Coacervation Microencapsulation of Talc Particles with a

Ind. Eng. Chem. Res. , 2006, 45 (18), pp 6162–6168. DOI: 10.1021/ie060403t. Publication Date (Web): July 29, 2006. Copyright © 2006 American Chemic...
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Coacervation Microencapsulation of Talc Particles with a Fluoropolymer by Pressure-Induced Phase Separation of Supercritical Carbon Dioxide Solutions Kiyoshi Matsuyama* and Kenji Mishima Department of Chemical Engineering, Faculty of Engineering, Fukuoka UniVersity, 8-19-1 Nanakuma Jonan-ku, Fukuoka 814-0180, Japan

We report a method for the coacervation microencapsulation of talc (Mg3Si4O10(OH)2) microparticles with the fluoropolymer poly(heptadecafluorodecyl acrylate) (poly(HDFDA)) by pressure-induced phase separation of a supercritical CO2 solution. A suspension of talc in CO2 and dissolved poly(HDFDA) were mixed in supercritical CO2. After the system pressure was slowly decreased to atmospheric pressure, the microcapsules were obtained in a high-pressure cell. Coacervation was achieved by the precipitation of poly(HDFDA) during the decrease in the pressure of CO2; the solubility of poly(HDFDA) in CO2 decreased with the pressure. The structure and morphology of the microparticles were investigated by using a scanning electron microscope (SEM) and an electron probe microanalyzer (EPMA) equipped with a wavelength dispersive X-ray spectroscope (WDX). This investigation revealed that the talc was thoroughly coated with poly(HDFDA). Furthermore, the effects of experimental conditions such as the depressurizing rate, polymer concentration, and temperature on particle morphology were investigated. 1. Introduction The field of particle formation is most likely to emerge as a commercial application area that uses supercritical fluids.1-5 Polymer microcapsules and/or composite particles containing inorganic materials are attracting considerable attention in the paint, cosmetic, and specialty chemical industries, and many applications have been reported such as those in the development of dry ink, powder coating, polymerized toner, and cosmetics.6-8 In these applications supercritical CO2 (scCO2) is the solvent of choice because it is readily available, inexpensive, and environmentally benign. Many investigators have attempted the formation of polymer microcapsules and/or composite particles using scCO2.1-14 In particular, rapid expansion from supercritical solutions (RESS) is a well-known process, and a variety of polymer microcapsules and/or composite particles have been produced with the help of this process by many investigators.9,10,12,13,15-19 However, the RESS process is limited by the low polymer solubility in CO2, caused by its low dielectric constant. Relatively few polymers are soluble in CO2 without a cosolvent.20,21 RESS of fluoropolymers such as perfluoropolyether, poly(1,1,2,2-tetrahydroperfluorodecyl acrylate), and poly(heptadecafluorodecyl acrylate), which are highly soluble in CO2 at temperatures near the ambient temperature, produces coating materials12,13,22 and submicrometer- to several micrometer-sized particles and fibers.23,24 In this work, we attempt the formation of microcapsules of talc (Mg3Si4O10(OH)2) and poly(heptadecafluorodecyl acrylate) (poly(HDFDA)) using scCO2. The microcapsules of talc and fluoropolymers are used as powder foundations for cosmetic use. The microcapsules of talc and fluoropolymers are used in cosmetic foundations. Talc finds use as a cosmetic (talcum powder) product and lubricant,25 while a fluoropolymer especially lends luster to coatings and imparts very high water repellency.26 Supercritical CO2 is advantageous for the formation of fluoropolymer products because a fluoropolymer is soluble in scCO2 but not in common liquid solvents.27 In a previous work, we reported the microencapsulation of proteins, medi* To whom correspondence should be addressed. Tel.: +81(92)871-6631. Fax: +81(92)865-6031. E-mail: [email protected].

