Kinetics, Thermodynamics, and Dynamics in Organosilane Self

Article pubs.acs.org/JPCC

Kinetics, Thermodynamics, and Dynamics in Organosilane SelfAssembly Lingli Ni,† Abraham Chemtob,*,† Céline Croutxé-Barghorn,† Jocelyne Brendlé,‡ Loïc Vidal,‡ and Séverinne Rigolet‡ †

Laboratory of Photochemistry and Macromolecular Engineering, and ‡Institut de Science des Matériaux de Mulhouse, Mulhouse, LRC-CNRS, 7228, ENSCMu, University of Haute-Alsace, 3 rue Alfred Werner 68093 Mulhouse Cedex, France S Supporting Information *

ABSTRACT: Organosilane self-assembly is a widely studied template-free approach to design organic−inorganic hybrids structured at the nanometer scale. The main emphasis has been focused so far on novel precursor architectures and sol−gel preparation methods to drive the self-assembly. This feature attempts for the first time a thermodynamic, kinetic, and dynamic description of the organosilane supramolecular assembly. Condensation and hydrolysis rates are the main kinetic parameters impacting the self-assembly, while organic moiety, alkoxy head, temperature, or relative humidity determine essentially the energetic contributions of the self-association, and therefore, form part of a thermodynamic description. In terms of dynamics, the gradual conversion of the isotropic precursor into a cross-linked hybrid nanostructure was assessed by time-resolved infrared spectroscopy combined with small-angle X-ray scattering. To reveal the mechanism of self-assembly, our system is simplified to the main ingredients: n-dodecyltrimethoxysilane (C12H25Si(OCH3)3) as a model organosilane building block and a photoacid generator ((C12H25)2Φ2I+ SbF6−), deposited as a photolatent micrometric film. UV light governs the sol−gel polymerization kinetics through the controlled liberation of Brönsted superacids.

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INTRODUCTION

(i) The R−Si(OH)3 organosilanes arising from hydrolysis consist of a condensable hydrophilic silanol head conjugated to a structural hydrophobic element R. Although the coupling of these two units yields an amphiphile, its ability to self-assemble is dictated by inter- and intramolecular interactions. For the generation of long-range ordered mesostructures, the preferred approach advocated by Moreau et al.7−9 in 2001 relies on the molecular design of the organic moiety with functionalized groups possessing strong cohesive forces. Hydrogen bondings (urea,8,10,11 amido,12,13 amino acids14) and π−π stacking15,16 are the most widespread attractive forces, while polar17,18 or nonpolar6,19−25 van der Waals interactions are also reported. In regards to the siloxy segment, symmetric bridged structures (O1.5Si−R−SiO1.5) promoting interactions between the organic units showed a higher ability for self-organization than their monosilylated counterparts (R−SiO1.5).2 In addition, Kuroda et al. demonstrated that a larger siloxy head was an effective way to promote a richer structural polymorphism, thus enabling the generation of various lyotropic hybrid phases.26−28 (ii) Attention has also been paid to the judicious selection of experimental parameters29 (temperature, catalyst, solvent, and precursor concentration) as a means to favor a long-range periodicity. For example, a change of catalyst from HF to HCl was found to drive the transition from an amorphous structure to a lamellar morphology.9 Corriu et al. prompted interest in

Sol−gel polymerization of organosilanes has proven increasingly popular as a single-component synthetic route toward periodically ordered organic−inorganic nanostructures.1−4 Proceeding without surface-directing agent, the intricate formation mechanism can be viewed as a three-step concurrent process. The amphiphile species first form in situ by precursor hydrolysis, which in a second step self-assemble, to organize ultimately into a solid cross-linked mesostructure by Si−O−Si condensation. Despite numerous investigations since the pioneering works of Huo5 and Parikh6 on lamellar siloxane/ alkyl hybrids, it is not yet been possible to predict the order and structure starting from a given organosilane building block unit. The complexity of a thorough description of organosilane selfassembly stems first from the diversity of the sol−gel process conditions with multiple experimental parameters, precursor structures, and reaction pathways. Not only are there a number of experimental issues, but there are many conceptual hurdles: the randomness of the Si−O bond network, the competitive hydrolysis−condensation reactions, and a starting precursor lacking amphiphilicity. Finally, a self-assembly process intertwined with a polymerization reaction implies a temporal change of the precursor structure, which complicates the thermodynamics of amphiphilic aggregation and confers a primordial role to reaction kinetics. To bring about selfassembly, many parameters already have been identified, in particular, (i) the precursor architecture, and (ii) the preparation method. © 2012 American Chemical Society

Received: July 23, 2012 Revised: October 15, 2012 Published: October 22, 2012 24320

