Spherical and lamellar octadecylsilane hybrid silicas

Journal of Non-Crystalline Solids 354 (2008) 5033–5040

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Spherical and lamellar octadecylsilane hybrid silicas Rodrigo Brambilla a, Gilvan P. Pires a, Nadya P. da Silveira a, João H.Z. dos Santos a,*, Márcia S.L. Miranda b, Ray L. Frost c a

Instituto de Química, UFRGS, Av. Bento Gonçalves, 9500, Porto Alegre 91501-970, Brazil Braskem S.A., III Pólo Petroquímico, Via Oeste, Lote 05, Triunfo 95853-000, Brazil c Inorganic Materials Research Program, Queensland University of Technology, 2 George Street, Brisbane, Queensland 4001, Australia b

a r t i c l e

i n f o

Article history: Received 14 September 2007 Received in revised form 22 July 2008 Available online 17 September 2008 PACS: 81.07.Pr 81.16.Be 81.20.Fw 81.70.Fy 82.80.Ej

a b s t r a c t Silicas bearing different contents of octadecylsilane groups were synthesized by the sol–gel method and characterized by solid-state magic angle spin 13C nuclear magnetic resonance spectroscopy, Raman spectroscopy, attenuated total reflectance infrared spectroscopy, small angle X-ray scattering, laser light scattering and atomic force microscopy. A structural model for such hybrid materials is proposed in which spherical or lamellar morphology, fern-like or Porod’s structure, are proposed depending on the tetraethyl orthosilicate (TEOS)/octadecylsilane (ODS) molar ratio. The effect of the ODS addition time on the chain conformation and on the particle morphology and texture was also investigated. The degree of organization lowered as the amount of ODS increased. The nanostructured xerogel obtained in the case of pure TEOS evolutes from spherical to lamellar patterns, as the amount of octadecylsilane is increased. Ó 2008 Elsevier B.V. All rights reserved.

Keywords: Hybrid silica SAXS ODS AFM Sol–gel

1. Introduction Hybrid inorganic–organic composites are an emerging class of nanostructured materials that permit the microstructural design and engineering of functional systems for a wide range of applications. Through versatile chemistry under mild synthetic conditions, the combination of organic moieties with inorganic oxide components forms a hybrid structure, allowing systematic control and modification of physical properties. Many applications are reported in the literature including nanocomposites [1], dielectric films [2], pharmaceutical carriers [3], optical materials [4], sorbents [5], sensors [6], catalytic supports [7,8], just to mention a few. Among the potential procedures to prepare hybrid materials, the sol–gel method has been one of the most investigated, due to its capacity to control composition, microstructure and morphology of the final products. In the case of sol–gel materials, hybrid silicas can be obtained from the hydrolysis and co-condensation of tetraethyl orthosilicate (TEOS) with other organosilanes (RxSi(OR)4x, where R is alkyl groups). In these materials, TEOS functions as building blocks to construct the framework while * Corresponding author. Tel.: +55 51 3316 7238; fax: +55 51 3316 7304. E-mail address: [email protected] (J.H.Z. dos Santos). 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.07.031

the organosilanes with non-hydrolysable organic groups contribute both to the framework silica units and to organic surface functional groups [9]. Among the employed organosilanes, octadecyltrimethoxysilane (ODS) and octadecyltrichlorosilane (OTS) have been largely used for surface chemical modification. Examples of applications of these hybrid materials in the development of self-assembled monolayers (SAMS) [10], mesoporous materials [11], nanocomposites [12], pharmaceutical carriers [13] and columns for electrochromatography [14,15] are found in the literature. The advance in the development of these materials has depended on the degree of characterization of such hybrid materials. Complementary techniques have to provide information about both domains (organic and inorganic moieties) in terms of structure, texture and morphology. Among the available instrumental techniques, small angle X-ray scattering (SAXS) [16–18], light laser scattering (LLS) [19–21], and atomic force microscopy (AFM) [22– 24] have been employed for the textural and morphological characterization of hybrid materials. In a previous study, we have investigated the effect of the TEOS/ ODS molar ratio and of the components addition time on the final material morphology by scanning electron microscopy (SEM). Depending on the synthesis conditions, the morphology can vary

