Influence of Surface Morphology on the Colloidal and Electronic

Jul 26, 2008 - A template approach to the synthesis of a series of conjugated polymer−mesoporous silica composite microspheres is described. Poly(3 ...
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Langmuir 2008, 24, 9809-9815

9809

Influence of Surface Morphology on the Colloidal and Electronic Behavior of Conjugated Polymer-Silica Microspheres Timothy L. Kelly,† Sara P. Y. Che,† Yuri Yamada,‡ Kazuhisa Yano,‡ and Michael O. Wolf*,† Department of Chemistry, UniVersity of British Columbia, VancouVer, BC V6T 1Z1, Canada, and Toyota Central R & D Laboratories, Inc., Nagakute, Aichi 480-1192, Japan ReceiVed May 1, 2008. ReVised Manuscript ReceiVed June 6, 2008 A template approach to the synthesis of a series of conjugated polymer-mesoporous silica composite microspheres is described. Poly(3,4-ethylenedioxythiophene) (PEDOT), poly(thiophene), and poly(N-methylpyrrole) composites were prepared. The surface morphology of the samples was analyzed by scanning electron microscopy, and it was found that well-defined, monodisperse colloidal materials could only be prepared when the monomer is insoluble in the polymerization medium. The filling of the mesopores was systematically varied from 0% to 100%, and powder X-ray diffraction and nitrogen adsorption studies were used to confirm the pore filling. Thermogravimetric analysis shows that the polymer loading tracks the monomer loading in an asymptotic fashion. Conductivity measurements show that the conductivity of the PEDOT materials is relatively constant at high polymer loadings but decreases exponentially at low loadings. Measurements of the electrophoretic mobility were made in order to explain this behavior. These data suggest that, at high polymer loadings, the particle surface is characteristic of the polymer, while at low polymer loadings it is characteristic of the silica host. These results identify important design criteria for the template synthesis of a variety of new colloidal materials. Importantly, these optimized parameters may open the door to the preparation of colloids and colloidal crystals of previously unprocessable materials.

* To whom correspondence should be addressed. E-mail: mwolf@ chem.ubc.ca. † University of British Columbia. ‡ Toyota Central R & D Laboratories, Inc.

As such, the number of materials that can be self-assembled in this way is currently quite limited. The most common approaches to dealing with these problems have been the Sto¨ber-Fink-Bohn method of synthesizing monodisperse silica spheres8 and the emulsion polymerization technique for preparing polystyrene or polyacrylate latexes.9 Both of these methods produce particles that are spherical, monodisperse, and stable in dispersion. Few other materials possess these characteristics. Therefore, the assembly of colloidal crystals has been almost exclusively accomplished by first preparing a silica or polystyrene opal as a template.7,10 Direct assembly of colloidal crystals from other colloidal particles would be of great interest due to the sheer simplicity of the methodology; however, relatively few examples have been reported. The majority of these are high refractive index inorganic semiconductors such as TiO2,11 ZnS,12,13 ZnSe,14 and CdS.15 Core-shell Se@Ag2Se particles were recently prepared by Jeong and Xia, and these particles exhibit a thermally switchable photonic band gap.16 Clusters of magnetite nanoparticles have also been assembled into a photonic band gap structure by assembly in the presence of an applied magnetic field.17 These examples illustrate the intriguing new properties that can result from expanding the range of colloidal materials to include a wider diversity of function. In the pursuit of new colloidal materials, the preparation of conjugated polymer colloids makes an attractive target. Conjugated polymers are a class of organic materials that often display

(1) Busch, K.; Wehrspohn, R. B.; Lo¨lkes, S. Photonic Crystals: AdVances In Design, Fabrication, And Characterization; Wiley-VCH: New York, 2004. (2) Choi, S. Y.; Mamak, M.; von Freymann, G.; Chopra, N.; Ozin, G. A. Nano Lett. 2006, 6, 2456–2461. (3) Ostinelli, O.; Almuneau, G.; Ebnoether, M.; Gini, E.; Haiml, M.; Baechtold, W. Electron. Lett. 2004, 40, 940–942. (4) Heiss, W.; Schwarzl, T.; Roither, J.; Springholz, G.; Aigle, M.; Pascher, H.; Biermann, K.; Reimann, K. Prog. Quantum Electron. 2001, 25, 193–228. (5) Noda, S.; Chutinan, A.; Imada, M. Nature 2000, 407, 608–610. (6) Chow, E.; Liu, S. Y.; Johnson, S. G.; Villeneuve, P. R.; Joannopoulos, J. D.; Wendt, J. R.; Vawter, G. A.; Zubrzycki, W.; Hou, H.; Alleman, A. Nature 2000, 407, 983–986. (7) Ni, P.; Dong, P.; Cheng, B.; Li, X.; Zhang, D. AdV. Mater. 2001, 13, 437–441.

