Dispersion Stability Enhancement of Titania Nanoparticles in Organic

Jul 13, 2009 - of Engineering, Sebelas Maret UniVersity, Jl. Ir. Sutami 36 A, Surakarta, Central JaVa 57126, Indonesia. A stable dispersion of titania...
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APPLIED CHEMISTRY Dispersion Stability Enhancement of Titania Nanoparticles in Organic Solvent Using a Bead Mill Process I Made Joni,† Agus Purwanto,†,‡ Ferry Iskandar,† and Kikuo Okuyama*,†

Ind. Eng. Chem. Res. 2009.48:6916-6922. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 01/28/19. For personal use only.

Department of Chemical Engineering, Graduate School of Engineering, Hiroshima UniVersity, 1-4-1 Kagamiyama, Higashi Hiroshima, Hiroshima 739-8527, Japan, Department of Chemical Engineering, Faculty of Engineering, Sebelas Maret UniVersity, Jl. Ir. Sutami 36 A, Surakarta, Central JaVa 57126, Indonesia

A stable dispersion of titania nanoparticles (primary size 15 nm) in an organic solvent of diethylene glycol dimethylether (diglyme) was successfully prepared using a bead milling process. In order to enhance dispersion stability of the solution, surface modification of dispersed titania particles was carried out during the centrifugal bead mill process. Surface modification was utilized with silane coupling agents, (3-acryl-oxypropyl)trimethoxysilane and trimethoxypropylsilane. The effects of surface modification at selected milling times on the morphology, mean particle size distribution, and degree of dispersion stability by means of zeta potential were investigated. The effect of titania concentration (1 wt %, 5 wt %, and 10 wt %) on the optical transmission properties was also examined. Finally, a solution with a high degree of dispersion stability with zeta potential up to 80 mV was obtained. The achievement of a decrease in average particle size to approximately the primary size of 15 nm and high dispersion stability has improved solution optical transparency at a corresponding concentration of titania particles. Surface-modified nanoparticles sustained their UV filter properties as desired in titania-based composites for optical applications. Introduction The study of titania-based nanocomposites is still in its infancy, and much research is needed to explore and improve synthesis techniques that yield different nanocomposite properties.1-3 Nanocomposites can be prepared either by synthesizing nanoparticles within a polymer matrix4,5 or by dispersing nanoparticles in a monomer and polymerizing the monomer in the presence of the nanoparticles.6-9 Although successful in many cases, the former method has several drawbacks. It builds up only relatively weak interaction forces between polymer and nanoparticles. In addition, the attractive force between nanoparticles in liquid suspensions is often sufficiently strong10,11 that nanoparticles tend to agglomerate in most monomers. Poorly dispersed nanoparticles in monomer solutions will not give rise to nanoparticle-polymer nanocomposites. The dispersion of nanoparticles has been successfully achieved only after modification of the surface of the particles.12 Difficulties have arisen because a particular polymer needed a specific type of surfactant to make the solution stable. A method for dispersing nanoparticles in organic solvent was therefore proposed to allow the resulting solution more flexibility to mix with polymerics related to the applications of the nanocomposite. The dispersion of titania nanoparticles in organic solvent was prepared by a centrifugal bead milling process10-12 that has prospective industrial applications. Titania nanoparticles were dispersed in organic solvent of diethylene glycol dimethylether (CH3OCH2CH2-)2O or diglyme. Diglyme was chosen as a solvent because of its distinctive structural property, made of a combination of ether, alcohol, and hydrocarbon chains. This property provides versatile solvency characteristics with both * To whom correspondence should be addressed. E-mail: Okuyama@ hiroshima-u.ac.jp. Tel.: +81-82-424-7716. Fax: +81-82-424-5494. † Hiroshima University. ‡ Sebelas Maret University.

