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Langmuir 2002, 18, 1098-1103
Multiparticle Effects on the Interactions of Complex Colloidal Dispersions S. C. Olugebefola, S. Y. Park, P. Banerjee,† and A. M. Mayes* Department of Materials Science and Engineering, MIT, Cambridge, Massachusetts 02139
C. M. B. Santini, J. Iyer, and P. T. Hammond Department of Chemical Engineering, MIT, Cambridge, Massachusetts 02139 Received September 24, 2001 A model polymer colloid with simple “recognition” ability was synthesized by dispersion polymerization using comb copolymer stabilizers that incorporate both 23-unit poly(ethylene oxide) and C18H37 side chains. Force interactions between beads of the resulting latex were examined using colloid-probe atomic force microscopy (CAFM) and Langmuir compression (LC). CAFM measurements showed an initial repulsion between beads on close approach that gives way to a strong bridging attraction and a minimum in the force-distance profile at bead surface separations comparable to twice the hydrocarbon side chain dimensions. By contrast, latex beads stabilized with combs that incorporate only the (EO)23 side chains exhibited purely repulsive behavior. LC experiments on the same systems produced pressure-area isotherms qualitatively similar to the CAFM data; however, significant forces were registered at monolayer bead densities corresponding to surface separation distances 1-2 orders of magnitude larger than the side chain dimensions and comparable to the bead size (∼1 µm). A simple model was developed to connect the data from these two experiments into a unified picture of how particle surface chemistry is expressed in the multiparticle behavior of complex colloidal dispersions.
Introduction Colloidal systems are currently being developed for an array of high-technology applications that demand the ability to tune and control their adsorption and aggregation behavior. In biotechnology, for example, dispersed gold and semiconductor nanoparticles are of interest for chromogenic labeling of specific proteins and other biopolymers.1-5 Other “smart” colloid systems have also shown potential for use as synthetic enzymes for remodeling of proteins,6,7 and vehicles for targeted drug and gene delivery.8,9 Current biocolloids typically carry proteins/ biopolymers immobilized by adsorption, spray coating, or covalent binding to the particle surface, or trapped within the core of liposomes or vesicles. Such approaches, however, often leave dispersed particles vulnerable to undesirable adsorption processes or other secondary interactions with elements in the surrounding medium, limiting their targeting efficiency and efficacy. Thus, a need exists for mechanisms to shield colloidal particles from unwanted interactions while maintaining their selective adsorption or binding behavior. * Please address correspondence to
[email protected]. † Current address:
[email protected]. (1) Delair, T.; Meunier, F.; Elaı¨ssari, A.; Charles, M.-H.; Pichot, C. Colloid Surf., A 1999, 153, 341-353. (2) Bruchez, M., Jr.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013-2016. (3) Chan, W. C. W.; Nie, S. Science 1998, 281, 2016-2018. (4) Mahtab, R.; Harden, H. H.; Murphy, C. J. J. Am. Chem. Soc 2000, 122 (1), 14-17. (5) Elganian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277 (22), 1078-1081. (6) Shimizu, H.; Fujimoto, K.; Kawaguchi, H. Colloid Surf., A 1999, 153, 421-427. (7) Caruso, F.; Schu¨ler, C. Langmuir 2000, 16, 9595-9603. (8) Nishiya, T.; Murata, M.; Handa, M.; Ikeda, Y. Biochem. Biophys. Res. Commun. 2000, 270 (3), 755-760. (9) Lu¨ck, M.; Paulke, B.-R.; Schro¨der, W.; Blunk, T.; Mu¨ller, R. H. J. Biomed. Mater. Res. 1998, 39 (2), 478-485.