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from spherical to lamellar [25]. In the present paper, we deepen the characterization of such materials combining other complementary technique, namely solid-state magic angle spin 13C nuclear magnetic resonance spectroscopy (13C MAS-NMR), Raman spectroscopy, attenuated total reflectance infrared spectroscopy, LLS, SAXS and AFM. A structural model for such hybrid materials is proposed based on the results provided by these techniques. 2. Experimental 2.1. Materials Octadecyltrimethoxysilane (ODS) (Acros, 90%) and tetraethyl orthosilicate (TEOS) (Merck, >98%) were used without further purification. Ethanol (Merck, >99.8%) was deoxygenated and dried by standard techniques before use. Ammonium hydroxide (Merck) was purchased as a 25% solution. 2.2. Synthesis of xerogel by hydrolytic alkaline route Xerogels were synthesized in accordance to Stöber synthesis [26]. In a typical preparation, 20 mL of ammonia solution were diluted in 100 mL of ethanol in a two-neck flask equipped with a mechanical stirrer. 5 mL of a TEOS: ethanol solution (1:4; volume ratio) were added to that solution. The mixture was let under stirring for 2 h. Then, the organosilane (ODS), 2.10 mmol diluted in ethanol (4 mL) were dropwisely added to the solution. The addition time last ca. 1 h 45 min. After addition, the mixture was let under stirring for more 2 h. The wet gel was then dried under vacuum and washed with 5  10 mL of ethanol. The xerogel was finally dried under vacuum for 16 h. 2.3. Characterization of silicas 2.3.1. Elemental analysis (CHN) Carbon content was determined in a Perkin Elmer M-CHNSO/ 2400 analyzer. The oxidation and reduction column temperatures were 925 and 640 °C, respectively. The sample mass laid between 1 and 2 mg. Measurements were made in triplicate and results were expressed as a mean. 2.3.2. Attenuated total reflectance infrared spectroscopy (ATR-IR) The samples were analyzed on a Perkin Elmer (Spectrum BX) coupled to a Miracle ATR accessory (Pike Technologies) using a diamond crystal/ZnSe. The spectra were obtained in the range of 4000–525 cm1. The number of scans was 32 at the resolution of 2 cm1.

Table 1 Particle size distribution measured by laser light scattering TEOS/ODS

d50 (lm)

0 2 10 100

21 ± 2.1 14 ± 1.4 30 ± 3.0 14 ± 1.4

2.3.4. 13C Magic angle spin nuclear magnetic resonance (13C MASNMR) Solid state NMR measurements were performed on a Chemagnetics CMX-300 (Varian, USA). Samples were transferred to zirconia rotors. Measurements were performed at 75.3 MHz for 13C. The number of scans was 5000 for 13C. NMR parameters for 13C were a contact time of 2 ms and a recycle time of 1 s. 2.3.5. Laser light scattering (LLS) The size distribution was measured by light scattering using a Malvern Mastersize 2000 coupled to a Hydro 2000 UM (A) accessory. Samples were diluted in paraffine (ca. 0.102%) (see Table 1). 2.3.6. Scanning electron microscopy (SEM) SEM experiments were carried out on a JEOL JSM/6060. The hybrid silicas were initially fixed on a carbon tape and then coated with gold by conventional sputtering techniques. The employed accelerating voltage was 10 kV for SEM. 2.3.7. Atomic force microscopy (AFM) Images were obtained using a Nanoscope IIIaÒ atomic force microscope (Digital Instruments Co.), using the contact mode technique with probes of silicon nitride. WS X M 4.0 software from Nanotec Electronic S.L. was used for the image treatment. Samples were compressed in the form of tablets or fragments of roughly 16 mm2. 2.3.8. Small angle X-ray scattering (SAXS) The SAXS experiments were carried out using synchrotron radiation at LNLS (Campinas, Brazil) with a wavelength k = 1.488 nm. The beam was monochromatized by a silicon monochromator and collimated by a set of slits defining a pin-hole geometry. A solid-stated CCD detector (MAR 160) was used to collect two-dimensional (2D) images with 2048  2048 pixels located at 6752.5 mm of the sample. The q-range of the scattering curves was 0.02 nm1 6 q 6 0.49 nm1, where q is the scattering vector ðq ¼ 4kp sinð2hÞÞ. The data were corrected for sample transmission and background scattering using an empty cell as reference. Samples were placed in stainless steel sample holders closed by two mica windows. 3. Results