polar and nonpolar properties. The chemical structure is a long hydrocarbon chain resistant to solubility in water, while ether or alcohol groups introduce improved hydrophilic solubility performance. Glycol ethers are characterized by their wide range of hydrophilic/hydrophobic balance. On the other hand, the titania particles used are very hydrophilic and cannot be dispersed directly in low-polar organic solvent. The attractive force between nanoparticles in liquid suspensions is often sufficiently strong13-16 that nanoparticles tend to agglomerate, producing poor dispersion of nanoparticles in organic solvents. The bead milling process is necessary to create surface-modified particles,11,12,17 to change the surface nature of the particles in ground slurry and improve slurry flowability to prevent agglomeration and sustain long-term dispersion stability. To achieve this, surface modification can be performed either by specific adsoption of long chain carboxylic acids18 or by employing selected silane coupling agents. Surface modification was conducted by employing selected silane coupling agents (SCAs) with the same hydrolyzable group and different organofunctional groups. The selected silane-coupling agents were (3-acryl-oxypropyl)trimethoxysilane and trimethoxypropylsilane in this article.19 Furthermore, the solution properties of various concentrations of dispersing agent and titania were experimentally evaluated with respect to their dispersion stability, finest particles distribution, and transparency. The obtained mean particle size of dispersed titania nanoparticles decreased to the primary size of approximately 15 nm after bead milling of agglomerated titania (size ranging from 1 to 3 µm). Dispersion stability of nanosized titania particles in diglyme of organic solvent was successfully synthesized, when examined with the aid of zeta potential observation. High dispersibility of titania particles improved transparency at visible light properties, even though the titania

10.1021/ie801812f CCC: $40.75  2009 American Chemical Society Published on Web 07/13/2009

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concentration increased up to 10 wt %, and it still had fairly good optical transparency and maintained UV filtering properties. Experimental Section Materials. Commercially produced titania nanoparticles, surface unmodified particles with a primary particle size of 15 nm (MT150A, rutile phase; Tayca Co. Ltd., Japan), were used in experiments. Two types of silane coupling agents, (3-acryloxypropyl)trimethoxysilane and trimethoxypropylsilane (Shin Etsu, Kagaku Co. Ltd., Japan), were used in nanoparticle suspensions. The bead mill contained 15 µm ZrO2 (zirconia) beads (Neturen Co. Ltd., Tokyo, Japan). Nanoparticles were dispersed in organic solvent of diethylene glycol dimethylether (CH3OCH2CH2-)2O (Kishida, Kagaku Co. Ltd., Japan). Milling Process. A detailed description of the bead mill is found elsewhere.11,12 The bead mill (Kotobuki Co. Ltd., Japan) is composed of a 170 mL vessel, a pump, and a mixing tank. Nanoparticle suspensions are pumped into the vessel, which contains zirconia beads and a centrifugation rotor at the speed of 73.8 Hz (6095 rpm). The beads are agitated in the lower portion of the vessel (dispersing section), which drives the breakup of agglomerated particles. The suspension is pumped from the dispersing section to the upper region (centrifugation section), where centrifugal force is used to separate the zirconia beads from the nanoparticle suspension. The nanoparticle suspension is then recycled back to the dispersing section. To prevent temperature increase in the system, the vessel is housed in a cooling water jacket and is completely sealed from the outside environment. The weight % was the base of titania concentration from the weight percentage of titania to the total weight of solution that consisted an organic solvent, titania particles, and SCAs. The % of SCA fraction was the concentration of SCAs compared with the wt % of titania concentration, i.e., for a total solution of 140 g, the 10 wt % of titania was 14 g, 100% of SCAs was then 14 g, and weight of the organic solvent (diglyme) was 112 g. The suspended titania particles in organic solvent with the titania weight fractions of 1 wt %, 5 wt %, and 10 wt % were pumped through the bead mill with an optimized recirculation mass flow rate of 10-20 kg/h. Centrifugal forces for bead separation in the upper part of the vessel were generated by rotating the outer cylindrical wall at a speed of 10 m/s. Milling operation conditions20-22 (mass flow rate and speed of centrifugation rotor) have been optimized to control the slurry rheology to get well-dispersed nanoparticle suspensions. Characterization. Particle size distribution after selected times was measured using dynamic light scattering with an HPPS-5001 Malvern Instrument. Field emission scanning electron microscopy (FE-SEM, S-5000, Hitachi Ltd., Tokyo, Japan) was used to obtain the images of titania dispersion evolution in organic solvent at various selected milling times. Particle morphology was examined visually using transmission electron microscopy (TEM, JEM-3000F Japan Electron Optics Laboratory Ltd., Tokyo, Japan). The dispersion stability was observed by measuring zeta potential of the solution with a Malvern ZS NanoS analyzer (Malvern Instrument Inc., London, U.K.). About 2-4 mL of titania suspensions was transferred into measuring cell. The measurement was run at V ) 10 V, T ) 25 °C, with switch time at t ) 50 s. Each experiment was repeated at least 10 times to calculate the mean value of the experimental data. To obtain the existence of surface modification on titania particles, the samples were prepared to obtain the only titania particles with the surface coverage of SCA (silane coupling agent). Thus, prior to the Fourier transform infrared spectroscopy

