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Langmuir 2003, 19, 1106-1113
Dual Function Surfactants for Carbon Dioxide Based Microencapsulation Hongwei Liu and Matthew Z. Yates* Department of Chemical Engineering, Laboratory for Laser Energetics, University of Rochester, Rochester, New York 14627-0166 Received September 26, 2002. In Final Form: December 5, 2002 The successful incorporation of dyes into sterically stabilized aqueous polystyrene colloids has recently been reported using a new microencapsulation technique in which liquid carbon dioxide is used to facilitate mass transport into the polymer particles. The sterically stabilized particles retain their original size and shape after impregnation with dye. However, electrostatically stabilized polystyrene latexes made by surfactant-free emulsion polymerization lose stability when exposed to liquid carbon dioxide due to pH and ionic strength changes caused by carbonic acid formation in the aqueous phase. When colloidal stability is lost, particles coalesce due to the plasticization induced by carbon dioxide. To solve this problem, a series of surfactants that simultaneously provide colloidal stability and enhance mass transport during microencapsulation have been identified. These “dual function” surfactants are active at both the CO2/ water and polymer/water interfaces. By adsorption onto the particle surface, colloidal stability is enhanced, and by adsorption at the carbon dioxide/water interface, emulsion formation is enhanced so that there is improved mass transport into the polymer particles. Light scattering, zeta potential measurements, and scanning electron microscopy are used to identify a series of amphiphiles that act as dual function surfactants during microencapsulation. All of the identified surfactants are commercially available hydrocarbonbased materials, some of which are currently used in food and drug formulations. The dual function concept demonstrates that the addition of a single low-cost additive can greatly enhance the utility of the CO2based microencapsulation technique by extending it to electrostatically stabilized polymer colloids.
Introduction A new microencapsulation technique has been reported recently in which liquid carbon dioxide (CO2) is used to facilitate the transport of additives into aqueous polymer colloids.1,2 In the CO2-based process, sterically stabilized polystyrene particles with a covalently bound polymeric surface layer were impregnated with additives using liquid CO2 as a plasticizer to facilitate mass transport in the polymer phase. The process holds promise as an organic solvent-free alternative to traditional microencapsulation techniques. The utility of CO2-based microencapsulation can be enhanced dramatically by extending it to electrostatically stabilized particles rather than just sterically stabilized particles. However, our own work and that reported by Otake et al.3 have found that liquid or supercritical CO2 can induce coagulation of aqueous electrostatically stabilized latexes when exposed to CO2 over long periods of time. Presumably the coagulation is caused by changes in pH and ionic strength due to carbonic acid formation in the CO2-saturated water. Since the particles are highly plasticized by CO2, coagulation is followed by coalescence of the particles to form large, irregularly shaped particles. For successful microencapsulation, it is imperative to maintain colloidal stability throughout the process so that particle size and shape can be controlled. There are two main mechanisms of stabilizing colloidal particles.4 One is steric stabilization due to polymer chains extending from the particle surface. Steric stabilization * To whom correspondence should be addressed. (1) Yates, M. Z.; Birnbaum, E. R.; McCleskey, T. M. Langmuir 2000, 16, 4757-4760. (2) Liu, H.; Yates, M. Z. Langmuir 2002, 18, 6066-6070. (3) Otake, K.; Webber, S. E.; Munk, P.; Johnston, K. P. Langmuir 1997, 13, 3047-3051. (4) Hiemenz, P. C.; Rajagopalan, R. Principles of Colloid and Surface Chemistry, 3rd ed.; Marcel Dekker: New York, 1997.
can be achieved by a thin surfactant layer formed during classical emulsion polymerization, but generally this effect is accomplished by adsorbing or grafting a polymer layer onto the surface of the particles.5 The other is electrostatic stabilization due to long-range repulsion from electrical double layers at the particle surface. Electrostatic stabilization is generally effective only in aqueous solvents and depends on the pH, dielectric properties, and ionic strength of the solvent.6 Steric stabilization can be effective in both aqueous and organic solvents and is usually unaffected by high salt concentration. Since steric stabilization depends on the thickness of the polymer adlayer, it is sensitive to changes in solvency and molar mass of the polymer.6 Adsorption of surfactants on the surface of colloidal particles is a widely used technique for modifying surface characteristics. The driving force for physical adsorption of surfactant on the surface can be hydrophobic interactions7,8 or electrostatic attractions.9,10 The purposes of surfactant adsorption are varied, such as screening the original surface properties to suppress unwanted reactions7,11 or improving colloidal stability against coagulation.8 By the choice of appropriate surfactants, the stabilization mechanism of colloidal particles may be changed from electrostatic to steric. Since sterically stabilized particles are less sensitive to changes in pH (5) Fritz, G.; Schadler, V.; Willenbacher, N.; Wagner, N. J. Langmuir 2002, 18, 6381-6390. (6) Ortega-Vinuesa, J. L.; Martin-Rodriguez, A.; Hidalgo-Alvarez, R. H. J. Colloid Interface Sci. 1996, 184, 259-267. (7) Tan, J. S.; Butterfield, D. E.; Voycheck, C. L.; Caldwell, K. D.; Li, J. T. Biomaterials 1993, 14, 823-833. (8) Martin-Rodriguez, A.; Cabrerizo-Vilchez, M. A.; Hidalgo-Alvarez, R. J. Colloid Interface Sci. 1997, 187, 139-147. (9) Fuchs, A.; Killmann, E. Colloid Polym. Sci. 2001, 279, 53-60. (10) Velegol, S. B.; Tilton, R. D. J. Colloid Interface Sci. 2002, 249, 282-289. (11) Miraballes-Martinez, I.; Martin-Rodriguez, A.; Hidalgo-Alvarez, R. J. Dispersion Sci. Technol. 1996, 17, 321-337.
