Modification of nano-fibriform silica by dimethyldichlorosilane

Applied Surface Science 255 (2009) 7542–7546

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Modification of nano-fibriform silica by dimethyldichlorosilane Lijuan Wang a,b,*, Anhuai Lu c, Zhiyong Xiao d, Junhong Ma a,b, Yuanyuan Li a,b a

National Laboratory of Mineral Materials, China University of Geosciences, Beijing 100083, China State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China c School of Earth and Space Science, Peking University, Beijing 100871, China d Beijing Municipal Station of Agro-environment Monitoring, Beijing 100029, China b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 16 August 2008 Received in revised form 8 April 2009 Accepted 8 April 2009 Available online 16 April 2009

The modification of nano-fibriform silica by dimethyldichlorosilane was studied by transmission electron microscopy, X-ray powder diffraction, infrared spectroscopy, Raman spectroscopy, physical N2 adsorption techniques, differential thermal and thermogravimetric analysis, scanning electron microscopy, and elemental analyzer. The results show that dimethyl silane derivatives have been successfully covalently grafted on nanofibriform silica. The polarity of the modified product decreases with the substitution of –OH groups by siloxyl groups. Therefore, the modified product can be easily dispersed in organic solvent and its compatibility with organic molecules is improved. After modification the pore volume decreases and the ductility greatly increases, indicating that the modified product is of a higher strength than before. The study demonstrates that the modified product can be used as an ideal additive to reinforce the strength of organic materials. Crown Copyright ß 2009 Published by Elsevier B.V. All rights reserved.

Keywords: Chrysotile Nano-fibriform silica Dimethyldichlorosilane Organic modification

1. Introduction Silica is widely used in rubber, plastics, adhesives, paint and other fields. In rubber industry it is usually used as an additive to reinforce the strength of rubber products. Nano-fibriform silica (magnesium-lost chrysotile) is a new type of silica, which is made from chrysotile by acid leaching method [1]. Compared to the ordinary silica, nano-fibriform silica is a mesoporous material with greater anti-aggregation ability favoring its application as additives. [1–3] In general, the hydrophilicity and surface polarity of silica are of great disadvantages in the application mentioned above. Fortunately, the surface modification of silica can decrease the hydrophilicity and surface polarity. After modification, the dispersion degree of silica in organic phase increases, and the compatibility and adhesion between silica and organic molecule are improved, so the strength of rubber products is enhanced. Some researches about the surface modification of ordinary silica and chrysotile had been carried out, and a lot of achievements and meaningful conclusions had been obtained [4–11]. In the present study, the nano-fibriform silica was modified by dimethyldichlorosilane and the chemical composition, structure, surface morphology and adsorption property of the modified product was characterized. Moreover, the ideal chemically bonded model and ideal chemical formula of the modified product were also studied,

which may work as a theoretical foundation in utilizing this modified nano-fibriform silica.

2. Materials and methods 2.1. Synthesis of modified product Chrysotile was obtained from Laiyuan chrysotile mine, China. Dimethyldichlorosilane (chemically pure), absolute ethyl alcohol (chemically pure), cyclohexane (analytically pure), and hydrochloric acid (analytically pure) were used as received. Chrysotile sample (chry) and nano-fibriform silica (chry-s) was prepared as literature method. [1]. The chry-s sample of 1.1482 g was dried at 105 8C for 3 h in an oven, and then mixed with 10 mL dimethyldichlorosilane. The mixture was mechanically stirred by a magnetic stirrer for 24 h in a sealed container, and the HCl gas produced in the reaction was absorbed by calcium hydroxide solution in order to accelerate the reaction rate. The remnant was extracted with cyclohexane and then washed with absolute ethyl alcohol and plenty of deionized water. Finally, the modified product (chry-m) of nano-fibriform silica by dimethyldichlorosilane was obtained. 2.2. Characterization

* Corresponding author. Tel.: +86 10 82322176; fax: +86 10 82322176. E-mail address: [email protected] (L. Wang).