Figure 1. Principles of the formation of polymer microcapsules of talc by (a) RESS and (b) pressure-induced phase separation of scCO2 solutions.

cines, and inorganic particles with a polymer using RESS.16-19 The core materials are coated thoroughly with a polymer. In the RESS process the core materials in the expanding jet serve as a nucleating agent for precipitating the polymer, thus aiding in a thorough encapsulation. However, polymer particles not containing core materials also precipitate in the expanding jet because supersaturation leads to homogeneous nucleation and particle formation during the RESS process. It is difficult to inhibit the precipitation of polymer particles not containing core material in the RESS process. Therefore, we investigated a production method for the fluoropolymer microcapsules of talc by pressure-induced phase separation of scCO2. Figure 1 provides a conceptual framework of our proposed process in comparison with the conventional RESS process. In RESS, a supercritical fluid solution is expanded across a nozzle, leading to rapid supersaturation and

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the production of small particles. After a suspension of talc in CO2 containing a dissolved fluoropolymer is sprayed through the nozzle at atmospheric pressure, microcapsules and small polymer particles are obtained as shown in Figure 1a. To use the microcapsules in cosmetic applications, we have to restrict the generation of polymer particles not containing talc because they degrade the cosmetic products. The following formation mechanism of polymer particles may be considered in the RESS microencapsulation process. First, the polymer solute precipitates on the talc surface because the talc particles serve as a nucleating agent. Second, supersaturation and nucleation occur in the expanding jet and polymer particles not containing talc precipitate. Therefore, to prevent nucleation (caused by the supersaturation through rapid depressurization) and precipitation of polymer particles not containing talc, the pressure is decreased slowly and microparticles are collected in the high-pressure cell as shown in Figure 1b. During the slow pressure decrease coacervation is induced by the decreasing solvent power of CO2 with regard to the fluoropolymer. As a result of the coacervation in CO2, fluoropolymer microcapsules of talc are obtained. The objective of this work is to apply the pressure-induced phase separation of the scCO2 solution to the formation of fluoropolymer microcapsules of talc and study the effect of several experimental conditions on particle morphology. 2. Experimental Section 2.1. Materials. Talc (Mg3Si4O10(OH)2) was obtained from Wako Pure Chem. Ind., Ltd., and carbon dioxide (CO2) (99.9% minimum purity) was purchased from Fukuoka Sanso Co., Ltd. The fundamental idea and synthesis of poly(HDFDA) was reported by DeSimone et al.,27 and a similar approach based

on their method is employed in the present study. The fluoropolymer poly(HDFDA) was synthesized in a high-pressure cell by the free-radical polymerization of a homogeneous solution of the 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluorodecyl acrylate (HDFDA) monomer with an azobis(isobutyronitnile) (AIBN) initiator in CO2 for 48 h at 333 K and 20 MPa. AIBN and HDFDA were purchased from Aldrich Co. Upon completion of polymerization, the polymer was precipitated from CO2 directly into a methanol bath. Subsequently, the poly(HDFDA) was washed several times and allowed to dry overnight. 2.2. Experimental Procedure. Known amounts of the fluoropolymer and talc were placed in the high-pressure cell (25 cm3) equipped with sapphire windows. The cell was placed in a water bath, and the system temperature was maintained at the desired value within (0.1 K. CO2 was pumped through a preheater to the high-pressure cell. The mixture was stirred by a magnetic agitator for 30 min. The system was slowly depressurized for approximately 30 min at the experimental temperature. Following the decrease in pressure, polymer microcapsules were obtained in the high-pressure cell. The structure and morphology of the products were analyzed using a scanning electron microscope (SEM, JEOL JSM6060) and an electron probe microanalyzer (EPMA; Shimadzu, EPMA 1610) equipped with a wavelength dispersive X-ray spectrometer (WDX). An EPMA equipped with WDX can identify elements through the use of a crystal monochromator to select X-rays of a particular wavelength. For the SEM sample preparation, polymeric microparticles were mounted on a small glass plate covered with a small piece of double-sided carbon conductive tape. The samples were then sputter-coated with silver palladium and imaged using SEM and EPMA.

Figure 2. SEM photographs of (a) poly(HDFDA) microcapsules of talc formed by the pressure-induced phase separation of scCO2 solutions and (b) talc. Preexpansion conditions: temperature, 313 K; pressure, 20 MPa; CO2, 97.9 wt %; poly(HDFDA), 0.19 wt %; talc, 1.9 wt %.

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Figure 3. EPMA images of (a) poly(HDFDA) microcapsules of talc formed by the pressure-induced phase separation of scCO2 solutions and (b) talc. See Figure 2 for preexpansion conditions. a-2 and b-2 show the F distribution, while a-3 and b-3 show the Si distribution.

Figure 4. WDX spectra of (a) poly(HDFDA) microcapsules of talc formed by the pressure-induced phase separation of scCO2 solutions and (b) talc. See Figure 2 for preexpansion conditions.