10.1021/la026614u CCC: $25.00 © 2003 American Chemical Society Published on Web 01/22/2003
Surfactants for CO2-Based Microencapsulation
and ionic strength, surface modification with surfactants is one possible means to maintain colloidal stabilization during CO2-based microencapsulation. The second issue in creating an effective CO2-based microencapsulation process is creating a high interfacial area between the CO2 phase and the aqueous latex. Our previous work has shown that the transport kinetics of additives into aqueous latex particles is greatly enhanced by increasing the CO2/water interfacial area through emulsification.2 Stable emulsions of water and CO2 have been recently studied.12-20 Fluorinated surfactants are generally more effective than hydrocarbon surfactants in maintaining a stable emulsion.16,20 The low cohesive energy of the fluorinated chain allows more favorable interaction with the CO2 phase.14,15,21 However, recent studies have demonstrated that some inexpensive hydrocarbon surfactants are also effective in stabilizing CO2/ water emulsions.18-20 In the present study, we have identified a class of surfactants that enhance CO2-based microencapsulation in two ways simultaneously. Namely, the surfactants are interfacially active at both the CO2/water and particle/ water interfaces. By adsorbing onto the particle surface, the surfactant enhances colloidal stability and allows particles that were originally electrostatically stabilized to be impregnated with additives. Adsorption at the CO2/ water interface increases emulsion stability and enhances the rate of mass transport during microencapsulation by increasing the interfacial area between CO2 and water. We refer to these surfactants as “dual function” surfactants and show several examples of surfactants that function in this manner. The examples include surfactants that are nontoxic, inexpensive, and commercially available. Several of the surfactants shown are currently used in food, drug, or personal care formulations. These dual function surfactants greatly enhance the utility of CO2based microencapsulation through the addition of a single low-cost surfactant that expands the types of particles that can be impregnated and improves the kinetics of microencapsulation. Experimental Section Materials. Styrene was obtained from Aldrich, and the inhibitor in the monomer was removed by passing through an inhibitor removal column (Aldrich) before polymerization. Poly(N-vinylpyrrolidone) (PVP, Mw ) 55 000 g/mol), 2,2′-azobisisobutyronitrile (AIBN), potassium persulfate (KPS), ethanol (reagent grade, denatured), Sudan Red 7B, ZONYL FSO-100, Brij 78, Brij 700, cetylpyridinium chloride monohydrate (CCM), and potassium chloride (KCl) were purchased from Aldrich and used as received. Triton X-100, sodium dodecyl sulfate (SDS), and (12) Hoefling, T. A.; Beitle, R. R.; Enick, R. M.; Beckman, E. J. Fluid Phase Equilib. 1993, 83, 203-212. (13) Yazdi, A. V.; Lepilleur, C.; Singley, E. J.; Liu, W.; Adamsky, F. A.; Enick, R. M.; Beckman, E. J. Fluid Phase Equilib. 1996, 117, 297303. (14) Harrison, K.; Goveas, J.; Johnston, K. P. Langmuir 1994, 10, 3536-3541. (15) O’Neill, M. L.; Yates, M. Z.; Johnston, K. P.; Wilkinson, S. P.; DeSimone, J. M. Polym. Mater. Sci. Eng. 1996, 74, 228-229. (16) Lee, C. T., Jr.; Psathas, P. A.; Johnston, K. P.; de Grazia, J.; Randolph, T. W. Langmuir 1999, 15, 6781-6791. (17) Lee, C. T., Jr.; Psathas, P. A.; Ziegler, L. J.; Johnston, K. P.; Dai, H. J.; Cochran, H. D.; Melnichenko, Y. B.; Wignall, G. D. J. Phys. Chem. B 2000, 104, 11094-11102. (18) Sarbu, T.; Styranec, T.; Beckman, E. J. Nature 2000, 405, 165168. (19) Johnston, K. P.; Cho, D.; da Rocha, S. R. P.; Psathas, P. A.; Ryoo, W. Langmuir 2001, 17, 7191-7193. (20) da Rocha, S. R. P.; Psathas, P. A.; Klein, E.; Johnston, K. P. J. Colloid Interface Sci. 2001, 239, 241-253. (21) DeSimone, J. M.; Guan, Z.; Elsbernd, C. S. Science 1992, 257, 945-947.