dx.doi.org/10.1021/jp307274d | J. Phys. Chem. C 2012, 116, 24320−24330

The Journal of Physical Chemistry C

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this area by coining the concept of “kinetically-driven” selforganization.1 In another emblematic example, a precise control of the sol aging in alcohol-free and basic conditions allowed Innocenzi et al. to produce lamellar films with 3-glycidoxypropyltrimethoxysilane.30 However, the preparation method is not only related to kinetic parameters, and can also affect the thermodynamics of self-assembly, as exemplified by the structuring role of an excess of water added to bridged alkylene precursors.31 By swelling the siloxy parts, water solvation can enhance the phase segregation and induce the formation of structures with higher interfacial curvature. Although the combined effect of structural and experimental factors on organosilane self-assembly is well recognized, there is almost no attempt to find a complete thermodynamic and kinetic description of this process.6 To reveal the mechanism of organosilane self-assembly, our system was simplified to the main ingredients: n-dodecyltrimethoxysilane (C 12 H 25 Si(OCH3)3, C12TMS) as a model organosilane building block and photoacid generator (C 12 H 25 ) 2 Φ2 I + SbF 6 − , PAG), deposited as a photolatent micrometric film. UV irradiation controls the liberation of Brönsted superacid catalysts via PAG decomposition, which then foster the sol−gel polymerization and self-assembly.20,25 Therefore, the number of components and variables is significantly reduced compared to a conventional sol−gel route in solution: solvent and even water are advantageously removed, while UV provides a means of controlling the sol−gel kinetics through light intensity modulation. The condensation/hydrolysis rates and the mobility of the amphiphilic species are the main kinetic parameters influencing the formation of supramolecular structures. In addition, parameters such as alkyl chain length, alkoxy structure, temperature, or relative humidity essentially determine the energetic contributions of the self-assembly by changing the enthalpy and entropy of association, and therefore, form part of a thermodynamic description. Point by point, this tutorial article first reviews their effect on the supramolecular structure and its level of ordering (amorphous, short- or long-range order). A second approach angle to study self-assembly is from the dynamics point of view, by evaluating the structural changes throughout the UV irradiation time, from an isotropic precursor solution to an organized organosilica mesostructure. Both parts should merge into a complete description of organosilane self-assembly. Sol−gel kinetics can be assessed in situ by a temporally resolved technique: real-time Fourier transformed infrared spectroscopy (RT-FTIR). For the characterization of the supramolecular structure, small-angle X-ray scattering (SAXS) is systematically used. When an organized nanostructure emerges from the n-alkylsilane polymerization, a lamellar alkyl/siloxane mesostructure alternating siloxane layer and dodecyl bilayer is generally obtained, as sketched in Scheme 1. In addition, the alkyl chain order at the conformational level is examined by the FTIR symmetric and asymmetric CH2 stretching modes (d+ and d−, respectively). Herein, we also endeavor a more rational approach toward the design of organosilica mesostructures. The properties of such hybrids have already been established and customized in a number of applications, such as polymer nanocomposites,32 heterogeneous catalysts,33 electronics,34 and nonlinear optics.35

Scheme 1. Representation of the Photoacid-Catalyzed Sol− Gel Process of C12TMSa

a The UV-driven self-assembly affords a lamellar hybrid mesostructure, consisting of alternating siloxane layers separated by docecyl moieties.

lane (C12TMS, 95%), and n-dodecyltriethoxysilane (C12TES, 95%) were purchased from ABCR. Benzophenone used as a photosensitizer and the photoacid generator UV1241 (Bis(dodecyl)diphenyliodonium hexafluoroantimonate salt) were provided by Aldrich and Deuteron, respectively. All chemicals were used as received without any further purification. Scheme 2 depicts the photolysis mechanism of the photoacid generator in the presence of benzophenone. Scheme 2. Photolysis Mechanism of the Photoacid Generator in the Presence of Benzophenonea

a

As diaryl iodonium salts weakly absorb light above 300 nm with a limited overlap with the light emission region of conventional medium- or high-pressure mercury arc lamps, a practical solution consists of the addition of a photosensitizer (PS), having an extended absorption range, and able to transfer its excitation to the photoacid generator. An electron transfer preferentially occurs when using benzophenone as PS. In addition to radical, radical cation species, H+SbF6− superacids, are generated, which demonstrated their capacity to catalyze sol−gel reactions.

Synthesis of Nanostructured Hybrid Films Derived from C12TMS. Photoacid generator UV1241 (2 wt %) and benzophenone (2 wt %) were dissolved in the C12TMS precursor to form a photolatent solution in the absence of UV light. Then, the resultant mixture was deposited on a CaF2 pallet (FTIR experiments) or silicon wafer (SAXS chacaterization) with use of an Elcometer 4340 automatic film applicator equipped with a wire wound bar to produce a reproducible 4 μm thick liquid film determined by profilometry. In a typical procedure, the UV-driven self-assembly was performed at a

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EXPERIMENTAL SECTION Materials. n-Octyltrimethoxysilane (C8TMS, 97%), ndecyltrimethoxysilane (C10TMS, 98%), n-dodecyltrimethoxysi24321



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a traditional solution method of preparing sol−gel organosilica films, two differences should be particularly emphasized: the absence of reaction without UV light (photolatency) and the removal of solvent and water from the film preparation. 1.1. Kinetics Controlled by Light Intensity. The most straightforward demonstration of the impact of kinetics on mesostructuration is to be found in an early study on alkyl/ siloxane lamellar films. Kuroda et al. showed brilliantly that a 2step methodology separating clearly the alkylsilanetriol CnH2n+1Si(OH)3 synthesis from their condensation was an effective pathway to crystalline structures.23,38 Figure 1 shows

controlled temperature, with a relative humidity (RH) maintained between 30% and 35%. UV irradiation was provided by the polychromatic light of a mercury−xenon lamp (Hamamatsu L8251, 200 W) fitted with a 365 nm elliptical reflector and coupled with a flexible light-guide. The end of the optical guide was placed vertically at a distance of 3 cm from the film and directed at an incident angle of 90° onto the sample window. The samples were irradiated at an incident light intensity of 20 mW/cm2 during 16 min to yield a transparent solid poly(n-dodecylsilsesquioxane) film. RH and temperature-dependent experiments were carried out in a customized environmental cell. The RH was controlled by using saturated salt solutions of NaCl. The light output also can be adjusted within 20 and 200 mW/cm2. Note that the elliptical reflector efficiently reflects the UV light, and lets heat rays and visible light pass through to prevent adverse effects from heat on the irradiated film. We checked that there was no temperature increase during the irradiation time, which could affect the self-assembly process. Characterization. Infrared spectra obtained by RT-FTIR were recorded in transmission with a Bruker Vertex 70 spectrophotometer equipped with a liquid-nitrogen-cooled mercury−cadmium telluride (MCT) detector. The resolution of the infrared spectra was 2 cm−1. In a conventional RT-FTIR analysis, the C12TMS-PAG film was simultaneously exposed to UV light from the light guide output end, and to the analyzing IR beam. The progressive hydrolysis of the methoxysilyl functions during the irradiation was followed by using the symmetric CH3 stretching vibration (νsym(CH3)) appearing as an isolated and sharp band, which tends to decrease gradually in intensity throughout UV irradiation. The in-phase CH3 stretching vibration of SiO−CH3 absorbs near 2840 cm−1, and can be distinguished from the comparable CH 3 absorption36 of (CH2)11−CH3 near 2870 cm−1. Films thicknesses were assessed by profilometry, using an Altisurf 500 workstation (Altimet) equipped with a 350 μm AltiProbe optical sensor. The SAXS patterns were obtained on a PANalytical X’pert Pro diffractometer with fixed slits, using Cu/Kα radiation (λ = 1.5418 Å) and θ−2θ mounting. Before analysis, films on silicon wafer were directly deposed on a stainless steel sample holder. Data were collected between 0.5 and 10° 2θ degrees with a scanning step of 0.01 deg s−1. At low angles, a square surface of 1−3 mm per side is analyzed and the light penetration is close to 100 μm. Film morphology was evidenced by scanning electron microscopy (SEM) (FEI Quanta 400 microscope working at 30 kV). The samples being nonconductive, they have been metalized with gold (15 nm thickness). The transmission electron microscopy (TEM) observations were performed with a Phillips CM200 microscope operating at 200 kV. The scratched powders were deposited at the surface of copper observation grids directly.