Figure 5. Stability of microcapsules in pure water. (a) Talc and (b) poly(HDFDA) microcapsules formed by the pressure-induced phase separation of scCO2 solutions. See Figure 2 for preexpansion conditions.

3. Results and Discussion

MPa to atmospheric pressure for approximately 30 min at 313 K. The feed concentrations of the talc and fluoropolymer were 1.9 and 0.19 wt %, respectively. The talc had a platelike configuration and a smooth surface, as shown in Figures 2b1,2 and 3b-1. Compared with the SEM photographs of the talc, the microcapsules of the fluoropolymer containing talc have a similar configuration, as shown in Figures 2, 3a-1, and 3b-1. The surface morphology of the microcapsules reflects the configuration of talc in the microcapsules because the coating thickness of talc is very small. The primary particle diameter (PPD) and particle size distribution (PSD) of talc and microcapsules were determined by a laser diffraction particle size analyzer (SALD-2000, Shimadzu Co. Ltd.). In this work, PSD,

3.1. Evolution of Microencapsulation. Prior to the experiment for microcapsule formation, the phase behavior of the CO2 + poly(HDFDA) system at 20 MPa and 313 K was confirmed visually using a high-pressure vessel equipped with sapphire windows. Without the talc, mixtures of CO2 and poly(HDFDA) form a single phase. Details of the phase behavior of the CO2 + poly(HDFDA) system were reported by Blasig et al.23 Similar phase behaviors for CO2 + poly(1,1-dihydroperfluorooctylacrylate)28 and CO2 + poly(1,1,2,2-tetrahydroperfluorodecyl acrylate)24 systems were reported. SEM photographs of the talc and the fluoropolymer microcapsule containing talc that was produced by the pressureinduced phase separation of scCO2 are shown in Figures 2a1,2 and 3a-1. The system was slowly depressurized from 20

σ was defined as σ ) x(1/n)Σk)1n(logDk-logD)2, where n is the number of particles, Dk is the particle diameter, and D is

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Figure 6. SEM photographs of poly(HDFDA) microcapsules of talc formed by RESS. Preexpansion conditions: temperature, 313 K; pressure, 20 MPa; CO2, 97.9 wt %; poly(HDFDA), 0.19 wt %; talc, 1.9 wt %.

the primary particle diameter. The PPD and PSD of talc are 20.7 µm and 0.402, respectively, and the PPD and PSD of microcapsules are 22.1 µm and 0.403, respectively. The value of PPD and PSD of talc and microcapsules is almost same. We cannot observe the remarkable change of particle size. Further evidence for the formation of fluoropolymer microcapsules of talc can be obtained using EPMA. WDX spectra of the talc and microcapsules are shown in Figure 4. Although the peak corresponding to F caused by the fluoropolymer can be observed for the microcapsules, it cannot be detected for talc (Mg3Si4O10(OH)2) because talc does not possess F. The surface distributions of F and Si were mapped in an EPMA image, as shown in Figure 3. Although the distribution of F in the microcapsules was fairly sharp (Figure 3a-2), it was not detected on the talc surface (Figure 3b-2). On the other hand, the distribution of Si on the talc surface was sharper and broader (Figure 3b-3). However, the distribution of Si on the microcapsule surface was poorer (Figure 3a-3) than that on the talc surface (Figure 3b-3). It can be considered that talc was completely encapsulated by a thin fluoropolymer film. This film inhibited the detection of Si. This revealed that the talc was uniformly encapsulated by the fluoropolymer. The coating thickness of the fluoropolymer was less than a micrometer; EPMA measures only up to a depth of 1 µm. It was difficult to check the coating performance for all the collected microcapsules using EPMA because in the proposed process an extremely large number of microcapsules were produced. To evaluate the performance of the polymer coating, we examined the stability of the microcapsules in pure water. The talc particles or microcapsules were added to pure water (particle concentration 1 wt %), and the suspended solution was shaken by a mechanical shaker. Figure 5 shows the stable conditions of the talc and microcapsules in water. Although the talc was dispersed in pure water for more than 5 min (Figure 5a), all the microcapsules floated on water (Figure 5b) because of the high water repellency of the fluoropolymer. The density of talc and microcapsules is almost same (about 2.7 g cm-3) because microcapsules contain more than 90% talc. Although the density of microcapsules is higher than that of water, the microcapsules floated on the water. It is inferred that bulk density of microcapsules is lower than that of water. It is difficult to penetrate the water to the void between the microcapsules because of the repellency of fluoropolymer. On the other hand, the talc was dispersed in water because the talc has hydrophilic surfaces. To check the stability of the microcapsules in pure water, a turbidity measurement was performed using an ultraviolet/visible (UV/Vis) spectrometer at 600 nm wavelength. The turbidity measurement was used to observe the stability of

