Effect of fluorocarbon addition on the structure and pore diameter of mesoporous materials prepared with a fluorinated surfactant

Microporous and Mesoporous Materials 87 (2005) 67–76 www.elsevier.com/locate/micromeso

Effect of fluorocarbon addition on the structure and pore diameter of mesoporous materials prepared with a fluorinated surfactant J.L. Blin *, M.J. Ste´be´ Equipe Physico-chimie des Colloı¨des, UMR SRSMC No. 7565, Universite´ Nancy-1/CNRS, Faculte´ des Sciences, BP 239, F-54506 Vandoeuvre-les-Nancy Cedex, France Received 6 April 2005; received in revised form 27 June 2005; accepted 27 June 2005 Available online 8 September 2005

Abstract Pore diameter of mesoporous materials has been tailored by adding fluorocarbons as organic auxiliaries. Characterization results show that perfluoroheptane and perfluorodecalin (PFD) can be effectively used as expander. The pore size is increased from 4.0 to 6.8 nm. However a transition from a well ordered hexagonal arrangement to a wormhole like structure is detected when the oil loading is higher than 2.5 wt.%. Neither the structure nor the pore sizes are influenced by the addition of perfluorooctane. To better understand the influence of these perfluorocarbons and to shed some light on the mechanism of pore size enlargement, we have studied the effect of this fluorocarbon addition on the surfactant phase behavior and the ternary RF8 ðEOÞ9 =oil=water systems have been investigated. Micellar phase (L1), direct hexagonal (H1), cubic (I1) and lamellar liquid crystals were identified. Phase behavior and SAXS measurements proved that only a weak fraction of oil can be incorporated into the hydrophobic core of micelles (L1). However, results also show that the swelling effect of H1 takes place with PFD, whereas the penetration effect occurs with perfluorooctane. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Mesoporous materials; Fluorocarbon; Synthesis; Characterization

1. Introduction Since the discovery of MCM materials by MobilÕs scientists in 1992 [1,2], numerous studies dealing with the preparation and characterization of these materials are reported in literature. Due to their properties, such as high specific surface area, uniform pore size distribution, these silica supports are in particular interest in the preparation of catalysts [3–5], drug delivery systems [6] and sensors [7,8]. The synthesis of these mesostructures consists in the polymerization of a silica source, the *

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22. E-mail address: [email protected] (J.L. Blin). 1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.06.041

tetramethoxysilane for instance in the presence of surfactant. Until now, a large series of ionic [1,2] and non ionic surfactant [9–12] have been used in the synthesis of ordered mesoporous silicas or non-silicas. Nonionic polyoxyethylene alkyl ether surfactants [Cm(EO)n] have attracted particular research attention in the synthesis of large pore mesoporous silicas [13–25], due to their interesting physico-chemical properties [26–29]. The substitution of hydrogen atoms by fluorine ones enhances the chemical and thermal stability of the surfactant [30]. As regards the synthesis of mesoporous molecular sieves, the main advantage of fluorinated surfactants compared to the hydrogenated ones is their high thermal stability. Replacing the hydrogenated surfactant by a fluorinated one, as the hydrothermal

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J.L. Blin, M.J. Ste´be´ / Microporous and Mesoporous Materials 87 (2005) 67–76