Langmuir, Vol. 19, No. 4, 2003 1107 cetyltrimethylammonium bromide (CTAB) were purchased from J. T. Baker 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 surfactant F108 and the Tetronic block copolymer surfactant T908 were 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. Surfactant-Free Emulsion Polymerization. Negatively charged polystyrene (PS) particles were synthesized by surfactant-free emulsion polymerization similar to the method reported by Tauer.22,23 Styrene (3.0 g) and 95 mL of water were added to a 200 mL round-bottom flask. The flask was sealed and purged with nitrogen for 30 min and then was put into an oil bath and heated to 70 °C under a nitrogen atmosphere. In another flask, 0.060 g of KPS was dissolved in 5 mL of water. The KPS/water solution was injected into the styrene/water mixture by a syringe to start the reaction. The polymerization was carried out for 24 h at 70 °C. The particle size is 328.6 nm and monodisperse as determined by dynamic light scattering. Dispersion Polymerization. Sterically stabilized polystyrene particles were made by dispersion polymerization following the procedure reported before.2 PVP (1.5 g) was dissolved in 85 mL of absolute ethanol under stirring and purged with nitrogen for 30 min. The PVP/ethanol solution was heated to 70 °C under a nitrogen atmosphere. The reaction was started by injecting 0.15 g of AIBN dissolved in 15 mL of styrene. The polymerization was carried out for 24 h at 70 °C. After reaction, ethanol was removed by evaporation at room temperature. The polystyrene particles produced were spherical and size monodisperse with an average diameter of 2.0 µm as measured by scanning electron microscopy. Incubation of Polystyrene Latex. Different surfactants were adsorbed onto the polymer latex made by surfactant-free emulsion polymerization. Their molecular structures are shown in Figure 1. The solid content of the polystyrene latex was 2.1 wt %. The particles were suspended in a 4.0 wt % solution of surfactant to ensure maximum adsorption.7 The surfactant adsorptions were performed by incubating for 24 h by gently stirring the surfactant/latex mixtures. Impregnation Process. The impregnation process for the sterically stabilized polystyrene latex was the same as reported before.2 For the electrostatically stabilized polystyrene latex, the impregnation process is as follows. Polystyrene latex (10 g) incubated with 0.4 g of a surfactant was added to a stainless steel variable volume cell (SC Machining, 28 mL maximum volume). The cell was sealed, and then 3.5 g of CO2 was added using a computer-controlled high-pressure syringe pump (ISCO, Inc., model 260D). The cell was pressurized to 310 bar using 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 and 310 bar for 24 h. 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 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 process for measuring dye loadings was reported elsewhere.2 Particle Size and Morphology. The particle size and size distribution were determined by dynamic light scattering using a 90 plus particle size analyzer (Brookhaven Instruments). The polydispersity is close to zero for monodisperse or nearly monodisperse samples. The morphology of the PS particles was characterized by scanning electron microscopy (SEM, LEO 982 FE-SEM). (22) Tauer, K.; Deckwer, R. Acta Polym. 1998, 49, 411-416. (23) Tauer, K.; Deckwer, R.; Kuhn, I.; Schellenberg, C. Colloid Polym. Sci. 1999, 277, 607-626.
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Figure 1. Molecular structures of the surfactants. Electrophoretic Measurement. Zeta potential measurements were made at 25 °C under different ionic strengths and pH conditions using a phase analysis light scattering (PALS) zeta potential analyzer (Brookhaven Instruments). One drop of the 2.1% latex was added to 3 mL of buffer to dilute the sample. All measurements were duplicated, and the average value was reported. Surfactant Adsorption. The adsorption of Triton X-100 on the polystyrene particle surface was obtained by comparing its concentration in aqueous solution before and after incubation for 24 h. The concentration after incubation was measured by a Perkin-Elmer Lambda 900 UV-vis spectrometer at 275 nm.24
Results and Discussion Surfactant Adsorption. The polystyrene particles made by surfactant-free emulsion polymerization have negative surface charge, arising from the initiator fragments. To modify their surface properties, different kinds of surfactants, including anionic, cationic, and polymeric surfactants (Figure 1), were adsorbed onto the polystyrene latex. Table 1 shows their effects on the particle size, size distribution, and zeta potential. The measurement of the zeta potentials was performed in 0.001 M KCl solution. SDS is a widely used anionic surfactant. The slightly larger particle size and more negative zeta potential indicated some SDS molecules adsorbed on the particle surface. It is assumed that the SDS molecules were adsorbed at the (24) Ma, C. Colloids Surf. 1987, 28, 1-7.