Figure 7. SEM photographs of poly(HDFDA) microparticles without talc formed by RESS. Preexpansion conditions: temperature, 313 K; pressure, 20 MPa; CO2, 99.8 wt %; poly(HDFDA), 0.19 wt %.

small particle dispersions.29 We could not observe the dispersed particles through the stability analysis of microcapsules in water because as in the case of pure water no turbidity was observed The stability analysis revealed that most of the talc particles were coated with the fluoropolymer and present inside the produced microcapsules. 3.2. Formation Mechanism of Microcapsules. To identify the advantage of the formation mechanism of microcapsules by the pressure-induced phase separation of scCO2 as compared with RESS, the microcapsules were prepared by RESS. Because RESS is one of the promising methods for the formation of polymer microcapsules and/or composites by using scCO2, several investigators have reported the formation of polymer microcapsules and/or composites by RESS.1,4 The particle formation mechanism by RESS was analyzed thermodynamically.9 In this work, we attempted the formation of microcapsules by RESS under the following experimental conditions. The preexpansion pressure was 20 MPa, and the temperature was 313 K. The feed concentrations of the talc and the fluoropolymer were 1.9 and 0.19 wt %, respectively. The feed composition in the RESS experiment was the same as that in the experiment on the formation of microcapsules by the pressure-induced phase separation of scCO2. The mixtures of scCO2, fluoropolymer, and talc were expanded across the capillary nozzle (L ) 500 mm, D ) 1.2 mm) to atmospheric pressure. After the expansion the microparticles were precipitated. SEM photographs of the fluoropolymer microcapsules produced by RESS and containing talc are shown in Figure 6.

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Figure 8. SEM photographs of the cross section of poly(HDFDA) foams formed by the pressure-induced phase separation of scCO2 solutions without talc. Preexpansion conditions: temperature, 313 K; pressure, 20 MPa; CO2, 99.8 wt %; poly(HDFDA), 0.19 wt %.

The polymer does not form a smooth surface at the talc particles but is adhered as small particles at the surface of the talc. To examine the coating performance of RESS, the obtained particles were analyzed by EPMA and by performing a stability test in water. Si and F were detected in the WDX spectrum of the microcapsules. Furthermore, we examined the stability of the microcapsules in pure water to evaluate the performance of the polymer coating. The WDX spectrum and stability test revealed that most of the talc was microencapsulated with the fluoropolymer. However, small polymer particles were precipitated on the surface through the RESS process. The formation mechanism of microcapsules and small polymer particles in the

RESS process may be considered as follows. During rapid depressurization both the talc and the polymer precipitate from the solutions, and the talc particles are formed in the expanding jet. Some polymer coated on the talc particles, and some fine polymer particles are generated during the deposition. Evidence for the formation of fine polymer particles by RESS can be obtained by performing the RESS experiment without talc, as shown in Figure 7. The mean particle diameter was less than 1 µm. With regard to the RESS experiment for the formation of fluoropolymer particles, similar particle morphology was reported by Blasig et al.23 and Mawson et al.24 These fine polymer particles precipitated on and adhered to the talc surface by the supersaturation and homogeneous nucleation of the fluoropolymer that was caused by rapid depressurization. To prevent the formation of polymer particles we have to inhibit the supersaturation of the solute and the homogeneous nucleation caused by the rapid expansion of CO2. However, it is impossible to prevent the supersaturation in RESS. Therefore, we discussed a new microencapsulation technique using scCO2. We can prevent the formation of polymer particles by the pressure-induced phase separation of CO2 proposed in this work. Because the depressurizing rate is very slow compared with the conventional RESS process, it is possible to inhibit the large supersaturation of the solute and the homogeneous nucleation of particles. During the slow depressurization the coacervation was achieved. On the other hand, after the pressure in the highpressure cell containing no talc decreased, polymer foams were obtained in the cell, as shown in Figure 8. With the experimental setup, no pure fluoropolymer particles were formed. It is inferred that the talc suspended in scCO2 acts as an accelerator for the precipitation of polymer particles and the occurrence of coacervation on the talc surface. Furthermore, it is very difficult for the microcapsules to produce forms because the microcapsules contain about 90% talc. In the conventional coacervation

Figure 9. Effect of polymer concentration on the morphology of particles formed by the pressure-induced phase separation of scCO2 solutions. Preexpansion conditions: temperature, 313 K; pressure, 20 MPa; talc, 1.9 wt %. Concentrations of poly(HDFDA) are (a) 0.19, (b) 0.4, (c) 1.2, and (d) 1.9 wt %.

