Formation of Nanostructured Silica Materials Templated with Nonionic

Article pubs.acs.org/Langmuir

Formation of Nanostructured Silica Materials Templated with Nonionic Fluorinated Surfactant Followed by in Situ SAXS Julien Schmitt,† Marianne Impéror-Clerc,*,† Florentin Michaux,‡,§,@ Jean-Luc Blin,‡,§ Marie-José Stébé,‡,§ Jan Skov Pedersen,∥ and Florian Meneau⊥ †

Laboratoire de Physique de Solides, UMR 8502, Bât. 510, Université Paris-Sud, F-91405 Orsay, France Université de Lorraine, SRSMC, UMR 7565, 54506 Vandoeuvre-lès-Nancy Cedex, France § CNRS UMR 7565, 54506 Vandoeuvre-lès-Nancy Cedex, France ∥ iNANO Interdisciplinary Nanoscience Center and Department of Chemistry, Aarhus University, DK-8000 Aarhus, Denmark ⊥ SWING, Synchrotron Soleil, BP 48, F-91192 Gif-sur-Yvette, France ‡

S Supporting Information *

ABSTRACT: The formation of two-dimensional (2D)-hexagonal (p6m) silica-based hybrid materials from concentrated micellar solutions (10 wt %) of two nonionic fluorinated surfactants, RF7 (EO)8 and RF8 (EO)9, is investigated in situ using synchrotron time-resolved small angle X-ray scattering (SAXS). The two surfactants form direct micelles with different structures prior to the silica precursor addition as demonstrated by SAXS and SANS. RF8 (EO)9 gives spherical micelles and RF7 (EO)8 more complex ones, modeled here as short wormlike micelles. The in situ SAXS experiments reveal that both surfactants form well-ordered 2Dhexagonal hybrid materials after the addition of the silica precursor, in coexistence with an excess of surfactant micelles. The structures of both 2D-hexagonal phases are compared just after precipitation, and it is found that more robust and larger silica walls are formed for RF8 (EO)9 than for RF7 (EO)8. This could explain why only the material obtained with RF8 (EO)9 is stable upon washing, as observed previously. Moreover, it is proposed that in both cases, only a part of the micelles interact with the silica oligomers and undergo structural modifications before forming the 2D-hexagonal mesophase. The obtained results are finally discussed in the more general framework of the templating mechanism for nonionic surfactants. species.5 The stages in the CTM route involve a structural transition from micelles in solution to a hybrid organic/ inorganic-ordered mesophase. The formation mechanism of the mesoporous materials has been studied using spectroscopic techniques,6−9 including in situ EPR investigations,7 TEM,6,10 and small angle scattering measurements.11−18 As ex situ investigations may induce modifications of the system, small angle scattering techniques have the main advantage to allow the measurement directly under in situ conditions. In the case of the SBA-15 materials (based on the Pluronic P123 as a structure-directing agent), it has been shown that, as a result of the interaction with the inorganic silica species, the shape of the micelles in solution evolves from spherical to rodlike before the formation of an organic/inorganic 2D-hexagonal mesophase.16 This result was also supported by a direct cryo-TEM study.10 The use of fluorinated surfactants for the preparation of wellordered 2D-hexagonal mesoporous materials was first reported

1. INTRODUCTION Since the early 90s, surfactant-based systems are used as templates for the synthesis of well-ordered inorganic mesoporous materials.1,2 These nanostructured compounds are mainly prepared using the cooperative templating mechanism (CTM) route by adding an inorganic precursor to a micellar solution of surfactant in water used as a structuredirecting agent.3 Many mesostructures (hexagonal, cubic, and lamellar, etc.) can be obtained using different kinds of surfactants and synthesis conditions. The formation of the mesostructure is due to interactions between the polar head of the amphiphile and the inorganic precursor. These interactions depend on the properties of the hydrophilic part of the surfactant molecule and on the pH of the micellar solution.4 For materials synthesized from nonionic surfactants, such as alkylpolyoxyethylene ether (CnH2n+1(OC2H4)mOH) or triblock copolymers (Pluronics P123, F127), in acidic medium,4 the interactions are effective owing to the counterions (X−) in solution, according to a S0/H+/X−/I+ mechanism. The synthesis pathway in neutral conditions involves the formation of hydrogen bonds between the polar head and the precursor © 2013 American Chemical Society

Received: November 21, 2012 Revised: January 9, 2013 Published: January 10, 2013 2007

dx.doi.org/10.1021/la304648j | Langmuir 2013, 29, 2007−2023

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in 2004.19 Fluorinated surfactants are of particular interest due to their specific properties such as thermal and chemical stabilities and their strong hydrophobic behavior. These properties allow their use in more severe conditions than their hydrogenated analogs. As a result of the good thermal stability of the perfluorinated chain, this kind of surfactant allows the formation of stable materials using a higher hydrothermal treatment temperature than commonly used with analogous hydrogenated compounds.20,21 Moreover, fluorinated surfactants are good candidates to explore the formation mechanism of mesostructured materials by SAXS. Indeed, hydrogenated surfactant, especially Pluronics triblock-copolymers (like P123), have a very low electron density contrast in water (less than 10 e/nm3 between the core of the micelles and water).22 In contrast, fluorinated surfactants have a much higher electron density than water (due to their perfluoroalkyl chains), leading to a very good contrast (about 230 e/nm3) for X-ray scattering. These surfactants are thus perfectly suited performing time-resolved in situ SAXS experiments. Two surfactants with different hydrophobic and hydrophilic chain lengths were investigated, C8F17C2H4(OC2H4)9OH and C7F15C2H4(OC2H4)8OH, labeled R8F(EO)9 and R7F(EO)8, respectively. The first one has previously been investigated, and its phase diagram shows mainly a large micellar domain and a hexagonal liquid crystal phase.19 Moreover, a previous study shows that this surfactant allows the preparation of wellordered hexagonal materials, using surfactant solution concentrations between 5 and 15 wt %.23 The second surfactant, RF7 (EO)8, presents a more hydrophobic character as shown by its phase diagram in water. It exhibits a cloud point curve at low temperature (34 °C) above a direct micellar phase and a lamellar mesophase for higher surfactant concentrations. This surfactant leads to the formation of disordered mesoporous materials also called wormlike materials.24,25 The micellar structures of these two systems have previously been studied by small angle neutron scattering (SANS), using the contrast variation method.26 It was shown that a model of core−shell (quasi-) spherical/globular micelles fits well the data of the RF8 (EO)9/water system. In contrast, for RF7 (EO)8 neither a model of spherical micelles nor a rodlike one was able to fit the SANS data. Moreover, for R7F(EO)8, dynamic light scattering (DLS) experiments gave a hydrodynamic diameter of 40 nm when assuming a spherical shape, which is not consistent with the surfactant dimensions (about 8 nm for two fully extended conformations). Then, one can already conclude that the micelles for RF7 (EO)8 are not spherical but large objects. And from these preliminary results, it seems that the presence of spherical micelles in solution prior to the silica source addition may be a condition for creating a stable 2Dhexagonal material. In addition, the rheophysical behavior of the RF7 (EO)8/water system has been studied under a wide range of temperatures and concentrations.27 The increase of viscosity with temperature highlighted the fact that the particles become more and more elongated. Moreover, flow birefringence (at 20 wt %) indicated the existence of large but fragile structures, which are easily broken under flow, even at a low shear rate. This behavior is unusual and does not correspond to the one expected for long wormlike micelles. Thus, the structure of RF7 (EO)8 is still under discussion, as neither spherical objects nor long wormlike ones are able to explain all the observations.

To bring new information on the mechanisms at stake in the mesophase formation, measurements were performed using small angle X-ray scattering (SAXS). Synchrotron radiation allows a very short accumulation time and makes it possible to follow in situ the evolution of the micelle shape during the material formation. Moreover, SAXS experiments prior to TMOS addition were necessary to shed some light on the micellar behavior of the surfactants. For this, different concentrations were studied for both surfactants, in order to separate the form factor and the structure factor contributions. The paper is organized as follows. First, the structure of the micelle formed by these two surfactants prior to TMOS addition by full characterization and modeling is described. Then, the evolutions by time after TMOS addition for these two surfactants are studied using time-resolved SAXS experiments and are compared.