Figure 1. Size distribution of titania particles dispersed in organic solvent after bead milling as a function of milling time.

(FTIR) and thermogravimetric-differential thermal analysis (TG-DTA) analysis, the dispersed titania sample was four times diluted with water and centrifuged at 15 000 rpm for 1 h at room temperature to remove the organic solvent. Subsequently, the samples of solid deposit were dried for evaporation of water and homocondensates of SCA (unused SCA) at 60 °C for 2 h. The titania sample of unmodified and modified trimethoxypropylsilane was observed by FTIR in the range of 600-4000 cm-1 (PerkinElmer, Spectrum One System). The existence of surface modification was also observed by gravimetric and differential thermal analysis (DG-DTA) using a thermogravimetric analyzer (TG-DTA 6200, Seiko Instruments Inc., Tokyo, Japan) with maximum temperature at 600 °C and temperature ramp rate at 10 °C/min in N2. The optical transmission properties of the solution were measured by UV-vis spectroscopy (U-2810, Hitachi, Japan) at different titania and dispersant concentrations. Results and Discussion Evolution of Particle Size and Morphology. SCAs are silicon-based chemicals that contain two types of reactivity, inorganic and organic, in the same molecule. A typical structure is (XO)3SiCH2CH2CH2-R, where XO is a hydrolyzable group and R is an organofunctional group. The evolution of the particle size distribution of titania with a concentration of 1 wt % and selected SCA of trimethoxypropylsilane was measured using dynamic light scattering (DLS) at selected milling times (Figure 1). Initially the suspension contained large agglomerated particles, and the size distribution was shifted to a smaller size distribution after 60 min of milling time. The size distribution was shifted to the smallest size distribution at milling time (120 min); even if the milling time increased, the distribution remained the same. Figure 2 shows the evolution of the mean particle size distribution of titania particle suspensions with weight fraction 1 wt % at several selected milling times, obtained by performance comparison of two types of selected SCA with the same hydrolyzable group and different organofunctional groups. Selected SCA of (3-acryloxypropyl)trimethoxysilane did not significantly prevent dispersed particles from reagglomeration because less ionic and steric density at the surface promoted the van der Waals forces to dominate. On the other hand, selected SCA of trimethoxypropylsilane successfully separated titania nanoparticle agglomerates in organic solvent after 120 min of milling time. Agglomerated

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Figure 2. Effect of the different silane coupling agents on the dispersion of titania particles with concentration of 1 wt %: (a) (3-acryloxypropyl)trimethoxysilane and (b) trimethoxypropylsilane.