Figure 1. SAXS patterns (down) of the photopolymerized C12TMS hybrid film obtained at different light intensities: 20 (a), 40 (b), 80 (c), and 200 mW/cm2 (d). The conversion−time curves (top) show the effect of light intensity on hydrolysis reaction kinetics. (Hg−Xe lamp, 16 min irradiation, RH = 30%, T = 25 °C.)

the different SAXS patterns of the hybrid films derived from C12TMS after 16 min irradiation upon varying the light intensity from 20 to 200 mW/cm2. Clearly, the irradiation conditions have a drastic effect on the level of ordering. At low intensity (20 mW/cm2), the SAXS data are indicative of a longrange organization with the presence of three discernible Bragg peaks at 2.7°, 5.2°, and 7.7° with associated d spacings of 32.8, 16.4, and 10.9 Å, respectively. The former peaks can be readily indexed as the (001), (002), and (003) directions of a lamellar mesostructure. The (001) sharp diffraction peak at 32.8 Å corresponding to the interlamellar distance matches relatively well with the theoretical value of 34.8 Å predicted for the bilayer structure CH3(CH2)11SiOx−OxSi(CH2)11CH3 with non-interdigitated alkyl chains in fully extended trans conformation (Scheme 1). Other evidence for the lamellar structure was given by electronic microscopy images displayed in Figure 2. Clearly, the SAXS data support a progressive disordering upon increasing the light intensity, manifested by the removal of the reflection orders and a sharp decrease in peak intensity. From a crystal organization at 20 mW/cm2, the

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RESULTS AND DISCUSSION 1. Kinetic and Thermodynamic Control over Organosilane Self-Assembly. When preparing nanostructured hybrids from organosilane, a number of kinetic and thermodynamic parameters are critical for the self-assembly process. Thermodynamics is related to energy differences between static states, while kinetics describes rates of change and thus time-dependent phenomena.37 On the basis of these two principles, several experiments were conducted upon investigating the influence of light intensity, temperature, alkyl chain length, alkoxy structure, and temperature. Compared with 24322



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Figure 2. SEM (a) and TEM (b) images of the self-assembled C12TMS film. (Hg−Xe lamp, 16 min irradiation, 20 mW/cm2, RH = 30%, T = 25 °C.)

film becomes short-range organized (local ordering) at 80 mW/cm2, then completely amorphous at 200 mW/cm2.39 There is usually a strong relationship between the mesoscopic organization, reflected in the SAXS data, and the alkyl chain conformational organization, which can be either fully extended (trans) or disordered (gauche defects). Taking this fact into consideration, it is relevant to study the impact of reaction kinetics (light intensity) on conformational order. Such information can be obtained from the film IR spectra by the position of the symmetric (d+) and antisymmetric (d−) CH2 infrared stretching modes in the 2800−3000 cm−1 region (Figure 3). As expected, the amorphous film irradiated at high

competitive chemical reactions are involved in sol−gel polymerization: hydrolysis and polycondensation, both presumably affected by light intensity change. Hydrolysis rate can be readily estimated by real-time FTIR through systematic deconvolution and integration of the symmetric CH3 stretching mode of the methoxysilyl groups at 2840 cm−1. (The IR spectrum is shown in Figure S1, Supporting Information.) Confirmation of the reliability of the method was given by the temporal evolution of the Si−O stretching vibrations near 905 cm−1 (characteristic of the silanol groups), which coincides precisely with that of the methoxy groups (Figure S2, Supporting Information). Note that hydrolysis is also accompanied by the formation of a broad band spanning ≈3000−3600 cm−1 assigned to the OH···O stretching of hydrogen-bonded hydroxyl compounds. An additional figure (Figure S2) was given in the Supporting Information showing a complete correspondence between methoxy moieties disappearance (revealed by νsym(CH3), 2840 cm−1) and the Si−OH band growth (υ(Si−O), 905 cm−1) during the hydrolysis stage, which is in line with the direct conversion of Si−OCH3 into Si−OH. Such an agreement is a confirmation of the reliability of the νsym(CH3) mode to assess hydrolysis kinetics. The effect of light intensity on hydrolysis kinetics is displayed in Figure 1. As expected, an acceleration of the hydrolysis reaction is observed from 20 to 40 mW/cm2. While there is no longer a marked difference in kinetics above 40 mW/cm2, the diffractogram profiles remain strongly dependent on light intensity. This suggests that the condensation kinetics should be a finer marker of the mesostructuration. Although the progressive formation of the siloxane network is difficult to probe by IR spectroscopy, information can be derived from the antisymmetric Si−O−Si stretching band in the region 1000− 1260 cm−1 (Figure S1, Supporting Information). At the beginning of the irradiation, the band is not resolved due to the overlapping νasym (Si−)O−C band (1090 cm−1) from the reactive functions. After 30 s of irradiation, the hydrolysis is complete and the changes in this broad envelope are assumed to be only representative of υa(Si−O−Si) mode. We then recognize the typical signature of pure silica, with a band centered at ∼1065 cm−1 related to the longitudinal optical (LO) component accompanied by a clearly visible shoulder at ∼1150 cm−1 associated to the transversal optical (TO) component. After deconvolution, the temporal evolution of the relative areas underlying the TO and LO bands can be estimated during the irradiation time, as shown in Figure 4. Regardless of the light intensity, there are respectively an increase and decrease of the TO and LO components, thus

