Preparation of Asymmetrically Nanoparticle-Supported

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Preparation of Asymmetrically Nanoparticle-Supported, Monodisperse Composite Dumbbells by Protruding a Smooth Polymer Bulge from Rugged Spheres Daisuke Nagao, Kanako Goto, Haruyuki Ishii, and Mikio Konno* Department of Chemical Engineering, Graduate School of Engineering, Tohoku University, 6-6-07 Aoba, Aramaki-aza Aoba-ku, Sendai 980-8579, Japan

bS Supporting Information ABSTRACT: A novel method is proposed to create asymmetrically nanoparticle-supported, monodisperse composite dumbbells. The method consists of the three steps of double soap-free emulsion polymerizations before and after a heterocoagulation. In the first step, soap-free emulsion polymerization was conducted to cover silica cores with cross-linked poly(methyl methacrylate) (PMMA) shells. Then, positively or negatively charged silica nanoparticles were heterocoagulated with the silica PMMA core shell particles. In the heterocoagulations, the nanoparticles surfacemodified with a cationic silane coupling agent, 3-aminopropyltriethoxysilane, were used as the positively charged ones, and silica nanoparticles without any treatment were used as the negatively charged ones. In the third step, soap-free polymerizations at different pH values were performed to protrude a polystyrene (PSt) bulge from the core shell particles supporting the charged silica nanoparticles. In the polymerization, the core shell particles heterocoagulated with the positively charged silica nanoparticles were aggregated in an acidic condition whereas the silica nanoparticles supported on the core shell particles were dissolved in a basic condition. For the negatively charged silica nanoparticle, a PSt bulge was successfully protruded from the core shell particle in acidic and neutral conditions without aggregation of the core shell particles. The protrusion of the PSt bulge became distinctive when the number of heterocoagulated silica nanoparticles per core shell particle was increased. Additional heterocoagulation experiments, in which positively or negatively charged magnetite nanoparticles were mixed with the asymmetrically nanoparticle-supported composite dumbbells, confirmed direct exposure of silica nanoparticles to the outer solvent phase.

1. INTRODUCTION Capsulation of polymeric microspheres with inorganic substances is widely used to functionalize the microspheres in various industrial applications. For instance, formation of hydrophilic silica shells on the polymeric microspheres improves the colloidal stability in polar media.1 Capsulation with inorganic shells resistant to high temperatures or apolar media is applicable to fabrication of hollow microspheres by removal of inner polymer components with heat treatment2 or soaking in apolar media.3 Surface decoration of polymeric microspheres with inorganic nanoparticles allows functionalization of microspheres with fluorescence,14 magnetism,11,15 and permittivity.16 Incomplete coverage with nanoparticles leaving the polymeric surface partially exposed provides an opportunity for a combined use of the polymeric surface and the nanoparticle functions. Currently, the surface decoration with inorganic nanoparticles is also regarded as an effective method to tune surface roughness on microspheres applicable to superhydrophobic coatings.4 r 2011 American Chemical Society

Another interesting phenomenon relating to surface roughness of microspheres is the depletion interaction between microspheres induced by the presence of nonadsorbing polymers.6,7 When nonadsorbing polymers are added to a dispersion of microspheres, attractive forces between microspheres are induced due to osmotic pressure. The attractive forces between the microspheres sterically stabilized are found to be weakened by roughening the surface of microspheres because the overlap volume of stabilization layers between the microspheres is reduced.5 When the control over attractive forces is applied to anisotropic particles such as Janus particles having both smooth and rough surfaces, approaches to assembling of anisotropic particles will be extended. In the literature on functionalizing particle surfaces, polymer dumbbells with two different polymer surfaces have been asymmetrically decorated with nanoparticles interacting with Received: July 29, 2011 Revised: September 21, 2011 Published: September 21, 2011 13302

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agents of 3-amnopropyltriethoxysilane (APS, Shinetsu Chemical, Japan) and N-triethoxysilylpropyl-N,N,N,-trimetylammonium chloride (TSA, 50% methanol solution, Gelest Ink., U.S.) and 3-methacryloxypropyltrimethoxysilane (MPTMS, 95%, Shinetsu Chemical, Japan) were used as received. Preparation of Silica Particles. Silica particles with an average size of 330 nm were used as cores incoporated into polymer dumbbells and prepared by hydrolysis and condensation of TEOS in ethanol solvent at 25 °C and [TEOS] = 0.4 M, [H2O] = 11 M, and [NH3] = 0.3 M. Silica nanoparticles supported on core shell particles were also prepared by the hyrolysis and condensation at 35 °C with a basic catalysis of methylamine in a mixed solvent of ethanol and acetonitrile (60:40 wt %). The concentrations of TEOS, H2O, and methylamine were 0.19, 4, and 0.05 M for 70 nm silica particles and 0.20, 2, and 0.05 M for 30 nm particles. In the case of preparation of cationic silica nanoparticles, APS was added to the reaction system at the molar ratio of TEOS:APS = 95:5 Figure 1. Schematic procedure for preparation of asymmetrically nanoparticle-supported composite dumbbells.