(8) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62–69. (9) Thickett, S. C.; Gilbert, R. G. Polymer 2007, 48, 6965–6991. (10) Norris, D. J.; Vlasov, Y. A. AdV. Mater. 2001, 13, 371–376. (11) Jiang, X.; Herricks, T.; Xia, Y. AdV. Mater. 2003, 15, 1205–1209. (12) Breen, M. L.; Dinsmore, A. D.; Pink, R. H.; Qadri, S. B.; Ratna, B. R. Langmuir 2001, 17, 903–907. (13) Velikov, K. P.; van Blaaderen, A. Langmuir 2001, 17, 4779–4786. (14) Zhong, H.; Wei, Z.; Ye, M.; Yan, Y.; Zhou, Y.; Ding, Y.; Yang, C.; Li, Y. Langmuir 2007, 23, 9008–9013. (15) Matijevic, E.; Wilhelmy, D. M. J. Colloid Interface Sci. 1982, 86, 476– 484. (16) Jeong, U.; Xia, Y. Angew. Chem., Int. Ed. 2005, 44, 3099–3103. (17) Ge, J.; Hu, Y.; Yin, Y. Angew. Chem., Int. Ed. 2007, 46, 7428–7431.

Introduction Photonic crystals have attracted significant attention due to their unique optical properties.1 They are dielectric materials that have a periodically modulated refractive index that varies on the length scale of visible light. These materials possess a photonic band gap, or range of frequencies for which the propagation of electromagnetic radiation within the crystal is forbidden. This is due to the phenomenon of Bragg diffraction by the lattice planes of the crystal and is essentially analogous to the electronic band gap observed in modern semiconductors. While 1-D (Bragg mirrors)2–4 and 2-D photonic crystals5,6 have been extensively explored in the literature, perhaps the most interesting optical effects can be observed when the periodicity is extended to three dimensions. These 3-D photonic crystals can be prepared by the self-assembly of monodisperse colloidal spheres.7 This technique has a number of advantages over other fabrication methods (such as laser holography) due to its low cost and simplicity. Unfortunately, the self-assembly process is particularly demanding of the colloidal material. The particles must be spherical; they must be monodisperse; and they must be mutually repulsive (i.e., form a stable colloidal dispersion).

10.1021/la8013688 CCC: $40.75  2008 American Chemical Society Published on Web 07/26/2008

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behavior typical of inorganic semiconductors.18 They possess a diversity of structure and function, from highly luminescent poly(p-phenyleneethynylene) and poly(p-phenylenevinylene) based materials to highly conductive p-type semiconductors such as polythiophene (PT), poly(3,4-ethylenedioxythiophene) (PEDOT), polypyrrole (PPy), poly(N-methylpyrrole) (PMePy), and polyaniline. Compared to their inorganic counterparts, they offer several advantages. Their structures can be fine-tuned synthetically, leading to differences in oxidation potential, band gap energy, and luminescent properties. A number of groups have been developing methodologies toward the preparation of conjugated polymer nanostructures and to date have produced examples of nanoparticles,19,20 nanowires,21,22 submicrometer hollow spheres,23,24 core-shell particles,25–29 and colloidal composites.30 Recently, we reported the synthesis of a poly(3,4-ethylenedioxythiophene) (PEDOT)-silica colloidal composite by a templated synthesis approach using mesoporous silica spheres (MSS).31 These materials are spherical, monodisperse and have a moderately large zeta potential. However, it was observed that well-formed colloidal crystals could only be prepared from composites with a lower PEDOT loading. The goal of the present study was to generalize the synthesis by determining what factors are integral to the synthesis of the conjugated polymer-silica composites. This should allow for the preparation of an even wider variety of colloidal materialssan important goal for the preparation of photonic crystals with novel properties. Even more importantly, we wanted to discover what factors govern the colloidal and electronic behavior of the composite particles. A thorough understanding of the factors that influence the colloidal behavior of such hybrid materials has to date been lacking and is investigated here by physical and electrophoretic measurements on a series of hybrid colloidal materials.