Figure 3. Influence of the light intensity on the symmetric (d+) and antisymmetric (d−) CH2 stretching vibration IR bands of the UVcured C12TMS film. (Hg−Xe lamp, 16 min irradiation, RH = 30%, T = 25 °C.)

intensity (200 mW/cm2) does not experience any shift of the methylene stretching vibrations, which remain at 2926 (d−) and 2854 cm−1 (d+), some values comparable to those of the conformationally disordered liquid precursor. In contrast, the d− and d+ of the film cured at 20 mW/cm2 are found to be shifted to lower energy and observed at 2923 and 2852 cm−1, respectively, which is suggestive of a high proportion in all-trans dodecyl chains, in agreement with the crystalline structure assessed by the SAXS data. The loss of organization at the conformational and supramolecular level in response to light intensity increase is entirely consistent with a kinetically driven self-assembly. Two 24323



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Figure 4. Relative evolution of TO and LO Si−O−Si asymmetric stretching modes during the irradiation time of C12TMS films illuminated at different light intensities: (○) 20 mW/cm2, (▲) 40 mW/cm2, ( ×) 80 mW/cm2, and (■) 200 mW/cm2. (Hg−Xe lamp, 16 min irradiation, RH = 30%, T = 25 °C.)

reflecting the progressive formation of siloxane bonds. While the hydrolysis kinetics remain poorly sensitive to light intensity, the condensation rates gradually accelerate when increasing the light intensity from 20 to 200 mW/cm2. Mechanistically, this scenario is consistent with condensation reaction kinetics determining the level of ordering, and can be understood in terms of a sufficient “lifetime” of the surfactant-like species (nC12H25Si(OH)3) arising from hydrolysis before their conversion into a static cross-linked siloxy network. At low intensity, the observed scenario supports a greater temporal discontinuity between hydrolysis and condensation favorable to the creation of molecular amphiphilic species, and then a slow condensation rate to enable a sufficient equilibration time for diffusion and self-assembly. The key observation that emerges is that fast siloxane cross-linking tends to disfavor the ordering and to create a range of “kinetically” trapped products that can be either short-range ordered (80 mW/cm2) or amorphous (200 mW/cm2). 1.2. Temperature. Figure 5 depicts a series of SAXS patterns for C12TMS-based films irradiated under similar conditions (20 mW/cm2) but at different temperatures: 25, 35, and 50 °C. Clearly, a temperature increase tends to destabilize the sensitive process of self-assembly. Upon increasing the temperature from 25 to 35 °C, the mesostructure becomes more disordered as reflected by the full width at half maximum (fwhm) of the (001) signal broadening from 0.13° to 0.2°. The crystalline order is definitely lost at 50 °C and the SAXS pattern no longer exhibits any sharp Bragg signals, a broad halo appears instead which is characteristic of a short-range order.40 As summarized in Table 1, the position of the CH2 mode IR peaks denotes the presence of a higher proportion in gauche defects upon increasing the temperature. At 50 °C, the d+ and d− modes localized respectively at 2854 and 2925 cm−1 are quite consistent with typically observed values for disordered alkane structures. The temperature effect on mesoscopic and conformational order can be interpreted on both kinetic and thermodynamic grounds.41 As shown in Figure 5, the evolution of the hydrolysis kinetics with temperature provides, as expected, the proof of an acceleration of the sol−gel reaction, which is presumably detrimental to the construction of a wellordered mesostructure.42 The “destructuring” effect of temperature also has a thermodynamic facet. Since interchain cohesion must overcome thermal agitation of the molecules, it is thus

Figure 5. SAXS patterns (down) of C12TMS hybrid films UV-cured at three different temperatures: 25 (a), 35 (b), and 50 °C (c). The methoxy conversion plots (top) give an overview of the temperature effect on the hydrolysis kinetics. (Hg−Xe lamp, 16 min irradiation, 20 mW/cm2, RH = 30%.)

Table 1. Evolution of the d+ and d‑ Stretching Modes before and after Light Exposure (Hg−Xe lamp, 16 min irradiation, RH = 30%) antisymmetric CH2 stretching mode (d−) temperaturea

chain lengthb

a

25 °C 35 °C 50 °C C8TMS C10TMS C12TMS

symmetric CH2 stretching mode (d+)

t=0

t = 16 min

t=0

t = 16 min

2926 2926 2926 2926 2926 2926

2923 2924 2925 2925 2925 2923

2854 2854 2854 2855 2855 2854

2852 2853 2854 2855 2854 2852

C12TMS sample. bT = 25 °C.

expected that the level of ordering decreases with increase in temperature. 1.3. Alkyl Chain Length. The alkyl chain length is another crucial feature for the effectiveness of alkylsilane to selfassemble into well-ordered mesostructures. It is clear that van der Waals attractive forces among alkyl chains enhance the aggregate stability, and their contribution to the enthalpic change will be discussed in another section (see part 1.6). Figure 6 compares the SAXS patterns of UV-cured films prepared from n-alkyltrimethoxysilane with saturated straightchain hydrophobic groups containing 8, 10, and 12 carbon atoms. Compared to the C12TMS crystalline structure, the C10TMS film leads to a weaker (001) signal exhibiting a significantly broader fwhm of 0.24° (0.13° with C12TMS), which is the sign of a marked disordering. A shorter alkyl group with only 8 carbons causes the fwhm to increase significantly 24324



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Figure 6. SAXS patterns of the samples C12TMS (a), C10TMS (b), and C8TMS (c) after UV irradiation. (Hg−Xe lamp, 16 min irradiation, 20 mW/cm2, RH = 30%, T = 25 °C.)