Table 1. Effect of Surfactant Incubation on Zeta Potential, Particle Size, and Polydispersitya
no.
surfactant
0
without surfactant SDS CTAB CCM F108 T908 Triton X-100 Brij 78 Brij 700 Zonyl FSO-100
1 2 3 4 5 6 7 8 9 a
repeat no. of particle zeta units PEO size potential of EO blocks (nm) polydispersity (mV)
0 0 0 129 120 9.5 20 100 10-15
0 0 0 2 4 1 1 1 1
328.6
0.005
-63.31
330.3 337.5 351.7 345.6 346.4 339.3 339.7 342.4 379.5
0.005 0.162 0.033 0.005 0.007 0.005 0.005 0.005 0.126
-65.34 +60.52 +62.00 -20.18 -21.46 -54.33 -51.64 -31.63 -23.20
PS, 2.1 wt %; surfactant, 4.0 wt %; incubation time, 24 h.
hydrophobic patches on the surface of polystyrene particles due to hydrophobic interactions between the hydrocarbon chain of SDS and the polystyrene surface. Cationic surfactants, CTAB and CCM, induced some particle aggregation by charge neutralization,25 followed by restabilization as the zeta potential increased in magnitude again but was of opposite sign.26 The driving force for the adsorption of the cationic surfactants is likely a combina(25) Somasundaran, P.; Healy, T. W.; Fuerstenau, D. W. J. Colloid Interface Sci. 1966, 22, 599-605. (26) Colic, M.; Fuerstenau, D. W. Langmuir 1997, 13, 6644-6649.
Surfactants for CO2-Based Microencapsulation
tion of electrostatic attraction to surface sulfate groups and hydrophobic interactions with the polystyrene surface. The increase of the particle size and polydispersity indicate that some aggregates remain after adsorption of the cationic surfactants. All the nonionic surfactants had PEO chains of different lengths as the hydrophilic moiety. The hydrodynamic diameter of the polystyrene particles increased with increasing PEO chain length of the adsorbed surfactant, while the zeta potentials decreased in absolute value. As the length of the adsorbed surfactant molecules increased, it is expected that the surfactant layer extends farther from the polystyrene particle surface and has a denser coverage, shifting the shear plane outward and thus lowering the zeta potential. The absorbed polymeric surfactants provide steric stabilization that will increase the critical coagulation concentration and thus improve the stability of the polystyrene latexes at higher ionic strengths.8 Triton X-100 has an adsorption peak at around 275 nm that allows adsorption to be quantified with UV-vis spectroscopy. Adsorption measurements showed that approximately 10% of the Triton X-100 was adsorbed on the polymer surface. The surface coverage is thus estimated to be 10.8 Å2 per surfactant molecule. This value is larger than that reported by Martin-Rodriguez et al.,8 perhaps because there are more hydrophobic patches on the surface of the polystyrene particles we made. It is also possible that the surfactant adsorbed in multiple layers due to the higher starting concentration. Multilayer adsorption may also enhance steric stabilization with low molecular weight surfactants such as Triton X-100. ZONYL FSO-100 did not adsorb on the surface of the polystyrene particles, and it destabilized the latex according to visual observation and the measured increase in size and size distribution. All of the other polymeric surfactants contained either alkanes or propylene oxide as the hydrophobic moiety. Adsorption is driven primarily through van der Waals forces between the surfactant and the polystyrene surface. In the case of ZONYL FSO-100, the hydrophobic moiety is a perfluoroalkane chain. Teflon and other perfluorinated compounds typically have weak van der Waals interactions. ZONYL FSO-100 therefore has much weaker interactions with the polystyrene surface leading to a poor driving force for adsorption. Carbon Dioxide Based Microencapsulation Process. 1. Latex Stability. The SEM micrographs of the polystyrene particles before impregnation and after impregnation are shown in Figure 2. Table 2 lists the results of the impregnation processes. Before the impregnation, the polymer particles were spherical and monodisperse as shown in Figure 2A. When no surfactant was added, the latex lost stability and big polymer chunks could be seen in the pressure cell during the impregnation. After the impregnation, the polymer melted together and stuck on the inner surface of the pressure cell. Figure 2B shows the SEM graphs of the polymer after the impregnation without surfactant. When the surfactants, including SDS, CCM, CTAB, Triton X-100, F108, T908, Brij 78, and Brij 700, were present, the latexes remained stable through the impregnation process. Figure 2C-J shows that the particle size and size distribution did not change after the impregnation. Surface modification by surfactant adsorption is thus an efficient method to stabilize the polystyrene latex in the CO2-in-water emulsion during the microencapsulation process. The results demonstrate that CO2-based microencapsulation may be extended to electrostatically stabilized polymer particles with the aid of appropriate surfactants to enhance stability. The fluorinated surfactant, ZONYL FSO-100, could not sta-
Langmuir, Vol. 19, No. 4, 2003 1109 Table 2. Effects of Different Surfactants on Microencapsulation of Sudan Red 7Ba
no.
