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Development of a Carbon Dioxide-Based Microencapsulation Technique for Aqueous and Ethanol-Based Latexes H. Liu and M. Z. Yates* Department of Chemical Engineering, University of Rochester, Rochester, New York 14627-0166 Received April 1, 2002. In Final Form: May 16, 2002 A new microencapsulation technique is presented in which carbon dioxide is used to facilitate the transport of additives into colloidal polymer particles. Aqueous latexes were impregnated with additives by emulsifying carbon dioxide and water with the aid of a surfactant, Pluronic F108. Ethanol-based latexes were also successfully impregnated by dispersing additives into an ethanol-carbon dioxide solution. Preexisting size monodisperse polystyrene microspheres were impregnated with dyes with varying solubility characteristics. The effects of particle size, surfactant concentration, dye amount, impregnation time, and pressure on the dye loadings were studied. Partitioning of the dye between the polymer phase and the medium surrounding the polymer particles is the driving force for the dye to enter the polymer phase. Consequently, the partition coefficient is the most important factor that determines the maximum dye loading at the thermodynamic equilibrium. Increasing the solubility of the dye in the medium improves the kinetics but results in low dye loading. Dye loading is improved by increasing the interfacial area, either by increasing surfactant concentration or by lowering particle size. Data reported in this paper illustrate high potential of this technique for microencapsulation at room temperature without organic solvents and with precise control of particle size.
Introduction Microencapsulation refers to a wide variety of techniques used to entrap an active agent or additive into a matrix material, which is usually an organic polymer. The active agent may form a core within a shell of polymer, or it may be dispersed throughout the polymer matrix. A wide range of materials have been encapsulated, including adhesives, agrochemicals, live cells, active enzymes, flavors, fragrances, dyes, and pharmaceuticals. Much recent attention on microencapsulation has been focused on its ability to produce advanced composite materials with novel optical, physical, or chemical properties.1,2 The most widespread use of microencapsulation is in drug delivery, where it is used to mask taste, increase bioavailability, or provide extended release of pharmaceuticals.3 Conventional microencapsulation techniques, such as the widely studied emulsification/solvent evaporation method4 and its modified versions,5 rely on emulsification to create particles and simultaneously impregnate them with additives. Creating particles by emulsification provides limited control over the particle size and size polydispersity. Better control of particle size is possible by decoupling the particle formation and impregnation steps. An early Xerox patent describes a process for incorporating dyes into preformed toner particles.6 In the Xerox process, a dichloromethane/dye solution is emulsi-
fied into an aqueous latex to plasticize the particles and facilitate the transport of dye into the particles. The size and shape of the particles is unchanged after incorporating the dye. More recently, Kim et al.7 presented a similar method that they refer to as “solute co-diffusion,” to incorporate indomethacin into preexisting monodisperse sized poly(methyl methacrylate) (PMMA) aqueous latex. Cross-linked PMMA particles were swollen by an indomethacin/dichloromethane solution emulsified into water. After the solvent was evaporated, indomethacin was entrapped in the PMMA particles to produce monodisperse sized microcapsules. While providing better control over the particle size, these techniques, like the emulsification/ solvent evaporation technique, use harmful organic solvents undesirable for drug delivery applications. In addition, low free volume in the cross-linked polymer particles results in low impregnation efficiency. Carbon dioxide (CO2) has been studied as an alternative solvent in a wide range of applications including material processing and chemical synthesis.8,9 CO2 is inert, nontoxic, nonflammable, naturally abundant, and easy to separate due to its high volatility. Compressed CO2 dissolves into a wide variety of polymers and acts as a plasticizer, reducing the glass transition temperature of polymers and viscosity of polymer melts. The plasticization of polymers with CO2 increases the rate of diffusion in the polymer phase and has been utilized in several polymer impregnation processes.10-12 The concept of microencap-
* Author to whom correspondence should be addressed. (1) Benita, S. Microencapsulation: Methods and Industrial Applications; Marcel Dekker: New York, 1996. (2) Park, N.-H.; Park, S.-I.; Suh, K.-D. Colloid Polym. Sci. 2001, 279, 1082-1089. (3) Langer, R. Nature 1998, 392, 5-10. (4) Watts, P. J.; Davies, M. C.; Melia, C. D. Crit. Rev. Ther. Drug Carrier Syst. 1990, 7, 235-259. (5) Soppimath, K. S.; Aminabhavi, T. M.; Kulkarni, A. R.; Rudzinski, W. E. J. Controlled Release 2001, 70, 1-20. (6) Ober, C. K.; Lok, K. P.; Hair, M. L.; Branston, R. E. U.S. Patent 4,613,559, 1986.