The functional groups of nano-fibriform silica and the modified product were measured with PE983G infrared spectrometer, using

0169-4332/$ – see front matter . Crown Copyright ß 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.04.024

L. Wang et al. / Applied Surface Science 255 (2009) 7542–7546

KBr as a dispersing agent. The measurement was performed in a wavenumber range of 4000–180 cm1 and at 27 8C with a humidity of 64%. X-ray diffraction (XRD) patterns were obtained using fully automatic X-ray diffraction apparatus (DMax-2400) with CuKa radiation, an acceleration voltage of 40 kV and an emission current of 100 mA at a scanning speed of 48/min by step size 0.028. The special surface area of sample was determined by using the Brunauer–Emmet–Teller (BET) method in a volumetric adsorption apparatus (ASAP 2010 M, Micrometritics Instrument Corp.). The chry-m sample of 30 mg was outgassed for more than 12 h at micropore absorption gas station and then transferred to an analysis station to carry out a nitrogen adsorption analysis (at liquid nitrogen temperature of 77 K), using a volume method to obtain adsorption isotherms. The thermal analysis was carried out using differential thermal and thermogravimetric analyzer (SDT2960, American Thermal Analysis Company), with a mass sensitivity of 0.1 mg. The samples may be heated from room temperature to 1300 8C. The contents of C and H elements in the chry-m sample were measured using Vario EL element analyzer. The major elements of samples were analyzed by energy dispersive spectrum (EDS) of scanning electron microscopy (XL30 SFEG). The micro-morphology of samples was observed with transmission electron microscopy (TEM) (Jeol 200CX). All measurements were made in the College of Chemistry and Molecular Engineering and the School of Physics, Peking University. 3. Results and discussion 3.1. Infrared spectrum and Raman spectrum analysis Fig. 1 is the infrared spectrogram of chrysotile, nano-fibriform silica and the modified product. The chrysotile is of a typical vibration mode of clinochrysotile, and contains Si–O–Si, O–Si–O, –OH and Mg–OH groups [1]. For nano-fibriform silica, the stretching vibration peak at 2750– 3600 cm1 is attributed to the physical absorbed water. The absorption peaks at 1096, 952, 802 and 466 cm1 correspond to the Si–O network (Si–O–Si and O–Si–O) vibration [12], Si–OH deformation vibration [10], –OH stretching vibration [13–15], and Si–O–Si bending vibration, respectively. The weak absorption peaks at 303, 401 and 403 cm1 are due to the coupling action of the Si–O bending vibration, Si–O–Mg and Mg–O vibration and OH

7543

Table 1 Raman spectrum analyses on chrysotile, nano-fibriform silica and the modified product. Sample chry

Wavenumber (cm1) 234.42 390.55 693.2

Vibration mode Si–O bending vibration, Mg–O vibration –OH plane vibration Si–O vibration

chry-s

1769.7

–OH bending vibration

chry-m

2907 2962.3

Asymmetrical stretching vibration

plane vibration. These results suggest that the chemical groups in nano-fibriform silica are Si–O–Si, O–Si–O, Si–OH, –OH and a little Si–O–Mg and Mg–O. Compared to nano-fibriform silica, the modified product appears some new absorption peaks at 2961, 1411 and 1261 cm1. According to the IR spectrum of dimethyldichlorosilane [14,15], these new peaks correspond to the C–H asymmetrical stretching vibration in –CH3, the asymmetrical and symmetrical bending vibrations in Si–CH3, respectively. Moreover, the absorption peak at 802 cm1 becomes strong due to the addition of bending vibration in Si–CH3. That is to say, these new absorption peaks in the modified product is derived from the functional groups of dimethyl silane, consistent with the result reported by Mendelovici et al. [9]. These validate that the dimethyl silane groups exist in the modified product. The Raman spectrum analysis results (Table 1) show that chrysotile is featured by the absorption peaks at 234.42, 390.55 and 693.2 cm1. However, these peaks disappear in nano-fibriform silica, because the hydrochloric acid dissolves the Mg–OH octahedron layer and partly destroys Si–O tetrahedron layer. For the modified product, new absorption peaks appear at 2907 and 2962.3 cm1, corresponding to the C–H asymmetrical stretching vibration in –CH3 [14]. These results also indicate that the dimethyl silane groups are grafted on nano-fibriform silica in the form of chemical bond. 3.2. Chemical composition The major elements of chrysotile, nano-fibriform silica and the modified product are semi-quantitatively analyzed by EDS. The wt% data listed in Table 2 are from the measured values by percent calculation, and the Mg/Si is an atomic ratio. It is found that only a few Mg occur in nano-fibriform silica and the Mg loss rate is 92.22% ((17.61–1.37%)/17.61%). The previous study [1] also indicates that nano-fibriform silica with the Mg loss rate of 92.73% is one kind of more pure silica, and it contains 94.34% SiO2 and 2.46% OH. In the modified product, a large number of C are detected, and the Mg and O contents decrease. The Mg/Si atomic ratio of chrysotile, nanofibriform silica and the modified product is 1.53, 0.05 and 0.02, respectively, showing a decrease trend. The first decrease from 1.53 to 0.05 is due to Mg loss from chrysotile by acid. The second decrease from 0.05 to 0.02 is attributed to the Si increase resulting from the dimethyl silane groups grafted on nano-fibriform silica. Since all samples are needed to be covered with a thin layer of carbon before EDS analysis, they can be contaminated by carbon. In Table 2 Major element contents in chrysotile, nano-fibriform silica and the modified product.