treatment can be performed at higher temperature, we could expect that the level of silica condensation would be enhanced and, thus, that the recovered materials would exhibit a higher hydrothermal stability. In order to synthesize silica with high hydrothermal stability, Xiao and coworkers [31] mixed an ionic fluorocarbon surfactant with a hydrocarbon triblock copolymer and this mixture was used as template to prepare mesoporous materials. By this way highly ordered mesoporous silicas with unusual hydrothermal stability have been prepared in strong acid media at high temperature. However, in the same paper the authors claimed that due to the rigidity and the strong hydrophobicity of the fluorocarbon chains, those fluorocarbon surfactants are not suitable as templates for the preparation of mesoporous materials. Nevertheless, taking advantage of our experience in the field of fluorocarbon surfactants [32,33], our group has reported the first example of mesoporous silica synthesized with only fluorinated surfactant [34]. Mesoporous materials with a hexagonal channel array are prepared at 80 °C via a CTM-type mechanism (cooperative templating mechanism) in a wide range of surfactant concentrations (5–20 wt.%). Decreasing the hydrothermal temperature leads to the formation of wormhole like structure. The recovered materials exhibits high specific surface area and uniform pore size distribution centered at around 4.5 nm. More recently Rankin and coworkers [35,36] and Antonietti and coworkers [37] also demonstrate that fluorocarbons surfactants are excellent candidates for the mesostructured silica preparation. Indeed using partially fluorinated octylpyridinium chloride as template, Rankin reported the first example of cationic fluorinated surfactant templating of an ordered porous ceramic material with 2D hexagonal structure, a specific surface area of 982 m2/g and a pore diameter of 2.6 nm. By mixing nonionic fluorocarbon surfactant and hydrocarbon block polymer Antionetti et al. have investigated the preparation of porous system with controlled pore size distribution. For some applications [3,6] of these mesoporous materials, control of the pore size is required. One way to tailor the pore diameter of mesostructured silica consists in incorporating swelling agents during the synthesis. Since fluorinated surfactants allow co-solubilization of water and perfluorocarbons [32,38,39], perfluoroheptane (C7F16), perfluorooctane (C8F18) and perfluorodecalin (C10F18, labeled PFD) have been employed as swelling agent in order to obtain large pore silica mesoporous materials. However, the addition of these oils will strongly affect the surfactant behavior in aqueous solution and, thus, the mesoporous structure. Therefore, in order to shed some light on the swelling mechanism of mesoporous materials, in a second part we have studied the effect of this fluorocarbon addition on the phase behavior and the ternary RF8 ðEOÞ9 =oil=water systems have been determined.

2. Materials and methods 2.1. Sample preparation 2.1.1. Mesoporous materials preparation Before the incorporation of oil, a micellar solution of RF8 ðEOÞ9 containing 10 wt.% in water was prepared at 40 °C. The concentration of oil, added to the micellar solution, was made to vary from 0 to 5 wt.%, which corresponds to a variation of the oil=RF8 ðEOÞ9 molar ratio from 0 to about 0.8, depending on the oil. The pH value of the solution was then adjusted to 2.0 using sulphuric acid. Tetramethoxysilane (TMOS), used as the silica source, was added dropwise. The surfactant/silica molar ratio was adjusted to 0.5. The obtained gel was 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 30 h. 2.1.2. Mesoporous materials characterization X-ray measurements were carried out using a homebuilt apparatus, equipped with a classical tube ˚ ). The X-ray beam was focused by means (k = 1.54 A 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 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 [40] using the adsorption branch of the isotherm. Although it is well known that this method gives an underestimated pore size and that some new methods have been developed [41], we use it here for the sake of simplicity and the use of this mathematical algorithm does not affect significantly our results as it is a systematic comparison. 2.1.3. Phase diagram determination The used fluorinated surfactant, which was provided by DuPont, had an average chemical structure of C8F17C2H4(OC2H4)9OH, labeled as RF8 ðEOÞ9 (or FSN100, its commercial name). The surfactant hydrophilic chain moiety exhibited a Gaussian chain length distribution. Perfluoroheptane (C7F16), perfluorooctane (C8F18) and perfluorodecalin (C10F18), which is a molecule with two saturated rings, were provided by Aldrich. The samples were prepared by weighting the required amounts of surfactant, oil and water in well-closed glass vials to avoid evaporation. They were left at controlled temperature for some hours in order to reach equilibrium. The phase diagram was established between 10 and 60 °C in the whole water-surfactant composition.

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Oil concentration was made to vary from 1 to 30 wt.%. Liquid crystal phase domains were identified by their texture observed with optical microscope equipped with cross-polarizers. In order to find the exact limits of these domains, additional SAXS measurements were also performed.

3. Results and discussion 3.1. Mesoporous materials 3.1.1. Perfluorooctane as swelling agent For the compounds obtained with C8F18 (Fig. 1A) as swelling agents, whatever the oil loading, in addition to a sharp peak at 5.4 nm, two peaks at 3.1 nm and 2.7 nm are detected on the SAXS pattern (Fig. 1Aa–g). The presence of these two last peaks is suggestive of a hexagonal organization of the channels. According to the BraggÕs 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 6.4 nm. The position of the first peak slightly varies from 5.4 to 5.6 nm when the oil loading is increased from 0.5 to 4.1 wt.%. The reflection lines become broader with the addition of C8F18, nevertheless the value of a0 remains almost constant with oil incorporation. From nitrogen adsorption–desorption measurements (Fig. 2A), we can observe that all the samples exhibit a type IV isotherm, characteristic of mesoporous materials. 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 4.3 nm.