Experimental Section Materials. 3,4-Ethylenedioxythiophene (EDOT) was donated by Bayer, vacuum distilled, and stored at 4 °C under a nitrogen atmosphere. N-Methylpyrrole was similarly purified after purchase from Sigma-Aldrich. 2-Bromothiophene, magnesium powder, and sodium persulfate were also purchased from Sigma-Aldrich and used as received. 1,3-Bis(diphenylphosphino)propane nickel(II) chloride was purchased from Strem Chemicals and used as received. 2,2′-Bithiophene was synthesized from 2-bromothiophene by a modified Kumada coupling using 1,3-bis(diphenylphosphino)propane nickel(II) chloride catalyst.32 The MSS hosts were synthesized by an established procedure33 and dried in Vacuo at 120 °C prior to use. (18) Heeger, A. J. Angew. Chem., Int. Ed. 2001, 40, 2591–2611. (19) Taranekar, P.; Park, J.-Y.; Patton, D.; Fulghum, T.; Ramon, G. J.; Advincula, R. AdV. Mater. 2006, 18, 2461–2465. (20) Mu¨ller, K.; Klapper, M.; Mu¨llen, K. Macromol. Rapid Commun. 2006, 27, 586–593. (21) Li, G.; Bhosale, S.; Wang, T.; Zhang, Y.; Zhu, H.; Fuhrhop, J.-H. Angew. Chem., Int. Ed. 2003, 42, 3818–3821. (22) Han, M. G.; Foulger, S. H. Chem. Commun. 2005, 3092–3094. (23) Liu, X.; Wu, H.; Ren, F.; Qiu, G.; Tang, M. Mater. Chem. Phys. 2008, 109, 5–9. (24) Tan, Y.; Bai, F.; Wang, D.; Peng, Q.; Wang, X.; Li, Y. Chem. Mater. 2007, 19, 5773–5778. (25) Khan, M. A.; Armes, S. P. Langmuir 1999, 15, 3469–3475. (26) Lascelles, S. F.; Armes, S. P. AdV. Mater. 1995, 7, 864–866. (27) Khan, M. A.; Armes, S. P. AdV. Mater. 2000, 12, 671–674. (28) Norton, J. C. S.; Han, M. G.; Jiang, P.; Shim, G. H.; Ying, Y.; Creager, S.; Foulger, S. H. Chem. Mater. 2006, 18, 4570–4575. (29) Han, M. G.; Foulger, S. H. Chem. Commun. 2004, 2154–2155. (30) Yang, Z.; Kou, X.; Ni, W.; Sun, Z.; Li, L.; Wang, J. Chem. Mater. 2007, 19, 6222–6229. (31) Kelly, T. L.; Yamada, Y.; Che, S. P. Y.; Yano, K.; Wolf, M. O. AdV. Mater. 2008, 20, 2616–2621. (32) Nakayama, J.; Konishi, T.; Hoshino, M. Heterocycles 1988, 27, 1731– 1754. (33) Yano, K.; Fukushima, Y. J. Mater. Chem. 2004, 14, 1579–1584.