Fig. 1. Infrared spectrograms of chrysotile, nano-fibriform silica and the modified product.

Sample

Si (wt%)

Mg (wt%)

O (wt%)

C (wt%)

Mg/Si

chry chry-s chry-m

13.24 31.35 15.66

17.61 1.37 0.25

35.49 38.31 22.73

33.66 28.97 61.36

1.53 0.05 0.02

L. Wang et al. / Applied Surface Science 255 (2009) 7542–7546

7544

Table 4 C and H contents in the modified product and ideal dimethyl silane derivatives.

Fig. 2. TGA and DTA curves of chrysotile, nano-fibriform silica and the modified product.

Table 2, the carbons in chry and chry-s derive from the contaminated carbon, and the carbon in chry-m includes the contaminated carbon and the carbon from the graft of dimethhyl silane groups, so it has a very high content. Furthermore, no Cl detected in chry-m implies that the chemical reaction in modification has been completely carried on and the Cl in dimethyldichlorosilane has been already displaced during modification. So dimethyldichlorosilane is suitable as a modifying agent. The thermogravimetric analysis (TGA) results (Fig. 2) show that the total weight loss rate of chry-s is 16.57% (including 9.59% absorbed water and 6.98% –OH groups). If all water (containing – OH) in the sample is completely lost and the residue is only SiO2, the molecular formula [(SiO2)x(OH)y] of nano-fibriform silica without absorbed water can be roughly calculated: x ¼ mass fraction of SiO2 =relative molecular mass of SiO2 ¼ ð1  16:57%Þ=60:1 ¼ 0:0139 y ¼ mass fraction of OH=relative molecular mass of OH ¼ 6:98%=17 ¼ 0:0041 According to the above formula, the molecular formula of nanofibriform silica is (SiO2)0.0139(OH)0.0041, namely SiO2 (OH)0.295. Table 3 lists the O/H atomic ratio values of materials with ideal Si– O tetrahedron structures including different number of H. The O/H atomic ratio value of chry-s is 7.8, between 6.5 and 8.5 (Table 3), suggesting that every three to four Si-O tetrahedrons in nanofibriform silica contain one H atom, that is to say, there is one H atom to be connected with one O atom in three or four Si–O tetrahedrons. In fact, the Si–O tetrahedron layer in chrysotile is more or less affected by acid leaching, so Si–O bond may be broken on some place in layer. Therefore, the obtained molecular formula of nano-fibriform silica by the above calculation is the molecular formula in ideal state. Differential thermal analyses (DTA) show that chry-m has an exothermic peak between 401.77 and 534.53 8C (Fig. 1) that does not appear in chry-s. The total weight loss rate of chry-m is 34.44%. The most weight loss happens in the range of 400–535 8C, indicating that the grafted dimethyl silane groups in chry-m are burned out in this temperature period. The TGA curve of chry-m

Sample and ideal derivatives

C (wt%)

H (wt%)

H/C

chry-m Derivative containing 2 methyl in unit structure: (Si3O5)(CH3)2 Derivative containing 4 methyl in unit structure: (Si4O6)(CH3)4