A

B 6.4 nm

g

e

Intensity (a.u.)

f e d

d 5.6 nm

5.4 nm

c

3.2 nm

c 3.1 nm

2.8 nm 2.7 nm

b

b a 0.1

0.2

0.3

a 0.1

0.2

0.3

q (Å-1) Fig. 1. Mesoporous materials: SAXS patterns of samples synthesized with a: 0, b: 0.5, c: 1.2, d: 1.8, e: 2.5, f: 3.2, g: 4.1 wt.% of C8F18 (A) and with a: 0, b: 1.2, c: 2.0, d: 2.5, e: 3.0 wt.% of C7F16 (B).

69

This value, which is very close to the one obtained for the mesostructured silica prepared without addition of C8F18 (4.6 nm), does not vary when the oil content is increased (Fig. 3a). The specific surface area is rather high, superior to 1000 m2/g (Table 1). From these observations it is obvious that neither the channel arrangement nor the pore size diameter are really affected by the incorporation of perfluorooctane. No swelling of the mesopore occurs. 3.1.2. Perfluoroheptane and perfluorodecalin as swelling agent When perfluoroheptane is employed as swelling agent the situation is quite different. Indeed, as regards SAXS patterns materials obtained with a C7F16, we can see that the hexagonal structure is kept until a loading of oil equal to 2 wt.% (Fig. 1Ba–c). If the content of perfluorohepatane is further increased, no secondary reflections are detected any longer (Fig. 1Be). 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 is observed on the XRD pattern is an indication of the average pore-to-pore separation in the disordered wormhole framework, which presents a lack of longrange crystallographic order. In this case, the addition of oil disfavors the formation of the hexagonal structure. Whatever the weight percent of C7F16, a IV-type nitrogen isotherm (Fig. 2B), characteristic of mesoporous compounds according to the BDDT classification, is obtained by nitrogen adsorption–desorption analysis. With increasing the oil content, 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 loading of oil is raised. This increase in pore diameter is further evidenced by the pore size distribution (Fig. 2B), whose maximum is shifted from 4.6 to 6.4 nm (Fig. 3c) when the C7F16 content is progressively increased from 0 to 3 wt.%. We have recently shown [42] that perfluorodecalin (PFD) adopts similar behavior than perfluoroheptane (Table 1). For both oils a swelling of the mesopore is observed (Fig. 3b); the hexagonal channel array is not disturbed for low content of oil (2 wt.%), however an increase of the oil loading leads to the disorganization of the channel array and wormhole-like mesostructure is obtained. In order to explain the difference of behavior between C8F18 in the one hand and C7F16, C10F18 on the other hand and to shed some lights on the swelling mechanism we have investigated the RF8 ðEOÞ9 =oil= water ternary systems.

J.L. Blin, M.J. Ste´be´ / Microporous and Mesoporous Materials 87 (2005) 67–76

70

A

B 1000

800

800 600 600 400

400

200

e

200

e

1000

800

800

600

600

Volume adsorbed (cm3/g-STP)

400

400 200

200

d

d

1000

1000

800

800

600

600

400

400

200

c

200

1000

1000

800

800

600

600

400

400

200

b

200

1000

1000

800

800

600

600

400

400

200 0.0

a 0.2

0.4

0.6

0.8

c

b

200 0.0

a 0.2

0.4

0.6

0.8

Relative pressure p/p0 Fig. 2. Nitrogen adsorption–desorption isotherms and pore size distribution of samples synthesized with a: 0, b: 1.2, c: 2.5, d: 3.2, e: 4.1 wt.% of C8F18 (A) and with a: 0, b: 1.2, c: 2.0, d: 2.5, e: 3.0 wt.% of C7F16 (B).