Balazs and co-workers10 have proposed several methods to tailor repulsive and attractive interaction regimes between colloidal particles. Using self-consistent mean field theory, they showed that intersurface interaction exhibiting both long-range repulsion and a local attractive minimum could be created by cotethering solvophilic and solvophobic polymer chains to opposing surfaces. Upon approach, the solvophilic chains, extended into solution, provide an initial steric barrier, while the solvophobic chains, clustered near the surface, engage in attractive bridging interactions at smaller separations. The use of “shielding” components along with “binding” components in this fashion suggests a simple scheme to enhance the efficacy of colloidal recognition processes. Experimentally, a number of groups have observed the effects of adding hydrophilic shielding molecules to the surface of colloidal therapeutic vehicles. Already, the most popular of these methods, pegylation (the covalent bonding of poly(ethylene glycol), PEG, units), is becoming relatively mainstream.11 Pegylation has been shown to increase solubility while reducing toxicity in DNA transfection agents12 and to reduce antibody recognition for bioactive proteins.13 There can be disadvantages to this process as well, such as loss of tissue selectivity for tumor therapies14 and loss of bioactivity.15 In general, however, the funda(10) Singh, C.; Pickett, G. T.; Balazs, A. C. Macromolecules 1996, 29, 7559-7570. (11) Kodeera, Y.; Matsushima, A.; Hiroto, M.; Nishimura, H.; Ishii, A.; Ueno, T.; Inada, Y. Prog. Polym. Sci. 1998, 23 (7), 1233-1271. (12) Choi, J. H.; Choi, J. S.; Suh, H.; Park, J. S. Bull. Korean Chem. Soc. 2001, 22 (1), 46-52. (13) Belcheva, N.; Woodrow-Mumford, K.; Mahoney, M. J.; Saltzman, W. M. Bioconjugate Chem. 1999, 10, 932-937. (14) Rovers, J. P.; Saarnak, A. E.; de Jode, M.; Sterenborg, J. C. M.; Terpstra, O. T.; Grahn, M. F. Photochem. Photobiol. 2000, 71 (2), 211217. (15) Pettit, D. K.; Bonnert, T. P.; Eisenmann, J.; Srinvasan, S.; Paxton, R.; Beers, C.; Lynch, D.; Miller, B.; Yost, J.; Grabstein, K. H.; Gombotz, W. R. J. Biol. Chem. 1997, 272, 2312-2318.
10.1021/la011466d CCC: $22.00 © 2002 American Chemical Society Published on Web 01/24/2002
Interactions of Complex Colloidal Dispersions
mental particle-particle and particle-substrate force interactions of these dispersions are not well characterized, nor are the effects of such forces on adsorption and aggregation behavior. To enhance the specific recognition ability of colloidal particles, a better understanding is needed of how interparticle forces translate to multiparticle phenomena in complex colloidal dispersions. Here we consider a model complex colloidal system of acrylic latex particles in aqueous solution, stabilized by comb polymers incorporating both hydrophilic and hydrophobic side chains as simple “shielding” and “binding” elements, respectively. In addition, a control system, stabilized with combs that incorporate hydrophilic side chains only, is studied for comparison. The force-distance profiles between individual latex particles in water are measured using colloid-probe atomic force microscopy16 (CAFM), which brings fixed particles into close proximity, analogous to surface-force apparatus (SFA) measurement techniques.17 To investigate multiparticle effects, pressure-area isotherms are measured for a 2D dispersion of the latex particles using a Langmuir compression (LC) technique. The combined results of these experiments, explained by a simple analytical model that connects the experimental findings, demonstrate that multiparticle effects play a central role in amplifying the expression of surface chemistry at moderate dispersion densities. Experimental Section Synthesis and Characterization. All reagents were purchased from Aldrich Chemical Co. unless otherwise noted. The comb copolymer stabilizers for both types of latexes were synthesized free radically in solution. For combs incorporated into the hydrophilic control latex, methyl methacrylate (MMA) and poly(ethylene glycol) methyl ether methacrylate, referred to herein as polyoxyethylene methacrylate (POEM, Mn ∼ 1100 g/mol, 23 EO units/mer), were added to excess benzene in equal weight fractions. The comb stabilizer for the model “binding” latex was prepared by adding stearyl methacrylate (StMA, Mn ∼ 339 g/mol, C18H37 side chain) with POEM and MMA in a 1:1:2 weight ratio in benzene. Azo(bis)isobutyronitrile (AIBN) initiator was subsequently added to each mixture at a molar ratio of 20:1 (monomer/initiator). The solutions were degassed under nitrogen for 15 min, followed by polymerization at 60 °C for 16 h. The resulting comb copolymers were purified by repeated precipitations in petroleum ether. Mole fractions of POEM/POEM + StMA along the comb backbones were 11-15%, as determined by proton nuclear magnetic resonance spectroscopy (NMR). NMR spectra were obtained on a Bruker Avance DPX400 proton NMR operating at 400 MHz on polymers in 0.01 g/mL deuterated chloroform solutions. The molecular weights of the purely hydrophilic and mixed side chain comb copolymers were ∼22 000 and ∼25 000 g/mol, respectively, as determined through gel permeation chromatography (GPC) measured against polymethyl methacrylate (PMMA) standards. GPC measurements were performed on 4 mg/mL solutions of the polymers in tetrahydrofuran (THF) using a Waters Associates GPC system with a Waters 510 pump, three Styragel columns connected in series, and a Waters 410 differential refractometer operated at 30 °C with a 1 mL/min flow rate. Latexes were prepared by dispersion polymerization of MMA using the comb copolymers as a stabilizing agent. For each system, approximately 1.25 g of comb copolymer was dissolved in 50 mL of 1:1 by volume ethanol/water solution, followed by addition of 10 mL of MMA monomer and ∼0.950 g of ammonium persulfate initiator. The solutions were degassed under nitrogen for 15 min and then allowed to polymerize at 60 °C for 16-24 h. The resulting latexes were purified by repeated centrifugation and resuspension in fresh 1:1 ethanol/water solution. (16) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 1831-1836. (17) Patel, S. S.; Tirrell, M. Annu. Rev. Phys. Chem. 1989, 40, 597635.