(7) Kim, J.-W.; Cho, S.-A.; Kang, H.-H.; Han, S.-H.; Chang, I.-S.; Lee, O.-S.; Suh, K.-D. Langmuir 2001, 17, 5435-5439. (8) Cooper, A. I. J. Mater. Chem. 2000, 10, 207-234. (9) Wells, S. L.; DeSimone, J. Angew. Chem., Int. Ed. 2001, 40, 518527. (10) Berens, A. R.; Huvard, G. S.; Korsmeyer, R. W. U.S. Patent 4,820,752, 1989. (11) Perman, C. A.; Bartkus, J. M.; Choi, H.-O.; Riechert, M. E.; Witcher, K. J.; Kao, R. C.; Stefely, J. S.; Gozum, J. E., U.S. Patent 5,508,060, 1996. (12) Shine, A. D.; Gelb, J., U.S. Patent 5,766,637, 1998.
10.1021/la0257945 CCC: $22.00 © 2002 American Chemical Society Published on Web 07/11/2002
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sulation using swelling of aqueous polymer colloids with carbon dioxide has been introduced recently by Yates et al.13 and was based on the process patented by Xerox with liquid carbon dioxide replacing dichloromethane.6 Preexisting colloidal PS particles were impregnated with a dye with CO2 as a plasticizer to facilitate mass transport in the polymer phase. A surfactant was used to emulsify CO2 and the dye into the aqueous latex. After impregnation, CO2 was vented off and the particles returned to their original size, entrapping the dye. This technique decouples the particle formation and microencapsulation steps, providing improved control of particle size and surface properties. Harmful organic solvents are replaced by CO2, which is nontoxic and easily removed. Since CO2 does not dissolve the polymers, cross-linking of the microspheres is not required. On the basis of the promising preliminary study,13 this paper investigates the kinetics and the effects of the operational parameters on the impregnation process with the aim of developing a microencapsulation process free of harmful solvents that is effective at room temperature. Experimental Section Materials. Styrene, poly(N-vinylpyrrolidone) (PVP, Mw ) 55 000 g/mol), 2,2′-azobis(isobutyronitrile) (AIBN), ethanol (reagent grade, denatured), Sudan Red 7B, Solvent Yellow 1, Solvent Violet 8, and Rose Bengal dye were purchased from Aldrich and used as received. Absolute ethanol (200 proof) was obtained from Pharmco Products, Inc. and used as received. The poly(ethylene oxide)/poly(propylene oxide) (PEO/PPO) Pluronic block copolymer [F108, (PEO)129-(PPO)58-(PEO)129] surfactant was donated by BASF and used as received. Carbon dioxide (SFC/SFE grade) was obtained from Air Products and Chemicals, Inc. Deionized water was used in the experiments. Dispersion Polymerization. Different size monodisperse polystyrene (PS) microparticles were synthesized by dispersion polymerization, similar to the method reported by Paine et al.14,15 Appropriate amounts of PVP and denatured ethanol were added to a 100 mL round-bottom flask. The mixture was stirred for 30 min to dissolve the PVP. The flask was then sealed and purged with nitrogen for 30 min. The PVP/ethanol solution was put into an oil bath and heated to 70 °C under a nitrogen atmosphere. In another flask, AIBN was dissolved in styrene. The AIBN/ styrene solution was injected into the PVP/ethanol solution by a syringe to start the reaction. The polymerization was carried out for 24 h at 70 °C. After reaction, ethanol was removed by evaporation at room temperature. Three different sized particles were produced (1.7, 2.3, and 4.5 µm diameter). For the majority of experiments, the 2.3 µm particles were used. Impregnation Process. The impregnation process was similar to that recently reported.13 Dried PS particles (0.300 g) were dispersed into 10 g of deionized water by use of an ultrasonic cleaning bath. Particles were dispersed in 10 g of ethanol instead of water in selected experiments. The resulting aqueous latex was transferred to a stainless steel variable-volume cell (SC Machining, 28 mL maximum volume) and appropriate amounts of dye and the surfactant F108 were added. The cell was sealed and then 3.5 g of CO2 was added via a computer-controlled highpressure syringe pump (ISCO, Inc., model 260D). The cell was pressured and maintained at the desired pressure by the syringe pump. Water and CO2 were emulsified with a magnetically coupled stir bar. The impregnation was carried out under continuous stirring at 25 °C. After impregnation, CO2 was vented off very slowly to minimize foaming by slightly loosening a fitting. It took 4-6 h for the pressure to decrease to atmospheric. Absolute ethanol (50 mL) was then added to the dyed aqueous latex to wash off excess dye. The dyed PS particles were collected by (13) Yates, M. Z.; Birnbaum, E. R.; McCleskey, T. M. Langmuir 2000, 16, 4757-4760. (14) Paine, A. J.; Luymes, W.; McNulty, J. Macromolecules 1990, 23, 3104-3109. (15) Paine, A. J.; McNulty, J. J. Polym. Sci., Part A: Polym. Chem. 1990, 28, 2569-2574.