3.2. Study of the RF8 ðEOÞ9 =fluorocarbon=water ternary systems In this paper we have investigated in detail the RF8 ðEOÞ9 =perfluorooctane=water system, results will be compared with those obtained previously [42], using perfluorodecalin as oil. 3.2.1. Ternary phase diagram Fig. 4 depicts the partial phase diagram of the ternary RF8 ðEOÞ9 =C8 F18 =water system at 20 °C (Fig. 4A) and at

40 °C (Fig. 4B). It is observed that the incorporation of C8F18 in the micellar phase L1 is limited to 1%. If the C8F18 loading is increased, a Winsor I-type biphasic system is formed. The upper phase is composed of the swollen micelles, whereas the C8F18 excess constitutes the lower phase. The liquid crystal phase domain is composed of cubic, hexagonal and lamellar phases. The existence range of these phases depends on the temperature, the surfactant/water ratio and oil loading. At 20 °C, in the RF8 ðEOÞ9 composition range from 38 to 48 wt.%, a cubic phase (I1) is formed between 3 and 12 wt.% of

J.L. Blin, M.J. Ste´be´ / Microporous and Mesoporous Materials 87 (2005) 67–76 8

71

C8F18

A

c

Pore diameter (nm)

b 0.25

6

0.25 V

L1

I1

H1

a Water

4

0.25

0.50

L2 RF8 (EO)9

0.75

C8F18

2 0

1

2

3

B

0.25

4

0.25 Lα

Oil content (wt.%) L1

Fig. 3. Variation of the pore diameter with the oil loading a: C8F18, b: PFD and c: C7F16. Water

I1 0.25

H1 0.50

V 0.75

L2 RF 8 (EO) 9

Table 1 Mesoporous materials: Structure, values of d-spacing (d), cell parameter (a0), specific surface area (SET) and pore diameter as a function of the oil loading

Fig. 4. RF8 ðEOÞ9 =C8 F18 =water ternary system: composition phase diagram (wt.%) at 20 °C (A) and 40 °C (B).

Oil

wt.%

Structure

d (nm)

a0 (nm)

SBET (m2/g)

Pore diameter (nm)

C8F18

0 0.5 1.2 1.8 2.5 3.2 4.1

Hexagonal Hexagonal Hexagonal Hexagonal Hexagonal Hexagonal Hexagonal

5.3 5.4 5.4 5.5 5.6 5.6 5.6

6.1 6.2 6.2 6.4 6.5 6.5 6.5

1044 1144 1122 1104 1063 1037 1076

4.6 4.1 4.3 4.2 4.5 4.1 4.2

Except the presence of the cubic phase (V), similar behavior is observed for the RF8 ðEOÞ9 = perfluorodecalin=water ternary system [42].

0 1.2 2 2.5 3

Hexagonal Hexagonal Hexagonal Wormhole Wormhole

5.3 5.3 5.7 6.1 6.4

6.1 6.1 6.6

1044 840 811 852 807

4.6 6.0 6.8 6.7 6.7

0 0.5 1.1 2.1 2.5 3.2 4.1

Hexagonal Hexagonal Hexagonal Hexagonal Wormhole Wormhole Wormhole

5.3 5.3 5.6 5.6 6.1 6.2 6.3

6.1 6.1 6.5 6.5

1044 978 865 889 871 858 833

4.6 5.0 5.4 5.8 6.0 6.3 6.4

C7F16

PFD (C10F18)

3.2.2. The H1 liquid crystal phase The distance d associated to the first peak is related to the hydrophobic radius RH by the relation: pffiffiffi 2 V B þ bV O 3pRH ¼ V S þ aV W þ bV O 2d where a and b respectively stand for the number of water and oil molecules per surfactant molecule, and VB, VS, VW, VO respectively stand for the molar volumes of the hydrophobic part of the surfactant (VB = 261 cm3/ mol), the surfactant (VS = 626 cm3/mol) water (VW = 18 cm3/mol) and oil values of VO are 237 and 247 cm3/mol, respectively, for C10F18 and C8F18. The cross-sectional area S can then be deduced from the following relation [43]: S¼

oil. When the weight percent of surfactant is increased from 57 to 76 wt.%, a characteristic optically anisotropic hexagonal phase is detected. The hexagonal symmetry is confirmed by SAXS measurements. If the weight percent of surfactant is further increased from 80 up to 90, for C8F18 concentration comprised between 3 and more than 20 wt.%, an other cubic phase is observed (V). This second cubic crystal domain progressively disappears when the temperature is raised and it is substituted by a lamellar phase (Fig. 4B). At 40 °C the domain of this cubic phase is limited to perfluorooctane concentration comprised between 3 and 7 wt.% (Fig. 4B).