2.3.3. Raman spectroscopy Measurements were performed at room temperature using in Via Renishaw Raman spectrometer equipped with NIR diode laser (785 nm), a charge-coupled detector, a 1200 lines/mm diffraction grating and edge filter. The samples were mounted on an XYZ manual stage of a Leica microscope and the laser beam was focused onto the samples through a 20  long-working distance objective. The spectra were recorded using a laser power adjusted between 15 and 300 mW and a slit width of 50 nm. Several scans were collected to improve the signal-to-noise ratio. The acquisition time was varied from sample to sample in the range of 10–60 s. The Raman spectrometer was calibrated prior to the measurements by exciting a Si wafer placed under the microscope and by performing an automatic offset correction. The data acquisition and analysis were accomplished using WiRE software. The wavenumbers are estimated to be accurate to at least ±1 cm1. TM

3.1. Effect of the TEOS/ODS molar ratio on the chain conformation In a previous study, we investigated the effect of the preparative route on the conformation of octadecysilane modified silicas. We observed that hybrid silicas prepared by grafting afforded ODS chains in liquid-like state, while those prepared by the sol–gel method, presented a crystalline conformation [27]. In the present work, we report the effect of the initial molar ratio TEOS/ODS and of the ODS addition time on the conformation of ODS chains in hybrid silicas prepared by the sol–gel method. Fig. 1 presents CP-MAS-NMR 13C of the silica produced at different TEOS/ODS ratios. According to Fig. 1, the signal assigned to methylene carbons is centered at 33.5 ppm. Therefore, the ODS chains in these systems are mainly in trans conformation. Nevertheless, the lower signal at 31.0 ppm indicates that there are some disorganized chains. In

R. Brambilla et al. / Journal of Non-Crystalline Solids 354 (2008) 5033–5040

Fig. 1. 13C NMR spectra of octadecylsilane hybrid silicas obtained with TEOS/ODS molar ratio of (a) 50, (b) 10 and (c) 0.

other to better understand this behavior, the systems were further analyzed by Raman spectroscopy. Raman is a powerful technique for the characterization of conformational changes in the alkyl chains [28–31]. In the Raman spectrum (not showed), the two bands centered at 1080 and 1062 cm1 are assigned, respectively, to m(C–C) for gauche e trans conformation of the ODS alkyl chains. The ratio in intensity of these two bands was evaluated in order to monitorate the influence of the TEOS/ODS molar ratio in the presence of trans or gauche conformation. For all TEOS/ODS molar ratio, the intensity between the two bands laid above 1, which means that for these hybrid silica, obtained by the sol–gel method, there is a predominance of trans conformation in comparison to gauche one, indicating therefore an intense molecular organization in the hybrid silica prepared by the sol–gel method. Besides, the increase in ODS content engenders a decrease in the alkyl chains organization, as shown by the decrease from 3.8 to 2.1 in the Im(C–C)trans/Im(C–C)gauche ratio. The molecular organization of the ODS alkyl chains can also be confirmed by attenuated total reflectance (ATR) in the infrared region. In the spectrum (not showed), three bands can be observed: 2956, 2917 and 2849 cm1, assigned to t(C–H)as from CH3, t(C–H)as and t(C–H)s from CH2, respectively. 3.2. Effect of the TEOS/ODS molar ratio on the morphology and texture In a previous work, the effect of TEOS/ODS ratio on the hybrid silica morphology was investigated. Spherical or lamellar patterns can be obtained according to the TEOS/ODS molar ratio [25]. Fig. 2 shows micrographies of a hybrid silica prepared with TEOS/ODS = 10. According to Fig. 2, spherical silica particles assemble with those of lamellar morphology forming conglomerates of 10–