Figure 1. SEM photographs of the PS particles (A) before dyeing; (B) after being dyed with Sudan Red 7B; (C) after being dyed with Solvent Yellow 1; (D) after being dyed with Solvent Violet 8; (E) after being dyed with Rose Bengal dye. centrifugation (30 min at 14 500 rpm) and then redispersed into 50 mL of absolute ethanol. The particles were repeatedly centrifuged and redispersed into 50 mL of ethanol until the supernatant was colorless, at least five cycles. The recovered PS particles were dried in a vacuum oven for several hours at 50 °C before characterization. Dye Loading Determination. The dyed PS particles were dissolved in dichloromethane, and the absorbance of the solution was measured by a Perkin-Elmer Lambda 900 UV-vis spectrometer. Sudan Red 7B, Solvent Yellow 1, and Solvent Violet 8 have absorbance peaks at approximately 538, 373, and 582 nm in dichloromethane, respectively. The weight percent dye loading was calculated from Beer’s Law by use of a standard curve prepared from known concentrations of the dyes in dichloromethane. Since Rose Bengal dye is insoluble in dichloromethane or other common organic solvents, the procedure for measuring its loading was different and is as follows: an amount of PS particles containing Rose Bengal dye was added to 10 mL of absolute ethanol and stirred for 2 days to extract the dye from the PS particles. The dye solution was analyzed by the spectrometer at a wavelength of 558 nm. The result was compared to a standard curve for Rose Bengal dye to determine the dye loading. Since the dye absorbed on the particle surfaces and incorporated near the surfaces was washed off by the dispersion/ centrifugation cycles, the dye loading measured was the core loading. Particle Size and Morphology. The size and morphology of the PS particles were characterized by scanning electron microscopy (LEO 982 FE-SEM).
Results and Discussion Particle Morphology. SEM photographs of the PS particles of 2.3 µm diameter before and after dyeing with different dyes for 24 h at 310 bar are shown in Figure 1. The PS particles produced by dispersion polymerization were of uniform size and spherical in shape. Panels B-E of Figure 1 show that the particle size and morphology
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did not change after dyeing, and thus the swelling of the particles is reversible. In the impregnation process, the particles were highly plasticized and swollen by CO2. At 310 bar and 25 °C, the solubility of CO2 in bulk PS can reach 10 wt %.16,17 Otake et al.18 studied the swelling of a 50 nm monodisperse PS aqueous latex by CO2 and found liquid CO2 swells PS latex spheres in water about 60% more than bulk PS because of the adsorption of CO2 into the interfacial region. The swelling and plasticization of polystyrene by CO2 causes the polystyrene to change from a hard glass to a liquid, thus greatly enhancing mass transport within the polymer phase. Previous control experiments have demonstrated that no dyeing of polystyrene occurs at 25 °C when CO2 is not present.13 After impregnation, CO2 was vented off and the particles went back to their original size and returned to the glass state, entrapping the dye. Reversibility of swelling of the particles makes it possible to maintain the original morphology of the particles. In addition, the SEM micrographs show that no coalescence or aggregation occurred during dyeing, which indicates that the surfacegrafted PVP provides steric stabilization of the PS particles in the CO2-in-water emulsion. One of the most important advantages of this technique compared to existing microencapsulation techniques such as the solvent evaporation method4 is control of particle size. By use of carbon dioxide-assisted impregnation, particles of a desired size can be synthesized prior to impregnation, and that size is maintained after impregnation. CO2 has been shown to act as a foaming agent when polymers swollen by CO2 are rapidly depressurized.19 Microcellular foams may be produced in which the cell size may be “tuned” by the rate of depressurization. For all of the latexes that were investigated, the rate of depressurization was very slow so as to minimize foaming of the polymer particles. SEM micrographs show that the surfaces of the PS particles after dyeing were smooth just like that before dyeing, indicating no apparent foaming took place. However, other techniques are needed in order to get more detailed