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Figure 11. Effect of the depressurizing rate on the morphology of particles formed by the pressure-induced phase separation of scCO2 solutions. Preexpansion conditions: temperature, 313 K; pressure, 20 MPa; CO2, 97.9 wt %; poly(HDFDA), 0.19 wt %; talc, 1.9 wt %. The system pressure was rapidly decreased to atmospheric pressure in 2 min.

Figure 10. Effect of the preexpansion temperature on the morphology of particles formed by the pressure-induced phase separation of liquid CO2 solutions. Preexpansion conditions: temperature, 288 K; pressure, 20 MPa; CO2, 97.9 wt %; poly(HDFDA), 0.19 wt %; talc, 1.9 wt %.

microencapsulation technique, coacervation is induced by a phase separation caused due to a pH change and the addition of a nonsolvent or electrolyte.30 In contrast, in the present experiment, coacervation was induced by a phase separation caused by a decrease in pressure. 3.3. Effect of Various Factors on the Particle Morphology. The effects of experimental conditions such as the polymer concentration, temperature, pressure, and depressurizing rate on the particle morphology were investigated. Figure 9 shows the effect of polymer concentration on the particle morphology of the microcapsules produced by the pressure-induced phase separation of scCO2. At low polymer concentrations, the microcapsules reflect the configuration of the talc surface because the coating thickness is very small. However, the microcapsules cohere to each other with increasing polymer concentration. If the feed composition ratio of the talc and the polymer is 1:1, we can obtain bulky solids. As shown in Figure 9d, the talc is completely buried in the fluoropolymer solid and we cannot obtain any microcapsule or microparticle. To verify the advantage of scCO2 as compared with liquid CO2, we attempted the formation of fluoropolymer microcapsules in liquid CO2 at 288 K. We shall now discuss the effect of temperature on the particle morphology. Figure 10 shows SEM photographs of products formed by the pressure-induced phase separation of CO2 solutions at 288 K. Flocculent polymer products were obtained. A liquid-vapor interface was formed

during depressurization at a temperature less than the critical temperature of CO2 (Tc ) 304 K). The fluoropolymer microcapsules were aggregated by capillary stress. On the other hand, the aggregation of particles was prevented under temperatures higher than the critical temperature because the liquid-vapor interface was not formed and it was possible to prevent capillary stress. The absence of capillary stress in the liquid-vapor interface is an advantage in the process using a supercritical fluid.31 Furthermore, the effect of the depressurizing rate on the particle morphology was studied. The process conditions were set to 20 MPa and 313 K. The system pressure was rapidly decreased to atmospheric pressure in 2 min. Close to the critical pressure (Pc ) 7.37 MPa) the system temperature was decreased to 293 K by the Joule-Thomson effect. The polymer microcapsules were aggregated, as shown in Figure 11. It is inferred that the temperature of the fluid was decreased by the JouleThomson effect and that the liquid-vapor interface appeared during the rapid decrease in pressure. The capillary stresses in the liquid-vapor interfaces accelerated the aggregation of particles. A slow depressurization to avoid the decrease in temperature and capillary stress is the key novel feature of the present work. Finally, the effect of operating pressure on the particle morphology was discussed. The system pressure was slowly depressurized from several pressures (10, 15, 20, and 25 MPa). However, the particle morphology is independent of the operating pressure because fluoropolymer is completely dissolved in scCO2 at our experimental conditions. 4. Conclusions The pressure-induced phase separation of scCO2 has been utilized to produce fluoropolymer microcapsules of talc. Prior to depressurization, the polymer and talc were mixed in scCO2. Fluoropolymer coacervation was achieved during the slow decrease in the pressure. Following the coacervation, we obtained the fluoropolymer microcapsules of talc. The products were analyzed by SEM and EPMA equipped with WDX. The talc was completely coated with a thin fluoropolymer film. Compared with the microcapsules formed by RESS, the obtained