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Fig. 2. SEM micrographies of octadecylsilane hybrid silica obtained in TEOS/ODS = 10. Magnification of (a) 1000 and (b) 3500.

Table 2 Particle size distribution of octadecylsilane hybrid silica obtained at different ODS addition times Addition time (min)

d50 (lm)

0 5 120

3.6 ± 0.3 4.0 ± 0.4 30 ± 3.0

40 lm. In order to complementarily evaluate the size of these systems, laser light scattering was employed, since this technique allows the statistic determination of spherical-lamellar aggregates, considering the mean diameter of the equivalent spherical particle. Table 2 presents the apparent particle (or aggregate) size distribution for the hybrid silicas prepared at different TEOS/ODS molar ratios, expressed in terms of d50, corresponding to cumulative frequencies of volume (50%), i.e., 50% of the particles (or aggregates) have lower mean diameter that a given value. According to Table 2, the mean diameter laid between 14 and 30 lm. No trend could be established between this value and the increasing in ODS content. It is worth noting that these d50 values are not valid for the individual particles, but for the particle aggregates, which can be considered as individual particles of big size. For instance, silica obtained with 100% of ODS, which was shown to be essentially lamellar [25], presented d50 = 21 lm. This value means that 50% of the lamellar conglomerates have mean particle diameter equivalent to spheres lower diameter lower than 21 lm. Atomic force microscopy (AFM) has been largely employed for textural analysis of silica gel [32,33], organic–inorganic hybrids prepared by the sol–gel method [34,24], and for the study the mechanism of self-assembled monolayer growing of octadecyltrichlorosilane on oxidized silicon substrate [35,36].

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Fig. 3. AFM images of octadecylsilane hybrid silicas obtained in TEOS/ODS molar ratio of (a) 100; (b) 20; (c) 10 and (d) 0.

In the present study, AFM was employed for the textural characterization in terms of surface morphology, and roughness of some silicas obtained by the sol–gel method. Fig. 3 presents some tridimensional AFM images of sílicas produced at different TEOS/ ODS molar ratios. A enhancement of lamellar patterns was observed by SEM as the ODS content increased [25]. The same trend could be expected to be observed in AFM images of Fig. 3. However, only some lamellar patterns could be identified. According to Fig. 3, the surface morphology of hybrid silicas is irregular and as the ODS content increased surface smoothness increased.

The AFM technique also allowed the calculation of the surface roughness, which is a measurement of the surface non-uniformity. The surface roughness can be quantitatively identified by the rootmean-squared roughness (Rrms), which is given by the standard deviation (SD) of the data from AFM images, and determined by means of software using the standard definition as follows:

Rrms

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi PN 2 n¼1 ðzn  zÞ ; ¼ N1

ð1Þ

where zn represents the height of the n data, z is equal to the mean height of zn in AFM topography, and N is the number of the data

0.5 -1

10

0.4

0.3 -3

10

I (a.u.)

Roughness (nm)

-2

10

0.2

-4

10

0.1

ODS 4.0 5: TEOS: 1 ODS 2.5 50 TEOS: 1 ODS 1.9

-5

10

0.0 0

10

20

30

40

50

60

70

80

TEOS 1.7

-6

10

-1

0

10

Carbon content (%) Fig. 4. Roughness versus carbon content of octadecylsilane-modified silicas obtained by sol–gel method. Error (±2%) in elemental analysis.