information on the surface properties of the dyed particles and investigate microscopic foaming. The effect of the rate of depressurization on particle morphology was not investigated. Kinetics of the Impregnation Process. The impregnation process was carried out with aqueous latexes and ethanol-based latexes of 2.3 µm diameter PS for different times at 310 bar. Figure 2 shows the change of Sudan Red 7B dye loading with time. In both cases, dye loading increased with increasing time and appeared to approach a limiting value asymptotically. The maximum dye loading was much larger for the aqueous latexes than for the ethanol-based latexes. On the other hand, the dye loading reached the maximum value much faster in ethanol than in water. For ethanol, the dye loading did not increase significantly after 24 h, but the aqueous latex showed a continual increase in dye loading even up to 100 h. Sudan Red 7B is insoluble in water, and it is soluble in ethanol, up to 0.4% at room temperature.20 The solubility of Sudan Red 7B in polystyrene is assumed to be similar to its solubility in aromatic solvents due to the aromatic (16) Sada, E.; Kumazawa, H.; Yakushiji, H.; Bamba, Y.; Sakata, K. Ind. Eng. Chem. Res. 1987, 26, 433-438. (17) Wissinger, R. G.; Paulaitis, M. E. J. Polym. Sci., Part B: Polym. Phys. 1987, 25, 2497-2510. (18) Otake, K.; Webber, S. E.; Munk, P.; Johnston, K. P. Langmuir 1997, 13, 3047-3051. (19) Goel, S. K.; Beckman, E. J. AIChE J. 1995, 41, 357-367. (20) Colour Index, 3rd ed.; American Association of Textile Chemists and Colorists: Yorkshire, England, 1971; Vol. 3.
Liu and Yates
Figure 2. Effect of processing time on dye loading. Dye, Sudan Red 7B, 0.050 g; F108, 0.60 wt %; particle diameter, 2.3 µm; pressure, 310 bar.
rings in the polymer chain. Since Sudan Red 7B has high solubility, up to 10% at room temperature, in toluene or benzene, the dye should also be highly soluble in polystyrene. It is expected that the partition coefficient of the dye into polystyrene from water, the ratio of the solubility in polystyrene to that in water, is very high, which favors the dye entering the polymer phase. Since the solubility of the dye is greater in ethanol than in water, the partition coefficient of the dye into polystyrene from ethanol will be somewhat lower than that for polystyrene/water. Therefore, the partitioning of dye into the polymer phase was higher for the aqueous latex than with the ethanolbased latex, resulting in the higher maximum dye loading shown in Figure 2. In the impregnation process in water, CO2 was emulsified into water with the aid of the surfactant F108. The dye had to cross the water phase to reach the polymer phase. Because Sudan Red 7B is insoluble in water, water acts as transfer barrier and lowers the kinetics of dye loading. As a result, the dye loading kept increasing with time even after impregnation times of several days. In the impregnation process carried out in ethanol, an emulsion did not form as it did with water because CO2 and ethanol are miscible.21 Thus, extra dye was suspended in CO2/ethanol solution and dispersed with the surfactant F108. Because the dye has some degree of solubility in ethanol, the barrier for the dye to cross the ethanol phase was reduced for the ethanol-based latex. Therefore, kinetics of dye loading was improved in ethanol and the maximum dye loading was reached faster than compared to the aqueous latex. However, the solubility of the dye in ethanol lowered partitioning of the dye into the particles and caused the maximum dye loading to be lower with ethanol-based latexes than with aqueous latexes. There is thus a balance between equilibrium partitioning and kinetics of mass transport of the additive for effective impregnation by this process. Dyes with Different Partition Coefficients. From the above kinetic study, we know that the solubility of the dye plays a very important role in the impregnation process. Three other dyes with different solubility characteristics were investigated. Table 1 lists their solubility in water and aromatic solvents. From the solubilities, the partition coefficient of the dyes into polystyrene from water is expected to be largest for Sudan Red 7B and decrease in the order Solvent Yellow 1 > Solvent Violet 8 > Rose Bengal dye. Aqueous latexes of 2.3 µm diameter PS (21) Dandge, D. K.; Heller, J. P.; Wilson, K. V. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 162-166.