functional groups on one side of dumbbell surface.8 On the other hand, here we propose a novel method to prepare asymmetrically nanoparticle-supported composite dumbbells with incorporation of an inorganic sphere into the nanoparticle-supported part. The synthetic route to the composite dumbbells is shown in Figure 1. What is profoundly different from ref 8 is that nanoparticles are hetrocoagulated with cross-linked spherical particles before a protrusion of polymer. The present method enabling incorporation of an inorganic sphere into one part of polymer dumbbells is based on our previous method9 that employed two-step soap-free emulsion polymerizations in the presence of inorganic spheres. The incorporation of inorganic spheres provides effective ways to interiorly functionalize the polymer dumbbells, to prepare monodisperse composite dumbbells when monodisperse inorganic spheres are used as a starting material, and to identify which is the protrusion polymer of dumbbells in electron microscopic observation. Preparations of the asymmetric dumbbells with the present method are performed at different number densities of nanoparticles supported on core shell particles to tune the surface roughness of one part of the dumbbells. Exposure of nanoparticles supported on the core shell particles is examined by other hetrocoagulations in which positively or negatively charged nanoparticles were mixed with the asymmetrically nanoparticle-supported composite dumbbells. It is first shown in the present work that polymer protrusion from the nanoparticle-supported particles with polymer shell is effective for preparation of asymmetrically nanoparticlesupported composite dumbbells with low polydispersity. The present work also provides a novel way to control the anisotropies in not only particle shapes but also surface compositions.

2. EXPERIMENTAL SECTION Chemicals. Tetraethyl orthosilicate (TTIP, 95%), ethanol (99.5%), acetonitrile (99.5%), methylamine aqueous solution (40%), ammonium hydroxide solution (25%), methyl methacrylate (MMA, 98%), styrene (St, 99%), p-vinylbensoic acid (VBA), potassium persulfate (KPS, 95%), and potassium hydroxide solution (KOH, 0.1 M) were purchased from Wako Pure Chemical Industries (Osaka, Japan). The inhibitors for monomers of MMA and St were removed by an inhibitor removal column. The other chemicals were used as received. Silane coupling

Coating of the Silica Cores with Cross-Linked PMMA Shell. Deionized water dissolving KOH was bubbled with nitrogen for 30 min, and then MPTMS and an aqueous suspension of silica cores were added to the water. After 30 min stirring, MMA and VBA were added to the silica suspension. After 2 h stirring at 35 °C, the mixed solution was heated to 65 °C and a KPS solution was added to initiate polymerization. The concentrations of MMA, VBA, MPTMS, KPS, and KOH were 0.2 M, 0.5 mM, 2 mM, 2 mM, and 1 mM, respectively. The volume concentration of silica cores was 0.15%. The polymerization was performed at 300 rpm for 2 h at 65 °C. Then, the PMMA-coated silica particles were washed and redispersed into deionized water. Electrostatic Heterocoagulation. A suspension of silica nanoparticles surface-modified with or without APS was mixed with a suspension of the silica PMMA core shell paticles, and the suspension pH values of the two particles were lowered by the addition of hydrochloric acid. The number ratio of silica nanoparticles to core shell particles in the heterocoagulation was varied in a range of 200 2000.

Protrusion of a PSt Bulge from the Core Shell Particles Supporting Silica Nanoparticles. Aqueuos suspension of the core shell particles supporting silica nanoparticles were bubbled with nitrogen, and St was added to the suspension. After 2 h stirring at room temperature, the mixture was heated to 65 °C and a KPS aqueous solution was added to initiate polymerization. The concentrations of KPS and silica cores were fixed at 2 mM and 0.01 vol %, respecctively, and St concentration was varied in a range of 0.1 0.2 M. After 5 h polymerization, the resultant suspension was washed by centrifugation and redispersion in deionized water. Characterization. The resulting particles were observed with STEM (Hitachi, HD-2700B). Zeta potentials of particles prepared were measured with electrophoresis light scattering (ELS-8000, Otsuka Electronics). The Smoluchowski equation was used to convert electrophoretic mobilities into the zeta potentials. The percentage of silica nanoparticles loaded on the core shell particles to the nanoparticles added to the hetrocoagulation system was calculated from the number of core shell particles and the increase in weight fraction of silica component by the heterocoagulation. The weight fractions of silica component were measured with thermal gravity analysis (TG/DTA 220, Seiko Instruments).