15.66 12.35

4.48 3.09

3.42 3

17.88

4.47

3

above 500 8C almost becomes a horizontal line (Fig. 1), implying that chry-m has no weight loss above 500 8C, which is different from chry-s. No weight loss shows that –OH groups in chry-m are very few, suggesting that the modification can reduce the number of –OH groups in chry-s. Table 4 lists the C and H contents in chry-m detected by Vario EL element analyzer and those in two ideal dimethyl silane derivatives ((Si3O5)(CH3)2 and (Si4O6)(CH3)4) calculated by us. According to the C content in chry-m (Table 4), it is obvious that the methyl group number in unit structure of chry-m lies between that of (Si3O5)(CH3)2 and (Si4O6)(CH3)4. The H/C ratio value of chry-m is slightly higher than both of ideal derivatives, inferring that there is still a little hydroxyl groups in chry-m. 3.3. Crystal structure Fig. 3 shows the XRD spectra of chrysotile, nano-fibriform silica, the modified product and silica (amorphous). The typical peaks in chrysotile completely disappear in nano-fibriform silica, the modified product and silica (amorphous). The full-widths at half-maximum (FWHM) at 2u = 228 of chry-s, chry-m and silica (amorphous) become wide in turn. All these indicate that during the process of chrysotile ! nano-fibriform silica ! the modified product, their crystallinities gradually drop. 3.4. Micromorphology feature Fig. 4 displays the TEM photographs of chrysotile, nanofibriform silica and the modified product. Chrysotile is a nanotube

Fig. 3. XRD patterns of chrysotile, nano-fibriform silica, the modified product and silica (amorphous).

Table 3 Molecular formulas of nano-fibriform silica and ideal silica with different number of OH. Sample and ideal silica

Molecular formula

Molecular formula (based on 1 Si atom)

O/H

chry-s Silica containing Silica containing Silica containing Silica containing

SiO2(OH)0.295 SiO2.5H Si2O4.5H Si3O6.5H Si4O8.5H

SiO2.295H0.295 SiO2.5H SiO2.25H0.5 SiO2.167H0.333 SiO2.125H0.25

7.8 2.5 4.5 6.5 8.5

1 1 1 1

OH OH OH OH

in in in in

1 2 3 4

Si–O Si–O Si–O Si–O

tetrahedron tetrahedrons tetrahedrons tetrahedrons

L. Wang et al. / Applied Surface Science 255 (2009) 7542–7546

7545

Fig. 4. TEM images of chrysotile (a), nano-fibriform silica (b) and the modified product (c). Table 5 Surface characteristics of chrysotile, nano-fibriform silica and the modified product. Sample

Specific surface area (m2/g)

Adsorptive capacity (cm3/g)

Pore volume (cm3/g)

Average pore diameter (nm)

chry chry-s chry-m

36.78 384.76 12.75

92.44 355.39 50.90

0.14 0.55 0.08

15.55 5.71 24.70

mineral and the tube is observed in Fig. 4a, which the inner and outer diameters are about 3 nm and 30 nm, respectively. Chrysotile is difficult to be dispersed and its surface is easily affected under electron beam radiation because of the occurrence of hydroxyl layer on chrysotile surface. Nano-fibriform silica is composed of short fibers with rugged surfaces (Fig. 4b) and may be dispersed in rubbing or solution [1], and its surface is not affected under electron beam radiation. The modified product also shows short fibers, and the fiber is thicker, smoother and tougher than that of nano-fibriform silica. It can be easily dispersed in organic solution, and the surface is not affected under electron beam radiation. 3.5. Specific surface area and volume adsorbance The nitrogen adsorption isotherms of chrysotile, nano-fibriform silica and the modified product are shown in Fig. 5. All isotherms belong to Type IV proposed by the International Union of Pure and Applied Chemistry (IUPAC), indicating that they are mesopore materials. Their specific surface area, adsorptive capacity, pore volume and average pore diameter are listed in Table 5. The values of specific surface area, adsorptive capacity, and pore volume for nano-fibriform silica are very higher than the corresponding values for chrysotile because of the destruction of Mg–OH octahedral layer in chrysotile (acid leaching), and nano-fibriform silica becomes to be a rough surface and porous material. The specific surface area, adsorptive capacity, pore volume of the modified product are much smaller than those of nano-fibriform silica, but the average pore diameter is bigger than that of nano-fibriform silica since many pores in nano-fibriform silica, especially for small pores, are jammed by the graft of the dimethyl silane groups during modification.

