Hydrophobic modification of silica nanoparticle by using aerosol spray reactor

Colloids and Surfaces A: Physicochem. Eng. Aspects 236 (2004) 73–79

Hydrophobic modification of silica nanoparticle by using aerosol spray reactor Yun Seup Chung, Shin Ae Song, Seung Bin Park∗ Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea Received 21 July 2003; accepted 30 January 2004

Abstract Surface of commercial silica particle was hydrophobically modified with octyltriethoxy-silane (OTES) in an aerosol spray reactor. When the reactor temperature was higher than 250 ◦ C, one liquid droplet produced one agglomerated particle. However, when the reactor temperature was lower than 250 ◦ C, the original droplet shape was completely disappeared and particles were produced in a dispersed state. Based on the Fourier transform-infrared (FT-IR), thermo gravimetric analysis (TGA) and scanning electron microscopy (SEM) results, competition between the evaporation of solvent and reaction of OTES with the silica surface was proposed to be responsible for the morphology dependency on the reactor temperature. Hydrophobicity of the modified particle was quantitatively measured by ethanol volume ratio (EVR) and contact angle. The hydrophobicity was increased as increasing the concentration of OTES and had the optimum point with the reaction temperature. It was found that the discrepancy of the hydrophobicity measured by the EVR and contact angle gave information on the degree of agglomeration of particles produced by the aerosol spray reactor. © 2004 Elsevier B.V. All rights reserved. Keywords: Surface modification; Coating; Silica; Nanoparticle; Aerosol spray reactor; Hydrophobicity

1. Introduction The coated particles draw attention because of their importance in a fundamental understanding of properties and performance in their applications. The surfaces of these materials possess the completely different characteristics compared with the core element because of the composition gradient imposed by coating. This difference gives the coated particle unique optical, electronic, magnetic, and mechanical properties [1,2]. Silica particle has been used as fillers, polishing materials, pigments, and reinforcement materials for its nonporous and hydrophilic characteristics. The hydrophilicity comes from the hydroxyl group (–OH) on the silica surface [3,4]. Especially, the hydroxyl group on the silica surface absorbs moisture and causes the particle to be agglomerated. Improvement of purity, crystallinity and hydrophobic charac-

∗ Corresponding author. Tel.: +82-42-869-3928; fax: +82-42-869-3910. E-mail address: [email protected] (S.B. Park).

0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.01.025

teristics are needed if more versatile and effective applications of silica particles are expected [5]. There are many processes for coating particles in liquid-phase such as reverse micelles method [6], liquid precipitation method [7] and sol–gel method [8,9]. The liquid-phase method is the most adaptable coating process because of low reaction temperature, low operating cost and wide range of operating conditions. However, the liquid-phase method requires large quantity of process solvent for washing and produces modified particles of heavy agglomeration. In the polymer and ceramic mixed matrix, this agglomerated ceramic particle serves as a defect or gives the negative influence on the product performance [1,10]. On the contrary, gas-phase coating method such as chemical vapor deposition (CVD), physical vapor deposition (PVD) and plasma coating method solve these problems at the expense of high operating cost. In this research, we investigated an aerosol spray reactor method for modifying the hydrophobicity of silica particle. The aerosol spray reactor method has all the advantages of liquid-phase process, while it produces agglomeration-free particle at lower operating cost than the gas-phase

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Droplet

OTES

Silica

EvaporationofEtOHandH2O

Fig. 1. Schematic diagram of coating process in aerosol spray reactor system.

process. Hydrophobicity of the modified particles was compared with the silica particle modified by liquid-phase process and gas-phase process. Hydrophobicity of commercial silica (Aerosil® R812, Degussa) was also measured as a reference. 2. Experiment Commercial fumed silica (Aerosil® 200, Degussa) was modified with OTES (Octyltriethoxysilane CH3 (CH2 )7 Si (OC2 H5 )3 , >96%, Aldrich). The aerosol spray reactor was composed of droplet generator, furnace reactor for surface modification reaction and particle collector. In the droplet generator, the precursor solution was atomized into 5 ␮m size droplets by the ultrasonic nebulizer [11]. The generated droplet was carried into the reactor by carrier gas, N2 . The reactor temperature was varied from 100 to 350 ◦ C and the residence time in the reactor was around 10 s when the carrier gas velocity was 1 l/min. The surface-modified silica was collected by the thimble filter (28 mm × 100 mm, Toyo Roshi Kaisha Ltd., Japan). The precursor solution for atomization was a 7:3 mixture of ethanol (C2 H5 OH, Aldrich) and distillated water. The 1 wt.% of silica particle (Aerosil® 200, Degussa) was dispersed in this diluted ethanol. A small amount of nitric acid was added for controlling the reaction time. The concentration of OTES was varied from 1.6 to 13 g/g silica. The Fig. 1 is the schematic diagram of coating process in an aerosol spray. The OTES molecules in the droplet react with H2 O and turns to hydroxide forms (CH3 (CH2 )7 Si(OH)3 ). These hydroxide modifiers react with the hydroxyl groups on the silica surface [12,13]. CH3 (CH2 )7 Si(OC2 H5 )3 + H2 O → CH3 (CH2 )7 Si(OH)3 + C2 H5 OH