(1.5°), which is indicative of a short-range organization in this case. Accordingly, a similar effect of the alkyl chain length on the alkyl conformational order is also found (Table 1). Since interchain cohesion increases with increasing length of the hydrophobic group, this may account for the improvement of organosilica mesostructure ordering. Efforts to find kinetic conditions suitable to the self-assembly of short C8TMS precursor failed, demonstrating that in this case the limitation to self-assembly is thermodynamic. In surfactants possessing a straight chain, the achievement of a long-range organization requires a sufficient chain length. The hydrocarbon portions of all the molecules are held together by van der Waals forces, which, if too short, bring forth a chain that probably produces insufficient cohesiveness.43 A word of caution is appropriate here: the reaction rates are also slightly sensitive to the length of the organic segment (Figure S5, Supporting Information). 1.4. Alkoxy head. The effect of changing the methoxy group to ethoxy group can be seen in the SAXS patterns gathered in Figure 7. A decreased structural ordering is observed, which is difficult to interpret kinetically in the light of the slower hydrolysis found with the triethoxy precursor (Figure 7). In addition, the alkoxide substitution does not affect the chemical structure of the in situ generated trisilanol surfactant molecule, which is an important thermodynamic driving force of the selfassembly. To understand this result, we need to take into account not the alkoxy function itself, but the sol−gel byproducts instead. In our case, the film is composed of organosilica species surrounded by adsorbed water and alcohol molecules released as byproducts of the sol−gel reaction. Change from methoxy to ethoxy implies the release of ethanol, in the place of methanol. First, ethanol has a higher boiling point (79 °C) than methanol (65 °C), leading to a slower evaporation speed, thereby creating a higher residual concentration in the film. Second, the change of alcohol induces a modification of the solvation properties. Abundant work has been performed to study the effect of cosolvent, mainly alcohol, on the self-assembly of nonionic amphiphiles.44,45 The addition of ethanol is known to disfavor micelle formation and results in a significant increase or even disappearance of the critical micellar concentration (CMC). Polar organic solvent dissolves the alkyl tails, thus decreasing their cohesive forces. The solubility of an amphiphile A (solute) in a solvent S is dependent on the difference between their

Figure 7. SAXS patterns (down) of C12H25(OCH3)3 (a) and C12H25(OCH2CH3)3 (b, C12TES) hybrid films after UV exposure. The upper curve shows the difference of hydrolysis kinetics between the two precursors. (Hg−Xe lamp, 16 min irradiation, 20 mW/cm2, RH = 30%, T = 25 °C.)

chemical potentials, which can be expressed in terms of solubility parameters. To discriminate the solubilizing ability of the two alcohols, an empirical and useful solubility parameter (RAS) proposed by Nagarajan46 can be used. RAS2 = (δAd − δSd) + (δSp)2 + 0.5(δSh)2

(1)

with δ , δ , and δ the dispersion, polar, and hydrogen-bonding components of the Hansen solubility parameter δ, respectively. In our case, the solubility parameter RAS2 is respectively 1151, 401, and 266 MPa for water, methanol, and ethanol. Thus, the solubility of surfactant molecules improves substantially from water to methanol, and even further in ethanol. Ethanol has increased nonpolar properties, improving its solubility with dodecyl chains. 1.5. Relative Humidity. It has been well documented that water content plays a critical role in the surfactant-templated mesostructuration of silica films.47 In contrast, the relative humidity (RH) is seldom investigated as a parameter likely to impair or promote the structuration in self-assembled organosilica films. The content of water in the film can be modulated during the UV process by adjusting the relative humidity (RH). In the SAXS patterns depicted in Figure 8, structural modifications of the C12PS film are enabled by increasing the relative humidity from an initial value of 30% to 50% and 60%. At higher humidity, the SAXS patterns display several peaks indicative of a retained mesoscale periodicity, but clearly different from the conventional lamellar structure. At 50%, the broad envelope includes two sharp peaks corresponding to a dspacing of 2.3 and 2.5 nm and a larger peak in the lower region (1.8 nm). TEM experiments are underway to assist us in the interpretation of these new mesophases. Despite that, the SAXS d

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1.6. Thermodynamic Description of the Self-Assembly Process. Although not sufficient if considered alone, thermodynamics of association play a crucial role in the selfassembly of organosilanes. The standard free energy change for the transfer of 1 mol of amphiphile to the mesophase (free energy of association), ΔGo, can be expressed in terms of standard enthalpy of association, ΔHo, and the standard entropy of association per molecule of surfactant, ΔSo, as

ΔGo = ΔH o − T ΔS o

A quantitative approach is out of prospect here as the determination of these thermodynamic quantities is not easily accessible. In contrast, the analysis of the latter experimental parameters can merge into a qualitative thermodynamic description. First of all, the association free energy values, ΔGo, are negative since thermodynamically stable lamellar mesophases can form spontaneously (under favorable kinetic conditions). The possibility of an entropy-driven self-assembly (positive ΔSo) is improbable because it is generally related to an entropy gain by the solvent molecules (absent in our case), and not by the solute molecules, “released” from the hydrophobic chains when they associate.49 Entropy-driven self-assembly is only found in excess water or polar solvents but not in apolar or bulk media.50 On the contrary, the disordering effect (ΔGo becomes less negative) caused by the temperature increase (part 1.2) suggests a negative entropic term, which is also consistent with less mobile alkyl chains aligned in ordered bilayers.51 As a result, the enthalpic forces (negative ΔHo) might govern the formation of the organosilica mesostructure. In our system, the enthalpy association force results only from van der Waals forces between alkyl chains and possibly solvophobic forces derived from solubility difference between the “solvent” (adsorbed water and alcohol molecules) and the amphiphile.48 Thus, the overall enthalpy change can be expressed by