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microcapsules had a smooth surface; fine polymer particles on the talc surface were not observed. Acknowledgment This work was partially supported by a Grant-in-Aid for Young Scientists (Grant No. 17760610) and the Iketani Science and Technology Foundation. Literature Cited (1) Jung, J.; Perrut, M. Particle Design Using Supercritical Fluids: Literature and Patent Survey. J. Supercrit. Fluids 2001, 20, 179. (2) Sun, Y.-P.; Rollins, H. W.; Bandara, J.; Meziani, M. J.; Bunker, C. E. In Supercritical Fluid Technology in Materials Science and Engineering: Synthesis, Properties, and Applications; Sun, Y.-P., Ed.; Marcel Dekker: New York, 2002; p 491. (3) Beckman, E. J. Supercritical and Near-critical CO2 in Green Chemical Synthesis and Processing. J. Supercrit. Fluids 2004, 28, 121. (4) Yeo, S. D.; Kiran, E. Formation of Polymer Particles with Supercritical Fluids: A Review. J. Supercrit. Fluids 2005, 34, 287. (5) Reverchon, E.; Adami, R. Nanomaterials and Supercritical Fluids. J. Supercrit. Fluids 2006, 37, 1. (6) Mandel, F. S.; Wang, J. D. Manufacturing of Specialty Materials in Supercritical Fluid Carbon Dioxide. Inorg. Chim. Acta 1999, 294, 214. (7) Weidner, E.; Petermann, M.; Blatter, K.; Rekowski, V. Manufacture of Powder Coatings by Spraying of Gas-enriched Melts. Chem. Eng. Technol. 2001, 24, 529. (8) Matsuyama, K.; Mishima, K.; Umemoto, H.; Yamaguchi, S. Environmentally Benign Formation of Polymeric Microspheres by Rapid Expansion of Supercritical Carbon Dioxide Solution with a Nonsolvent. EnViron. Sci. Technol. 2001, 35, 4149. (9) Tom, J. W.; Debenedetti, P. G.; Jerome, R. Precipitation of Poly(L-lactic acid) and Composite Poly(L-lactic acid)-Pyrene Particles by Rapid Expansion of Supercritical Solutions. J. Supercrit. Fluids 1994, 7, 9. (10) Kim, J.-H.; Paxton, T. E.; Tomasko, D. L. Microencapsulation of Naproxen Using Rapid Expansion of Supercritical Solutions. Biotechnol. Prog. 1996, 12, 650. (11) Howdle, S. M.; Watson, M. S.; Whitaker, M. J.; Popov, V. K.; Davies, M. C.; Mandel, F. S.; Wang, J. D.; Shakesheff, K. M. Supercritical Fluid Mixing: Preparation of Thermally Sensitive Polymer Composites Containing Bioactive Materials. Chem. Commun. 2001, 109. (12) Meziani, M. J.; Sun, Y. P. Protein-conjugated Nanoparticles from Rapid Expansion of Supercritical Fluid Solution into Aqueous Solution. J. Am. Chem. Soc. 2003, 125, 8015. (13) Meziani, M. J.; Pathak, P.; Harruff, B. A.; Hurezeanu, R.; Sun, Y. P. Direct Conjugation of Semiconductor Nanoparticles with Proteins. Langmuir 2005, 21, 2008. (14) Matsuyama, K.; Mishima, K. Formation of TiO2/Polymer Composite Microparticles by Rapid Expansion of CO2 Saturated Polymer Suspensions with High Shear Mixing. J. Supercrit. Fluids, in press. (15) Matson, D. W.; Fulton, J. L.; Petersen, R. C.; Smith, R. D. Rapid Expansion of Supercritical Fluid Solutions: Solute Formation of Powders, Thin Films, and Fibers. Ind. Eng. Chem. Res. 1987, 26, 2298. (16) Mishima, K.; Matsuyama, K.; Tanabe, D.; Yamauchi, S.; Young, T. J.; Johnston, K. P. Microencapsulation of Proteins by Rapid Expansion of Supercritical Solution with a Nonsolvent. AIChE J. 2000, 46, 857.

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ReceiVed for reView March 31, 2006 ReVised manuscript receiVed June 16, 2006 Accepted July 5, 2006 IE060403T