(1)

CH3 (CH2 )7 Si(OH)3 + Silica → Silica–Si(OH)2 (CH3 (CH2 )7 + H2 O

(2)

Other surface modified silica particles were prepared by using gas-phase reaction and liquid-phase reaction in order to compare the hydrophobicity of modified particle by using aerosol spray reactor. For the gas-phase coating, nano-size

CHx stretching and deformation mode -(CH2)n- (n > 3)

(e) (d) (c) (b)

(a)

0

500 1000 1500 2000 2500 3000 3500 4000 4500

cm

-1

Fig. 2. FT-IR spectra of OTES-coated silica particle (a); pure silica (b); 1.6 g OTES/g silica (c); 3.2 g OTES/g silica (d); 6.5 g OTES/g silica (e). 13 g OTES/g silica.

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Fig. 2 is the FT-IR spectra of bare silica and OTES-coated silica prepared at 150 ◦ C of the reactor temperature. The CHx stretching and deformation modes are observed at 1450 and 2950 cm−1 and (CH2 )n (n > 3) peak appears at 720 cm−1 . The peak intensity of CHx in the FT-IR spectra and the fraction of weight loss, calculated from the TGA data, are plotted against OTES concentration and reactor temperature in Fig. 3(a) and (b), respectively. In Fig. 3(a), the total 0.08

0.12

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o

Reaction Temperature : 150 C 0.09

0.04 0

3

6

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0.08

6.5 g of OTES per g of silica 0.12

0.07

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0.09

Height of -CHx peak [A. U.]

Weight loss fraction of modified particle

3. Results and discussion

0.13

(a)

0.04 150

(b)

scanning electron microscopy (SEM; Model 535M, Philips) and the coated OTES layer was observed by TEM (Model 2000EX, JEOL). The functional groups and the amount of polymer on the silica surface were detected by Fourier transform-infrared (FT-IR; Model MB-100, BOMEN) and thermo gravimetric analysis (TGA; Model 92-16, Setaram).

Height of-CHx peak [A. U.]

Weight loss fraction of modified particle

fumed silica was suspended in the gas-phase reactor and the same amount of OTES to the sample prepared in an aerosol spry reactor was injected into the reactor in gas-phase. Other experimental conditions were same to the aerosol spray reactor. For the liquid-phase preparation, nano-size fumed silica was reacted with the OTES in a water bath at 80 ◦ C for 1–16 h. Other experimental conditions were same to preparation in the aerosol spray reactor. The modified silica particles were washed twice to eliminate the unreacted OTES. The hydrophobicity of the modified silica was measured by the ethanol volume ratio (EVR) and the contact angle. The EVR is a volume ratio of ethanol to water at which the modified particles dispersed in a 50 ml of pure water start to sink as the amount of ethanol is increased. Contact angle on a pellet was measured by a machine manufactured by Erma co. Ltd. (Model GI type). The pellet was prepared by compressing a disk of 1.5 cm in diameter with a pressure of 14 MPa for 20 s. The morphology of coated silica was measured by

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200

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o

Reactor temperature [ C]

Fig. 3. Effect of OTES concentration and reactor temperature on the fraction of weight loss and CHx intensity of coated particle OTES. (a) OTES concentration, (b) reactor temperature.

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silica

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98

96

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o

250 C o

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Temperature [ C] Fig. 4. TGA curves of OTES-coated silica prepared at different reactor temperatures.

amount of hydrocarbon attached to the silica surface was increased with the OTES concentration. In Fig. 3(b), the fraction of weight loss and the CHx intensity reaches to the minimum point at around 250 ◦ C. In the low temperature range (<250 ◦ C), the modifier OTES of which boiling point is 85 ◦ C, vaporizes vigorously and the amount of OTES incorporated into the particle is reduced as the reactor temperature increases. In the high temperature range (>250 ◦ C), however, the amount of total CHx groups is increased because the rate of hydrolysis and condensation of OTES is increased. This high rate of reaction results in high surface reaction between hydroxide form (CH3 (CH2 )7 Si(OH)3 ) of OTES and hydroxyl group (OH) on a silica surface. This interpretation is backed by the TGA curves in Fig. 4. The samples prepared at lower than 250 ◦ C are monotonically decreasing curves and the decomposition is completed at around 500 ◦ C. The other samples prepared at 300 and 350 ◦ C have two steps in the TGA curves. The first step of the decreasing curve corresponds to the vaporization of hydroxide form (CH3 (CH2 )7 Si(OH)3 ) of OTES at around 300 ◦ C. The second step of the decreasing curve corresponds to the decomposition of hydrocarbon at around 500 ◦ C. The SEM micrographs of silica particle prepared at different OTES concentrations and reactor temperatures are in the Figs. 5 and 6. All the samples in Fig. 5 are prepared at 150 ◦ C of the reactor temperature. The sample, in Fig. 5(a), modified with 1.6 g OTES/g silica is in well-dispersed state with the particle size of 20–50 nm. As the OTES concentration is increased from 1.6 to 13 g/g silica, the agglomerates ranging from 50 to 200 nm in diameter are formed because of the