Figure 8. SAXS patterns (down) of UV-irradiated C12TMS hybrid film at different relative humidity: 30% (a), 50% (b), and 60% (c). As the atmospheric moisture diffusion ensures the hydrolysis, the change in RH affects significantly the hydrolysis kinetic profiles (top). (Hg−Xe lamp, 16 min irradiation, 20 mW/cm2, T = 25 °C.)

data show the potential of accessing new hybrid mesostructures by simply increasing the RH. The concept of richer structural diversity enabled by hydrophilic conditions already has been hinted at by Corriu et al., who reported that xerogels derived from bis(trimethoxysilyl)alkene precursors could evolve from lamellar to 2-D hexagonal phases upon increasing the water concentration.31 There have been very few examples of nonlamellar hybrid mesophases produced by the sol−gel polymerization of organosilanes, mostly by varying the size of the siloxane head.26−28 In aqueous solution, the cohesive forces may be supplemented by solvophobic forces (“hydrophobic effect”) that emanate from the less favorable interactions between surfactant alkyl chains and water molecules compared to the more favorable water−water interactions.48 The richer polymorphism induced by humidity also can be interpreted in terms of geometrical packing parameter g g=

V a 0l

(3)

ΔH o = ΔH o(solvophobic forces) + ΔH o (van der Waals forces)

(4)

The role of van der Waals forces is straightforwardly evidenced by the loss of order when decreasing the alkyl chain length (part 1.3). Although the medium is devoid of solvent, the experiments showed conclusively the “structuring” or “destructuring” role of water (part 1.5) and alcohol molecules (part 1.4) respectively on self-assembly. This agrees with the interference of solvophobic forces altering in certain conditions the magnitude of ΔHo. At high humidity or with the addition of cosolvent, the contribution of the solvophobic forces to the free energy of association is no longer negligible. The proposed idea is that the immiscibility water/hydrocarbon chains (interfacial tension ca. 45 mN/m) and the strong cohesive energy of water molecules (surface tension ca. 72 mN/m) can make the process enthalpically favored (decreased ΔHo). In contrast, these two properties are strongly attenuated with alcohols, in particular with ethanol exhibiting a good solubility with alkanes (RAS2 = 266 MPa), which is not appropriate for promoting an effective self-assembly (increased ΔHo). 2. From the Isotropic Precursor to a Mesostructured Organosilica Film: A Dynamic Study of the Organosilane Self-Organization. Of high significance in organosilane self-assembly is the gradual and time-dependent construction of a supramolecular solid structure from initially

(2)

with V the effective volume of the hydrophobic chain, a0 the effective area of the head group, and l the hydrophobic chain length. The lamellar phase (g = 1) corresponds to a minimal micelle interface resulting from a low value of a0, i.e., a poor water swelling of the hydrophilic head. In contrast, rod-like (g = 0.5) and spherical (g = 0.2) structures are associated with more curved interfaces. In our case, it is expected that an increased RH can promote the intercalation/solvation of water molecules between the polar trisilanol heads, resulting in an artificial increase of a0, thereby decreasing g. 24326



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the evolution of the d modes throughout the irradiation. A clear shift of the d− to downward frequency is observed after only 10 s (15% hydrolysis) while d+ requires more than 30 s (70% hydrolysis) to start its downward displacement. Then, both d− and d+ values move gradually to their definitive position at 2923 and 2852 cm−1 reached after 160 s of irradiation. This result argues for a gradual conformational change of the dodecyl chains with more and more C12H25 chains in gauche ordering into trans. Despite different behaviors between d− and d+, a strong correlation between hydrolysis and conformational order is emphasized, proving that a sufficient fraction of hydrolyzed amphiphilic species (different for d− and d+) is required to foster the self-assembly. Before 10 s, hydrolysis has already started but it is not sufficiently advanced to permit the conformational ordering of the alkyl chains. 2.2. SAXS. To obtain structural information during irradiation time, a complementary approach employs ex situ SAXS. Figure 11 shows a sequence of SAXS profiles at three different reaction times (20, 160, and 960 s) in a way to freezein the structure of the C12TMS film during the order-todisorder phase transformation. At time t = 20 s, the SAXS pattern already shows a series of diffraction peaks, consistent with the disappearance of the precursor isotropic phase. As the hydrolysis is significantly advanced (50%), the photogenerated surfactant molecules are no longer disordered and can organize rapidly. A thorough observation of the Bragg peaks position reveals the coexistence of two distinct mesophases: lamellar and micellar body-centered-cubic (Im3̅m), as is evident from the array of peaks indexed in Figure 11. The (001), (002), and (003) diffraction peaks are the signatures of a lamellar mesophase, while the (110), (200), (310), and (321) reflections suggest an Im3̅m space group. The surimposition of the diffraction peaks provides evidence for a direct disorderto-order phase transformation, and a dominant cubic structure for short irradiation times. At t = 20 s, the formation of an incipient lamellar mesophase is already apparent, but the formation of the final lamellar structure seems to occur through the formation of an intermediate transient (cubic) phase. For longer times, we note a gradual disappearance of the cubic diffraction pattern, as exemplified by (200) Bragg reflection of the Im3̅m structure decreasing in intensity, and vanishing completely in the last pattern taken at 660 s. Further details can be obtained by deconvolution of the dominant diffraction peak at 2.5−3° (2θ), as revealed in Figure 12. The multicomponent broad envelope obtained at 20 s can be decomposed into two intense Gaussians centered at 32.8 and 31.6 Å corresponding to the d-spacings of the lamellar (001) and cubic (110) structures, and two minor Gaussians55 at 34.8 and 30.9 Å. Clearly, there is a progressive reduction of the number of Gaussians as the self-assembly continues, which indicates a decreasing number of kinetically trapped (solidified) mesostructures. At t = 160 s, the multi-Gaussian peak disappears and we can identify a bimodal distribution by observing a twin-peak resulting only from the lamellar (32.8 Å) and cubic phases (31.6 Å). The final sample obtained after 960 s exhibits a single sharp peak arising from the crystalline lamellar structure. We suppose that the change of mesophase is driven mainly by the condensation reactions. It already has been reported that condensation can induce the restructuring of surfactant molecules, and mesophase transformation in surfactant-templated silica films.56,57 The influence of condensation may be even more critical in our system as it affects directly the chemical structure of the surfactant, while it