surfactant
1 2 3 4 5 6 7 8 9
SDS CTAB CCM F108 T908 Triton X-100 Brij 78 Brij 700 Zonyl FSO-100
repeat excess dye zeta units CO2 stable loading potential of EO phase? latex? (wt %) (mV) 0 0 0 129 120 9.5 20 100 10-15
yes yes yes yes yes yes yes yes no
yes yes yes yes yes yes yes yes no
0.8854 0.6988 0.8310 0.9206 0.8461 1.2520 1.0280 0.8914 0.9590
-72.98 -66.43 -65.18 -69.75 -70.84 -54.76 -72.56 -69.10
a The average diameter of the bare PS particle is 328.6 nm. Polymer latex, 10.0 g (PS, 2.1 wt %); Sudan Red 7B, 0.050 g; surfactant, 0.40 g; temperature, 25 °C; pressure, 310 bar; impregnation time, 24 h.
bilize the polymer particles in the CO2-in-water emulsion, resulting in agglomeration of the particles (Figure 2K), consistent with the data in Table 1 that show ZONYL FSO-100 does not adsorb onto the latex particles and destabilizes them. The fact that ionic surfactants enhance the stability of the polystyrene latex is at first surprising since the mechanism of stability both before and after surfactant adsorption is electrostatic repulsion. However, the charge density and composition of surface charged groups is likely very different after surfactant adsorption. The surface charge of the polystyrene particles made by surfactantfree emulsion polymerization using KPS as the initiator comes mainly from the sulfate groups, resulting from the initiator fragments.27 However, during polymerization, carboxyl groups originate from oxidation of intermediate alkanol groups.28-30 The so-called Kolthoff reaction occurring in the polymerization results in the loss of sulfate groups and the appearance of hydroxyl groups.31 In addition, hydrolysis of some of the sulfate groups in the polymerization and upon storage produces carboxylic acid and hydroxyl groups.32,33 Therefore, the surface of the polystyrene latex has sulfate, carboxylic acid, and hydroxyl groups. Figure 3 shows the zeta potential of the latex as a function of pH. The ionic strength of the buffer solutions used in the measurement was fixed at 0.002 M. The zeta potential decreased in absolute value with decreasing pH. The trend is the same as reported by Pefferkorn et al.34 Sulfate groups have a pKa in the range of 1-2,35 while the pKa of carboxyl groups is between 4 and 6.36 The dissociation of the carboxylic acid groups contributes to the increase of the zeta potential with increasing pH.34 At 25 °C and 310 bar, the pH of the CO2-in-water emulsion is about 3.37 At pH 3, the latex surface is composed of neutral (27) Hul, H. J. V. d.; Vanderhoff, J. W. Br. Polym. J. 1970, 2, 121127. (28) Goodwin, J. W.; Hearn, J.; Ho, C. C.; Ottewill, R. H. Br. Polym. J. 1973, 5, 347-362. (29) Laaksonen, J.; Lebell, J. C.; Stenius, P. J. Electroanal. Chem. 1975, 64, 207-218. (30) Stone-Masui, J.; Watillon, A. J. Colloid Interface Sci. 1975, 52, 479-503. (31) Kolthoff, I. M.; Miller, I. K. J. Am. Chem. Soc. 1951, 73, 30553059. (32) Hearn, J.; Wilkinson, M. C.; Goodall, A. R. Adv. Colloid Interface Sci. 1981, 14, 173-236. (33) Furasawa, K. Bull. Chem. Soc. Jpn. 1982, 55, 48-51. (34) Pefferkorn, E.; Widmaier, J. Colloids Surf., A 1998, 145, 25-35. (35) James, R. O. In Polymer Colloids; Buscal, R., Corner, T., Stageman, J. F., Eds.; Elsevier: Amsterdam, 1985. (36) Ottewill, R. H.; Shaw, J. N. Kolloid Z. Z. Polym. 1967, 218, 34-40. (37) Holmes, J. D.; Steytler, D. C.; Rees, G. D.; Robinson, B. H. Langmuir 1998, 14, 6371-6376.