2ðV B þ bV O Þ 0:6RH

In the RF8 ðEOÞ9 =water binary system, we recently showed that the hydrophobic chains in the H1 liquid crystal phase are completely extended [34]. Indeed, the size of the hydrophobic core cylinders corresponds to the length of an alkyl chain with 10 carbon atoms adopting a zigzag conformation. It can be seen from Table 2 that in the RF8 ðEOÞ9 = C8 F18 =water system, for a given RF8 ðEOÞ9 =water ratio (R), RH remains almost constant. For example RH varies from 1.9 to 2.0 nm when b is changed from 0.19 to 0.97, for R = 1.52. Nevertheless, a slight increase of RH is

J.L. Blin, M.J. Ste´be´ / Microporous and Mesoporous Materials 87 (2005) 67–76

72

Table 2 Hexagonal liquid crystal of RF8 ðEOÞ9 =C8 F18 =water system: repetition distance (d), hydrophobic radius (RH) and cross-sectional area (S) as a function of b, the number of oil molecules per surfactant molecule, for various RF8 ðEOÞ9 =water ratios (R) b

d (nm)

RH (nm)

S (nm2)

1.00

0.405 0.628 0.380 0.593 0.191 0.338 0.543 0.731 0.974 0.296 0.450 0.14

8.43 8.83 8.76 8.14 6.19 6.35 6.16 6.08 6.25 6.14 6.43 5.60

2.4 2.7 2.6 2.6 1.9 2.0 2.0 2.0 2.0 2.1 2.2 1.9

0.49 0.52 0.45 0.52 0.55 0.57 0.65 0.71 0.75 0.54 0.55 0.52

1.23 1.52

2.34 2.95

noted when perfluooctane is added. Indeed, it should be reminded that, in the RF8 ðEOÞ9 =water binary system, the value of RH is equal to 1.8 nm. In the H1 domain, the cross-sectional area increases with the loading of oil. For example for R = 1.52, S varies from 0.49 to 0.75 nm2 when b is changed from 0 to 0.974 (Fig. 5). Looking at the hexagonal liquid crystal phase H1 in the ternary RF8 ðEOÞ9 =perfluorodecalin=water system, we have reported [42] that the value of the cross-sectional area is found to be constant and equal to 0.49 ± 0.01 nm2. Since this value remains unchanged compared to the binary system [34], we can assume that the hydrophobic chains in the H1 liquid crystal phase also adopt a zigzag conformation. Moreover, for a given RF8 ðEOÞ9 =water ratio the value of the hydrophobic radius RH increases with the PFD loading (Fig. 6). For example for R = 2.28, RH is increased from 1.8 to 2.6 nm when b is raised from 0 to 0.542. Whatever the RF8 ðEOÞ9 =water ratio, it should be noted that the increase of the size of the hydrophobic core is propor-

R = 1.52

0.75 0.70

S (nm²)

0.65 0.60 0.55 0.50 0.45 0.0

0 .2

0. 4

β

0 .6

0.8

1 .0

Fig. 5. RF8 ðEOÞ9 =C8 F18 =water system cross-sectional area (S) as a function of b, the number of oil molecules per surfactant molecule, in H1 phase for R = 1.52.

R = 2.28 R = 1.85 2.4

R = 1.5

RH (nm)

R

2.8 R=3

2.0

1.6 0.0

0.2

β

0.4

0.6

Fig. 6. RF8 ðEOÞ9 =PFD=water ternary system: Variation of the hydrophobic radius (RH) as a function of b, the number of oil molecule per surfactant molecule, in H1 phase for various RF8 ðEOÞ9 =water ratios (R).