Kelly et al. Characterization. Pore volumes were measured on a Quantachrome Autosorb-1 analyzer at 77 K using nitrogen gas. Field emission scanning electron microscopy (FE-SEM) images were taken on a Hitachi S-4700 microscope, and energy dispersive X-ray (EDX) analyses were conducted using a Hitachi S-3000N microscope. Particle size distributions were determined by FE-SEM analysis, with a minimum sample size of 200 particles. Powder X-ray diffraction data were recorded on a Bruker D8 Advance X-ray diffractometer in the Bragg-Brentano configuration, using Cu KR radiation at 40 kV, 40 mA. UV-vis/NIR spectra were recorded on a Cary 5000 spectrophotometer. Thermogravimetric analysis (TGA) was carried out on a Pyris 6 system. Conductivity measurements were made on drop-cast films using the four-point probe technique. The film thicknesses were determined by cross-sectional analysis in the field emission scanning electron microscope. Electrophoresis measurements were carried out on a Beckman Coulter Delsa 440SX instrument. Measurements were made in 10 mM NaCl(aq) at a particle concentration of 0.01% (w/w), and the pH was adjusted by adding HCl or NaOH as required. PEDOT-n@MSS Synthesis. A sample of MSS (0.100 g) was loaded in a Schlenk tube and placed under vacuum. The silica was heated to 120 °C for 2 h to remove adsorbed water and then cooled to room temperature. The flask was backfilled with nitrogen gas. A solution of n% (v/v) EDOT in toluene was prepared (n ) 10-100). Enough of the solution to completely fill the pore volume of the silica was then added to the Schlenk tube by using a micropipet, and the mixture was agitated with a spatula for 6-7 min. The powder was transferred to a vial and dried at 60 °C for a period of 1 h, after which a solution of sodium persulfate (0.15 g) in water (3.00 mL) was added. The mixture was stirred overnight, and the solid was isolated by centrifugation. The solid was washed with water (3 × 15 mL) and ethanol (1 × 15 mL) by redispersion/centrifugation cycles and dried at 60 °C to yield a midnight blue powder. PT-n@MSS Synthesis. A sample of MSS (0.100 g) was dried as before. A solution of n% (v/v) 2,2′-bithiophene in toluene was prepared (n ) 10-100). The solution addition, drying, oxidation, isolation, and purification steps were the same as those for the PEDOT materials, except for the amount of oxidant added (0.30 g). Also, for higher volume percentages of bithiophene (>50%), the sample was first melted at 100 °C prior to transfer. PMePy-n@MSS and PPy-100@MSS Synthesis. These materials were prepared, isolated, and purified in the same way as the PEDOT samples, except that methanol was used as the solvent for the N-methylpyrrole solutions and the samples were dried at 45 °C instead of 60 °C.

Results and Discussion Synthesis of PEDOT-100, PT-100, PPy-100, and PMePy100@MSS Composites. Composite conjugated polymer-silica particles were prepared in a manner similar to that recently reported.31 The MSS hosts used in this work had an average diameter of 746 ( 30 nm. Briefly described, the monomer is drawn into the pores of the mesoporous silica host by capillary action and subsequently polymerized by addition of an aqueous oxidant. This is illustrated in Scheme 1. The mechanism by which the polymer remains isolated inside the pores during the polymerization process was unknown up to this point. One possibility is the insolubility of the organic monomer in the aqueous reaction medium. Other possibilities include the capillary forces of the mesoporous silica, which act to trap the monomer inside the host, or a high rate constant for polymerization, whereby the kinetics of polymerization are more rapid than the kinetics of monomer diffusion. Understanding the reason by which the monomer polymerizes inside the pores, rather than diffusing into the bulk solution, is important in the design of other host-guest colloidal materials. Thus, in order to better understand this process, a series of different conjugated polymer-silica composites was prepared. PEDOT-100@MSS (where 100 refers

Conjugated Polymer-Silica Microspheres Scheme 1. Synthesis of Conjugated Polymer-Silica Composites

to the volume percentage of EDOT in the filling solution) colloidal materials were prepared as a reference, since they have been reported previously.31 Unsubstituted polythiophene was also chosen as a guest polymer. This was due to the higher oxidation potential and insolubility in water ( 100, and the Smoluchowski equation should yield a reasonable approximation of the zeta potential. These potentials are plotted as a function of pH in Figure 6 and as a function of pore filling fraction in Figure 7. As can be seen for the PEDOT-100@MSS sample, the zeta potential is relatively pH independent. The value ranges from approximately 30-40 mV across the entire pH range studied. The lack of an isoelectric point (IEP) and pH independence suggests that the origin of the positive charge is not derived from acid-base equilibria, as is often the case with typical colloidal materials. A more plausible explanation of this behavior is that the surface charge is derived from the positive charge present in the conjugated polymer due to the formation of polarons or bipolarons. Further evidence for this can be obtained from the variation of ζ with pore filling fraction. As shown in Figure 7, the zeta potential for the PEDOT-100@MSS material is +39 mV. The materials with 90%, 70%, and 50% pore filling fraction also display a zeta potential of the same sign and roughly the (37) Elaissari, A. Colloidal Polymers; Marcel Dekker, Inc.: New York, 2003; Vol. 115. (38) Kosmulski, M. Chemical Properties of Material Surfaces; Marcel Dekker, Inc.: New York, 2001; Vol. 102.