2. EXPERIMENTAL METHODS 2.1. Sample Preparation. The fluorinated surfactants used, provided by DuPont, have an average chemical formula of C8F17C2H4(OC2H4)9OH (MW = 870 g mol−1) and C7F15C2H4(OC2H4)8OH (MW = 744 g mol−1), and are labeled RF8 (EO)9 and RF7 (EO)8, respectively. Their hydrophobic chain moieties exhibited a Gaussian chain length distribution. The in situ experiments are performed at 40 °C for RF8 (EO)9 and at room temperature (below the cloud point) for RF7 (EO)8 using 40 mL of a micellar solution at 10 wt % of surfactant in water. This surfactant concentration corresponds to the usual synthesis conditions to form the hybrid mesophase.21,28 The pH is adjusted using a small amount of sulfuric acid. Tetramethoxysilane (TMOS), used as the silica source with a molar ratio R = surfactant/TMOS = 0.5,23 was added dropwise to the micellar solution. The corresponding compositions of the synthesis solution in molar ratio are R8F(EO)9:H2O:Si = 0.5:218:1 and R7F(EO)8:H2O:Si = 0.5:186:1. For RF8 (EO)9, one additional experiment was also performed with a doubled amount of TMOS, R = 0.25 (RF8 (EO)9:H2O:Si = 0.25:109:1) and it is more briefly discussed in the results section. All syntheses are done under permanent stirring, and the mixture is placed on a hotplate to control the temperature. With the use of an alreadydescribed setup13 with a peristaltic pump, part of the synthesis solution was continuously circulating (flow rate of about 10 mL min−1) through a 2 mm quartz capillary. Because of the continuous circulation and permanent stirring, the composition of the sample in the beam is the same as that of the solution in the reaction beaker, even after the precipitation of the material. 2.2. SAXS (Small Angle X-ray Scattering). The time-resolved SAXS measurements were carried out at the D2AM ESRF beamline (energy 11 keV) and at the SWING SOLEIL beamline (energy 12 keV). At D2AM, the measured q range was 0.06−0.60 Å−1, where q = 4π sin(θ)/λ is the modulus of the scattering vector, 2θ is the scattering angle, and λ is the X-ray wavelength. At SWING, the sample-CCD camera distance was 1.50 m, and the q range was from 6 × 10−3 to 0.5 Å−1. At both beamlines, the q-range calibration was made using a silver behenate standard sample (dref = 58.38 Å). For the absolute intensity calibration, scattering patterns of the empty capillary and the capillary filled with deionized water were always recorded first. The value of the constant intensity contribution of water is equal to 0.016 cm−1 on the absolute scale.16 Then, the signal of the same capillary filled with the solvent solution was recorded for subtraction purposes before the introduction of the synthesis solution. During each kinetics experiment, the capillary was kept fixed in the beam using a capillary holder, so that all patterns were recorded at the same position. Complementary measurements of the micellar solutions were carried out at different temperatures using a SAXSess mc2 apparatus (Anton Paar) equipped with a CCD detector at the Université de Lorraine. The micellar solutions were introduced into a capillary having a diameter of 1 mm before being placed inside an evacuated 2008


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Figure 1. SAXS data of the micellar solutions at 2 (green dots), 5 (blue dots), and 10 (red dots) wt % surfactant concentrations. The black dots correspond to the fits. The curves are rescaled in intensity to the same equivalent concentration of 10 wt %. For RF8 (EO)9: (A) Normalized experimental SAXS spectra, Inorm(q), and best fits in a log−log representation (black dots). (B) Radial density profiles. For RF7 (EO)8: (C) normalized experimental SAXS spectra, Inorm(q), and fits in a log−log representation. The inset is the corresponding Holtzer plot, q × I(q) versus q. (D) Density profiles along the cross section of the micelles. Sketches of the spherical micelle model used for RF8 (EO)9 and of the short wormlike micelle model for RF7 (EO)8 are also given. For the later, the Kuhn length, bKuhn, is about one-third of the overall contour length, LC. micelles via a smearing σshell of the shell interface and/or a polydispersity, σ, over the radii. As those two parameters are both used to smooth the oscillations of the form factor, we tried as much as possible to favor smearing over polydispersity in order to get an ideal model of the micelles with a radial repartition of shell hydration more in agreement with a radial gradient of hydration of the EO chains. For wormlike micelles, this form factor can be factorized as following: P(q) = PCS(q)Pworm(q), where PCS(q) is the form factor of the cross section (depending on the parameters presented above), and Pworm(q) is the form factor related to the length of the micelle. Pworm(q) depends on the contour length, LC (overall length of the micelle) and the Kuhn length, bKuhn (equal to two times the persistence length of the micelle, which characterizes the length scale over which the micelle can be considered as stiff). In a classical wormlike micelle model, the three length scales are assumed to be well-separated (Rt ≪ bKuhn ≪ LC). It gives rise to three different regimes for I(q) for a dilute system. The wide-q region is related to the average cross section of the micelle via the PCS(q) term. The intermediate q range is sensitive to the local stiffness of the objects and then exhibits a q−1 slope. In the low-q region, corresponding to higher length scales, I(q) is sensitive to the flexibility of the objects and follows a q−5/3 slope (by analogy with the scattering by a polymer chain in good solvent).29,30 S(q) is the structure factor characterizing the interactions between these micelles. At a large concentration, the structure factor contribution is important, especially in the low-q range. The interactions were modeled for spherical micelles by a repulsive term using the Percus−Yevick model for hard spheres for the structure factor with the hard-spheres radius, RHS, and the volume fraction of hard spheres, φHS. For wormlike and cylindrical micelles, a PRISM-type approximation is used for the structure factor.29,30 Repulsive intermicelle interactions are modeled by the RPA approximation using the parameter νRPA > 0. Moreover, a direct correlation function term, c(q), is also included. The term c(q) is the Fourier transform of a spherical excluded volume of radius Rex, which was fixed in the present study to 4 nm. The idea is to describe the excluded volume along the micelle as a series of hard

chamber equipped with a temperature-controlled sample holder unit. Typical acquisition times are 30 min, and the q range is from 9 × 10−3 to 0.5 Å−1. Scattering data, obtained with a slit collimation, contain instrumental smearing. Therefore, the beam profile has been determined and used for the desmearing of the scattering data. All data were corrected for the background scattering from the filled capillary with solvent (water). The scattered intensities were evaluated on an absolute scale using water as a reference. The electron density of the solvent (H2O) is equal to 334 e/nm3. For both surfactants, RF8 (EO)9 and RF7 (EO)8, the estimated values of the electron densities are 543 e/nm3 for C8F17, 537 e/nm3 for C7F15, and 292 e/nm3 for the C2H4 spacer. This gives a hydrophobic core of 511 e/nm3 for RF8 (EO)9 and 502 e/nm3 for RF7 (EO)8. As for the hydrophilic EO groups, the electron density is estimated at 394 e/nm3 (without hydration).22 2.3. SANS (Small Angle Neutron Scattering). Micellar solutions (10 wt %) for four different H2O/D2O ratios were studied by SANS. The experiments were carried out at the PAXE spectrometer of the Laboratoire Léon Brillouin (CEA Saclay, FRANCE) and were already published.25 The sample−detector distance was 3 m with a wavelength of 5 Å, which correspond to the following q range: 0.013 ≤ q ≤ 0.24 Å−1. 2.4. Fitting Models. The results have been interpreted using models previously reported in the literature16,17 developed in the case of the formation of SBA-15 materials. All data have been treated with a least-squares fitting procedure. Notations for these models are summarized in this paragraph. The intensity is modeled by the sum of the following contributions:

Itot(q) = Imicelle(q) + Icluster(q) + IBragg(q) + IPorod(q) Imicelle(q) = A P(q)S(q) is the contribution of micelles in the solution, where P(q) is the form factor of the micelles that can be either spherical or wormlike and which are both described using a core/shell model with a core radius Rc, a total radius Rt, and a contrast term, α = (ρshell − ρ0)/(ρcore − ρ0), where ρ0, ρshell, and ρcore are the electronic density of the solvent, the hydrophilic shell, and the hydrophobic core, respectively. Moreover, we also described the 2009


2010

H2O

SANS

D2 O

48% D2O

32% D2O

H2O

SAXS

D2 O

40% D2O

2 5 10 5 10 5 10 5 10 5 10 2 5 10 5 10 5 10 5 10 5 10

wt %

wormlike, Rc = 0.8 nm, Rt = 2.8 nm, LC = 50 nm, bKuhn = 35 nm

wormlike, Rc = 0.8 nm, Rt = 1.9 nm, LC = 50 nm, bKuhn = 35 nm

sphere, Rc = 1.5 nm, Rt = 3.4 nm

sphere, Rc = 1.0 nm, Rt = 2.5 nm

model

1.9

−0.02

0.48 0.48 0.48 0.12 0.15 0.018 0.007 −0.19 −0.19 0.75 0.78

0.8 0.8 0.8 0

0.5

1.7

−0.18

1.21

1.0 1.1 1.2 0.0

σshell (nm)