tional to the quantity of oil incorporated and that it varies in the same way whatever R value. According to previous work, when oil is added, it either penetrates into the amphiphilic film (penetration effect) or forms a core (swelling effect). Both effects can also simultaneously occur [44,33]. In a paper dealing with the effect of oil on the structure of liquid crystals in polyoxyethylene dodecylether-water systems, Kunieda et al. [45] reported that saturated hydrocarbons such as decane favor a swelling of the hexagonal structure core cylinders, whereas aromatic hydrocarbons such as m-xylene tend to penetrate the surfactant palisade layer. The penetration effect involves an expansion of the cross-sectional area without increasing the hydrophobic volume of the cylinders, whereas the swelling effect corresponds to an increase of the volume of the hydrophobic part without changing the cross-sectional area. Concerning the RF8 ðEOÞ9 =PFD=water system, since the cross-sectional area value remains unchanged, the increase in the RH value with oil loading, for a given RF8 ðEOÞ9 =water ratio indicates that PFD is truly entrapped into the hydrophobic core of the rods, whereas penetration of perfluorodecalin between the hydrophobic chains is not favored. For the RF8 ðEOÞ9 =C8 F18 = water system, the situation is quite different. Indeed, we have evidenced that for a given R, with the value of the hydrophobic radius remains almost constant when the oil content is raised, whereas the one of the cross-sectional area increases. So, even when regarding the slight increase of RH values between the binary RF8 ðEOÞ9 =water (RH = 1.8 nm) and the ternary RF8 ðEOÞ9 =C8 F18 =water systems, we cannot exclude a slight swelling effect of C8F18. Nevertheless, comparing with the values of RH obtained for the RF8 ðEOÞ9 =PFD=water we can conclude that the swelling

J.L. Blin, M.J. Ste´be´ / Microporous and Mesoporous Materials 87 (2005) 67–76

effect is not the main one and that C8F18 molecules rather penetrate between the hydrophobic chains. The penetration effect of C8F18 is further confirmed by the increase of the values of the cross-sectional area with the oil content. 3.2.3. The cubic liquid crystal phases There are two classes of cubic phases. The first one consists in bicontinuous cubics, denoted V and their basic structure is fine. The segregation of the aqueous and nonaqueous domains is obtained by the constitution of two overlapped but not connected networks. The second family deals with the micellar cubics (I), which consist in discrete micellar aggregates arranged on cubic lattices. The position of the cubic domain, noted I1, in the RF8 ðEOÞ9 =perfluorooctane=water phase diagram represented in Fig. 4 strongly suggests that it belongs to the family of direct micellar. Reflection lines have been indexed in the Fm3m space group. This structure has also been encountered for hydrogenated systems [46] and was also evidenced in the fluorinated RF8 ðEOÞ9 = PFD=water system [42]. In thispcase, the relation between the cell parameter ffiffiffi ða ¼ d 111 3Þ and the hydrophobic radius RH becomes:

3.2.4. The lamellar liquid crystal phase A large domain of lamellar phase (La) is detected in the RF8 ðEOÞ9 =C8 F18 =water system (Fig. 4B) when the temperature is increased. We have also determined the structural parameters of this liquid crystal phase at 40 °C. The surfactant chains, which are assume to be ordered on average perpendicularly to the bilayer planes, can be represented in the form of cylinders with a cross-sectional area S derived from the following relation [43]: S¼

2ðV S þ aV W þ bV O Þ 0:6d 001

where d001 corresponds to the layer spacing, which comprises the water and oil films separated by the surfactant bilayer. The hydrophobic thickness (dB) is inferred by the relation: dB ¼

2ðV B þ bV O Þ S

5.5 5.0

V B þ bV O 16pR3H ¼ V S þ aV W þ bV O 3a3

d 001 , R = 3.88 d 001 , R =7.77

The I1 cubic domain is limited and the structural parameters have been determined only for one RF8 ðEOÞ9 =water ratio and for two loadings of oil. From Table 3, it appears that in the RF8 ðEOÞ9 =perfluorooctane=water system cross-sectional area increases with the oil loading. The increase in RH with oil incorporation reveals that C8F18 is probably entrapped in the hydrophobic core of the micelles (I1). The second cubic domain evidenced at higher surfactant concentration in the RF8 ðEOÞ9 =C8 F18 =water system belongs to the V family. Two space groups Pn3m, Ia3d have been unambiguously identified in this family. This kind of cubic structure is usually observed in Cm(EO)n/ water, CFm ðEOÞn =water or lipid/water systems. The reflections lines observed by SAXS are in good agreement with the Ia3d space group.