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Figure 6. Zeta potential as a function of pH for the PEDOT-100@MSS (black circles), PT-100@MSS (black squares), and PMePy-100@MSS (black triangles) materials as well as the MSS hosts (hollow circles).

Figure 7. Zeta potential as a function of pore filling percentage for the PEDOT-n@MSS (black circles), PT-n@MSS (black squares), PMePyn@MSS (black triangles), and MSS (hollow circle) materials. All measurements were made at pH ) 6.7.

same magnitude. The zeta potential of the PEDOT-30@MSS material, however, is essentially zero. At lower loadings, the sign of the zeta potential reverses; the negative potential indicates negatively charged surface groups. These results are consistent with the conductivity measurements (Vide supra). At greater than 30% pore filling fraction, films of the particles display relatively high conductivity. This indicates that a p-doped surface polymer layer is likely present. The positive charges localized in the polymer (in the form of either polarons or bipolarons) give rise to the positive zeta potential. At the 30% filling level, the resistivity of the sample increases sharply; this indicates that the surface polymer layer has been greatly reduced and that the sample is at the percolation threshold. The surface of the sphere is now likely a mixture of both positively charged polymer and negatively charged silanol groups. The two charges roughly balance, leading to a zeta potential of zero. Below this threshold, the PEDOT-10@MSS material displays essentially zero conductivity and a negative zeta potential characteristic of surface silanol groups. Further evidence for this hypothesis can be derived from both UV-vis spectroscopy and particle size analysis by SEM. As discussed previously, the lack of a π-π* absorption

Conjugated Polymer-Silica Microspheres

band and the presence of pronounced bands in the near-infrared are good evidence for polaron or bipolaron formation. Also, the diameter of the PEDOT-30@MSS materials was determined to be 741 ( 23 nm. The fact that the diameter of the PEDOT30@MSS sample is essentially indistinguishable from that of the silica host (746 ( 30 nm) is good evidence that it is at the 30% pore filling level that the silanol groups of the underlying silica host begin to become exposed. The behavior of the polythiophene analogues is markedly different from that of their PEDOT counterparts. As indicated by the negative zeta potential, nonmeasurable conductivity, and pronounced π-π* absorption, the polymer exists in its pristine, undoped state. Since the polymer itself is electrically neutral, and contains no particularly acidic functional groups, the origin of the negative zeta potential is at first unclear. As shown in Figure 6, the value of ζ is strongly negative across the pH range studied. Even if the low pH region was heavily extrapolated, it is unclear whether an IEP would ever be reached. This behavior is atypical for the MSS host, which displays an IEP at a pH of approximately 3.5, and also for other examples of colloidal silica, which display an IEP in the pH range of 1.8-4.0.38–40 This suggests that the origin of the negative zeta potential is not the silanol groups of the silica host. Rather, the pH independence of ζ for this sample is similar to that observed for polystyrene latexes synthesized using an anionic initiator such as ammonium persulfate.37,41 In these cases, the negative surface charge is due to either sulfonic acid polymer end-groups or residual sulfate anions tightly adsorbed to the latex surface. Since a stoichiometric amount of oxidant is required to facilitate the thiophene polymerization, it is very likely that a similar mechanism is operative here. Due to their nature as extremely weak bases, the sulfonic acid groups or sulfate anions remain negatively charged even at the low pH end of the region studied. The poly(N-methylpyrrole) samples offer another interesting contrast. The samples are still negatively charged; however, the pH dependence is more pronounced than that in the case of either the PEDOT or PT materials. Also, if the low pH end of Figure 6 is extrapolated slightly, these materials may be expected to display an IEP at a pH of approximately 2.0. Due to the low polymer content in these samples (see Figures 3 and 4), and the fact that there is no significant polymer overlayer observed in the SEM images (Figure 1b), it seems reasonable to suggest that the negative zeta potential is at least partially due to the deprotonation of surface silanol groups. Both the gradual decrease in ζ with increasing values of n and the shift of the IEP to more acidic pH compared to the silica host are likely due to a small contribution to ζ from adsorbed sulfate anions. This effect is far less pronounced than that in the case of the polythiophene analogues due to the much lower surface coverage of the microspheres by poly(N-methylpyrrole). Also of note is the fact that all three data sets shown in Figure 7 begin to converge on a zeta potential of approximately -16 mV for low polymer loadings. This is quite similar to the potential observed for the silica host, indicating that, at low loadings, the zeta potential is dominated by the surface silanol groups of the (39) de Keizer, A.; van der Ent, E. M.; Koopal, L. K. Colloids Surf., A 1998, 142, 303–313. (40) Kosmulski, M.; Matijevic, E. Langmuir 1992, 8, 1060–1064. (41) Kettlewell, S. L.; Schmid, A.; Fujii, S.; Dupin, D.; Armes, S. P. Langmuir 2007, 23, 11381–11386.