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Table 1. Dye Loadings of Different Dyesa dyes
solubility in water
solubility in benzene
dye loading, wt %
Sudan Red 7B Solvent Yellow 1 Solvent Violet 8 Rose Bengal dye
insoluble slightly soluble very good very good
3-10 wt % soluble slightly soluble insoluble
0.5613 0.1908 0.1193 0.0188
a Medium, water; dye amount, 0.050 g; F108, 0.60 wt %; particle diameter, 2.3 µm; time, 24 h; pressure, 310 bar.
Table 2. Effect of Pressure on Dye Loadinga pressure, bar
dye loading, wt %
103 138 207 310
0.5131 0.5235 0.5592 0.5613
a
Medium, water; dye, Sudan Red 7B, 0.050 g; F108, 0.60 wt %; particle diameter, 2.3 µm; time, 24 h.
particles were impregnated with each of these dyes at 25 °C and 310 bar for 24 h. The dye loadings are also shown in Table 1 and follow the same trend as the partition coefficients. A lower partition coefficient made dye partitioning shift to water, resulting in lower dye loading. This result indicates that the partition coefficient is the crucial factor determining the ultimate dye loading. Effect of Pressure. Table 2 lists Sudan Red 7B dye loadings at different pressures for a fixed impregnation time of 24 h at 25 °C with 2.3 µm diameter PS particles. Dye loadings increased with increasing pressure, but the increase was small. Compressed carbon dioxide dissolves into polystyrene in increasing amounts as pressure is increased. As a result, the glass transition temperature of polystyrene is lowered as pressure increases. Polystyrene becomes a liquid at 25 °C when exposed to CO2 at pressures above 100 bar.18 Diffusion in the polymer phase is highly facilitated in the liquid state compared to the glassy state. Since polystyrene was already in the liquid state at 103 bar, further increasing pressure did not dramatically increase the rate of diffusion. Therefore, dye loading did not increase greatly. To ensure that the polymer was highly plasticized, all other experiments were carried out at 310 bar in this paper. However, this technique could be carried out at lower pressures down to 103 bar without decreasing dye loadings significantly. Effect of Surfactant Concentration. The main difference in the present technique from other polymer impregnation studies with supercritical fluids10-12 is that surfactant was added to emulsify CO2 into water to form CO2-in-water emulsion. Because Sudan Red 7B is insoluble in water and sparingly soluble in liquid CO2, the dye was mainly located at the interface between CO2 droplets and water. The transport rate of dye into polymer phase was highly increased by forming an emulsion with a large interfacial area.13 It is expected that increasing the interfacial area of the emulsion, either through higher energy input or increased surfactant concentration, should result in improved transport kinetics. Increasing surfactant amount resulted in increasing dye loading, but the increase leveled off when the surfactant concentration was 0.95% or higher, as shown in Figure 3. The impregnation processes were carried out at 25 °C and 310 bar for 24 h. Studies of emulsion formation have shown that, for a fixed energy input, the interfacial area of an emulsion increases with increasing surfactant concentration up to a certain point and then remains constant as additional surfactant is added.22 Higher (22) Walstra, P. Chem. Eng. Sci. 1993, 48, 333-349.
Figure 3. Effect of surfactant concentration on dye loading. Medium, water; dye, Sudan Red 7B, 0.050 g; particle diameter, 2.3 µm; time, 24 h; pressure, 310 bar.
Figure 4. Effect of dye amount on dye loading. Dye, Sudan Red 7B; particle diameter, 2.3 µm; F108, 0.60 wt %; time, 24 h; pressure, 310 bar.