L B , R = 7.77

4.0

L B , R = 3.88

3.5 3.0 2.5 2.0 0.2

0.3

0.4

0.5

A

β

0.6

0.7

0.8

0.9

0.6

0.7

0.8

0.9

0.68 R = 3.88

0.66

R = 7.77 0.64 0.62

S (nm²)

3ðV B þ bV O Þ S¼ 0:6RH

d001, LB (nm)

4.5

The cross-sectional area S can then be deduced from the following relation [43]:

73

0.60 0.58 0.56 0.54 0.52

Table 3 Cubic liquid crystal (I1) of RF8 ðEOÞ9 =C8 F18 =water system: repetition distance (d), hydrophobic radius (RH) and cross-sectional area (S) as a function of b, the number of oil molecules per surfactant molecule R

b

d (nm)

RH (nm)

S (nm2)

0.651

0.321 0.554

8.95 9.29

3.33 3.61

0.51 0.55

0.50 0.2 B

0.3

0.4

0.5

β

Fig. 7. RF8 ðEOÞ9 =C8 F18 =water system: (A) Variation of the repetition distance (d001), hydrophobic thickness (LB) and (B) variation of the cross-sectional area (S) as a function of b, the number of oil molecules per surfactant molecule, in lamellar phase.

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The hydrophilic thickness (dA) is obtained by subtracting dB from d values and the hydrophobic thickness (LB) of the surfactant chain is equal to dB/2. Obtained results are reported in Fig. 7. It is observed that d001 remain constant when the quantity of oil is increase, whereas the value of LB slightly increases (Fig. 7A). It can be seen from Fig. 7B that the increased in cross-sectional area with the addition of C8F18 is well pronounced. For example for R = 3.88, it value varied from 0.55 to 0.67 nm2 when b is changed from 0.257 to 0.887. Even if we cannot exclude the existence of an interstitial film of oil, it appears that the main part of the perfluooctane added penetrates between the hydrophobic chains of the surfactant.

4. Discussion Comparing pore diameter of silica prepared with and without oil (Table 1), it is clear that the pore size enlargement occurs when C7F16 and PFD are used as organic auxiliaries, whereas no effect on pore diameter is noted with C8F18. Mesoporous materials have been prepared with a concentration of RF8 ðEOÞ9 at 10 wt.% in water. According to the binary phase diagram established, this concentration corresponds to a micellar phase (L1). Moreover, all the molecular sieves have been prepared in the same synthesis conditions. Thus the difference in behavior can only be related to the effect of oil on the surfactant behavior in solution. Several groups attempt to expand the pore size of mesoporous materials, but only few of them have tried to suggest a detailed swelling mechanism. Moreover few studies deal with pore size enlargement of mesostructure using a nonionic surfactant, most of them are related to ionic template. For example, Ulagappan et al. [47], dealing with alkane incorporation and micellar system organization using a cationic surfactant in the process of mesoporous silica formation, indicated, on the basis of XRD and nitrogen adsorption results, that, for short alkanes, the solubilizing agent penetrates between the tails of surfactant molecules. On the contrary, for higher alkanes, such as decane, the molecules form a core which is then surrounded by a layer of the cationic surfactant molecules and the swelling mechanism involves one molecule of surfactant for one molecule of expander. Galarneau and coworkers [48] used trimethylbenzene (TMB) to enlarge the size of the micelles of cetyltrimethylammonium bromide surfactant in order to prepare large pore MTS (micelle template silica) materials. Based on EPR and 1 H NMR analysis, the authors conclude that TMB partly localizes around the headgroup of the surfactant. To explain the pore size enlargement of MCM-41 or SBA materials, Namba and coworkers [49] consider that hydrophobic organic species, such as methyl-substituted benzene, isopropyl