0.38 0.41 0.49 0.13

α

0.14

0

0.13

0.15

0.15

0.15

0

σ 0.03 0.07 0.13 0.08 0.14 0.1 0.14 0.19 0.14 0.07 0.14 − − − − − − − − − − −

ϕHS 4.3 4.3 4.4 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 − − − − − − − − − − −

RHS (nm)

χ2r 1.5 0.5 0.3 5.3 1.1 8.6 7.1 >20 >20 4.6 4.4 1.7 0.8 0.9 13.3 7.3 >20 8.7 >20 13.6 4.0 3.9

νRPA − − − − − − − − − − − 0.13 1.6 5.9 8.1 11. 10. 10. 5.4 4.1 5.4 7.7

A core-shell model is used for the micelles, with spherical micelles for RF8 (EO)9 and wormlike micelles for RF7 (EO)8. Rc is the core radius and Rt the total radius (of the spheres or the cross section of the wormlike micelles). Wormlike micelles are also defined by their Kuhn length, bKuhn, and their contour length, LC. The contrast term between the micelles and the solvent is α = (ρshell − ρ0)/(ρcore − ρ0), where ρ0, ρshell, and ρcore are the electronic density of the solvent, the hydrophilic shell, and the hydrophobic core, respectively. σ is the polydispersity over the radii. σshell is the smearing between the shell and the solvent. Repulsive interactions are taken into account via a Percus−Yevick term for spherical micelles with RHS the hard sphere radius and φHS the volume fraction of hard spheres (RF8 (EO)9) or the random phase approximation for wormlike micelles with the νRPA parameter, (RF7 (EO)8). Best fits are found by minimizing χ2r − 1/N ∑i N= 1 [(Iexp(qi) − Imod(qi))/(σi)]2 with Iexp(qi), the data points measured at the diffusion vector qi, Imod(qi), the model intensities depending on the parameters of the fits, σi, the statistical uncertainties, and N, the number of points.

a

RF7 (EO)8

H2O

SANS

32% D2O

H2O

SAXS

RF8 (EO)9

solvent

technique

surfactant

Table 1. Fit Parameters for the RF8 (EO)9 and RF7 (EO)8 Micelles Deduced from the Fits of the SAXS and SANS Dataa

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In the low-q region (q < 0.05 Å−1), the overall decrease of the intensity with the concentration is attributed to the interaction term, S(q). In the wide-angle region (0.05 to 0.5 Å−1), the curves obtained at the three concentrations are superimposed for both systems. In this q-range, it can be assumed that only the form factor contributes to the scattering. It gives rise to a small oscillation, at q = 0.25 Å−1 for RF8 (EO)9 and q = 0.3 Å−1 for RF7 (EO)8, superimposed for the three concentrations, meaning that the smallest dimensions of the micelles do not vary with the concentration. For the more dilute micellar solutions (2 wt % of surfactant), where there is only a very small influence of the structure factor (green curves), a noticeable difference is observed on the scattering profiles between the two systems as a plateau at low angles (q < 0.015 Å−1) is observed for RF8 (EO)9 and not for RF7 (EO)8. In the latter system, instead of a plateau, the behavior is close to a q−1 dependence (q < 0.05 Å−1). Consequently, a spherical core−shell model can apply only to the case of RF8 (EO)9, and a short wormlike micelle model is appropriate for RF7 (EO)8, as discussed hereafter. 3.1.2. SAXS RF8(EO)9. For the RF8 (EO)9/water system, a spherical core−shell with a large corona smearing model fits the data very well (see Figure 1A, where the black dots correspond to best fits). Using only a smearing term of the outer shell and no polydispersity on the size for the form factor is suitable for nonionic surfactants systems, since EO groups have a hydration gradient among the shell.22 The same form factor is used for the three concentrations and is adjusted using the high-q region (q > 0.1 Å−1). The large smearing is in agreement with the low oscillation present at high-q values (q = 0.25 Å−1). The hydrophobic radius and the total micelle radius are determined to be equal to Rc = 1.0 ± 0.1 nm and Rt = 2.5 ± 0.1 nm, respectively, with a corona smearing σshell = 1 nm for the outer interface (see Table 1). These values are in rather good agreement with the dimensions of the surfactant molecule. The core is composed of the perfluorinated chains and the spacer. Rc is slightly below the value of a fully extended C8F17 chain (1.1 nm) plus the spacer, C2H4 (0.25 nm). The hydration gradient of the EO groups is shown on the radial electron density profile (Figure 1B). For R = Rc, the electron density is close to the one of pure EO (394 e/nm3). The maximum extension of the shell of Rt − Rc + σshell = 2.5 nm shows that the (EO)9 chains are not completely fully extended, as it would give an extended length of 3.1 nm. On increasing the surfactant concentration to 5 and 10 wt %, (Figure 1A), the experimental intensities are strongly influenced by the structure factor contribution, S(q). At 10 wt % (red curve), a broad interaction peak is observed at q = 0.055 Å−1. This interaction peak is well-modeled via a Percus− Yevick structure factor for a hard sphere, with an interaction radius RHS = 4.3 nm = 1.72 Rt and with increasing hard-sphere volume fractions of 0.03 (2 wt %), 0.07 (5 wt %), and 0.13 (10 wt %) (see Table 1). These values are about twice the estimated “dry” volume fraction of 0.015, 0.037, and 0.075, respectively. It is expected because the micellar volume in the solution includes the hydration of the micelle and is always smaller than the interaction volume. Moreover, a significant increase of the intensity at very low q (q < 0.01 Å−1) is observed. It is assumed to be due to sample heterogeneities, as it was used without any process of purification. It is taken into account using a Porod term (q−4 law). 3.1.3. SAXS RF7(EO)8. As already mentioned, the RF7 (EO)8 micelles are more complex to model than those of RF8 (EO)9,

spheres, each hard sphere having a radius of excluded volume equal to Rex. This gives the following expression for the structure factor: S(q) = SPRISM(q) =

1 1 + vRPAc(q)Pworm(q)/Pworm(0)

IBragg(q) is the contribution of the Bragg peaks from the long-rangeordered 2D-hexagonal mesophase. Two complementary approaches are used to extract information from the intensities of these Bragg peaks, either via a direct fitting method or via an inverse method. In the latter case, the peak intensities are extracted separately and are used to make a reconstruction of the electron density.31 To simplify the analysis, it is assumed here that the influence of disorder terms within the lattice is negligible. The fact that the intensity is measured in absolute units is used to estimate ϕ, the volume fraction of the mesophase and contrast terms as detailed hereafter. With the use of the direct fitting approach, fits are performed using IBragg(q) = BP(q)[Z0(q)], where B is a scaling factor, Z0(q) is the lattice term without disorder that depends mainly on the lattice parameter a and the domain size D″, and P(q) is the core/shell cylindrical form factor of the micelles ordered within the mesophase, where the cross section is described with the same parameters as for the wormlike model above (with the subscript 2 to indicate it is linked to the lattice Rc2, Rt2, α2, etc.). (See Figure S.I.6 of the Supporting Information for the detailed expression) The reconstruction of the electron density (inverse method) in the unit cell is performed in two stages. First, the individual Bragg intensities |Fhk|2 are derived without assuming any model. Then, the 2D-reconstruction of the electron density ρ(r⃗) in the unit cell is performed with various phase combination sets (φhk= 0 or +π) and the more suitable phase combination is selected by comparing the results with those obtained via the direct fitting method (see Figure S.I.6 of the Supporting Information). ρ( r ⃗) = ρ0 +

1 Sc

∑

|Fhk| cos(φhk) cos[2π(hx + ky)]

hk ≠ 00

The silica clusters contribution Icluster(q) = C[exp(−q2R2g) − 1 + is modeled using the Debye expression of a polymer chain, with a gyration radius, Rg (typically 2−5 Å), and its overall intensity is given by the constant, C. Lastly, the Porod contribution, IPorod(q) = K/q4, takes into account the scattering by the interface of the grains of the material or a largescale inhomogeneity. Here, this term contributes only for the very large-length scales, thus in the low-q region. This is because the grains of material have sizes on the order of a micrometer. After fitting the data, we were able to propose density profiles of the micelles along the (cross-section) radius both for the micelles in solution and for those inside the mesophase. q2R2g]/(q2R2g)2

