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Controlled Heterocoagulation of Platelets and Spheres D. J. Voorn,†,‡,§ W. Ming,*,‡ A. M. van Herk,*,† P. H. H. Bomans,| P. M. Frederik,| P. Gasemjit,⊥ and D. Johanssmann⊥ Laboratories of Polymer Chemistry and of Materials and Interface Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, The Netherlands, Department of Pathology, Electron Microscopy, University of Limburg, P.O. Box 616, 6200 MD Maastricht, The Netherlands, and Institute of Physical Chemistry, Clausthal University of Technology, Arnold-Sommerfeld-Strasse 4, D-38678 Clausthal-Zellerfeld, Germany Received March 7, 2005. In Final Form: May 12, 2005 We report the controlled heterocoagulation of platelets and spheres, leading to the formation of colloidally stable, anisotropic hybrid particles. Anionically charged, nanosized polymer latex spherical particles were heterocoagulated on the surface of cationically charged hexagonal gibbsite platelets via the adsorption of a single layer of spheres onto both sides of the hexagonal platelets. The latex particles were annealed at a temperature above the Tg of the latex polymer, resulting in a thin polymer layer covering the gibbsite platelets. This heterocoagulation approach enabled the encapsulation of hydrophilic inorganic particles with polymer latexes and the formation of anisotropic hybrid particles.
Introduction Aggregation of colloidal particles in dispersions is the basis of many different processes such as water purification, pigment formulation, and preparation of pharmaceutical and drug delivery systems. These processes are of technological as well as scientific significance, and it is therefore essential to have precise control over the dispersion stability. The colloidal stability of these dispersions is determined by many factors such as particle size, ionic strength, and the presence of specific stabilizers.1 Colloidal particle properties, charging effects, mutual interaction, and aggregation behavior between spherical particles2-10 or between spheres and rods11 have been systematically studied, but there are still major challenges for colloidal scientists.12-14 * To whom correspondence should be addressed. E-mail:
[email protected] (W.M.);
[email protected] (A.M.v.H.). † Laboratory of Polymer Chemistry, Eindhoven University of Technology. ‡ Laboratory of Materials and Interface Chemistry, Eindhoven University of Technology. § DPI. | University of Limburg. ⊥ Clausthal University of Technology. (1) Tombacz, E.; Csanaky, C.; Illes, E. Colloid Polym. Sci. 2001, 279, 484-492. (2) Vincent, B.; Young, C. A.; Tadros, T. F. Faraday Discuss. Chem. Soc. 1978, 65, 296-305. (3) Luckham, P.; Vincent, B.; Hart, C. A.; Tadros, T. F. Colloids Surf., A 1980, 1, 281-293. (4) Vincent, B.; Young, C. A.; Tadros, T. F. J. Chem. Soc., Faraday Trans. 1980, 1, 665-673. (5) Vincent, B.; Jafelicci, M.; Luckham, P. F.; Tadros, T. F. J. Chem. Soc., Faraday Trans. 1980, 1, 674-682. (6) Harley, S.; Thompson, D. W.; Vincent, B. Colloids Surf., A 1992, 62, 153-162. (7) Okubo, M.; Lu, Y. Colloids Surf., A 1996, 109, 49-53. (8) Ottewill, R. H.; Schofield, A. B.; Waters, J. A.; Williams, N. S. J. Colloid Polym. Sci. 1997, 275, 274-283. (9) Serizawa, T.; Taniguchi, K.; Akashi, M. Colloids Surf., A 2000, 169, 95-105. (10) Maruyama, K.; Kawaguchi, M.; Kato, T. Colloids Surf., A 2001, 189, 211-223. (11) Koenderink, G. H. Rotational and translational diffusion in colloidal mixtures. Ph.D. Thesis, University of Utrecht, The Netherlands, 2003.