substituted benzene or alkane, added as organic auxilaries are absorbed into the hydrophobic region of the rod-like micelles, causing an increase in pore diameter. Using nonionic polyoxyethylene surfactant, Boissie`re et al. [50] also consider a swelling effect of the micelle core to explain the effect of the addition of a swelling agent (TMB) on the pore size of MSU mesoporous silica. However, in both cases no study was performed to understand the effect of TMB addition on micelles in water. The results obtained in the present study lead us to conclude that such a mechanism is not universal since it is not applicable for the mesoporous silica prepared from RF8 ðEOÞ9 and the various perfluorocarbons used as organic auxiliary. Indeed, we have evidenced that whatever the oil is, only a very small quantity (1 wt.%) of oil can be incorporated into the micelles (L1) of RF8 ðEOÞ9 in water. Increasing the amount of oil leads to a biphasic system (Winsor I), as the excess of oil is in equilibrium with the swollen micelles in water. Therefore, a very weak fraction of oil is incorporated in the hydrophobic micelle core (1 wt.%), and a separation occurs between these slightly swollen micelles and excess oil. So, the swelling effect of the mesoporous pore diameter observed with C7H16 and PFD cannot be attributed to the formation of a core of oil in the micelles. When TMOS is added to the surfactant, water and oil mixture, a hexagonal hybrid mesophase, whose features are analogous to the H1 liquid crystal is formed, through a CTM (cooperative templating mechanism)type mechanism. It should be reminded that C7F16 and PFD can swell the rod of the hexagonal liquid crystal phase whereas no effect on the hydrophobic radius of the rod is noted when C8F18 is added. Since oil cannot be incorporated inside the micelles (L1), as regards the pore size of mesoporous molecular sieves, to explain the difference of behavior of the various perfluoroalkanes we rather should have a look at the effect of oil on the hexagonal liquid crystal phase. As mentioned previously the perfluorodecalin is truly incorporated into the core of the rods (swelling effect). So concerning the swelling mechanism of mesoporous materials we propose that PFD can be incorporated in this hybrid hexagonal mesophase, involving a channelswelling (Fig. 8A). However, at higher loading of solubilizer, the expansion of the pore size still occurs but the recovered materials adopt a wormhole-like structure. Same arguments are proposed to explain the pore size enlargement observed with perfluoroheptane. The complete study of the RF8 ðEOÞ9 =perfluoroheptane=water system was not performed. However, we can shown that the incorporation of oil in the core of the micelles is limited to loading inferior to 1 wt.% and that perfluoroheptane can be incorporated into hexagonal liquid crystal phase. When perfluorooctane is used as organic auxiliary to prepare mesoporous materials the hexagonal

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75

Fig. 8. Illustration of the swelling (A) and penetration effects (B) of the hybrid-mesophase.

hybrid mesophase is also formed, however perfluorooctane only penetrates between the hydrophobic chains of the hybrid mesophase (Fig. 8B), no swelling of the channel occurs and neither the structure nor the pore diameter are modified. The behavior of perfluorooctane in the hybrid hexagonal mesophase is similar to the one of this oil in the hexagonal liquid crystal H1.

5. Conclusions In the present paper we have investigated the ability of perfluorocarbons to be used as organic auxiliaries in the preparation of large pore mesostructured silica. Results obtained by SAXS and nitrogen adsorption– desorption analysis evidence that perfluoroheptane and perfluorodecalin can be used as expander. However, if the oil content is too high (>2.5 wt.%), the pore size enlargement occurs but the hexagonal structure is lost and recovered materials adopt a wormhole-like structure. The behavior of perfluorooctane is different, indeed neither the pore diameter nor the structure are influence by the addition of this oil. To better understand the effect of the addition of these fluorocarbons and to shed some light on the swelling mechanism of C7F16 and PFD we have investigated the RF8 ðEOÞ9 = oil=water ternary systems. We have delimited the different phase domains and determined the structural parameters of the liquid crystals. We have shown that only a low quantity of oil can be incorporated in micelles (L1) of fluorinated surfactant in water. Results also evidenced that in the hexagonal liquid crystal phase, C8F18 molecules are located between the hydro-

phobic chains. Thus the penetration effect occurs. As a consequence, for a given RF8 ðEOÞ9 =water ratio the value of the cross-sectional area increases with the addition of perfluorooctane, whereas the one of the hydrophobic radius remains constant. In the contrary, previous results have evidenced that PFD can swell the hydrophobic core of the rods. The value of the cross-sectional area remains constant to 0.49 nm2, so the swelling effect occurs for this oil. On the basis of the RF8 ðEOÞ9 =oil=water system, we suggested a mechanism to account for the enlargement of the pore size of mesoporous materials by PFD and perfluoroheptane.

Acknowledgements Authors would like to thank C. Brioual and A. Grandmougin, students of the IUT ge´nie biologique agro-alimentaire of the university of Nancy-1, for their contribution to this work as well as DuPont de Nemours Belgium for providing the RF8 ðEOÞ9 (FSN-100).

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