3. RESULTS Prior to the SAXS in situ experiments, the structure of the micelles formed in the RF8 (EO)9/water and RF7 (EO)8/water systems was first investigated at room temperature by SAXS at three different concentrations, 2, 5, and 10 wt %, and by SANS, at 5 and 10 wt %. The last concentration corresponds to the concentration used for the synthesis of the nanostructured silica materials.26,21 3.1. Micelle Structure. 3.1.1. General Observations. Figure 1 presents the SAXS scattering curves obtained at 2, 5, and 10 wt % in water for both systems in a log−log plot (Figure1, panels A and C). To better visualize the influence of the concentration on the scattering curves, the curves are rescaled in intensity to the same equivalent concentration of 10 wt %. This concentration corresponds to a dry volume fraction, ϕdry, of 0.075, and the intensity is rescaled as follows: Inorm(q) = I(q)(0.075/ϕdry). This way to present the data allows one to directly visualize the strong influence of the interactions between micelles onto the scattered intensity at low angles. 2011


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since spherical, simple rodlike, or “standard” wormlike models do not describe successfully this system. Hence, in this paper, a model of “short wormlike” micelles is proposed. The objects are considered as stiff, at length scales smaller than their persistence length, whereas they are flexible at higher length scales. However, unlike a classical wormlike model, which describes long and flexible objects, here, the total length, LC, is slightly higher or comparable to the Kuhn length, bKuhn (two times the persistence length). The fact that the objects have some flexibility can well be seen in the Holtzer plots (q × I(q) versus q) of the data (inset in Figure 1C). For the lowest concentration, a plateau and an upturn at low angles are observed, characteristic of wormlike behavior. For the two other concentrations, this upturn is screened by the rather strong interactions between micelles. In Figure 1C, for q values above 0.05 Å−1, the curves are superimposed and correspond to the contribution of the form factor PCS(q) of the circular core−shell cross section of the micelles. Best fits for the cross section give Rc = 0.8 ± 0.1 nm and Rt= 1.9 ± 0.1 nm, with a smearing of 0.8 ± 0.1 nm for the corona, which gives a maximal extension of the corona of 1.9 nm (Figure 1D). Once again, these values are in good agreement with the molecular dimensions (fully extended C7F15 chain is ∼1.0 nm, (EO)8 is ∼2.8 nm). The radial electron density profiles (Figure 1D) reveal that for R = Rc, the electron density corresponds to the one of pure EO as in the RF8 (EO)9/ water system. The low q region (q < 0.05 Å−1) is influenced both by the interactions between the micelles [S(q)] and the length of the micelles (wormlike form factor Pworm(q)). The fact that the intensity is decreasing at low q with increasing concentration is attributed essentially to the interaction term S(q). As expected, the interaction term νRPA is found to increase with the concentration: 0.13 (2 wt %), 1.6 (5 wt %), and 5.9 (10 wt %) (see Table 1). As for the length of the objects, best fits are obtained with a contour length fixed at LC = 50 nm and a Kuhn length, bKuhn = 35 ± 5 nm. The contour length can be determined when a plateau at low q is reached; however, the q range used in this study does not allow it, so the value was chosen to be in accordance with other experimental results (DLS and rheophysical studies) and performing complementary SAXS studies at different temperatures described just below. In order to confirm this short wormlike model, we performed complementary measurements at different temperatures between 10 and 35 °C and at very low concentrations (1.25 wt % and 2.5 wt %) to get rid of the influence of the interaction term (see Figure 2). In Figure 2A, between 0.02 ≤ q ≤ 0.05 Å−1, the slope follows the q−1 behavior, which means that the micelles are locally rodlike. For smaller q values (q < 0.02 Å−1), the slope increases with the temperature and no longer follows this q−1 behavior. Figure 2B gives the corresponding Holtzer plots, where the plateau is clearly seen for all temperatures above 25 °C. The parameters LC and bKuhn for the fits of these data are given in Table S.I.1 of the Supporting Information. LC increases with the temperature, whereas the stiffness is unchanged. Indeed, the upturn at q = 0.02 Å−1 between the very low angles and the q−1 regime is related to bKuhn, so as this upturn does not shift, bKuhn stays constant. Very near to the cloud point, a critical behavior in temperature is expected with a growth of the micelles. However, the investigated temperatures are below the cloud point, so the interaction term is supposed to be weak, still corresponding to a weakly repulsive regime

Figure 2. RF7 (EO)8 water solutions at 1.25 and 2.5 wt %: study in temperature. (A) Experimental SAXS spectra I(q) vs q on an absolute scale at different temperatures (10, 15, 20, 25, 28, 30, 32, and 35 °C) for both surfactant concentrations drawn in a log−log representation. (B) Corresponding Holtzer plots q × I(q) vs q. The black lines are drawn to localize the plateau.

between the micelles. Then, the value of νRPA is fixed, and LC is the only term adjustable to fit this region. The more pronounced slope with the temperature at very low angles is related to an elongation of the object. Hence, we can estimate, at least qualitatively, the variations of LC with the temperature. Below 25 °C, the micelles are getting shorter and essentially rodlike, as LC and bKuhn have comparable values. In summary, for the RF7 (EO)8 surfactant, our results show that at 25 °C, the micelles are elongated in the investigated concentration range (2 to 10%). At 5% and 10%, the PRISM interaction term is used to model the decrease of the intensity in the low-q range. The limit in the qmin value imposes us to fix LC, the overall length of the objects, based on other studies such as DLS and not to determine it from the SAXS data. The increase of LC with temperature is in agreement with the rheophysical studies27 predicting that the particles become more and more elongated. Also, due to a comparison with similar nonionic surfactants with hydrogenated or fluorinated chains, it has already been described that micelles near to the cloud point are growing in size with temperature.32−34 To confirm the model of short wormlike micelles, SAXS experiments at ultrasmall angles would be necessary for determining LC reliably and to relate it with the overall gyration radius of the micelles. Indeed, in the present 2012


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Figure 3. 3D plots of the temporal evolution of the scattering patterns (log (I) = f(log (q))). (A and B): The RF8 (EO)9/water system at pH = 7 and 2, respectively. The syntheses were performed at 40 °C. (C and D): The RF7 (EO)8/water system at pH = 7 and 2, respectively. The syntheses were performed at 25 °C. The molar ratio, R = surfactant/TMOS equals 0.5 for all four experiments. One pattern was recorded every 30 s for the fastest reactions done at pH = 7 (A and C) and every 60 s for the slowest reactions at pH = 2 (B and D). The measuring time per frame was 0.5 s for all experiments. The data shown in (B) were recorded on the D2AM beamline (ESRF) and the three others on the SWING beamline (SOLEIL) which explains the difference in q range. Note than the 10 reflection of the 2D-hexagonal mesophase is unusually weak because of the specific contrast with fluorinated surfactants.

experiments the change of regime between the global behavior and the local stiffness appears near the qmin value. 3.1.4. Comparison SANS and SAXS. In order to validate the models of spherical and short wormlike micelles proposed here, it is important to test it on several sets of data. Hence, we have also studied the SANS data previously acquired on the same systems, using the contrast variation method with four different mixtures of H2O and D2O.26 The spectra and associated fits and the evolution of the radial scattering length density profile as a function of the water composition are presented in Figures S.I.1 and S.I.2 of the Supporting Information and the parameters of the fits in Table 1. For each surfactant, all spectra are fitted with a fixed form factor (see Table 1), changing only the contrast term α with the nature of the solvent. One can note that the fits obtained for the two

mixtures of H2O and D2O are not as good as the others. This is due to a very low scattered intensity for these contrasts, due to their proximity to the matching point of the system. Moreover, at these conditions, we observe that the solvent nearly matches the corona (see Figures S.I.1 and S.I.2 of the Supporting Information). Nevertheless, the calculated curves follow well the experimental data. The models developed in this study were able to fit these data successfully. Interestingly, the fits of the SANS curves always give higher values for the dimensions than those obtained by SAXS. For example, for RF8 (EO)9, Rc equals 1.5 nm (SANS) and 1.0 nm (SAXS), whereas Rt − Rc equals 1.9 nm (SANS) instead of 1.5 nm (SAXS). Nevertheless, those differences can probably be explained due to the differences in contrast between the two methods, which therefore do not 2013