particles ranging from 1 to 3 µm in size were broken up into primary particles as small as 15 nm. Surface modification of dispersed titania particles prevented particle surfaces from coming into close contact by a number of forces, including electrostatic attraction, covalent bonding, hydrogen bonding, and solvation of various species. Those various mechanisms of force and their effect on the electrical double layer have been explained elsewhere in detail.13-16,23-26 The hydrophilic-lipophilic balances (HLBs) of selected SCAs calculated using Griffin’s method for nonionic surfactants are 10.3 and 14.7 for (3-acryloxypropyl)trimethoxysilane and trimethoxypropylsilane, respectively. The selected SCA with a high HLB value may have promoted the particle surfaces to form a very stable structure of siloxane bond27 (Si-O-Si) and improved the steric barrier at the double layer. The modified surface of titania particles is capable of overcoming the interparticle interaction energy due to van der Waals forces, and hence reduces the shear rate in the lowest state to bring the shear stress equal to the yield value28,29 and to promote better dispersion in each cycle of the milling process. Visual analysis from pictures of particle suspension in Figure 2 provides further evidence of the degree to which agglomerate particles were dispersed by the bead mill utilized with selected SCAs. For SCAs of (3-acryloxypropyl)trimethoxysilane, even after 300 min of milling time, particles were poorly dispersed, which was confirmed by a poor degree of transparency (Figure 2a). On the other hand, an additional SCA of trimethoxypropylsilane shows high transparency of solution, providing evidence of the improvement of solution dispersion and clearness (Figure 2b). For further analysis, only an SCA of trimethoxypropylsilane was investigated. Particle morphology in Figure 3 shows field emission-scanning electron microscopy (FE-SEM) images of the evolution of dispersed titania particles at several selected milling times. Before milling, the solution was not transparent, as visually observed in the pictures in Figure 2b. FE-SEM images shows clear features of particle morphology where the solution was composed of agglomerate suspensions (Figure 3a). However, after a milling time of 60 and 90 min, the dispersion of particles improved, which was indicated by smaller agglomerate size. In this state, there were some fragments of relatively larger size and more numerous irregular smaller and fine fragments (Figure 3 parts b and c). In addition, the primary particles appeared to have undergone a morphology change from rod-shape to more spherical (Figure 3 parts b-d). These agglomerates were nearly totally dispersed in organic solvent after 120 min of milling

Figure 3. FE-SEM images of titania particles dispersed in organic solvent after bead milling as a function of milling time: (a) 0 min, (b) 60 min, (c) 90 min, and (d) 120 min.

time, which is identifiable in Figure 3d where the average particle size was reduced nearly to the primary size. Figure 4 shows the TEM images of nanoparticles of before beads milling and after a dispersing time 90 and 120 min. Prior to milling, agglomerated titania nanoparticles with dimension from 1 to 3 µm were observable, and primary titania particles were rodshaped (Figure 4a). Furthermore, the primary particles appeared to have undergone a morphology change from rod-shape to more spherical after dispersing time 90 and 120 min. The concentration of suspended titania particles is important to the interaction of particles and bead to produce effective bead forces during the milling process. The SCA coverage of the particles surface depends on the weight fraction of suspended titania compared to the SCA concentration. For those aims, various titania concentrations with the same concentration of SCA were prepared and their average size at various selected milling times was measured. Figure 5 shows higher weight fraction of suspended titania; a longer milling time is needed to produce an average-sized reduction. This may be due to reduction of the bead force during the milling process in a higher concentration of suspended titania.27-29 Thus, successful dispersion by bead mill method depends on the time necessary to effectively carry out comminuation and surface modification. Dispersion Stability Properties. The DVLO (Derjaguin, Verwey, Landau and Overbeek) theory describes the interparticle potential of suspensions as useful for the prediction of stability of a colloidal suspension. In practice, if attractive interaction forces dominate the interface, the particles aggregate, and if repulsive forces dominate, the dispersion is stable. The surface modification by dispersant agent displays a number of dynamic processes during the milling procedure. Interactions between dispersant agent and

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Figure 4. TEM images of titania particles dispersed in organic solvent at a selected milling time: (a) 0 min, (b) 90 min, and (c) 120 min.

Figure 5. Effect of titania concentration on the average size of dispersed particles as a function of milling time.

nanoparticles differ by their nature and intensity and might be displayed simultaneously.23-25 It is difficult to distinguish between the effects generated by various forces, as changes in the properties of dispersing agent upon contact with nanoparticles reflect to a certain degree the total effect of interaction on the surface of dispersed-phase particles.26 The surface charge density and steric barrier at the double layer characterized the stability of the dispersion, examined by measurement of zeta potential. The stability of the dispersion began to occur at a milling time of 30 min, where the average size of particles was drastically reduced to nearly 100 nm (Figure 6). The high value of zeta-potential above 40 mV indicates that high charge density and micellization in the double layer at the surface of titania particles leads to the tendency of particles in

Figure 6. Zeta potential analysis of dispersed titania 1 wt % in organic solvent.