10 -1

q (nm ) Fig. 5. SAXS profiles plotted as I(q)  q to hybrid silicas obtained at different TEOS/ ODS ratio.

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[37]. Fig. 4 presents the correlation between silica roughness of the hybrid silica and carbon content. According to Fig. 4, increasing the carbon content (i.e., the ODS content), a reduction in surface roughness is observed. This means that for high ODS contents, silica particles seems to be more uniform. In the present study, fractal geometry of the hybrid silicas was determined by small angle X-ray scattering (SAXS). Fig. 5 shows the SAXS curves, plotted as I(q)  q, for the hybrid silicas prepared at different TEOS/ODS molar ratios. Taking into account the q intermediate region in the SAXS curve, a coefficient values (slopes) were calculated for the different systems. This coefficient is related to the fractal geometry of the silica particles, as shown in Scheme 1. According to Scheme 1, for a between 1 and 2, the silica particle presents a structure described as fern-like. If the a value lays between 2 and 3, the silica particles are formed by dense nucleus with rough surface, while for a = 4, the silica are constituted of dense and uniform particles related to typical Porod structures. Fig. 6 presents a correlation between a, determined by SAXS, and the carbon content of the hybrid silica. According to Fig. 6, increasing the carbon content in the hybrid silicas, the a value grew from 1.7 to 4.0, suggesting that silica particle uniformity increased. For systems without or with lower ODS content, a lays between 1 and 2, which characterizes massic fractal particles, that are open, as low density polymeric structures (fernlike). For the hybrid system obtained by the co-hydrolysis of ODS and TEOS in 1:5 ratio, a lays between 2 and 3, which is typical of surface fractal particles with dense nucleus and rough surfaces. Finally, for the system obtained by ODS hydrolysis (a = 4), the particle are described as a Porod structure, corresponding to dense, spherical and uniform particles, as shown in Scheme 1. The SAXS profiles (Fig. 5) allows to evaluate the radius of gyration (Rg) of the particles from the Guinier regime that obey the condition qRg  1, i.e., q ? 0, the intensity (I) could be approximated by the Gaussian function given by Eq. (2)

IðqÞ ¼ I0 e

q2 R2 g 3

:

ð2Þ

The radius of gyration (Rg) could be assessed by fitting the experimental data using the linearized Eq. (2) and algebraic manipulations. Table 2 presents Rg values for the hybrid silicas prepared at different TEOS/ODS molar ratios. According to Table 2, the Rg values for the hybrid silicas were lower that that for bare silica. For systems with high ODS content, the increase in ODS loading showed no influence on the Rg values of silica particles.

5

4

3

α 2

1

0 0

10

20

30

40

50

60

70

Carbon content (%) Fig. 6. Alpha value versus carbon content of octadecylsilane-modified silicas obtained by sol–gel method. Error (±2%) in elemental analysis.

3.3. Effect of the ODS addition time on the conformation of ODS chains Fig. 7 shows MAS 13C NMR spectra of the hybrid silicas prepared by the sol–gel method, in which ODS was added at different reaction time. According to Fig. 7, the ODS addition time influences on the alkyl conformation chains. When the addition is concomitant with TEOS (spectrum a), the ODS chains are predominantly disorganized (gauche), which is shown by the intense signal at 31.0 ppm. Nevertheless, the addition of ODS 2 h after the begin of TEOS hydrolysis and condensation reactions, the resulting solid present chain in more organized conformation (trans), as evidenced by the presence of the signal at 33.5 ppm in spectrum b of Fig. 7. 3.4. The effect of the ODS addition time on the texture and the morphology of ODS chains The effect of ODS addition time on the morphology of the hybrid silicas was also investigated. According to SEM-EDX analysis, when the TEOS and ODS are concomitantly reacted, the silica sphere produced by TEOS hydrolysis are deposited on the ODS lamellas. If ODS is added 2 h after the begin of TEOS hydrolysis and condensation, the lamellar structure is formed between the sphere already precipitated [25]. In the present study, this effect was further stud-

Scheme 1.