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All the sets of experiments present some common features. For both systems, the addition of the silica source leads to an increase of the scattered intensity at low angles and to the appearance of a q−2 slope at large angles, the signature of silica oligomers, before the appearance of the Bragg peaks of the hexagonal hybrid mesophase. Then, the signal at low q values decreases slowly, but the shape of the curve is not modified. Changing the pH of the micellar solution has only an influence on the rate of reaction. The same stages in the time evolution are observed whatever the pH of the solution. Hence, we will focus only on the experiments made at pH = 7 in the following. One last experiment was made with the RF8 (EO)9 system at pH = 7, with a doubled amount of TMOS (R = 0.25, molar ratio surfactant/TMOS) (Figure S.I.3 of the Supporting Information). The use of twice the amount of TMOS induced a much faster kinetics, as the appearance of the Bragg peaks is observed already 2 min after TMOS addition. However, because of the larger amount of silica, it allowed us to obtain more information about the interactions between the micelles and the silica species at the beginning of the kinetics. Figure 4 gives the evolution of the intensity and gyration radius of silica oligomers during the synthesis. It is seen that,

allow for the observation of exactly the same particle. Moreover, a higher value of polydispersity in size (∼15% for RF8 (EO)9 and 14% for RF7 (EO)8) is required to fit the SANS data. This is due to the fact that the SANS resolution function of the instrument is not taken into account in the fitting procedure. In addition, smaller values of σshell, the shell smearing parameter, are obtained with SANS. This is probably because the upper limit in the q range for SANS was less than that for SAXS, making the parameter σshell less relevant to fit the data. It can be concluded that these results compare well with those obtain in SAXS, giving a supplementary proof of the validity of the models used. A last remark can be done about the model used in the previous study of the SANS data.26 Indeed, an ellipsoidal model was proposed for the micelles of RF8 (EO)9, instead of the spherical model used here. The ellipticity allowed having much bigger objects than the simpler model of spheres here. Nevertheless, the spherical model allows to fit satisfactorily both the SANS and SAXS data, in accordance with the molecular dimensions of the surfactants, and it is thus preferred to the ellipsoid model because it contains one less parameter, the ellipticity. 3.2. In situ SAXS during the Material Formation. 3.2.1. General Observations. 3D plots of the in situ timeresolved SAXS data are shown in Figure 3 (R8F(EO)9 and RF7 (EO)8 at R = 0.5, molar ratio surfactant/TMOS). The measurements were started at the addition of the TMOS to the micellar solution. For the RF8 (EO)9/water/TMOS experiments, the Bragg peaks of the hexagonal hybrid mesophase appear after 5 and 60 min of reaction at pH 7 and 2 (Figure 3, panels A and B), respectively. The same evolution is reported for the R7F(EO) 8/water/TMOS reaction. The formation of the mesostructure is detected on the scattering patterns after 14 and 95 min after the addition of the silica source at pH 7 and 2 (Figure 3, panels C and D). These results are explained by the kinetics of hydrolysis and condensation of the silica source.35 The stability of the silica sol is at maximum at pH 2−3 (isoelectric point of the silica), which leads to a longer precipitation time. At lower or higher pH values, the condensation is catalyzed by acidic or basic species. Moreover, the kinetics of RF8 (EO)9/water/TMOS systems is faster than that of the RF7 (EO)8/water/TMOS one. This difference is related to the temperature used for the reaction. The kinetics studies performed with the RF8 (EO)9/water systems were done at 40 °C to increase the reaction rate and corresponds to the standard conditions for materials preparation used in the laboratory.23 In contrast, the experiments on the RF7 (EO)8based system were performed at 25 °C in order to be in the micellar phase. At most five Bragg reflections are observed. The (10) Bragg peak intensity is very low in both systems compared to that in the higher-order reflections. This observation can be explained by the specific contrast of fluorinated surfactants, with the highest electron density level located in the core region. This point is detailed hereafter (Figures 9 and S.I.6 of the Supporting Information), using the relative intensities of the five observed Bragg peaks to determine the electron density repartition within the 2D-hexagonal mesophase. In addition, the scattering signal of the micelles contributes strongly all along the experiment, which is due to the high concentration of surfactant used for the synthesis. From this observation, we conclude that a large amount of the micelles do not participate in the mesostructure formation.

Figure 4. Silica clusters contribution vs time deduced from the fits. The (A) constant C and (B) gyration radius (Rg) are plotted vs time. The blue curves correspond to RF8 (EO)9 with R = 0.5, the black curves to RF8 (EO)9 with R = 0.25, and the violet to RF7 (EO)8 with R = 0.5. For all curves, a plateau is obtained when the precipitation occurs. For RF7 (EO)8, no values are given between 10 and 28 min as no complete fits were made. For RF8 (EO)9 with R = 0.25, the oscillations at very short times are due to the very low signal at the beginning of the kinetics, making it difficult to really estimate these values before 3 min.

prior to the appearance of the mesophase, those oligomers increase in size and number, and a plateau is reached at the precipitation for those values. After, the growth is stopped (more details in the Discussion). 2014


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the appearance of a q−2 slope at large angles and also by small changes in the micelle form factor (oscillation at q = 0.25 Å−1). First, both the increase of the intensity and the q−2 slope are the consequences of TMOS addition. Indeed, the q−2 slope at large angles is the signature of silica oligomers that rapidly form within the solution. This contribution to the signal, fitted by the Icluster(q) term, strongly increases to arrive at a plateau value of C = 0.026 cm−1 and Rg = 3.7 Å at the appearance of the mesophase (t = 5 min) and does not vary much after that (see Figure 4, blue curves). With respect to the micelle form factor, we can still obtain good fits using a spherical shape for the micelles. The change observed in the oscillations at q > 0.1 Å−1 is mainly due to a change in the contrast term, α=(ρshell − ρ0)/ (ρcore − ρ0), whereas the other parameters of the micelle form factor (radii, smearing) stay relatively constants. Figure S.I.4 of the Supporting Information shows the electron density profiles for the first five spectra, corresponding to the first 3 min of the kinetics. For these profiles, it is assumed that both the density inside the core of the micelles and the density of the solvent (ρ0= 334 e/nm3) are constant during the kinetics. Indeed, even if the TMOS hydrolysis produces methanol during the first minute of the synthesis, which is completely solubilized in water, its amount does not induce a significant change of solvent electron density. A slight decrease of the shell electron density is observed versus time. This shows that the majority of the surfactant micelles do not incorporate silica clusters at their interface, as this would increase the density of the shell. On the contrary, the PEO chains appear to be slightly better solvated, an effect that could be attributed to a change of the solvent quality. Lastly, if we focus on the signal at low angles (q < 0.1 Å−1), we see that the interaction peak (q = 0.055 Å−1) gets weaker with time, suggesting that repulsive interactions between spherical micelles become weak. Moreover, at q < 0.05 Å−1, instead of a plateau characteristic of spherical micelles, an increasing signal is appearing. Following the SBA synthesis,16 it is assumed that a portion of the micelles are incorporating silica in their corona and start to elongate to form cylindrical hybrid micelles. Those hybrid micelles are also present in the solution, changing the signal especially at small angles (see more details in the Discussion). Here, as the amount of hybrid micelles formed is really low, it is difficult to isolate this contribution. However, the kinetics performed with twice the amount of TMOS (R = 0.25) shows it clearly (Figure 6). Figure 6A gives the two first spectra of the kinetics, before the Bragg peaks appear. At 1 min and 30 sec after TMOS addition, a characteristic behavior at low angles, (for 0.02 ≤ q ≤ 0.08 Å−1) is observed. It proves the coexistence of two types of micelles in the solution prior to the appearance of the mesophase. More precisely, we were able to fit successfully the data with a model of coexistence of spherical micelles plus hybrid cylindrical micelles with a length, L, of 20 nm. The cross section of these hybrid cylindrical micelles (Rc = 0.9 ± 0.1 nm and Rt = 2.6 ± 0.1 nm) is comparable with those found for the spherical micelles. Slightly smaller values would be expected; however, the contribution of the cylindrical cross-section form factor is difficult to separate at large angles because of the mixture. The second stage (Figure 5B) starts at the Bragg peaks appearance and goes for 26 min after the TMOS addition. It is characterized by a decrease in the micelle signal, and an increase in the Bragg peaks signal. The solution is made of a mix of the hybrid material (Ibragg(q) term) and the excess micelles (Imicelles(q)). This stage mainly corresponds to the

3.2.2. RF8(EO)9. For the RF8 (EO)9 system at pH = 7, the kinetics after TMOS addition can be described by three main stages versus time, as shown in Figure 5, presenting selected

Figure 5. RF8 (EO)9 at pH = 7 and a ratio R = 0.5: time evolution of the SAXS signal during the kinetics. The 3D plot of this experiment is shown in Figure 3A. Every spectra are given in absolute scale I(q) vs q in a log−log representation. Three stages are observed: (A) Stage 1, from the beginning of the kinetics until the Bragg peaks apparition, spectra at 1, 2, 3, 4, and 5 min after TMOS addition. The inset gives spectra 1 and 5 at large angles to focus on the signal of silica oligomers. (B) Stage 2, between 5 and 26 min after TMOS addition, spectra at 6, 11, 16, 21, and 26 min. (C) Stage 3, from 27 min after TMOS addition until the end of the experiment, spectra at 30, 45, 60, 75, and 90 min. The blue arrows give the way of the temporal evolution. The black dots correspond to the fits.