suspension to repel one another.23-25 The average particle size of titania dispersion did not change within the milling time range of 30-100 min. This stage level of evolution of particle size shows high dispersibility with zeta potential higher than 40 mV. The size-reduction process (bead milling) in this stage level was effectively to break up the agglomerated particles without further reagglomeration of the particles. If larger agglomerated particles successfully break up into smaller particles because of shear force of beads, there are two later possibilities that may occur. The first possibility is the breakup of agglomerated particles followed by reagglomeration or the particles remaining large agglomerated particles. The second possibility is that the particles break up into smaller size and remain separate due to successful surface modification. Usually, the finer agglomerated particles that result from the milling process lead to a domination

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Figure 7. FTIR measurement of bare titania particles and titania nanoparticles modified by trimethoxypropylsilane.

of interparticle forces that cause the particles to reagglomerate. However, the existence of surface-modified titania particles prohibited the interparticle forces or attractive London-van der Waals forces from dominating and prevented the particles from reagglomerating. This condition was also conformed in size distribution of titania in Figure 1, where the distribution was shifted to the lower size of distribution. The simultaneous processes of surface modification and break up of the agglomerate particles lead to conditions where the smaller size of particles probably gives rise to higher concentrations of the selected SCA being adsorbed. Therefore, two processes of breaking up particles and surface modification are time-dependent to finally disperse all particles into smaller size with high dispersibility. A remarkable logarithmic increase of zeta potential began to occur at 30 min milling time, which is related to the time necessary for surface modification. At 120 min milling time, particle size was again reduced drastically to the primary size of 15 nm. This was due to an effective milling process with continuous breaking up of the agglomerate particles within the level stage of milling time range 30-100 min without being followed by reagglomeration. There was a time-dependence for surface modification and also for the deagglomeration or milling process. This observation indicated that the comminuting process and reaction time for surface modification is a coupling mechanism to bring particles to a higher degree of dispersion. Both FTIR spectroscopy and thermogravimetric analysis have been employed to prove the surface modification at the solid surface of titania particles. There is an appreciable difference between the spectra of bare titania particles and titania modified by trimethoxypropylsilane as shown in Figure 7. FTIR spectrum of titania modified by trimethoxypropylsilane indicates the bands at 2865 and 3457 cm-1 are the stretching vibration of C-H (in methoxy functional group) and Ti-OH bonds, respectively. The wide band at around 946 cm-1 may mainly result from the vibration of Ti-O-Si bonds and the absorption band of Si-OH group. The siloxane (Si-O-Si) spectrum was observed at 1093 cm-1. The TG-DTA (Figure 8) of titania particles modified by trimethoxypropylsilane shifted toward higher temperature in comparison to unmodified titania particles. DTA curve showed an endothermic peak at around 100 °C and an exothermic peak at around 500 °C (Figure 8b). The exothermic peak corresponds to the dehydration of Si-OH or Ti-OH, which does not appear in the DTA curve of unmodified titania particles. FTIR spectra and TG-DTA analysis therefore show that the SCA coated the titania particles.

Figure 8. TG-DTA analysis of (a) bare titania particles and (b) titania particles modified by trimethoxypropylsilane.

Figure 9. Optical transmittance properties: (a) optical transmittance of dispersed titania milled from different titania concentrations and (b) effect of dispersant concentration on the transmittance of solution at the excited wavelength of 430 nm.

Optical Transmission Properties. Figure 9a shows the examination of light transmittance of dispersed titania nanoparticles at different titania concentrations. Before bead milling, the solution was not at all transparent at visible light. It was generally observed that the higher dispersibility of titania nanoparticles improved the optical transmission of visible light and maintained UV filtering properties. The increase of titania concentration reduced transmission of visible light, but with titania up to 10 wt % there still was fairly good optical transmittance of visible light. This phenomenon is in agreement with the visual evidence shown in Figure 2b. The dispersant concentration effect on optical properties of solutions containing surface-modified titania particles at different concentrations was obtained from excitation of the solution with a wavelength of 430 nm (Figure 9b). The enhancement of UV absorption at higher concentrations of dispersed titania (as required for optical application) may be caused by increasing numbers of interactions by means of multiple absorption between titania particles