Fig. 5. Silica particle morphology with different amount of modifier by SEM micrograph. (a) 1.6 g OTES/g silica, 150 ◦ C, (b) 6.5 g OTES/g silica, 150 ◦ C, (c) 13 g OTES/g silica, 150 ◦ C.

extra-added OTES (Fig. 5(c)). Despite of this agglomeration, all the samples modified at 150 ◦ C are in dispersed state and the original shapes of the droplets are not observed. However, as the reaction temperature is increased, the nano-size particles turn into the micrometer-sized agglomerates as shown in Fig. 6. The agglomeration occurs because the solvent in the droplet evaporates so fast that the OTES does not have enough time to be coated the silica surface. Instead, the hydroxide form of OTES (CH3 (CH2 )7 Si(OH)3 ) and unreacted OTES remains in between the particles and keeps the

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Fig. 6. SEM micrographs of silica particles prepared at 2 different reaction temperatures: (a) 6.5 g OTES/g silica, 250 ◦ C; (b) 6.5 g OTES/g silica, 350 ◦ C.

silica nanoparticles together. In Fig. 6(a), the morphology of the sample prepared at 250 ◦ C is a mixture of dispersed particles that lose the original droplet shape and agglomerates that are same as the original droplets produced in the aerosol generator. In Fig. 6(b), silica particle modified at 350 ◦ C, only agglomerates that are same as the original droplets are observed. The surface of one agglomerate particle is expanded and placed inside the Fig. 6(b). It clearly shows that the agglomerate is a collection of particles bound together. The hydroxide form of OTES and unreacted OTES is likely to be a binder in between the nano-size silica particles. Fig. 7 is a TEM micrograph of the coating layer prepared at 200 ◦ C from a solution that contains 6.5 g OTES/g silica. The original silica particle size is around 10–20 nm and the coating layer is 2–5 nm thick. Fig. 8 is a summary of the hydrophobicity of surface modified silica measured by EVR and contact angle. The EVR value decreases with increasing the reactor temperature. OTES concentration dependency is also not a simple function of OTES loading. This discrepancy is inevitable because both contact angle and EVR represent hydrophobicity of particles but the interpretation is different depending on the particle morphology. The contact angle represents the bulk characteristics rather than powder characteristics

Fig. 7. TEM micrograph of coated silica particle (6.5 g OTES/g of silica, 200 ◦ C).

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Contact angle [degree]

Gas-phase reaction

1.2

EVR

120

Contact angle [degree]

1.2

3

6

9

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15

OTES [g / g of silica]

Fig. 8. Hydrophobicity of coated silica with different operating conditions. (a) Reactor temperature; (b) OTES concentration.

because the hydrophobicity is measured on the pelletized film. It means that the contact angle reflects the total amount of hydrocarbon on the silica surface without taking into account the degree of agglomeration. The EVR data, however, represents the hydrophobicity of agglomerated particle. The hydrophobicity measured by EVR is lower for the agglomerated particle than the individually separated particle, because the hydrophobic groups inside the agglomerated particles do not contributes to the hydrophobicity. In Fig. 8(a) EVR data is decreased with reaction temperature because of agglomeration of particles. In Fig. 8(b) EVR data shows the maximum value at the 6.5 g OTES/g silica because the coating of OTES is dominant when the loading is less than 6.5 g OTES/g silica and but agglomeration of modified silica particles is dominant process when the OTES loading is higher than 6.5 g OTES/g silica. The EVR value of a coated silica particle by using aerosol spray reactor is similar to that of particles prepared by the gas-phase reaction and two times higher than commercial powder

(Aerosil® R812, Degussa) and that of particles prepared by the liquid-state reaction. It should be noted that the reason why silica particles modified by using the aerosol spray reactor have high EVR value is because the aerosol spray reactor produces particles of agglomeration-free morphology.

4. Conclusion The hydrophobically modified silica particles were prepared by using an aerosol spray reactor and characterized with FT-IR, TGA and SEM. When the reactor temperature was lower than 250 ◦ C, silica particles were coated with OTES, dried, and individually dispersed. The original droplet shapes were disappeared. However, when the reactor temperature was higher than 250 ◦ C, fast evaporation of the solvent prohibits the OTES from coating the silica surface. Instead, the OTES served as binding material and agglomerates of silica particles were produced. The

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hydrophobicity of surface modified particle was measured by EVR and contact angle. EVR value of the particle prepared by an aerosol spray reactor was similar to the value of the particles modified by a gas-phase reaction and two times higher than that of the particles prepared by the liquid-phase reaction and commercial process. Acknowledgements This work was partially supported by Center for Ultramicrochemical Process Systems and Ministry of Science and Technology (Strategic Research – Chemistry 99–02). References [1] G.P. Fotou, T.T. Kodas, B. Anderson, Aerosol Sci. Technol. 33 (2000) 557.

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