spectra in a log−log representation. All stages can be fitted via the model of free spherical micelles, plus the Bragg contribution once the formation of the mesophase begins. The first stage (Figure 5A) goes from the beginning of the kinetics until the Bragg peaks appearance 5 min after the TMOS addition. This stage covers the changes in the solution prior to the precipitation. It is characterized by an increase in the intensity and a change of the curve shape at low angles, by 2015


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is negative, as the contrast is made between micelles and silicate walls, and decreases with time, in agreement with an increase of silica concentration within the material walls. Moreover, the domain size increases to reach a value of D = 200 nm. Figure 9A shows the Bragg peak signal (red curve) for the last spectra of the kinetics (90 min after TMOS addition) and the associated fits. The dark gray curve is obtained using the expression for IBragg(q) with the P(q) term (see Fitting Models), and the light gray curve corresponds to the measure of the Bragg peak intensities, from which the individual |Fkh|2 values are extracted, to perform a 2D electron density reconstruction of the hybrid material given in Figure 9C. More details are also given in Figure S.I.6 of the Supporting Information. In Figure 9E, the radial density profile is also shown and compared with the one given by the fitting procedure. Both methods allow for the conclusion that the walls are well-formed between the cylindrical micelles of the lattice. Nonetheless, it has to be noted that the fitting procedure gives smaller walls than the reconstruction procedure (wall thickness of 0.6 nm instead of 1 nm for the reconstruction). This comparison also allows for the conclusion that the volume fraction of the material is about 0.05, which is slightly above 0.03, the maximum volume fraction of SiO2 that can be formed in solution by knowing the initial amount of TMOS. This, plus the low intensity of the background term, which does not vary significantly since the formation of the mesophase, indicates that a large amount of the silica introduced participate in the material formation. With a focus on the micelles in excess, we can conclude that their shapes do not vary much during the kinetics (Rc is kept at 0.9 nm and Rt = 2.5 nm), and especially that they do not accumulate silica oligomers within their corona, proving that only those micelles that incorporate silica participate to the lattice formation. At t = 90 min, it can be seen that the intensity of the signal of the spherical micelles is still high. By comparing with the starting intensity (before TMOS addition), it is found that, for R = 0.5, about 80% of the surfactant spherical micelles do not interact with the silica and remain in solution, whereas this value is about 50% for R = 0.25. Again, we conclude that a large amount of the micelles do not participate in the material formation. However, with the doubled amount of initial TMOS (R = 0.25), more micelles are incorporating silica. Nevertheless, at the end of both experiments (t = 90 min), the signals of the material present the same intensities (BP(q = 0) = 2.2 for R = 0.5 and 1.9 for R = 0.25). Moreover, the cell parameter a is the same for both systems as is the domain size, D. This shows that neither the amount nor the quality of the material formed is increased by adding more silica source, even if more hybrid cylindrical micelles were formed before precipitation. Indeed, for R = 0.25, it has been found that nearly 15% of the hybrid micelles formed at the beginning of the kinetics do not participate in the material formation and stay in solution. This may indicate that R = 0.25 is not optimum to form the material, as probably too much silica is added in the solution. A previous study focused on the final material characteristics after surfactant extraction. This study showed that for R = 0.25, the silica surplus formed a silica gel instead of participating in the mesophase.21 This element is in agreement with the larger silica clusters formed in the solution for R = 0.25 (Rg = 5.5 Å, instead of 3.7 for R = 0.5, see Figure 4B, black curve), with an overall intensity, C, two times higher (Figure 4A, black curve). More details are given after in the discussion.

Figure 6. Experiment with RF8 (EO)9 at pH = 7 and a ratio R = 0.25 (doubled amount of TMOS). (A) The two spectra recorded prior to the formation of the 2D-hexagonal mesophase. The first spectrum (t = 1 min after TMOS addition) is in red and the second one (t = 1 min and 30 s) in orange. (B) Evolution vs time after the formation of the 2D-hexagonal mesophase: after 2 min (yellow), 5 min and 30 s (green), and 58 min and 30 s (blue, end of the experiment). The black dots correspond to the fits. The 3D plot of this experiment is shown in Figure S.I.3 of the Supporting Information.

consumption of the cylindrical micelles to form the lattice. It is shown once again more clearly in the kinetics with R = 0.25 (Figure 6B). With the material formation, the extra signal at low angles slowly decreases. We also observe an increase in the q−4 term at very low angles, characteristic of the grains of material forming. In the last stage (Figure 5C) an increase of the Porod term with a q−4 slope at very low angles is observed. This is expected as it corresponds to the scattering by the interface of the grains of the 2D-hexagonal mesophase. Upon time, the grains are growing in size and their contrast with respect to the solvent is increasing because the silica regions are more and more condensed. The micelle form factor does not vary during this stage and is still well-modeled by the spherical core−shell model. The interactions between micelles are weaker than they were prior to the TMOS addition, with RHS = 3.8 nm, and φHS = 0.08 (respectively equal to 4.3 nm and 0.13 prior to TMOS addition), in agreement with the consumption of the surfactant to form the lattice. The evolution of the parameters linked to the lattice is given in Figure 8. It is seen that an interface smearing term is needed to fit the data, in agreement with the formation of a silica wall between the micelles. Besides, the lattice is a rather compact one (the lattice parameter a = 6.6 nm), with a/2 ≈ Rt2 + σshell2 (Figure 8C). Values of Rc2 and Rt2 are in agreement with the ones of micelles. The scale of the Bragg peaks (B) contribution stays relatively constant (Figure 8A), in agreement with the fact that all hybrid micelles have precipitated at this stage. The contrast term in the mesophase α2 = (ρshell − ρwall)/(ρcore − ρwall) is given in Figure 8E. Its value 2016


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b-3). RF7(EO)8. With this system, the kinetics at pH = 7 can also be described by three main stages (Figure 7). The first

micelles with a gradual decrease of the interaction term, which could be associated with a decrease in micellar concentration. However, during the second stage (Figure 7B), from 8 min after the TMOS addition until the Bragg peaks appearance at 14 min, the low angle slope keeps increasing further, until it reaches q−1.58. This behavior cannot be linked with the one of stiff objects (q−1 behavior), nor the one of totally flexible ones (q−5/3 = q−1.66) (see Fitting Models). A change in the middle angles is also observed. Hence, this stage cannot be satisfactorily described in the framework of the present model, and several qualitative interpretations are discussed later in the discussion part. The last stage (Figure 7C) corresponds to the material formation. Good fits can be obtained using a combination of free short wormlike micelles in coexistence with the hybrid 2Dhexagonal mesophase. Figure 8 gives the parameters describing the lattice versus time (from t = 25 min), whereas Figure S.I.5 of the Supporting Information gives the evolution of the scale and interactions of the micelles within the solution. In this stage, Bragg peak intensity increases (see Figure 7B), whereas micelle intensity decreases with time (Figure S.I.5A of the Supporting Information), once again corresponding to an increase in the amount of micelles that precipitate to form the material. Besides, the micelle intensity diminution is also connected to a lowering of the low angle slope, until it reaches a q−1 behavior. This shows that hybrid objects were formed prior to the precipitation and were responsible for the extra intensity at low angles, whereas micelles were still unchanged. Indeed, once the low angle slope is close to a q−1 dependency, the micelles are still well-modeled with the short wormlike model (Rc = 0.8 nm, Rt = 1.6 nm, σshell = 0.8 nm, with LC = 50 nm and bKuhn = 26.9 nm). Furthermore, at the end of the kinetics, νRPA = 2.5. This value is still much lower than found prior to TMOS addition (νRPA = 5.87), in agreement with the interpretation that a part of the micelle has precipitated to form the material. On the other hand, the mesophase shows differences with the one observed using R8F(EO)9 as a surfactant. Figure 8D shows that, if Rc2 is the same as for micelles, Rt2 is much larger (3.0 nm instead of 1.6 nm). Moreover, no smearing of the corona is needed to fit the data, and Rt2 is really close to a/2 (a = 6.2 nm at the end of the kinetics). Figure 8F also shows that the contrasting term within the lattice α2 has values close to zero, pinpointing a low contrast. All these elements indicate that the contrast between the shell and the wall is not pronounced, and that we do not form visible and large silica walls. This result is confirmed by focusing on the last spectra of the kinetics. Figure 9B gives the last spectra (at 90 min). As for RF8 (EO)9, Bragg peaks intensities were extracted to perform an electron density profile reconstruction. Both the 2D density profile (Figure 9D) and the radial density profile (Figure 9E, orange) indicate that the silica walls were not well-formed. Radial density profiles given by the reconstruction method (orange curve), and fitting procedure (green curve) are also in good agreement. Besides, this study gave a volume fraction of material of 0.03 at the end of kinetics, so lower than that for RF8 (EO)9. Furthermore, the background term is found to have increased until the appearance of the mesophase (see Figure 4, violet curves), and the following values are obtained at the end of the reaction: C = 0.046 cm−1 and Rg = 4.8 Å. Both elements, strong clusters signal, and low volume fraction of material, indicate that less silica is used here to form the material.