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Figure 2. SEM micrographs of the polystyrene particles before and after impregnation using different surfactant stabilizers.
hydroxyl and carboxyl acidic groups (not dissociated at pH 3), so that the electrical charges mainly result from the dissociated sulfate groups.34 Thus, the surface charge density decreases in the CO2-saturated water, resulting in lower stability of the latex. Moreover, polystyrene is in the liquid state at 25 °C when exposed to CO2 at pressures above 100 bar,3 making it easier for the polystyrene particles to coalesce. Because of the two reasons specified above, the bare polystyrene particles made by surfactant-
free emulsion polymerization did not maintain stability and melted together in the impregnation process. When the polystyrene latex is incubated with SDS, the SDS molecules adsorbed on the particle surface increase the number of surface sulfate groups. The sulfate groups remain ionized at pH 3 in the CO2-saturated water. The zeta potential of the polystyrene latex incubated with SDS was measured in different pH buffer solutions in which the SDS concentration was maintained above its critical
Surfactants for CO2-Based Microencapsulation
Figure 3. Effect of pH on the zeta potential of emulsion polymerization polystyrene latex. Open squares are for bare particles, and filled squares are for SDS-coated particles. The ionic strength was kept constant at 0.002 M. Particles coated with SDS were maintained in buffer solutions containing SDS above its critical micelle concentration.
micelle concentration. The results are also shown in Figure 3. The particles incubated with SDS have larger zeta potentials than the bare particles in the same pH, and the effect of pH on the zeta potential is not as significant as for the bare particles. SDS adsorption increased the surface charge density of the polystyrene particles, and the surface charge density had a smaller decrease at pH 3, improving the stability of the particles. Therefore, the polystyrene latex remained stable in the microencapsulation process and was effectively impregnated with dyes when SDS was present. Similarly, when the polystyrene latex was incubated with cationic surfactants, CCM and CTAB, the negative surface charge changed to positive and the cationic groups on the surface were less sensitive to changes in pH than carboxylate groups. Figure 2D,E shows that the positive surface charge could provide sufficient protection to the polymer latexes, making them stable in the impregnation process. The nonionic surfactants stabilize the particles through a steric mechanism resulting from the PEO chains extending out from the particle surface. All the polymeric surfactants except ZONYL FSO-100 were adsorbed on the surface of the polystyrene particles. Sterically stabilized particles are less sensitive to changes in pH and ionic strength than electrostatically stabilized particles and can therefore remain stabilized throughout the impregnation process. The fluorinated surfactant, ZONYL FSO-100, did not adsorb on the polystyrene particle surface and therefore the polymer latex lost stability in the impregnation process. As seen in Figure 2K, the particles coalesced in the presence of ZONYL FSO-100 when exposed to CO2. Modification of the surface increases the flexibility and utility of the CO2-based microencapsulation process. Surfactant adsorption offers a simple, inexpensive route to alter surface functionality that requires no reaction chemistry. For example, all of the nonionic surfactants introduce poly(ethylene oxide) onto the surface. In drug delivery, poly(ethylene oxide) surface coatings have been shown to increase the blood circulation time of biodegradable nanoparticles by reducing protein adsorption.38 Modification of the surface also offers a route to stabilize biodegradable particles that would otherwise flocculate during the microencapsulation process. For example, many biodegradable and biocompatible particles are (38) Muller, R. H.; Wallis, K. H. Int. J. Pharm. 1993, 89, 25-31.
Langmuir, Vol. 19, No. 4, 2003 1111
stabilized by surface carboxylate groups.39 As discussed above, carboxylate functionalities cannot maintain colloidal stability at pH 3. Surfactant adsorption offers a route to introduce surface functionalities such as sulfate or polymers that are effective at maintaining colloidal stability during microencapsulation. 2. Dye Loading. Sudan Red 7B is insoluble in water and sparingly soluble in liquid CO2, so it was mainly located at the interface between CO2 droplets and water. It has been shown that the transport rate of dye into the polymer phase can be highly increased by forming a CO2-in-water emulsion with a large interfacial area, resulting in high dye loading.2 Because Sudan Red 7B is nonionic and insoluble in water, it is expected that acid-base or electrostatic interactions of the dye with the surfactants or the particle surface will play no role in mass transport. Table 2 lists the dye loadings obtained by the impregnation processes using different surfactant stabilizers. An excess upper CO2 phase could be seen in the impregnation processes using all the selected surfactants except ZONYL FSO-100. The CO2-in-water emulsion was highly unstable with ionic surfactants, so the dye loadings for SDS, CCM, and CTAB were somewhat lower than with the nonionic surfactants. The other polymeric surfactants can emulsify CO2 into water to different degrees. The PEO and PPO block copolymer surfactants, F108 and T908, can produce more stable CO2-in-water emulsions than Triton X-100, Brij 78, and Brij 700 according to visual observation. A highly stable CO2-in-water emulsion forms with the fluorinated surfactant, ZONYL FSO-100.20 But ZONYL FSO-100 cannot stabilize the polymer latex in the impregnation process, resulting in coalescence of particles. The relatively high dye loading measured when ZONYL FSO-100 was used is attributed to dye entrapped in the space between polystyrene particles upon coagulation because the color of the dyed polymer did not look homogeneous as it did with other dyed samples. In addition to the surfactant’s effectiveness at forming an emulsion, it is also possible that the hydrophilic PEO chains adsorbed on the particle surface act as a transport barrier for dye entering the polymer phase. Triton X-100, Brij 78, and Brij 700 were all about equally effective in emulsion formation, yet the dye loadings for the three were significantly different. From entries 6-8 of Table 2, it can be seen that the dye loadings increased as the PEO chain decreased. The surfactants F108 and T908 were both more effective in forming emulsions and the dye loading for F108 was higher than that for T908 because each T908 molecule has four PEO blocks producing a denser polymer layer than with F108 that only has two PEO chains per molecule. The data suggest that the PEO layer thickness and packing density play a role in the transport of dye across the polymer/water interface. However, because a large excess of surfactant was used in latex incubation, it is possible that the surfactants were absorbed on the surface in multiple layers. Multilayer adsorption could also be influencing transport across the polymer/water interface. Further investigation is needed to verify this hypothesis. The dual function surfactants simultaneously stabilize the polymer colloid and stabilize the CO2/water emulsion to enhance the transport of the additives into the polymer phase while maintaining colloidal stability. It is possible that two surfactants may be more effective in achieving these goals, for example, an ionic surfactant for stabilizing the polymer colloid and a fluorinated surfactant like (39) Stolnik, S.; Garnett, M. C.; Davies, M. C.; Illum, L.; Bousta, M.; Vert, M.; Davis, S. S. Colloids Surf., A 1995, 97, 235-245.