’ RESULTS AND DISCUSSION Heterocoagulation of Core Shell Particles with Silica Nanoparticles Surface-Modified with APS. Figure 2 shows

electron microscope (EM) images of the particles used for heterocoagulation and the resultant particles. Silica nanoparticles surface-modified with APS (Figure 2B) were first heterocoagulated with the core shell particles (Figure 2A) at the number 13303

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Figure 2. Electron microscope images of silica-PMMA core shell particles (A) used for heterocoagulation and the nanoparticle-supported, core shell particles (D G). Silica nanoparticles surface-modified with APS (B) were used for the preparation of particles (D) and (E). Nonmodified silica nanoparticles (C) were used for the preparation of particles (F) and (G). The number ratios of silica nanoparticles to the core shell particles in the heterocoagulations were 200 and 2000 for silica nanoparticles of (B) and (C), respectively.

Figure 3. Zeta potentials of silica PMMA core shell particles (circles, Figure 2A) and silica particles surface-modified with APS (squares, Figure 2B) and the core shell particles heterocoagulated with the silica particles (triangles, Figure 2D, E).

Figure 4. Zeta potentials of nonmodified silica particles (squares, Figure 2C), silica PMMA core shell particles (circles, Figure 2A), and the core shell particles heterocoagulated with the nonmodified silica particles (triangles, Figure 2F, G).

ratio of silica nanoparticles to the core shell particles of 200. To find suitable pH for the heterocoagulation, zeta potentials of the particles A and B were measured. As shown in Figure 3, the core shell particles (A) had negative zeta potentials in the whole pH range, whereas the silica nanoparticles B had positive potentials at pH lower than 8. According to these profiles of zeta potentials, the heterocoagulation was conducted by decreasing the suspension pH of particles A and B in Figure 2 from a basic pH of 10 to an acidic pH of 3 with HCl. The heterocoagulates obtained are shown in TEM and SEM images of Figure 2D, E, which indicate that approximately 70 nm particles were homogeneously adsorbed by the core shell particles. Zeta potentials of the heterocoagulates, which were separated from the nonadsorbed silica nanoparticles by several centrifugations, are plotted in Figure 3. The potential profile of the heterocoagulates well agreed with the one of silica nanoparticles B, which verified that the core shell particles were completely covered with the APS-modified nanoparticles. Polymerizations of St in the presence of the heterocoagulates were conducted with an anionic initiator of KPS to protrude a PSt bulge from the heterocoagulates. However, during the polymerization, the stable dispersion could not be maintained. In the polymerization, solution pH decreased from 8 to 3 owing to generation of sulfuric acid from KPS, and secondary PSt

particles were formed in the aqueous phase. Since the particle surface charge arising from KPS was opposite to that of the heterocoagulates in acidic pH, the PSt particles were adsorbed by the heterocoagulate, which destabilizes the reaction system (see Figure S1). To improve stability of the reaction system, polymerization in the presence of the heterocoagulates was also carried out in a basic condition. Although a PSt bulge was protruded from spherical particles, the silica nanoparticles supported onto the heterocoagulates were dissolved in the polymerization. The basic condition at the elevated temperature in the polymerization promoted the dissolution of silica.10 Heterocoagulation of Core Shell Particles with Nonmodified Silica Nanoparticles. Since difference in particle sizes is another important factor in the heterocoagulation between two particles,20,21 it is possible that the large difference in particle size induces a uniform heterocoagulation between two particles without opposite charges on their surfaces. Then, silica nanoparticles without surface modification (Figure 2C) were used for heterocoagulation with the core shell particles of Figure 2A. Zeta potentials of the particles C measured at different pH values were presented together with the ones of the core shell particles A in Figure 4. The silica particles (Figure 2C) have an isoelectric point at pH 2 that is consistent with the literature value.11,17 13304