isotropic precursors. Intuitively, a sufficient time is necessary for the creation of silanol surfactant molecules, diffusion, and then organization. The issue of equilibration and structuration time raises numerous questions. What is the minimum time required for the observation of mesophases? Is the first mesostructure crystalline or does it evolve from transient locally ordered phases to highly ordered structures? What is the relationship between hydrolysis kinetics and the formation of a mesostructured organosilica film? To answer these questions and investigate the self-assembly process throughout irradiation time, we employed two techniques: First, in situ RT-FTIR which is suitable to probe dynamically with a high temporal sensitivity the conformational ordering of the dodecyl segments throughout the irradiation. Second, SAXS analysis was performed (ex situ) at different irradiation times. 2.1. Real-Time FTIR Spectroscopy. As seen previously, RTFTIR is a powerful technique to monitor the sol−gel reaction kinetics and the conformational order. First introduced in the early nineties to probe the ultrafast organic cross-linking reactions in photocurable films,52 Innocenzi et al. showed recently its analytical utility in the self-assembly of epoxy trimethoxysilane precursor to investigate in situ the continuous water evaporation. 53 In numerous studies, FTIR was instrumental to evidence conformation and H-bonds of organic moieties54 in the final self-assembled hybrid, but dynamic aspects are generally overlooked. RT-FTIR is an ideal probe to study the evolution with time of the frequency, width, and intensity of the symmetric (d+) and antisymmetric (d−) CH2 stretching vibrations, which are sensitive to the gauche/trans conformer ratio. Figure 9 shows the temporal evolution of the C−H stretching region between 2800 and 3000 cm−1 upon irradiating the C12TMS film (20 mW/cm2). From their initial positions at 2926 (d−) and 2854 cm−1 (d+), we note a progressive shift toward lower energy during the 16 min of UV exposure. To derive structural and compositional information, Figure 10 shows in the same plot the methoxy conversion and

Figure 9. Temporal evolution of the antisymmetric (d−) and symmetric (d+) CH2 IR stretching vibration band during the UV irradiation of the C12TMS film. Also visible in the IR spectra is the gradual absorbance decrease of the CH3 symmetric stretching vibration band centered at 2840 cm−1. (Hg−Xe lamp, 16 min irradiation, 20 mW/cm2, RH = 30%, T = 25 °C.) 24327



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Figure 10. Wavenumber shifts of the d− (a) and d+ (b) CH2 stretching bands during the irradiation of the C12TMS film. The conversion curves of the methoxysilyl functions are plotted in the same curve. (Hg−Xe lamp, 20 mW/cm2, RH = 30%, T = 25 °C.) (c) Schematic representation showing the transformation of the mixed trans−gauche bilayer environment into a fully trans conformation consisting of extended zigzag dodecyl chains.

Figure 11. Temporal evolution of the SAXS patterns during the UV irradiation of the C12TMS hybrid film: (a) 20, (b) 160, and (c) 660 s. (Hg−Xe lamp, 20 mW/cm2, RH = 30%, T = 25 °C.)

concerns only the silicic species surrounding the surfactant in the first case. The onset of condensation decreases the hydrophilicity and the area occupied by the head group (a0), and this leads to an increase in the packing parameter g.58 Presumably this change drives the transformation of the micellar cubic phase into the lamellar phase. Examination of the temporal evolution of the (001) peak shows an increasing intensity and narrowing width, which translates into a continuous reorganization of the surfactant molecules within the lamellar mesostructure due to siloxane condensation. We think that the system evolves toward an enhanced spatial correlation through the increasing density number of self-assembled molecules and to a greater alkyl chain ordering. Although the evolution of the peak width and intensity are in line with the transformation of a proto-lamellar mesostructure into a highly ordered mesophase, there is unexpectedly no change of the peak position (32.8 Å). This result goes against the idea of a progressive contraction as cross-linking and densification of the silica network occurs. It is plausible that the contraction may be obscured or compensated by the alkyl chain elongation resulting from the conformational ordering, with the progressive transfer of gauche to trans conformers, as proved by FTIR data.

Figure 12. Deconvoluted diffraction peak at 2.5−3° (2θ) extracted from the SAXS patterns of the C12TMS film irradiated during 20 (a), 160 (b), and 660 s (c).

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CONCLUSION The UV-driven self-assembly of C12TMS into lamellar siloxane/ alkyl films represents a simple, general, and comprehensive approach to illuminate the kinetics and thermodynamics of organosilane self-assembly. Experiments on light intensitydependent polymorphism were successfully interpreted on the basis of hydrolysis and condensation kinetics: hydrolysis generates in situ molecular amphiphilic species, but a slow enough condensation is necessary for their effective selfassembly. In addition, the effects of temperature, alkoxy head, alkyl chain length, and relative humidity on the mesoscopic and conformational orders enabled a qualitative thermodynamic description. The self-assembly was found to be enthalpically driven through the van der Waals attractive forces between alkyl chains and the solvophobic forces arising from residual 24328