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Table 3. Zeta Potential of Electrostatically Stabilized Polystyrene Latexes Produced by Emulsion Polymerization after Washing with Water and Ethanol
Table 4. Zeta Potential of Sterically Stabilized Polystyrene Latexes Produced by Dispersion Polymerization after Washing with Water and Ethanola
zeta zeta conductance of pH of potential potential supernatant supernatant of PS of PS when washing when particles particles washing with water washing washed with washed with times (µS) with water water (mV) ethanol (mV)
zeta zeta conductance of pH of potential potential supernatant supernatant of PS of PS when washing when particles particles washing with water washing washed with washed with times (µS) with water water (mV) ethanol (mV)
0a 1 2 3 4 5
3250 629 28 29 30 29
2.68 3.42 8.30 8.46 8.51 8.33
-57.68 -68.54 -55.22 -47.33 -55.23 -60.96
-72.56 -67.79 -69.44 -64.54 -66.34
a
The supernatant and the PS particles were obtained by centrifuging the original polymer latex. PS latex (solid content, 2.1 wt %), 4.0 mL; solvent (each time), 25 mL; centrifuge speed, 14 500 rpm; centrifuge time, 30 min. The pH of the DI water is 7.94. The zeta potentials were measured in 0.001 M KCl solution.
ZONYL FSO-100 for stabilizing the CO2/water emulsion. Since no hydrophilic PEO chains are around, which may act as a transport barrier for the additive to enter the polymer phase, it is possible that the dye loading might be enhanced. The fluorinated surfactant is the most effective in stabilizing the CO2/water emulsion, providing a high interfacial area for mass transport. But considering the high price of the fluorinated surfactants, one inexpensive and dual function surfactant is more preferable. 3. Surfactant Removal. From Table 2, it can be seen that washing with ethanol causes the zeta potentials of the dyed polystyrene particles to become large negative values. This is the case even for particles incubated with cationic surfactants whose zeta potentials were positive prior to washing (entries 2 and 3 in Table 2). It is possible that washing with ethanol removes the adsorbed surfactant layer. Ethanol is a solvent with a low dielectric constant that can reduce hydrophobic interaction and make it impossible for the surfactant to physadsorb on the particle surface.26 However, the same result (-71.22 mV) was obtained with the dyed dispersion polymerization polystyrene particles with a covalently bound stabilizing layer that cannot be removed by washing. Triton X-100 has an absorbance peak at around 275 nm when dissolved in dichloromethane as determined by UVvis spectroscopy. The absorbance of Triton X-100 allows it to be used as a probe to investigate surfactant removal by ethanol. The polystyrene particles incubated by Triton X-100 were washed once with ethanol, dried, and then dissolved in dichloromethane. UV-vis spectra showed there was no absorbance peak for Triton X-100, indicating that there was no Triton X-100 on the polystyrene particle surface even after one wash with ethanol. It is clear that ethanol is capable of causing the surfactant to desorb from the polystyrene particle surface. Unfortunately, Triton X-100 is the only one of the chosen surfactants that can easily be monitored with UV-vis spectroscopy, so conclusive evidence for desorption was not obtained for the other surfactants. To further investigate the reason for the change in zeta potential upon washing with ethanol, a polystyrene latex produced by surfactant-free emulsion polymerization was washed with water and ethanol immediately after polymerization and without adding any external surfactant. The zeta potentials of the polymer particles after each wash were measured in 0.001 M KCl solution. The results are shown in Table 3. The zeta potentials of the polystyrene particles washed with water did not have a significant change, while the zeta potentials of those washed with
1 2 3 4 5
31 27 28 26 26
7.66 8.35 8.48 8.43 7.98
-17.37 -21.36 -23.62 -26.67 -27.34
-58.73 -71.91 -73.40 -69.61 -72.33
a PS particles, 0.20 g; solvent (each time), 25 mL; centrifuge speed, 14 500 rpm; centrifuge time, 30 min. The zeta potential of the PS particles before washing is -28.69 mV. The pH of the DI water is 7.94. The zeta potentials were measured in 0.001 M KCl solution.