Figure 7. RF7 (EO)8 at pH = 7: time evolution of the SAXS signal during the kinetics. The 3D plot of this experiment is shown in Figure 3C. Every spectra are given in absolute scale I(q) vs q in a log−log representation. Three stages are observed: (A) Stage 1, spectra at 1, 2, 3, 4, 5, 6, 7, and 8 min after TMOS addition. (B) Stage 2, spectra at 9, 11, 14, 15, and 16 min after TMOS addition. (C) Stage 3, from 19 min until the end of the experiment, spectra at 19, 29, 41, 51, 61, 71, 81, and 90 min after TMOS addition. The blue arrows give way of the temporal evolution. The black dots correspond to the fits.

stage (Figure 7A), covering the first eight minutes of the kinetics, can be fitted with the model of free short wormlike micelles described above. In a similar way as for RF8 (EO)9, this stage is characterized by the q−2 slope of silica oligomers appearing at large angles (with C = 0.04 cm−1 and Rg = 3.8 Å) and a slight decrease of α, the contrast parameter in the form factor of the micelles. Moreover, we observe an increase of the slope at low angles. At 7.5 min after TMOS addition, the low q region follows a q−1 slope, which is the characteristic behavior of stiff objects without interactions. Hence, until 7.5 min, the micelle contribution can be described by the short wormlike 2017


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Figure 8. Evolution vs time of the hybrid material deduced from the fits at pH = 7 for RF8 (EO)9 (on the left side) and RF7 (EO)8 (on the right side): (A and B) BPcyl(0), scale parameter. (C and D) a/2, Rt2, Rc2, and σshell2, and (E,F) contrast parameter, α2 = (ρshell − ρwell)/(ρcare − ρwell). The χ2 value (quality of the fit) is between 0.8 and 1.5.

Can we explain why the wall thickness is smaller for RF7 (EO)8 than for RF8 (EO)9? At first sight, the two surfactants are very similar, with the RF7 (EO)8 molecule only slightly smaller than RF8 (EO)9 (8 EO groups compared to 9, 7 fluorine atoms compared to 8, and the same C2H4 spacer for both molecules). Grandjean et al.36 have proposed a model to predict the silica wall thickness based on molecular force balance between hybrid micelles. The authors pointed to the fact that the silica wall thickness increases with the length of the polar head of the surfactant used, whereas it decreases with the length of the hydrophobic chain increase. Then R8F(EO)9 should, as observed, form thicker walls than RF7 (EO)8 (9 EO groups instead of 8), but it has on the other hand an additional CF2 group, which on the contrary is in favor of thinner walls. Then it seems that, in the molecular force balance, the dominant effect that stabilizes thicker walls is the increase of the EO chain length by one EO group, which overcomes the increase of the fluorinated chain length by one CF2 group. However, the model proposed by Grandjean et al. may not be enough to explain the differences between these two surfactants. The shape and size of the micelles seem to be also an important element, as already pointed out previously.26 As also highlighted by Grandjean et al.,36 a strong decrease of the wall thickness is expected when using fluorinated surfactant

This study of the material formation thus gives interesting clues for the understanding of why no ordered porous material is recovered after thermal treatment in this system.

4. DISCUSSION The main result is that, unexpectedly, a 2D-hexagonal mesophase is formed for the RF7 (EO)8/water system, although the recovered materials obtained with this surfactant after hydrothermal treatment and solvent extraction are disordered (see Figure S.I.7B of the Supporting Information).24 Complementary SAXS experiments have shown that the hexagonal structure is maintained after the hydrothermal treatment at 80 °C for 24 h and during the ethanol extraction of the surfactant, and the mesostructure is lost during the drying of the silica at room temperature at the end of the extraction. From this result, we can conclude that the silica walls are not robust enough to preserve the structure during the drying. This may be due to a too small wall thickness, a suggestion which is in agreement with what is observed during the kinetics. Indeed, from the Bragg peaks intensity analysis, we find that the walls are wellformed in the 2D-hexagonal mesophase only for RF8 (EO)9 and not for RF7 (EO)8 (Figure 9). 2018


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Figure 9. 2D-hexagonal mesophase obtained 90 min after TMOS addition for the two experiments at pH = 7 with R = 0.5 (see also Figures 3, 5, 7, and Figure S.I.6 of the Supporting Information). (A and B) Zoom on the Bragg peak region for (A) RF8 (EO)9 and (B) RF7 (EO)8. The experimental data are shown in red. The fits with IBragg(q), including the form factor expression, are the dark gray lines, and the light gray lines correspond to the fit of the five Bragg peaks used for the electron density reconstruction. (C and D) 2D electron density reconstruction in the hexagonal plane for (C) RF8 (EO)9 and (D) RF7 (EO)8. The different colors correspond to different iso-density levels; bright colors stand for high electronic densities. (E and F) Electron density profile between two lattice nodes obtained via the electron density reconstruction (orange) and the fitting procedure (green) for (E) RF8 (EO)9 and (F) RF7 (EO)8. Silica walls are indicated by black arrows.

compared to the hydrogenated equivalent, due to an increase of the Hamaker constant. To our knowledge, no in situ experiments are available to investigate this point. Such experiments should be done with the equivalent nonionic hydrogenated surfactants, like C16EO10, in terms of hydrophobic/hydrophilic balance. It is interesting, however, to compare with the case of the well-known SBA-15 materials, where the surfactant is a nonionic Pluronic triblock copolymer

(P123 = EO20PO70EO20) and for which many in situ studies by SAXS or SANS have been performed, even if the syntheses are often made at lower pH values than in the present study.13,15−16,37−40 Nonetheless, for the fluorinated surfactants, it has been shown that well-ordered materials are obtained in a wide range of pH values (from 0.025 up to 11).28 The length of the EO groups is much larger than for the fluorinated surfactants discussed here, and the nature of the core is 2019


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min for RF8 (EO)9 with R = 0.5). With R = 0.5, larger silica clusters are obtained for RF7 (EO)8 than for RF8 (EO)9 (Rg = 5.0 Å instead of 3.7 Å), along with overall slower kinetics. For the experiment at R = 0.25 (black curves), the plateau is already reached after 2 min, with a cluster size of Rg = 5.3 Å. Note that these gyration radii correspond to clusters containing about 5 and 16 SiO2 units, respectively. This can be understood because the increase of the TMOS concentration induces a faster hydrolysis and condensation. Hence, larger silica clusters rapidly form within the solution without interacting with the micelles. Without the presence of a surfactant, these clusters would keep on growing and ultimately form a silica gel via a sol gel transition. The fact that a plateau is reached for the three experiments may indicate a competition between two phenomena: the material formation on one hand and a silica gel formation on the other hand. This competition may also explain why adding more TMOS do not induce systematically a larger amount of meso-structured material, favoring instead the growth of the clusters. In order to describe with accuracy the interactions between silica species and micelles in solution that will lead to the material formation, the moment just before the precipitation, is crucial. For the SBA-15 material formation, a “sphere-to-rod” shape transformation of the micelles prior to the formation of the mesophase has been proposed. The question is to know if such a “sphere-to-rod” model can apply here, especially for RF7 (EO)8 as the initial micelles are not spherical. As already mentioned, the high concentration of micelles (10 wt %) at the beginning of the synthesis, necessary to reproduce in situ the usual synthesis conditions, make the SAXS analysis more difficult than in the case of SBA-15. First, we find that the excess of micelles essentially dominates the signal and to some extent hides what happens to the portion of surfactant molecules that participate in the material formation. In addition, the contribution of the structure factor S(q) always has to be taken into account, making the determination of the form factor much more difficult than without interaction terms. What we conclude is that the kinetics for RF8 (EO)9 follows the same main stages as the ones of SBA-15.16 Indeed, we assume a model of “sphere-to-rod” transition of some micelles, the ones that will participate in the material formation. Another route, called the reservoir mechanism, is proposed in the literature for the formation of 2D-hexagonal material from spherical micelles. This is, for example, the route used to describe MCM-41 formation by Patarin et al.9,42 This mechanism is based on the idea that the surfactant monomers present in the solution interact with silica oligomers and start to aggregate, whereas the spherical micelles break to restore the monomer/micelle balance in the solution. If this route was followed here, we should observe at small angles only a gradual decrease of repulsive interactions with the material formation, as no hybrid micelles are formed via this route. But here, in contradiction with this route, an additional contribution is observed prior to the formation of the mesophase. This contribution is well-explained by a signal of elongated hybrid micelles that adds to the signal of the spherical micelles in excess. The complementary experiment made with twice the amount of TMOS (R = 0.25), even if it is not the “ideal” synthesis conditions (kinetics too fast, larger amount of silica oligomers), allows more micelles to interact with the silica species in the solution and to clearly show the presence of such hybrid micelles (Figure 6).