In recent years, aqueous dispersions of charged colloids have been the subject of intense investigations. Various composite materials have been prepared via heterocoagulation of small polymer latex particles with a low Tg onto the surface of oppositely charged large polymer particles with a high Tg.2-10,15-17 Heterocoagulation is generally driven by electrostatic interactions of oppositely charged species that generate a stable composite particle. The smaller shell-forming particles (SFPs) are adsorbed onto the surface of the larger core-forming particles (CFPs). Eventually, a monolayer of small particles forms on the surface of the large particle. In the subsequent step of the formation of core-shell composite particles, the heterocoagulated particles are heated to a temperature higher than the Tg of the SFPs but lower than the Tg of the CFPs.8,18 The method of heterogoagulation enables the formation of well-defined morphologies for composite particles either by engulfment (incomplete coverage and spreading of the small particles) or complete encapsulation of the core particles.19,20 In the past few decades, substantial progress has been made in theoretical description of the heterocoagulation experiments, including the prediction of the morphology of composite particles.18,21-26 (12) Russel, W. B.; Saville, D. A.; Schowalter, W. R. Colloidal dispersions; Cambridge University Press: Cambridge, U.K., 2001. (13) Hiemenz, P. C.; Rajagopalan, R. Principles of colloid and surface chemistry, 3rd ed.; Marcel Dekker Inc.: New York, 1997. (14) Hunter, R. J. Introduction to modern colloid science; Oxford Science Publications: Oxford, U.K., 1996. (15) Yamaguchi, K.; Ito, M.; Taniguchi, T.; Kawaguchi, S.; Nagai, K. Chem. Lett. 2002, 31, 1188-1189. (16) Li, H.; Kumacheva, E. Colloid Polym. Sci. 2003, 281, 1-9. (17) Yamaguchi, K.; Ito, M.; Taniguchi, T.; Kawaguchi, S.; Nagai, K. Colloid Polym. Sci. 2004, 282, 366-372. (18) Li, H.; Han, J.; Panioukhine, A.; Kumacheva, E. J. Colloid Interface Sci. 2002, 255, 119-128. (19) Waters, J. A. European Patent 0327199, 1990. (20) Waters, J. A. U.S. Patent 5210113, 1993. (21) Reerink, H.; Overbeek, J. Th. G. Discuss. Faraday Soc. 1954, 18, 74-84. (22) Schenkel, J. H.; Kitchener, J. A. Trans. Faraday Soc. 1960, 56, 161-173. (23) Marotoa, J. A.; de las Nieves, F. J. Colloids Surf., A 1998, 145, 121-133. (24) Stoll, S.; Pefferkorn, E. J. Colloid Interface Sci. 1993, 160, 149157. (25) Hogg, R.; Healy, T. W.; Feuerstenau, D. W. Trans. Faraday Soc. 1966, 62, 1638-1651.
10.1021/la050605e CCC: $30.25 © 2005 American Chemical Society Published on Web 06/18/2005
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Table 1. Recipes for the Preparation of Latex Particles by Emulsion Polymerization entry
monomer
massa (g)
surfactant
mass (g)
water mass (g)
APS mass (g)
3-MPA mass (g)
SHC mass (g)
C156 C206 C208 C210
BMA BMA S iBMA
8.5 8.4 8.5 8.7
SDS Emulan NP3070 SDS Emulan NP3070
0.81 0.51 0.79 0.75
132.4 130.9 124.6 128.7
0.35 0.39 0.29 0.37
0.019 0.023
0.005 0.006 0.006 0.006
aThe initial reactor charge included 0.5 g of monomer for all the recipes, and the remainder of the monomer was added at a rate of 0.01 mL min-1.
Experimental Section
Figure 1. Schematic illustration of the heterocoagulation between gibbsite platelets and spheres: (A) stable gibbsite platelets covered with one layer of spherical particles and (B) multilayer aggregation.
Moreover, heterocoagulation of polymer latex particles onto the surface of spherical inorganic core materials has resulted in numerous nanostructures with promising applications.16,27,28 Extension of the heterocoagulation to plate-sphere interactions, however, has only been scarcely investigated.1,29-32 Direct visualization of heterocoagulated plate-sphere structures by microscopy is very difficult due to the thickness of the single platelets. These two challenges have motivated us to systematically investigate the heterocoagulation process of spherical particles onto the surface of larger platelets. In this paper, we report the heterocoagulation between anionically charged spheres and cationically charged gibbsite platelets. Synthetic gibbsite platelets have a thickness of about 9 nm and a width in the range of 100400 nm, depending on the processing conditions. The thickness of gibbsite allows direct visualization with electron microscopy and atomic force microscopy (AFM). Snapshots of the heterocoagulation process with spherical particles can be taken. In the first step, the characteristics of the gibbsite platelets and different anionic spherical particles were studied. Subsequently, the controlled formation of stable composite particles of gibbsite covered by spherical particles was investigated (Figure 1). Finally, the heterogoaculated particles were annealed above the Tg of the polymer spheres. (26) Derjaguin, B. V. Discuss. Faraday Soc. 1954, 18, 85-98. (27) Caris, C. H. M.; van Elven, L. P. M.; van Herk, A. M.; German, A. L. Br. Polym. J. 1989, 21, 133-140. (28) Caris, C. H. M. Polymer encapsulation of inorganic submicron particles in aqueous dispersion. Ph.D. Thesis, Technishe Universiteit Eindhoven, The Netherlands, 1990. (29) Ferreiro, E. A.; Helmy, A. K.; de Bussetti, S. G. Clay Miner. 1995, 30, 195-200. (30) Lagaly, G.; Mecking, O.; Penner, D. Colloid Polym. Sci. 2001, 279, 1097-1103. (31) Ji, Y.-Q.; Black, L.; Weidler, P. G.; Janek, M. Langmuir 2004, 20, 9796-9806. (32) Xu, Y.; Brittain, W. J.; Xue, C.; Eby, R. K. Polymer 2004, 45, 3735-3746.
