5 0 0.50 0.80 0.90 1 RH12(EO)8 RH12(EO)12 0 0.50 0.70 0.80 0.90 1

Vp φ SBET VA/VB structure (m2/g) (cm3/g) (nm) 1.57 1.64 1.71 1.79 1.89 1.57 1.54 1.50 1.45 1.40 1.57 1.50 1.42 1.33 1.31 1.28 1.24 1.40 1.52 1.64 1.76 1.89 1.24 1.38 1.54 1.70 1.89 0.90 1.31 1.63 1.76 1.89 2.41 1.97 1.77 1.70 1.64 1.57

hexagonal hexagonal hexagonal hexagonal wormlike hexagonal hexagonal hexagonal hexagonal hexagonal hexagonal hexagonal hexagonal hexagonal wormlike wormlike wormlike hexagonal hexagonal hexagonal wormlike wormlike wormlike hexagonal hexagonal hexagonal wormlike disordered hexagonal hexagonal wormlike wormlike wormlike wormlike wormlike hexagonal hexagonal hexagonal

765 836 879 836 1124 768 945 1000 970 985 765 1050 1003 1035 997 1010 989 985 905 850 870 1124 989 872 1138 872 1124 740 966 750 947 1124 780 876 928 907 989 765

0.27 1.20 1.22 1.20 1.09 0.27 0.52 0.82 0.92 1.30 0.27 1.17 1.32 1.31 1.39 1.33 1.22 1.30 0.79 0.98 0.80 1.09 1.22 1.12 1.73 1.12 1.09 0.33 1.16 1.16 1.19 1.09 2.6 0.41 0.62 0.63 0.79 0.27

3.4 4.3 4.4 4.3 3.2 3.4 3.0 3.5 3.8 4.1 3.4 3.5 4.0 3.8 4.2 4.3 3.9 4.1 4.1 4.2 3.8 3.2 3.9 4.3 4.6 4.3 3.2