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Langmuir According to the profiles of zeta potentials, the suspension pH in the heterocoagulation was decreased from a basic pH of 10 to an acidic pH of 2 to lower electrostatic repulsion between the two particles of A and C. The number ratio of the particle C to the particle A, N, was 2000. The decrease in suspension pH could heterocoagulate the particles A and C, as shown in TEM and SEM images of Figure 2 F and G. The coverage of core shell particles with the silica nanoparticles were also confirmed by zeta potentials of the heterocoagulates in Figure 4. It is expected that the anionic heterocoagulates in Figure 4 are stably suspended in the polymerization with KPS initiator that lowers solution pH during polymerization. Figure 5 shows EM images of particles obtained by polymerization of St in the presence of heterocoagulates. A PSt bulge was successfully protruded from the heterocoagulates accompanied by the generation of small secondary PSt particles that can be readily seen in the wide views of Figure S2 and completely removed by centrifuge processes. The TEM images in Figure 5B and Figure S2B indicate that each silica core was contained at the center of one of the bulges of asymmetric dumbbells. The asymmetric dumbbells separated from the secondary particles with sonication and centrifugation are shown in a TEM image of Figure 6A where the dumbbells had a structure of a spherical bulge connected with a rugged bulge that contains a silica core with homogeneous shell thickness. The average sizes of PSt bulges and core shell particles supporting the nanoparticles in Figure 6A were 460 and 480 nm, respectively, and their coefficients of variation were 4.3% and 7.1% for each. The separation treatment scarcely detached the silica nanoparticles from the dumbbells. However, when the silica-nanoparticlesupported core shell particles (Figure 2F, G) before the second soap-free polymerization were subject to sonication, more than 75% of the nanoparticles were detached (see Figure S3). This means that the nanoparticles on the dumbbells were strongly

Figure 5. Asymmetrically nanoparticle-supported, composite dumbbells (A, SEM image and B, TEM image) obtained by protrusion of a smooth polymer bulge from core shell particles supporting silica nanoparticles. Relatively small, spherical particles in the images are secondary particles formed in the polymerization for the protrusion.

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fixed to the dumbbell surface through the soap-free polymerization. The possible reasons for fixing the nanoparticles on the PMMA shell include condensation of the silanol group on the nanoparticles with hydrolyzed MPTMS that was incorporated in PMMA shell, esterification of the silanol group with carboxyl groups that would be generated from hydrolysis of PMMA,18 and a decrease in molecular mobility19 with lowering temperature from the polymerization temperature of 65 °C. Figure 6B shows a TEM image of particles formed in the second polymerization at a styrene concentration of 0.2 M. Since the concentration became twice of that in Figure 6A, the sizes of PSt bulges were increased and became larger than the core shell part. Figure 6C shows the TEM image of particles formed with 70 nm silica particles used in the heterocoagulation. The larger silica nanoparticle of 70 nm could also be supported onto the core shell part of the dumbbells. Figure 7 shows EM images of asymmetric dumbbells on which different number of silica nanoparticles were supported. In the preparations, the heterocoagulations were conducted with 30 nm silica nanoparticles at the number ratios of N = 0, 250, and 1000, and then soap-free polymerizations in the presence of the heterocoagulates (see EM images in Figure S4) were performed at a monomer concentration of 0.1 M. Thermogravimetric analysis (TGA) revealed that 40% of the silica nanoparticles were loaded on the asymmetric dumbbells for N = 1000, and 30% of silica nanoparticle were loaded for N = 250. It is interesting to note that the contact angle between the two bulges of the dumbbells, θ, was increased with N value. It is known that hydrophilicity of the surface of seed particles in the emulsion polymerization affects the extent of polymer protrusion and then increases the contact angle of the anisotropic polymer particles finally obtained.12 Since the silica nanoparticles are more hydrophilic than the supporting polymer (PMMA), the adsorption of the silica nanoparticles onto the polymer can enrich the hydrophilicity of overall surface property. For the presently proposed method, it is important to examine the surface properties of the silica nanoparticles supported onto the dumbbells because the nanoparticles might be covered with other components than silica after the soap-free polymerization for the PSt bulge protrusion. The asymmetric composite dumbbells of Figure 7C were treated with a cationic silane coupling agent (TSA) that can selectively provide silica surfaces with positive charges. The asymmetric composite dumbbells treated with TSA were heterocoagulated with anionic or cationic Fe3O4 nanoparticles at neutral pH. The anionic Fe3O4 nanoparticles were prepared with the conventional coprecipitation method of Fe2+ and Fe3+, while the cationic Fe3O4 nanoparticles were

Figure 6. Asymmetrically nanoparticle-supported composite dumbbells obtained after centrifuges and sonications to remove the secondary particles. St concentrations in the second polymerization of (A) (C) were 0.1, 0.2, and 0.1 M, respectively. Thirty nanometer silica nanoparticles were used in preparations for (A) and (B), and 70 nm ones used for (C). The particles in Figure 5 were used for Figure 6A. 13305

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Figure 7. Effect of the number of supported silica nanoparticles on the contact angle of composite dumbbells. The composite dumbbells in (A) (C) were prepared by protrusion of PSt from nanoparticle-supported core shell particles obtained at the different N values of 0, 250, and 1000 in the heterocoagulation.