Langmuir, Vol. 18, No. 4, 2002 1099 The morphology and size distributions of the smaller bead dispersions used for LC were assessed by examining beads cast on a substrate, lightly coated with ∼100 Å of gold, with a JEOL 6320 field emission scanning electron microscope (SEM) operating at 4.0 kV accelerating voltage. Larger bead size batches used for CAFM measurements were imaged using optical microscopy to confirm spherical regularity of the particles. Force Measurements with AFM. The “colloid probe” AFM technique, first developed by Ducker, Senden, and Pashley,18 involves the modification of an AFM cantilever tip by attaching to it a colloidal particle. The interactions of the particle with a sample surface can then be recorded in the same manner as for a normal cantilever probe, yielding a force-distance profile. This technique has been used previously for studying polymer latexes19 as well as surfaces with adsorbed and grafted chains.20-23 Force-distance measurements were performed on a Nanoscope III (Digital Instruments) atomic force microscope operating in “force mode”. In force mode the X-Y translational motion of the cantilever is suspended and the sample is moved toward and away from the cantilever by the application of a sawtooth voltage to a piezoelectric part in the base beneath the sample. During this modulation, deflection of the cantilever is measured.24 The acquired data in the format of deflection signal versus z-piezo movement are then translated into force versus distance data25 using determined values for the spring constant of the cantilever. The cantilevers were commercially obtained “v-shaped” tipless silicon nitride (Digital Instruments) in order to eliminate potential interference from the standard probe tip. Spring constants were determined by measuring the native resonant frequency of the cantilevers using the method described by Cleveland et al.26 The colloid probe was prepared by attaching a single latex particle (diameter ∼ 20 µm) to the end of a cantilever using a microdroplet of standard epoxy resin. Attachment was performed under a Nikon type 104 optical microscope using a M3301R manipulator (World Precision Instruments) with one tungsten probe used for placing the droplet and a separate probe for picking up and placing a single colloidal particle onto the droplet. The resin was left to set overnight before measurements were performed. Sample bead surfaces were readied by spin coating a thin layer of PMMA onto glass followed immediately by a dilute concentration of latex particles ( 0.5. To obtain the interaction between colloids of radius R with separation distance D, we use the Derjaguin approximation (D , R) in 2-D:40
U(D) ≈
∫0RUpl dx
2 A0
R1/2 ≈ A0
∫D
D+R
dZUpl(Z) (Z - D)1/2
∫
hard disk, gHD is the hard disk radial distribution function, η ) A0F is the dimensionless packing parameter, F is the surface density of colloids, and the center-to-center distance r ) 2R + D. The above approach is well-known to reproduce good fits for various perturbations to the hard sphere potential.41 The Weeks-Chandler-Anderson (WCA) cutoff is introduced for the interparticle potential,42 that is, rm is the value of r for which U(r) ) UHD + U1(r) is a minimum, or the force f(r) ) 0.
UHD(r) ) U(r) + , ) 0, U1(r) ) -, ) U(r),
r < rm
r > rm r < rm r > rm
(6)
Here, rm approximately corresponds to 2(R + RB). The hard disk pressure PHD from scaled particle theory43 is
PHDA0 η ) kBT (1 - η)2
(7)
(4) and the hard disk radial distribution function44 is
where the area per particle A0 ) πR2. The pair interaction energy and force are plotted in Figure 4 for parameters that are close to those used in the experiments, that is, χBS ) 1.28, a ) 0.25 nm, R ) 1000a, σA ) σB ) 0.1/a, NA ) 60, and NB ) 20. A first-order perturbative 2-D equation of state can be written
PA0 PHDA0 η2 ∂U ) r gHD(r,η)2πr dr kBT kBT 4 ∂r
Figure 4. Dimensionless pair interaction energy (a) and interaction force (b) for χ ) 1.28, NA ) 60, NB ) 20, σA ) σB ) 0.1/a, R ) 1000a, and a ) 0.25 nm.
(5)
where P is the pressure, the subscript HD refers to the
gHD(2R) )
2-η 2(1 - η)2
(8)
(39) De Gennes, P.-G. Adv. Colloid Interface Sci. 1987, 27, 189-209. (40) Israelachvili, J. N. Intermolecular and surface forces, 2nd ed.; Academic Press: San Diego, 1991. (41) See for example: Barker, J. A.; Henderson, D. Rev. Mod. Phys. 1976, 48, 587-671. (42) Weeks, J. D.; Chandler, D.; Andersen, H. C. J. Chem. Phys. 1971, 54, 5237-5347. (43) Helfand, E.; Frisch, H. L.; Lebowitz, J. L. J. Chem. Phys. 1961, 34, 1037-1042. (44) Further, a cutoff value for gHDc ≈ 3 is introduced when gHD > gHDc to numerically handle η f 1.