ethanol increased slightly in absolute value. A similar experiment was done for particles produced by dispersion polymerization that have a covalently bound nonionic stabilizer (PVP). The dried particles were redispersed in water, and the zeta potentials were measured. Then, the particles were repeatedly washed with ethanol and water, and the zeta potential was measured after each wash. Table 4 shows the results. The zeta potential of the particles washed with water did not change much, while the zeta potential of those washed with ethanol increased to a larger negative value after one wash. Since the stabilizing layer is covalently bound and is unaffected by ethanol,40 it is apparent that ethanol itself has some influence on the zeta potential. In the zeta potential data presented in Table 2, it is unclear whether surfactant desorption or ethanol itself is the dominant cause of the large negative zeta potential. During the microencapsulation process, the particles become liquified by the plasticizing effect of CO2; then they return to a hard glass when CO2 is removed. The additive is entrapped in the glassy polystyrene and cannot be easily washed away. A similar effect may be possible with the surfactant as well. When the particle is liquified, the hydrophobic portion of the surfactant may diffuse into the polymer core. Then, when CO2 is removed, the hydrophobic portion of the surfactant may be entrapped in the glassy core if it has penetrated deeply. Permanent surface modification of polymers is thus possible through surfactant impregnation.41 However, the data show that Triton X-100 is completely removed by ethanol. It is still unclear if all of the other surfactants investigated are completely removed by washing. With appropriately designed surfactants that interact very favorably with the particle core, it should be possible to permanently modify the particle surface by exposure to CO2. Conclusions The development of a CO2-based microencapsulation technique is limited by the ability to maintain colloidal stability in the acidic CO2-saturated water in which polymer particles are highly plasticized. Electrostatically stabilized polystyrene latexes made by surfactant-free emulsion polymerization cannot maintain stability in the microencapsulation process because of the protonation of surface carboxylate groups that provide colloidal stability when ionized. By choosing proper surfactants to modify (40) Paine, A. J.; Luymes, W.; McNulty, J. Macromolecules 1990, 23, 3104-3109. (41) Ma, X.; Tomasko, D. L. Ind. Eng. Chem. Res. 1997, 36, 15861597.
Surfactants for CO2-Based Microencapsulation
the surface characteristics of the colloidal particles, we have successfully solved this problem. These surfactants improve the microencapsulation process by simultaneously enhancing colloidal stability of the polystyrene particles and increasing CO2/water interfacial area by enhancing emulsification. The selected ionic surfactants SDS, CCM, and CTAB and nonionic polymeric surfactants Triton X-100, F108, T908, Brij 78, and Brij 700 can be adsorbed on the surface of the polystyrene particles and stabilize the particles in the impregnation process. The ionic surfactants introduce less pH-sensitive charged groups to the polymer surface, improving electrostatic stability under acidic conditions. Adsorption of the nonionic surfactants onto the surface causes the particles to be sterically stabilized and thus less sensitive to pH and ionic strength changes. In addition to modifying the polymer surface, the surfactants can enhance the emulsification of CO2 and water. Emulsification results in improved kinetics of impregnation of the dye by increasing the interfacial area. The nonionic surfactants provide more stable emulsions of CO2 into water, resulting in high dye loading. The dye loading increases with decreasing PEO chain length. With
Langmuir, Vol. 19, No. 4, 2003 1113
the aid of the surfactant, the electrostatically stabilized size monodisperse polystyrene latex particles have been successfully incorporated with additives without changing particle size or morphology. After impregnation, UV-vis spectroscopy shows that one of the surfactants, Triton X-100, may be easily removed by washing with ethanol. It is unclear if all surfactants investigated may be removed by washing with ethanol. These dual function surfactants are all inexpensive, commercially available hydrocarbon surfactants, some of which are used in food, drug, and personal care applications. Addition of dual function surfactants is a simple low-cost method that greatly enhances the versatility of CO2-based microencapsulation. The dual function surfactant concept will be applied in the encapsulation of biodegradable nanoparticles with drugs, which is our present work. Acknowledgment. We acknowledge support from the University of Rochester and the Laboratory for Laser Energetics. The electron microscopy facility at the University of Rochester is supported by NSF CTS-6571042. LA026614U