Materials. Butyl methacrylate (BMA; 99%, Aldrich), isobutyl methacrylate (iBMA; 99% Aldrich), and styrene (S; 99%, Aldrich) were distilled under reduced pressure before use. The initiator ammonium persulfate (APS; 99+%, Aldrich), surfactants sodium dodecyl sulfate (SDS; 98%, Fluka) and an alkylphenol poly(ethylene oxide) (Emulan NP3070, BASF), chain transfer agent butyl 3-mercaptopropionate (3MPA; 98%, Aldrich), and buffer sodium hydrogen carbonate (SHC; 99%, Merck) were used as received. Hydrochloric acid (HCl; 32% pa, Aldrich), aluminum sec-butoxide (ASB; 95%, Aldrich), and aluminum isopropoxide (AIP; 99+%, Fluka) were all used without further purification for the synthesis of gibbsite platelets. Doubly deionized (DDI) water was obtained from a Milli-Q water system (Millipore) with a resistivity of 18.3 MΩ cm. Ludox HS40 and CL, kindly provided by Grace Davison, are aqueous dispersions of silica particles with negative and positive charges, respectively. Latex Preparation. Anionic latex particles were produced by a feed emulsion polymerization.33,34 Emulsion polymerizations were performed in a 250 mL three-neck round-bottom flask. The reactor was equipped with an argon inlet, a reflux condenser with an outlet to a bubble counter, a thermometer, and a septum through which the monomer feed was added using a Metrohm Dosimat 776 autotitrator. A typical polymerization is as follows (C156). The initial reactor charge (IRC) containing 0.50 g of BMA, 0.81 g of SDS, 0.02 g of 3MPA, 0.005 g of SHC, and 128 g of deionized water was vigorously stirred. A solution of 0.35 g of anionic initiator APS in 4 g of water was added after the system had been flushed with argon for 1 h at 60 °C. Five minutes after the initiator had been added, the remaining 8.0 g of BMA was fed to the reactor at a constant rate of 0.01 mL min-1. After the addition was complete, the flask was kept at 60 °C for another 2-4 h to ensure full monomer conversion. Multiple cycles of dialysis against DDI water were performed to clean the latexes using Visking dialysis tubes (MWCO 12000-14000) until the conductivity was almost identical to that of the DDI water. The polymerization recipes of the latex particles are listed in Table 1. Gibbsite Preparation. Gibbsite platelets (γ-Al(OH)3) were synthesized according to a method described by Wieringa et al.35 DDI water (1 L) acidified by 0.08 M HCl solution, 0.08 M ASB, and 0.08 M AIP were dissolved, stirred for 10 days at room temperature, and afterward heated to 85 °C for 3 days. The resulting suspension was dialyzed against DDI water for a week to remove nonreacted compounds. Sedimentation (15 min, 3000 rpm, Mistral 3000E) and redispersation of the supernatant in DDI water to remove the large platelets, followed by sedimentation (3 h, 10000 rpm, Kontron Instruments Centrikon T-2060) and redispersation of the sediment, was performed to remove very small gibbsite particles. Heterocoagulation. Heterocoagulation experiments of different spheres and gibbsite platelets were performed according to the recipes in Table 2, at solid contents between 0.005 and 0.02 wt %. Small amounts of gibbsite aqueous dispersions were added to continuously stirred aqueous dispersions of anionic particles with a number concentration of 1.0 × 1015 mL-1 at ambient temperature. The noncoagulated particles were removed by centrifuging the dispersion for 30 min at 3000 rpm. The (33) Voorn, D. J.; Ming, W.; van Herk, A. M. Macomolecules 2005, 38, 3653-3662. (34) Ming, W.; Jones, F. N.; Fu, S. Macromol. Chem. Phys. 1998, 199, 1075-1079. (35) Wierenga, A. M.; Lenstra, T. A. J.; Philipse, A. P. Colloids Surf., A 1998, 134, 359-371.