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silica host. This may explain the previously observed phenomenon whereby good quality colloidal crystals could be formed from PEDOT-30@MSS materials with diameters of 550 nm but only poor ordering was observed for higher polymer loadings. Even though these samples have a relatively small zeta potential, and would therefore be expected to flocculate by conventional Derjaguin-Landay-Verwey-Overbeek (DLVO) theory, it is the anomalously high stability of relatively uncharged colloidal silica surfaces42–47 that appears to stabilize the dispersions. Some researchers initially postulated that this colloidal stability is due to a highly ordered layer of water at the particle interface,45,47 while others have more recently postulated that the presence of a “hairy” partially dissolved polysilicilic acid layer gives rise to increased steric stabilization.42–44 Regardless of the exact mechanism of stability, however, the fact that insoluble, unprocessable materials with large Hamaker constants48 can be encapsulated by mesoporous silica spheres and subsequently processed as stable colloidal dispersions is an important advance.

Conclusions The major goals of this research were to identify the key design criteria for the synthesis of polymer-mesoporous silica composites, generalize the synthesis to a range of other conjugated polymers, and subsequently understand how to balance the desirable colloidal properties of the silica host with the electronic properties of the polymer guest. Clearly, the insolubility of the monomer and polymer in the aqueous polymerization medium is important for the preparation of the polymer-silica composites. This is a particularly interesting point in that it turns a major disadvantage of organic conjugated polymers, that is, their inherent hydrophobicity and insolubility, into a decisive advantage in terms of their processing. This development could potentially lead to greener approaches to materials synthesis and the replacement of volatile organic solvents with aqueous media. Additionally, it opens up a route into colloidal preparations of previously unprocessable materials, such as unsubstituted polythiophenes. The understanding of the factors affecting the colloidal properties for the PEDOT, PT, and PMePy composites, namely, the polymer doping, anion adsorption, and silanol deprotonation, respectively, is expected to aid in the preparation of more stable colloids. This in turn may lead to enhanced self-assembly into colloidal crystals and therefore higher quality crystalline films. Acknowledgment. The financial support of Toyota Central R & D Laboratories, Inc. is gratefully acknowledged. T.L.K. thanks the Natural Sciences and Engineering Research Council of Canada (NSERC) for a scholarship. Supporting Information Available: SEM micrographs, TGA curves, elemental analyses, and UV-vis/NIR spectra for all samples. This material is available free of charge via the Internet at http://pubs.acs.org. LA8013688 (42) Sˇkvarla, J. Langmuir 2007, 23, 5305–5314. (43) Kobayashi, M.; Skarba, M.; Galletto, P.; Cakara, D.; Borkovec, M. J. Colloid Interface Sci. 2005, 292, 139–147. (44) Kobayashi, M.; Juillerat, F.; Galletto, P.; Bowen, P.; Borkovec, M. Langmuir 2005, 21, 5761–5769. (45) Kihira, H.; Matijevic, E. Langmuir 1992, 8, 2855–2862. (46) Depasse, J.; Watillon, A. J. Colloid Interface Sci. 1970, 33, 431–439. (47) Allen, L. H.; Matijevic, E. J. Colloid Interface Sci. 1969, 31, 287–296. (48) Markham, G.; Obey, T. M.; Vincent, B. Colloids Surf. 1990, 51, 239– 253.