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and photons of electromagnetic waves in the UV wavelength region. On the other hand, higher concentrations of SCA changed the optical properties of the solution. The solution absorption of UV light not only depends on the titania concentration but also on the concentration of SCA. The increase of dispersant concentrations up to 100% did not disturb the UV filter capability, but levels higher than 100% changed the UV filter properties. However, a dispersant fraction of 200% (twice the titania concentration) still resulted in excellent UV filter properties. Conclusions Particle surface modification with a bead mill successfully dispersed titania nanoparticles down to approximately their primary size of 15 nm in organic solvent. This solution therefore allows further tailoring of titania-based nanocomposite properties mixed with polymerics related to application purposes. Modification of the titania surface produced high zeta potential of a solution up to 80 mV and brought the solution into high dispersion stability. A high degree of titania nanoparticle dispersion improved the solution transparency. Because of the high particle stability, with high concentrations of titania particles (up to 10 wt %), the solution was still transparent at visible light. The existence of a selected SCA dispersing agent at the modified surface of titania particle suspensions did not affect their UV absorption capability. Acknowledgment The authors thank Dr. Eishi Tanabe from Western Hiroshima Prefecture Industrial Research Institute, for TEM measurement. We also thank Mr. Takuya Nishiwaki for his assistance with the experiment. We acknowledge the Ministry of National Education Republic of Indonesia for providing a doctoral scholarship (I.M.J). Appendix Appendix: Calculation of Zeta Potential in Aqueous and Nonaqueous Solvent. The zeta potential calculated base on the electrophoresis methods of measurement. When an electric field is applied across an electrolyte, charged particles suspended in the electrolyte are attracted toward the electrode of positive charged. Viscous forces acting on the particles tend to oppose this movement. When equilibrium is reached between these two opposing forces, the particles move with constant velocity. The velocity of a particle in a unit electric field is referred to as its electrophoretic mobility. Zeta potential is related to the electrophoretic mobility by the Henry equation: UE )

2εz f(κa) 3η

where UE ) electrophoretic mobility, z ) zeta potential, ε ) dielectric constant, η ) viscosity, and f(κa) ) Henry’s function. The unit of κ, termed the Debye length, are the reciprocal length κ-1 is often taken as a measure of the “thickness” of the electric double layer. The parameter a refers to the radius of the particle and therefore κa measures the ration of the particle radius to the electric double layer thickness. Electrophoretic determinations of zeta potential are most commonly made in aqueous media and moderate electrolyte. The f(κa) in this case is 1.5, and this referred as the Smoluchowski approximation. For those small particles in low dielectric constant media (i.e., nonaqueous