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10

5

10

4

I (a.u.)

10

3

10

2

10

0h

4.0

2h

2.9

1

10

0

10

-2

10

-1

0

10

10 -1

q (nm ) Fig. 9. SAXS profiles plotted as I(q)  q to hybrid silicas obtained at different ODS addition times.

Fig. 7. Solid-state MAS 13C NMR spectra of octadecylsilane hybrid silicas obtained at different ODS addition time (a) 0 min and (b) 2 h.

Table 3 Rg values determined by SAXS for hybrid silicas obtained on different TEOS/ODS molar ratio TEOS/ODS

Rg (nm)

0 2 5 10 50 100

1.7 ± 0.1 1.7 ± 0.1 1.7 ± 0.1 1.7 ± 0.1 2.5 ± 0.1 2.8 ± 0.1

ied by laser light scattering. Table 3 shows the particle size distribution of the silica produced at different ODS addition time. Data are expressed in terms of d50 (lm). According to Table 3, there is only a little increasing in particle size when ODS is added 5 min after the beginning of the sol–gel reactions with TEOS. Nevertheless, the addition of ODS 2 h later the beginning of the reaction, affords a considerable increasing in particle or agglomerate size (30 lm). Fig. 8 shows AFM images of these hybrid silicas. According to Fig. 8, when both reactant are concomitantly added silica particles with irregular surfaces are formed, while the addition of ODS 2 h after the beginning of the TEOS sol–gel reactions, engenders the formation of some lamellar domains in the resulting hybrid silica. Fig. 9 shows the SAXS curve, plotted as I(q)  q, for the hybrid silicas obtained at different ODS addition time. According to Fig. 9, the ODS addition time influences the fractality of the silica particle, being a = 2.9 (rough fractal). When ODS and TEOS are concomitantly added, the resulting silica particles present typical Porod’s structure, i.e., uniform spherical particles with high density (a = 4), as illustrated in Scheme 1. 4. Discussion Solid state 13C NMR spectroscopy with cross polarization and magic angle spin (CP-MAS 13C NMR) allows the characterization

Fig. 8. AFM images of octadecylsilane-modified silicas obtained by the sol–gel method (a) ODS and TEOS concomitant addition and (b) ODS addition 2 h after TEOS reaction.

R. Brambilla et al. / Journal of Non-Crystalline Solids 354 (2008) 5033–5040

of chemically-modified silicas in terms of structure, organization and molecular dynamic. In the literature, many studies reported the use of this technique in the characterization of alkyl-modified silica gel in many situations such as evaluation of the synthetic route [27,38], of the support [39], of the chain length [40], of the coating content [41], and of the effect of the temperature on the conformation of the chemical species on the surface [38–40]. The chemical shift of the central methylene groups in an alkyl chain in a 13C CP-MAS-NMR spectrum depends on the conformation of the chains on the solid surface, i.e., on the c (gauche) conformation. For the alkyl chain segment °CH2–CH2–CH2cCH2, the distance between the observed carbon °C and the cC depends on the conformation (or on the isomeric rotational state). It is reduced from 4 to 3 Å in changing from trans to gauche and alter the electronic shielding on °C, which in turn modifies the chemical shift in the 13C NMR spectrum. For instance, in all trans conformation alkyl chains (in paraffin and crystalline polyethylene), the chemical