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Table 2. Heterocoagulation Recipes of Spheres and Gibbsite Plateletsa gibbsite mass (g) shell particle materialb a
HC01
HC02
HC03
HC04
HC05
HC06
HC07
HC08
HC09
0.18 C156 PBMA
0.19 C206 PBMA
0.18 C208 PS
0.37 CL Ludox
0.39 HS40 Ludox
0.39 C156 PBMA
0.38 HS40 Ludox
0.37 C206 PBMA
0.21 C210 PiBMA
The NaCl concentration was kept at 6.4 × 10-4 M, unless otherwise stated. b The amount was varied to tune the NS/NL ratio.
Table 3. Properties of Latex Particles Including Polystyrene (PS), Poly(butyl methacrylate) (PBMA), and Poly(isobutyl methacrylate) (PiBMA), Silica Particles (Ludox HS and Ludox CL), and Gibbsite Platelets monomer Dn (nm) (TEM) Dz (nm) (DLS) PDI (-) (DLS)c Mn (105 g/mol) Mw/Mn ζ potential (mV) pH of dispersion solid content (-)
C156
C206
C208
C210
gibbsite
BMA 32 ( 4 33 0.07 2.9 1.9 -24 5.3 0.03
BMA 56 ( 5 57 0.06 3.7 2.1 -36 4.8 0.05
S 30 ( 4 32 0.08
iBMA 28 ( 5 29 0.07
Ludox HS40
Ludox CL
213 ( 35a 223b 0.12
16 ( 3 15 0.05
12 ( 3 12 0.05
-37 5.2 0.03
-23 4.9 0.02
33 5.4 0.05
-21 9.7 0.19
42 5.0 0.19
a Width of the hexagonal platelet. b Equivalent diameter as determined by DLS. c The polydispersity index (PDI) is calculated from cumulant analysis as described in the International Standard ISO 13321.
precipitated heterogeneous particles were redispersed in DDI water, and similar separations were repeated two times. A 0.1 M aqueous solution of sodium chloride (NaCl; 99+%, Aldrich) was used to change the ionic strength of the dispersions. Heterocoagulation experiments were conducted by varying the number ratio between the small and large particles (NS/NL) at a constant ionic strength. The NS/NL ratio is calculated according to
NS 3(x3)MSwSFLWL2HL ) NL 8πMLwLFSRS3 where MS and ML are the weights of the sphere and platelet dispersions, respectively, wS and wL are the weight fractions of the particle dispersions, FS and FL are the densities of the spheres and platelets, WL and HL are the width and the thickness of the hexagonal platelets, and RS is the radius of the spherical particles. In addition, the effect of the fractional coverage of gibbsite platelets by spherical particles was investigated by adjusting the ionic strength of the dispersions at a constant NS/NL ratio. The fractional coverage of the gibbsite platelets by spherical particles is 2nπRS2/(x3)WL2, where n is the number of small particles. Characterization. The ζ potential of various particles together with its dependency on pH was determined on a Malvern Zetasizer Nano ZS instrument. The ζ potential was calculated from the electrophoretic mobility (µ) using the Smoluchowski relationship, ζ ) ηµ/, where κa . 1 (η is the solution viscosity, is the dielectric constant of the medium, and κ and a are the Debye-Hu¨ckel parameter and the particle radius, respectively). The solution pH was adjusted by adding a solution of either HCl or NaOH using a Malvern MPT2 autotitrator. The average particle size and particle size distribution were determined by dynamic light scattering (DLS) performed on a Malvern 4700 light scattering instrument (λ ) 488 nm) equipped with a Malvern Multi-8 7032 correlator at a scattering angle of 90° at a temperature of 23 °C. A JEOL 2000 FX transmission electron microscope was used for the particle size analysis of the Ludox spheres and gibbsite platelets. Samples of 0.05 wt % were air-dried on a 400-mesh copper grid with a Formvar-supported film. A 3 µL portion of dispersion was placed on a carbon-coated lacy substrate supported by a transmission electron microscopy (TEM) 300-mesh copper grid (Quantifoil R2/2). Excess sample was blotted with filter paper, and the resulting thin film was vitrified in liquid ethane at its melting temperature using a Vitrobot vitrofication robot. The vitrified specimen was then transferred in liquid nitrogen into the transmission electron microscope (Philips TEM CM12).