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media), f(κa) becomes 1.0 and allows an equally simple calculations. This referred as the Huckel approximation. Literature Cited (1) Sezer, E. Conducting Nanocomposite Systems. In The New Frontiers of Organic and Composite Nanotechnology; Victor, E., Manoj, K. R., Ozlem, Y., Eds.; Elsevier, Ltd.: Oxford, 2008; pp 143-235. (2) Thomas, P. J.; O’Brien, P. Recent Developments in the Synthesis, Properties and Assemblies of Nanocrystals. In Nanomaterial Chemistry: Recent DeVelopments and New Directions; Rao, C. N. R., Muller, A., Cheetham, A. K., Eds.; Wiley-VCH Verlag GmbH & Co. KgaA: Weinheim, Germany, 2007; pp 1-43. (3) Pramanik, P. Synthesis of Nanoparticles of Inorganic Oxides by Polymer Matrix. Bull. Mater. Sci. 1995, 18, 819. (4) Sambhy, V.; MacBride, M. M.; Peterson, B. R.; Sen, A. Silver Bromide Nanoparticle/Polymer Composites: Dual Action Tunable Antimicrobial Materials. J. Am. Chem. Soc. 2006, 128, 9798. (5) Yang, M. J.; Dan, Y. Preparation of Poly(methyl methacrylate)/ Titanium Oxide Composite Particles via in-situ Emulsion Polymerization. J. Appl. Polym. Sci. 2006, 101, 4056. (6) Pavel, F. M.; Mackay, R. A. Reverse Micellar Synthesis of a Nanoparticle/Polymer Composite. Langmuir 2000, 16, 8569. (7) Raula, J.; Shan, J.; Nuopponen, M.; Niskanen, A.; Jiang, H.; Kauppinen, E. I.; Tenhu, H. Synthesis of Gold Nanoparticles Grafted with a Thermoresponsive Polymer by Surface-Induced Reversible-AdditionFragmentation Chain-Transfer Polymerization. Langmuir 2003, 19, 3499. (8) Artelt, C.; Schimed, H. J.; Peurkert, W. J. On the Impact of Accessible Surface and Surface Energy on Particle Formation and Growth from the Vapour Phase. J. Aerosol Sci. 2005, 36, 147. (9) Barna, E.; Bommer, B.; Kursteiner, J.; Vital, A.; Trzebiatowski, O. V.; Koch, W.; Schmid, B.; Graule, T. Innovative, Scratch-Proof Nanocomposites for Clear Coatings. Composites A 2005, 36, 473. (10) Inkyo, M.; Tahra, T.; Iwaki, T.; Iskandar, F.; Hogen, C. J.; Okuyama, K. Experimental Investigation of Nanoparticles Dispersion by Beads Milling with Centrifugal Bead Separation. J. Colloid Interface Sci. 2006, 304, 535. (11) Inkyo, M.; Tahara, T. Dispersion of Agglomerated Nanoparticles by Micromedia Mill, Ultra Apex Mill. J. Soc. Powder Technol. Jpn. 2004, 41, 578. (12) Inkyo, M.; Tokunaga, Y.; Tahara, T.; Iwaki, T.; Iskandar, F.; Hogan, C. J.; Okuyama, K. Beads Mill-Assisted Synthesis of Poly Methyl Methacrylate (PMMA)-Titania Nanoparticles Composites. Ind. Eng. Chem. Res. 2008, 47, 2597. (13) Israelachvili, J. Intermolecular and Surface Forces; Academic Press: London, 2000. (14) Muller, F.; Peukert, W.; Polke, R.; Stenger, F. Dispersing Nanoparticles in Liquids. Int. J. Miner. Process 2004, 74S, S31. (15) Adamson, A. W. Physical Chemistry of Surfaces; John Wiley and Sons: Berlin, 1976. (16) Hans-Jurgen, B.; Karlheiz, G.; Michael, K. Physical and Chemistry of Interface; Wiley-VCH Verlag GmbH & Co. KgaA: Weinheim, Germany, 2006. (17) Zurcher, S.; Vital, A.; Dittmann, R.; Trottmann, M.; Bomber, B.; Graule, T.; Apel, E.; Holand, W. Ultrafine Comminution of Dental Glass in a Stirred Media Mill. Chem. Eng. Sci. 2008, 63, 484. (18) Zurcher, S.; Graule, T. Influence of Dispersant Structure on the Rheological Properties of Highly Concentrated Zirconia Dispersions. J. Eur. Ceram. Soc. 2005, 25, 863. (19) Barna, E.; Reutsch, D.; Bommer, B.; Vital, A.; Trzebiatowski, O. V.; Graule, T. Surface Modification of Nanoparticles for Scratch Resistant Clear Coatings. KGK-Kautschuk Gummi Kunststoffe 2007, 1, 49. (20) Tomohiro, I.; Munetake, S.; Tomokazu, T. Estimation of Net Energy Applied to Powders in a Newly Designed Bead Mill. Powder Technol. 2001, 119, 95. (21) Tomohiro, I.; Munetake, S.; Takaomi, K. Analysis of Collision Energy of Bead Media in High-Speed Elliptical-Rotor-Type Powder Mixer Using the Discrete Element Method. Powder Technol. 2001, 121, 239. (22) Tomohiro, I.; Jeong, H. K.; Munetake, S. Characterization of Bead Mills Based on Mechanical Energy Applied to Particles. Chem. Eng. Sci. 2006, 61, 1065. (23) Santanu, P.; Kartic, C. K. A Review on Experimental Studies of Surfactant Adsorption at Hydrophilic Solid-Water Interface. AdV. Colloid Interface Sci. 2004, 110, 75. (24) Duval, J.; Lyklema, J.; Kleijn, J. M.; Herman, P. L. Amphifunctionally Electrified Interface: Coupling of Electronic and Ionic SurfaceCharging Processes. Langmuir 2001, 17, 7573.

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ReceiVed for reView November 26, 2008 ReVised manuscript receiVed June 22, 2009 Accepted June 29, 2009 IE801812F