different (PPO instead of perfluorinated chains). For SBA-15 calcined materials, the wall thickness ranges from 3 to 6 nm, depending on the synthesis conditions. For the RF8 (EO)9/water system, the wall thickness is smaller and equal to 2.4 nm, as deduced from the cell parameter a0 calculated from the d10 reflection (a0 = 2d10/√3) and the pore diameter determined from nitrogen adsorption/desorption measurements (see Figure S.I.7 of the Supporting Information). From the SAXS patterns of the materials prepared from the RF7 (EO)8 surfactant, the pore thickness cannot be calculated due to the lack of pore arrangement. A simple way to compare different EO nonionic surfactants is to consider the EO:Si molar ratio used for the synthesis. This molar ratio is given first by the amounts of the different reactants and may have a different value inside the hybrid mesophase. Usually, for a given surfactant, an optimum value of the EO:Si ratio to obtain well-ordered materials is experimentally determined by performing synthesis at a different ratio. Then, the value of EO:Si alone already gives a good indication of how much silica can be incorporated inside the mesophase, if all the surfactant molecules participate in the formation of the mesophase. Here, a previous study has shown that the optimum synthesis ratio EO:Si is around 4 for both surfactants.23 For SBA-15, the optimum ratio EO:Si is about 1, that is 4 times more silica. For SBA-15, based on the SAXS in situ experiment, it has been established that all the Pluronics molecules are incorporated in the mesophase, with no extra micelles left in solution.16 This means that the ratio EO:Si is still around 1 in the mesophase. Indeed, this is also in agreement with the value of the electron density in the shell, 380 ± 10 e/nm3, and which corresponds to the silica oligomers interacting with the EO chains, still with the same molar ratio. Here, with the fluorinated surfactants, the value of the electron density obtained in the mesophase inside the “wall” region for RF8 (EO)9 is similar (Figure 9). Even if it is difficult to quantify precisely the value of the EO:Si ratio inside the mesophase, it means that it is much smaller than 4, the value calculated from the initial proportions of the reactants. Recall that a large excess of micelles is observed in solution. This tends to show that, inside the mesophase, the EO:Si is in fact not very different from that of SBA-15, and that a value of EO:Si close to 1 could correspond to a critical value for the precipitation of the mesophase for all EO nonionic surfactants. Here, the length of the terminal EO chains for RF8 (EO)9 is smaller than for that of P123, 9 instead of 20, and this explains essentially why the wall thickness is less. Again, this simple comparison shows that a long EO chain is a crucial element to incorporate large quantities of silica in order to get thick silica walls. Interestingly, let us just mention to conclude on this point that a mixture of P123 and RF8 (EO)9 has been recently used, leading to robust bimodal porous silica materials.41 When studying kinetics of such material formations, the question of the interactions between the surfactant and the silica species is crucial to the understanding of the self-assembly mechanisms at stake. Hence, it is important to evaluate the evolution of the silica clusters formation in the solution and study the micelles form factor before the precipitation. Figure 4 gives the evolution with time of the silica clusters for the three different experiments performed at pH = 7: RF8 (EO)9 with R = 0.5 (blue curves), R = 0.25 (black curves), and RF7 (EO)8 with R = 0.5 (violet curves). First, one can follow the growth of the clusters during the first stage, until a plateau is obtained when the precipitation occurs (after 5 min for RF8 (EO)9 with R = 0.5, 2 min for RF8 (EO)9 with R = 0.25, and 14 2020


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For RF7 (EO)8, it is more difficult to give a completely clear picture of the mechanism, as the second stage of the kinetics (Figure 7B) cannot be fitted by the model of short wormlike micelles because of the slope evolution at low angles. The maximum slope (q−1.58) is in agreement with neither stiff objects (q−1) nor flexible ones (q−1.66 for wormlike micelles in a good solvent). Moreover, even if a q−1.66 behavior was observed in the low-q range, it should coexist with a q−1 behavior in the intermediate q range.29,30 By comparing these results with the SBA-15 and RF8 (EO)9 results, one could expect that hybrid cylindrical micelles are forming before precipitation. But this cannot explain the slope evolution. Another possibility to explain these data would be a fractal model, with a fractal dimension reaching 1.58 at pH = 7 (1.55 at pH = 2). Then, we have to answer the question: What kind of particles in the solution could have a fractal structure? One interpretation could be that the silica clusters are forming aggregates of increasing size versus time, and that the observed exponent would correspond to their fractal dimension. In fact, it has been known for a long time that the condensation of TMOS in water solutions give rise to a sol−gel transition with fractal exponents in the same range of 1.5−1.7.43,44 Note that the clusters contribution then contains two parts: the q−2 behavior at high q values, already included in the model and corresponding to the elementary clusters and the fractal exponent at low q corresponding to the aggregates.44 But, then it would imply that the silica clusters interact together to form silica aggregates rather than with the surfactant molecules present in a large amount in the solution. Another weak point for this interpretation is also that the exponent decreases again after the formation of the material (during the third stage), which is inconsistent with this idea of silica gel formation. Another possible interpretation for this fractal model could be the formation of branching points between the wormlike micelles, and the slope at low angles would then be related to the connectivity of the branching points.45 This could be put in relation with the birefringence studies showing the existence of a connected structure at higher concentration in surfactant (20 wt %).27 The formation of branching points would be driven by an attractive interaction between the wormlike micelles. In favor of this interpretation is that the modification of the micelles induced by the silica oligomers would follow a similar trend as that in the “sphere-to-rod” transition: a decrease of the average curvature of the micelles along with an increase of their overall size. It is nevertheless difficult to figure out how branched wormlike micelles could arrange afterward into a 2Dhexagonal mesophase. Anyway, the most important fact is that this uncommon behavior is localized in time. This behavior is observed shortly before the Bragg peaks appearance (that is shortly before the formation of the mesophase) and disappears with the material formation and is totally gone once the material is formed, allowing us, during stage 3 (Figure 7C), to obtain good fits using free wormlike micelles in solution in coexistence with the precipitated mesophase. Hence, this slope is probably the signature of hybrid objects, which are the premises for the material formation. It can be suggested here that in situ rheological measurements would aid in the understanding of the very nature of these hybrid objects.

Article

CONCLUSION This study provides first a detailed description of the micellar state of the two investigated surfactants, RF7 (EO)8 and RF8 (EO)9. Small spherical micelles are found with RF8 (EO)9, whereas RF7 (EO)8 gives much larger micelles, which can be described as short wormlike micelles. Even if the micelles have different shapes, in situ SAXS experiments reveal that a 2D-hexagonal mesophase is obtained for both systems after TMOS addition. The proposed mechanism is that only a part of the micelles interact with silica oligomers to form hybrid micelles. These hybrid micelles result from a “sphere-to-rod” transition [RF8 (EO)9] or are more complex aggregates difficult to fully characterize [RF7 (EO)8]. In both systems, these hybrid micelles are believed to act as nucleation centers for the formation of the 2D-hexagonal mesophase. Then the 2D-hexagonal mesophase coexists with a large excess of free surfactant micelles in solution. It happens that the mesophase formed by RF7 (EO)8 is not stable after solvent extraction, and this can be explained by the fact that the silica walls are very thin. This result confirms the fact that EO nonionic surfactants should have a long enough hydrophilic chain to induce robust silica walls in the final material.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

SANS data in Figures S.I.1 and S.I.2, fit parameters of the data in Table S.I.1, 3D plot in Figure S.I.3, radial density profiles in Figure S.I.4, evolution versus time in Figure S.I.5, modeling of the 2D hexagonal phase in Figure S.I.6, and characteristic of the recovered materials in Figure S.I.7. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author

*E-mail: [email protected]. Present Address @ Laboratoire d’Ingénierie des Biomolécules (LIBio) ENSAIA, Université de Lorraine 2, avenue de la Forêt de Haye, BP-172, 54505 Vandœuvre-lès-Nancy, France.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the LLB (PAXE), the ESRF (CRG-D2AM), and the SOLEIL synchrotron (SWING) for beam-time allocation. The support from the Danish Council for Independent Research: Natural Sciences is gratefully acknowledged.

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REFERENCES

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