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shift is ca. 33 ppm, while for disorganized conformation (in amorphous polyethylene) a chemical shift of 30.5 is expected [39]. According to Fig. 1, for the systems prepared in TEOS/ODS molar ratios 50 and 10 (respectively containing 3.7% and 20% C), the signal at 31.0 ppm is very weak (shoulder). Yet, for the silica produced only with ODS (resulting hybrid silica containing 67% C), this signal at 31.0 ppm becomes sharper, being roughly 50% of the intensity of the signal corresponding to trans conformation. In the literature, it was reported that increasing the ODS coating content (and other alkyl groups content), enhances the degree of alkyl chain organization [41]. In the present work, conversely, it is observed the degree of disorganization increased as the ODS employed in the synthesis increased. The same trend was observed by the mean of Raman and ATR techniques. Besides, according to ATR measurements the position of peaks at 2956, 2917 and 2849 cm1 provides insight into the intermolecular environment of the alkyl chains in these assemblies, i.e., the location of these peaks are sensitive indicators

Scheme 2.

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for the extent of the lateral interactions between long n-alkyl and polymethylene chains. For instance, according to Ref. [42], the peak position for the mas(C–H) mode of a crystalline polymethylene chain (2920 cm1) is 8 cm1 lower than that for the liquid state (2928 cm1). Therefore, the band detected at 2917 cm1 (t(C–H)as) suggests an organized conformation of ODS in extended state with low mobility [27]. Taking into account texture and morphology data of the hybrid silicas prepared at different TEOS/ODS molar ratios, a model can be proposed to describe the characteristic of such systems (Scheme 2). The hydrolysis of pure TEOS affords silica agglomerates of 14 lm, consisted of spherical particles of ca. 0.7 lm. These spherical particles present fern-like fractal structures of 2.8 nm. In the case of co-hydrolysis of both components in TEOS/ODS = 10, the resulting hybrid silica assume the form of 30 lm agglomerates, showing both spherical and lamellar domains, which present rough surface fractal structure, i.e., condensed cores and rough surfaces of 1.7 nm. Finally, in the case of ODS pure hydrolysis, the resulting material is formed of lamellar structures with agglomerates of 21 lm. These structures are typical Porod structures of 1.7 nm. In sum, increasing the TEOS/ODS molar ratio, the presence of lamellar domains in comparison to spherical ones increases. Besides, the uniformity and the degree of condensation of the primary and secondary particles which constitute these systems grow. The ODS addition time showed to influence the alkyl chain conformation. (Fig. 6). As the ODS addition time increases, alkyl chain conformation of hybrid silicas increased. It was evidenced by the high peak intensity at 33.5 ppm in NMR spectrum. The variation on ODS time addition in the synthesis of hybrid silicas showed to influenced significantly the texture of systems. The addition of ODS concomitant with TEOS affords silica agglomerates of 3.6 lm, consisted of typical Porod structures of 0.7 nm. In the case of ODS addition 2 h after silica formation, the hybrid silica present agglomerates of 30 lm, consisted of spherical and lamellar particles. These particles present rough surface fractal structures of 1.7 nm. In sum, increasing ODS addition time, the presence of lamellar domains in comparison to spherical ones and fractality of primary particles increase.

5. Conclusions The textural and morphological characteristics of hybrid silica containing ODS groups were shown to be influenced both by the TEOS/ODS molar ratio, as well as by the ODS addition time. Increasing the ODS content, there is a reduction in the organization of ODS chains, and a modification in the size and morphology of the agglomerates, as well as an increase in the uniformity and degree of condensation of primary and secondary particles which constitute the hybrid systems. On the other hand, increasing the ODS addition time enhances the organization of ODS chains, the size of the agglomerated particles, nevertheless, reducing the degree of condensation and uniformity of the fractal structures. The set of complementary techniques (namely, LLS, SEM and SAXS) allows to propose a model to describe the effect of the ODS content in the ODS/TEOS sol–gel copolymerization on the texture properties of the resulting hybrid xerogel. The degree of organization lowered as the amount of ODS increased. The nanostructured material obtained in the case of pure TEOS evolutes

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