A detailed description of the vitrification technique and the corresponding protocols can be found elsewhere.36 A highly diluted dispersion droplet (0.005 wt %) was placed on mica and dried under atmospheric conditions at room temperature. The AFM measurements were performed with a MultiMode scanning probe microscope (NanoScope III controller) equipped with a temperature control unit (Nanoscope MMHTRS) from Digital Instruments. The tapping mode with cantilevers of 125 µm length (Nanosensors) was used. All images were taken under light tapping conditions.
Results and Discussion Properties of Spherical and Platelet Particles. For the heterocoagulation experiments different anionic latex particles were prepared using a feed emulsion polymerization.33,34 The properties of the latex particles, together with gibbsite and silica particles, are summarized in Table 3. All latex particles have a polydispersity index lower than 0.10 (which is sufficiently low for the heterocoagulation experiments). The anionic charges come from an anionic initiator and/or an anionic surfactant. The numberaverage molecular weight of the polymers is about 3 × 105 and the polydispersity around 2. The particle size for spherical particles is significantly lower than the width of the gibbsite platelets. All latexes were extensively cleaned by dialysis against deionized water for more than two weeks. Gibbsite platelets were formed as a result of a reaction among AIP, ASB, and HCl at 85 °C. Buining et al.37 showed that the structure of the aluminum colloids strongly depends on the reaction temperature. Boehmite rods are produced at approximately 150 °C, whereas the formation of platelet particles occurs at lower temperatures.35,38 The polydispersity of the platelet size depends strongly on the HCl concentration. At a HCl concentration of 0.055 M the average particle size was much larger than 300 nm with a polydispersity of 0.25. By increasing the HCl concentration to 0.086 M, the average particle size decreased to 213 nm, and the polydispersity index was lowered to 0.12. The solid content of the platelets after dialysis and centrifuge was 5-6 g/L. (36) Frederik, P. M.; Hubert, D. H. W. Methods in Enzymology; Volumes in Liposomes; Academic Press: New York, 2005; Vol. 391, Part E. (37) Buining, P. A.; Pathmamanoharan, C.; Jansen, J. B. H.; Lekkerkerker, H. N. W. J. Am. Ceram. Soc. 1991, 94, 1303-1307. (38) Philipse, A. P.; Nechifor, A. M.; Patmamanoharant, C. Langmuir 1994, 10, 4451-4458.
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Figure 2. Effect of pH on the ζ potential and electrophoretic mobility (µ) for (a) gibbsite (4), latex C156 (O), and Ludox HS40 (9) at an ionic strength of 10 mM and (b) gibbsite covered with (O) anionic PBMA latex particles (HC02) and (9) Ludox HS40 particles (HC05).
Figure 3. Particle size distribution during the heterocoagulation process: (a) HC01, addition of gibbsite platelets to PBMA latex particles (NS/NL ) 204) at 23 °C at different time intervals; (b) HC03, addition of gibbsite platelets to PS latex particles (NS/NL ) 131) at 23 °C at different time intervals.
ζ Potential Analysis. The electrophoretic mobility of the Ludox particles, the polymer latex particles, and the synthesized gibbsite platelets was investigated. Figure 2a shows the ζ potential of the individual colloidal particles as a function of pH (the electrophoretic mobility, µ, demonstrates a similar dependence on pH). ζ potentials of the anionic Ludox HS40 particles and the PBMA latex particles (entry C156) are negative over the entire pH range. The ζ potential of the gibbsite platelets is positive in acidic medium and becomes negative in alkaline medium; the isoelectric point (iep) of gibbsite is at pH 8.5. This finding is in agreement with earlier findings of Wierenga et al.35 At pH values lower than the iep, the gibbsite platelets are cationically charged (reaction 1). Gibbsite becomes negatively charged when the pH exceeds the iep, given by reaction 2.
Al-OH + H+ / Al-OH2+
(1)
Al-OH + OH- / Al-O- + H2O
(2)
Furthermore, Wierenga et al. reported a difference in the iep for the faces and edges of the gibbsite (pH ≈ 10 and pH ≈ 7, respectively).35 Although the iep indicates that the gibbsite platelets are cationically charged at pH values lower than 8.5, when the face-edge difference is included, complete cationic platelets are only available below pH 7. Heterocoagulation experiments of different spheres and gibbsite platelets were performed according to Table 2. It
appeared from visual observations that stable dispersions were obtained for the sphere-plate heterocoagulation experiments. ζ potential measurements at different pH values of the stable structures (HC02 and HC05) are shown in Figure 2b. The gibbsite platelets covered with small spheres demonstrated a decrease in the ζ potential at low pH (