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(13) Nunes, S. C.; Silva, N. J. O.; Hummer, J.; Ferreira, R. A. S.; Almeida, P.; Carlos, L. D.; De Zea Bermudez, V. RSC Adv. 2012, 2, 2087−2099. (14) Arrachart, G.; Creff, G.; Wadepohl, H.; Blanc, C.; Bonhomme, C.; Babonneau, F.; Alonso, B.; Bantignies, J. L.; Carcel, C.; Moreau, J. J. E. Chem.Eur. J. 2009, 15, 5002−5005. (15) Muramatsu, H.; Corriu, R. J. P.; Boury, B. J. Am. Chem. Soc. 2003, 125, 854−855. (16) Okamoto, K.; Goto, Y.; Inagaki, S. J. Mater. Chem. 2005, 15, 4136−4140. (17) Jiang, J.; Lima, O. V.; Pei, Y.; Zeng, X. C.; Tan, L.; Forsythe, E. J. Am. Chem. Soc. 2009, 131, 900−901. (18) Menaa, B.; Takahashi, M.; Innocenzi, P.; Yoko, T. Chem. Mater. 2007, 19, 1946−1953. (19) Bourlinos, A. B.; Chowdhury, S. R.; Jiang, D. D.; An, Y.-U.; Zhang, Q.; Archer, L. A.; Giannelis, E. P. Small 2005, 1, 80−82. (20) Chemtob, A.; Ni, L.; Demarest, A.; Croutxe-Barghorn, C.; Vidal, L.; Brendlé, J.; Rigolet, S. Langmuir 2011, 27, 12621−12629. (21) Fujii, K.; Fujita, T.; Iyi, N.; Kodama, H.; Kitamura, K. J. Mater. Sci. Lett. 2003, 22, 1459−1461. (22) Ke, Q. P.; Li, G. L.; Liu, Y.; He, T.; Li, X. M. Langmuir 2009, 26, 3579−3584. (23) Shimojima, A.; Sugahara, Y.; Kuroda, K. Bull. Chem. Soc. Jpn. 1997, 70, 2847−2853. (24) Shimojima, A.; Sugahara, Y.; Kuroda, K. J. Am. Chem. Soc. 1998, 120, 4528−4529. (25) Ni, L.; Chemtob, A.; Croutxe-Barghorn, C.; Vidal, L.; Brendlé, J.; Rigolet, S. J. Mater. Chem. 2012, 22, 643−652. (26) Sakamoto, S.; Shimojima, A.; Miyasaka, K.; Ruan, J.; Terasaki, O.; Kuroda, K. J. Am. Chem. Soc. 2009, 131, 9634−9635. (27) Shimojima, A.; Kuroda, K. Angew. Chem., Int. Ed. 2003, 42, 4057−4060. (28) Shimojima, A.; Liu, Z.; Ohsuna, T.; Terasaki, O.; Kuroda, K. J. Am. Chem. Soc. 2005, 127, 14108−14116. (29) Lerouge, F.; Cerveau, G.; Corriu, R. J. P. New J. Chem. 2006, 30, 1364−1376. (30) Takahashi, M.; Figus, C.; Kichob, T.; Enzo, S.; Casula, M.; Valentini, M.; Innocenzi, P. Adv. Mater. 2009, 21, 1732−1736. (31) Alauzun, J.; Mehdi, A.; Reyé, C.; Corriu, R. J. P. J. Mater. Chem. 2005, 15, 841−843. (32) Ijdo, W. L.; Lee, T.; Pinnavaia, T. J. Adv. Mater. 1996, 8, 79−83. (33) Alauzun, J.; Mehdi, A.; Reye, C.; Corriu, R. J. P. Chem. Commun. 2006, 347−349. (34) Peng, H.; Zhu, Y.; Peterson, D. E.; Lu, Y. Adv. Mater. 2008, 20, 1199−1204. (35) Cariati, E.; Ugo, R.; Cariati, F.; Roberto, D.; Masciocchi, N.; Galli, S. Adv. Mater. 2001, 13, 1665−1668. (36) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to infrared and Raman spectroscopy; Academic Press: New York, NY, 1990. (37) Gutzler, R.; Cardenas, L.; Rosei, F. Chem. Sci. 2011, 2, 2290− 2300. (38) Fujimoto, Y.; Shimojima, A.; Kuroda, K. Chem. Mater. 2003, 15, 4768−4774. (39) Relatively long irradiation times (16 min) were chosen indeed to afford at the end of the insulation process films displaying a similar degree of condensation whatever the light intensity. Although mesostructures can be obtained with shorter irradiation times, such conditions are important to prove that the difference of ordering arises from kinetics difference and not from a difference in the siloxane network microstructure. To analyze the Si−O−Si network, we rely on FTIR spectroscopy and solid-state NMR. Similar IR spectra in the 900−1200 cm−1 region (antisymmetric Si−O−Si stretching) were obtained in Figure S3 of the Supporting Information whatever the light intensity. More quantitative information on the degree of condensation can be accessed from the 1H MAS NMR spectra of films irradiated at different intensities, as shown in Figure S4 in the Supporting Information. We observed a similar concentration of residual uncondensed OH groups for the films irradiated at 200 and 20

water and alcohol (byproducts). To derive structural and compositional information during irradiation time, we employed time-resolved RT-FTIR and SAXS. This complementary dynamic approach revealed the “order-to-disorder” transition, from an isotropic medium to a lamellar structure, and the gradual conversion of the gauche defects into trans conformers. Obviously, our approach encompassing kinetics, thermodynamics, and dynamics remains primarily qualitative, but, its interest is for the first time to rationalize and simplify the surfactant-free synthesis of organosilica mesostructures. Despite its intrinsic complexity, the organosilane self-assembly mechanism can be reconciled with other well-known amphiphile aggregation processes such as self-assembled monolayers, bilayer membranes, surfactant micellization, or surfactant-templated silica self-assembly.

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ASSOCIATED CONTENT

S Supporting Information *

Time-resolved FTIR spectra of C12TMS-based film during UV irradiation time (Figure S1), temporal evolution of the integrated absorbance of the FTIR bands during the UV irradiation of a C12TMS film (Figure S2), temporal evolution of the FTIR spectra of C12TMS-based film under different UV light intensity in the range 900−1200 cm−1 (Figure S3), 1H MAS NMR spectrum of the UV-cross-linked C12PS films cured in different conditions (Figure S4), and chain length effect on methoxy hydrolysis kinetics of C8TMS, C10TMS, and C12TMS (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +33 3 8933 5030. Fax: +33 3 8933 5034. Notes

The authors declare no competing financial interest.

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REFERENCES

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Kinetics, Thermodynamics, and Dynamics in Organosilane Self

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