Figure 8. Electrostatic heterocoagulation to examine exposure of silica surface of the nanoparticles in the second polymerization for protruding a PSt bulge.

prepared with surface modification of the anionic Fe3O4 nanoparticles with TSA.11 Figure 8A shows EM images of heterocoagulates with the cationic Fe3O4 nanoparticles together with a schematic drawing for adsorption profile of the cationic Fe3O4. The magnified image in Figure 8A indicates that the Fe3O4 nanoparticles were not adsorbed by the silica nanoparticles. The TEM image in Figure 8A indicates that the Fe3O4 nanoparticles were distributed on the both sides of bulges of the dumbbells. Electrostatic interactions caused preferential adsorption of the cationic Fe3O4 nanoparticles onto the anionic polymer surface of

the dumbbells. The TEM image in Figure 8B shows the asymmetric composite dumbbells heterocoagulated with the anionic Fe3O4 nanoparticles that have negative charges in neutral pH due to an isoelectric point of approximately 5.513 (see Figure S5). In contrast to the distribution of cationic nanoparticles in Figure 8A, the Fe3O4 nanoparticles were selectively adsorbed by the silica nanoparticles surface-modified with TSA. These additional experiments on electrostatic heterocoagulations elucidated the exposure of silica surface of the nanoparticles to the outer solvent phase. 13306

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Langmuir As shown in Figures 7 and 8, the present work provided an effective way to control the anisotropies in not only particle shapes but also surface compositions of the dumbbells. These controls over the factors to determine the characteristics of dumbbells are new findings and important to diversify functional colloids in their applications.

’ CONCLUSION The proposed method consisting of the three steps of double soap-free emulsion polymerizations before and after a heterocoagulation succeeded in preparation of asymmetrically nanoparticle-supported, monodisperse composite dumbbells. Negatively charged core shell particles supporting nonmodified silica nanoparticles were chosen to successfully protrude a PSt bulge from core shell particles in the soap-free polymerization with the anionic initiator of KPS. The soap-free polymerizations in the presence of core shell particles with the different number N of the heterocoagulated silica nanoparticles revealed that the degree of PSt protrusion could be enhanced by an increase in N value. The silica nanoparticles supported on the composite dumbbells were adhesive enough to stand against ultrasonic irradiation. Additional heterocoagulation experiments in which positively or negatively charged magnetite nanoparticles were mixed with the composite dumbbells confirmed direct exposure of silica nanoparticles to the outer solvent phase. ’ ASSOCIATED CONTENT

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Supporting Information. TEM images to indicate particles formed in St polymerization in the presence of core shell particles supporting APS-modified silica nanoparticles; SEM and TEM images to show the generation of secondary particles in the polymerization for PSt protrusion; the influence of ultrasonic irradiation on core shell particles supporting nonmodified silica nanoparticles; TEM images of the core shell particles with the different number ratio N; zeta potentials to show isoelectric points for different particles. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel. +81-22-795-7239; fax +81-795-7293; e-mail konno@ mickey.che.tohoku.ac.jp.

’ ACKNOWLEDGMENT This research was partially supported by the Ministry of Education, Culture, Sports, Science and Technology (22651053, 23246134 and 23681020). ’ REFERENCES (1) Graf, C.; Vossen, D. L. J.; Imhof, A.; van Blaaderen, A. Langmuir 2003, 19, 6693–6700. (2) Zhang, H.; Zhang, X.; Yang, X. J. Colloid Interface Sci. 2010, 348, 431–440. (3) Hu, H.; Zhou, H.; Liang, J.; An, L.; Dai, A.; Li, X.; Yang, H.; Yang, S.; Wu, H. J. Colloid Interface Sci. 2011, 358, 392–398. (4) Li, Y.; Lee, E. J.; Cho, S. O. J. Phys. Chem. C 2007, 111, 14813– 14817. (5) Michiel, H.; Ph.D. Thesis, Utrecht University, 2010. (6) Asakura, S.; Oosawa, F. J. Chem. Phys. 1954, 22, 1255. 13307

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