surfactant concentration makes interfacial tension lower, which results in smaller CO2 droplets and larger interfacial area in the emulsion. However, the maximum interfacial area for a given energy input is reached and additional surfactant does not increase the interfacial area. Since the transfer of Sudan Red 7B into the polymer phase is facilitated by increasing interfacial area, it is expected that dye loading is proportional to the interfacial area of the emulsion. The shape of the curve in Figure 3 is consistent with the expected trend that interfacial area increases at first and then reaches a maximum value as surfactant is added. Effect of Dye Amount. Figure 4 shows that increasing the amount of dye results in higher dye loading at the same impregnation time, 24 h. This is also the case for the impregnation carried out in ethanol (Figure 4), though the dye loading was lower due to the lower partition coefficient. The increase in dye loading with increased dye amount (slope of line in Figure 4) is much lower for ethanol-based latexes than for aqueous latexes. In fact, the increase in dye loading is almost negligible for the ethanol-based latex. Figure 4 suggests that the dye dissolved in the ethanol-based latex is in thermodynamic equilibrium with excess solid dye. Adding additional excess solid dye does not shift the partitioning of the dye between PS and ethanol in the three-phase system (PS, CO2/ ethanol, and excess solid dye). However, for the aqueous latex, there is a significant increase in dye loading with increased dye amount. Excess solid dye is present in the aqueous system as well. Since there is an increase in
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Table 3. Effect of Particle Size on Dye Loadinga particle size, µm
dye loading, wt %
1.7 2.3 4.5
0.7081 0.5613 0.3271
Conclusions
a Medium, water; dye, Sudan Red 7B, 0.050 g; F108, 0.60 wt %; time, 24 h; pressure, 310 bar.
loading with increased dye amount, it appears that the aqueous system is not in thermodynamic equilibrium with the excess dye after 24 h of impregnation. This is consistent with the kinetic data presented in Figure 2. Adding additional excess dye to the aqueous system improves the kinetics of dye loading, but the very low solubility of the dye in the aqueous phase causes dye loading to be overall much slower than with the ethanol-based latex. Effect of Polymer Particle Size. Polystyrene particles of different sizes were used in the impregnation processes carried out at 310 bar for 24 h. Table 3 shows the dye loadings with different size PS particles. Dye loading increases as particle size decreases. The surface area of the monodisperse polymer particles can be obtained from
A)
m/F 3m (4πr2) ) 3 Fr (4/3)πr
(1)
where A is the surface area of the polymer particles, m is the mass of the polymer particles, F is the density of the polymer particles, and r is the radius of the particles. Decreasing particle size increases the surface area per unit mass, which increases the transfer rate of dye into the polymer particles, resulting in higher dye loading. By applying eq 1 to the data in Table 3, it can be seen that the dye loading was approximately inversely proportional to the radius of the polymer particles or directly proportional to the surface area of the latex. Since the transport of dye occurs across the surface of the polymer particles, it is expected that increasing the surface area of the latex will result in higher dye loading. The increased dye loading with smaller particles obtained with CO2-based impregnation is particularly attractive for the loading of biodegradable nanoparticles with drugs and is the focus of our current research.
A polymer impregnation technique that can be applied to incorporate dyes into preexisting monodisperse polystyrene particles was presented. Liquid carbon dioxide is used to plasticize the polymer particles and facilitate mass transport in the polymer phase at room temperature. CO2 is emulsified into the aqueous latex with the aid of a surfactant, which accelerates the transport of dye into the polymer phase. This technique has several advantages over existing microencapsulation techniques such as solvent evaporation or hot melt-freezing, most notably the elimination of harmful organic solvents and the ability to incorporate thermally labile additives into polymers. Scanning electron micrographs show that particle morphology is unaffected by the impregnation process. By decoupling the particle formation and impregnation steps, the CO2-based technique provides more flexibility in controlling particle morphology and composition. Such control is very important for producing monodisperse size polymer microcapsules or microcapsules with specific surface chemistries. Both ethanol-based and aqueous latexes were successfully impregnated with additives. It was determined that there is a trade-off between equilibrium partitioning of the additive and kinetics of impregnation. Partition coefficient is the most important factor that determines the limit of dye loading at the thermodynamic equilibrium. Increasing solubility of the dye in the medium surrounding the particles can shorten the time to attain the equilibrium, but the dye loading decreases because partitioning of the dye shifts to the medium. The kinetics of dye loading was shown to be improved by increasing the amount of added surfactant, pressure, or amount of dye added. The dye loading was also found to be approximately directly proportional to the surface area of the latex. Kinetics of dye loading was improved as particle size decreased. This technique can be extended to incorporate other small molecular weight additives into other polymer systems. In the future, it will be applied to incorporate drugs into biodegradable polymer or copolymer nanoparticles. Acknowledgment. We acknowledge support from the University of Rochester. LA0257945