3.6. Chemical reaction during modification The reaction between chrysotile and hydrochloric acid results in the destruction and dissolution of Mg-OH octahedral layer in chrysotile, and Mg and most of –OH groups are lost [4]. The remnant is nano-fibriform silica, which is mainly composed of Si–O tetrahedrons [1]. Nano-fibriform silica and dimethyldichlorosilane are mixed to produce a dimethyl silane derivative – the modified product. The composition analyses in Table 4 demonstrate that the ideal molecular formula of the modified product should be more similar to (Si4O6)(CH3)4. Since the methyl group is connected with Si in the form of covalent bond, the modified product is a stable material. 4. Summary The FTIR spectral analyses show that the dimethyl silane groups are grafted in nano-fibriform silica in the form of chemical bond, suggesting that the organic modification for nano-fibriform silica is feasible. During the process of chrysotile ! nano-fibriform silica ! the modified product, their crystallinities gradually drop according to the XRD spectral results. The composition analyses indicate that the ideal molecular formula of modified product is similar to (Si4O6)(CH3)4. There are so little –OH groups in the modified product that the polarity is greatly reduced. And as a result of that the modified product is easily dispersed in the organic solvent and its compatibility with organic molecules is improved remarkably. After modification, the pore volume significantly decreases and the ductility greatly increases, suggesting that the modified product may be used as a good filling reinforcing agent with great strength. Acknowledgements This project was financially supported by the National Key Program for Basic Research of China (no. 2001CCA02400), the Open Research Program of the National Laboratory of Mineral Materials of China (06014, 06008), and a Special Program of the Beijing Municipal Station of Agro-environment Monitoring. References

Fig. 5. N2 adsorption isotherms of chrysotile, nano-fibriform silica and the modified product.

[1] L. Wang, A. Lu, C. Wang, X. Zheng, D. Zhao, R. Liu, Journal of Colloid and Interface Science 295 (2006) 436. [2] L. Wang, A. Lu, C. Wang, X. Li, X. Zheng, D. Zhao, R. Liu, Acta Geologica Sinica 80 (2006) 180. [3] X. Li, Z. Xiao, A. Lu, L. Wang, X. Ouyang, J. Ma, Y. Li, Colloids and Surfaces A: Physicochemical and Engineering Aspects 324 (2008) 171.

7546

L. Wang et al. / Applied Surface Science 255 (2009) 7542–7546

[4] L. Zapata, J.J. Fripiat, J.P. Mercier, Polymer Letter 11 (1973) 689. [5] B. Martsinets, V. Duchmali, Acta Polymerica 34 (1983) 747. [6] E.F. Vansant, P. Van Der Voort, K.C. Vrancken, Characterization and Chemical Modification of the Silica Surface, Elsevier, Amsterdam, 1995. [7] Y. Sun, H. Yang, Silicone Material and Applications 13 (1999) 15. [8] Q. Wang, Q. Li, Z. Wang, H. Yang, C. Kang, T. Fang, Journal of East China University of Science and Technology 27 (2001) 626. [9] E. Mendelovici, R.L. Frost, J. Theo Kloprogge, J. Colloid and Interface Science 238 (2001) 273. [10] M.G. Fonseca, A.S. Oliveira, C. Airoldi, Journal of Colloid and Interface Science 240 (2001) 533.

[11] A. Fachini, I. Joekes, Colloids and Surfaces A: Physicochemical and Engineering Aspects 201 (2002) 151. [12] R.A. Nypuist, R.O. Kagel, Infrared Spectra of Inorganic Compounds, Academic Press, New York and London, 1971. [13] G. Wen, The Infrared Spectroscopy of Minerals, Chongqing University Press, Chongqing, 1988. [14] Y. Chen, Technology of Infrared-photoacoustic and Roman-photoacoustic Spectroscopy, Weave Industry Press, Beijing, 1988. [15] X. Jing, S. Chen, E. Me, Applied Directory of Infrared-photoacoustic Spectroscopy, Tianjin Science and Technology Press, Tianjin, 1992.