Interactions of Complex Colloidal Dispersions
Figure 5. Dimensionless pressure (solid) and hard-disk pressure (dotted) vs 1/η: χ ) 1.28, NA ) 60, NB ) 20, σA ) σB ) 0.1/a, R ) 1000a, and a ) 0.25 nm.
For a range of χBS values, eqs 7 and 8 substituted into eq 5 give a pressure isotherm (see Figure 5) resembling the experimental data in Figure 3. In particular, multiparticle effects are shown to greatly magnify the influence of particle surface chemistry at moderate packing densities (0.25 < η < 0.75). At packing densities representing a random-packed state (η ≈ 0.5), the net attraction between hydrophobic chains on opposing surfaces leads to a minimum in the pressure-area profile similar to that seen in the LC decompression isotherm. It should be noted that the experimentally measured osmotic pressure Π roughly corresponds to PA0/AW, where AW is the area per water molecule.45 Using AW ) 10-19 m2, the theoretical magnitudes for Π agree well with those obtained from experiments. As mentioned in the Introduction, the use of “shielding” molecules, for example, pegylation, increases the solubility of the particles by significantly reducing flocculation. We can define flocculation as P(η≈0.9) e P(η≈0.1) and, using the model calculation, predict flocculation for the relevant parameter space. Namely, for large enough χ values and NB g NA, the “shielding” is insufficient and flocculation is observed. Conversely, for small χ values and NB e 10 (for NA ) 60), only hard disk-type behavior is observed, since the tethered “binding” molecules are effectively screened by the “shielding” molecules. Rather than approximating a form for the forces between two particle surfaces, experimental force profiles such as those in Figure 1 can also be used to generate an estimate for the 2-D surface pressure by employing eq 5. Using the data from Figure 1b, predicted Π-A isotherms for the average particle diameters 0.2-20 µm are plotted in Figure 6. We consequently expect a system of small colloidal particles exhibiting such force versus distance profiles to display complex multiparticle behavior arising from the balance between molecular shielding and binding interactions, whereas colloids an order of magnitude larger are predicted to exist in a flocculated state. Conclusion Together with the theoretical analysis developed above, the CAFM and LC experiments described in this work provide a unified picture of how colloidal particle surface chemistry is expressed in the behavior of complex colloidal (45) Hunter, R. J. Foundation in colloid science; Oxford University Press: New York, 1987.
Langmuir, Vol. 18, No. 4, 2002 1103
Figure 6. Dimensionless pressure vs 1/η using data from Figure 1b for the average particle diameters 2R ) 0.2 (solid), 0.5 (‚ ‚ ‚), 1 (- - -), and 20 (- - -) µm from top to bottom, respectively.
dispersions. By preparing a polymer latex stabilized with comb polymers incorporating both hydrophilic and hydrophobic side chains, a model colloid with “shielding” and “binding” interactions was created, as suggested by Balazs and co-workers.10 Multiparticle effects in these colloidal dispersions were found to extend the effective range of these force interactions by 1-2 orders of magnitude, an aspect of colloidal systems not readily appreciated when measuring force profiles between isolated particles or surfaces at a fixed distance. Interpreted in the context of designing colloidal systems with specific recognition ability, the results of the present study suggest a potentially important, though little considered, role of multiparticle effects on recognition processes. Although our first order perturbative calculation uses a hard disk correlation function, the change in this weighting factor as a function of particle density expressly accounts for multiparticle interactions. This effect becomes progressively more important as particle density increases. In cellular environments, for example, approximately 25% of cell weight is comprised of macromolecules and approximately 5% of smaller molecules.46 More importantly for biomedical applications, 40-50% of blood volume consists of red blood cells.47 Colloidal dispersions developed for use in such high-density environments need to exhibit the proper behavior in a crowded system. Here we have shown that model interactions or experimentally measured ones can be used to predict, and hence select, colloidal systems with the desired suspension in moderate density regimes. Acknowledgment. This work was supported by the National Science Foundation under Award DMR-9817735, by the MRSEC program of the National Science Foundation under Award DMR-98-08941, and by the Environmental Protection Agency under Award R82522401-0. The authors acknowledge A. C. Balazs for helpful discussions. LA011466D (46) Alberts, B.; Bray, D.; Lewis, J.; Raff, M.; Roberts, K.; Watson, J. D. Molecular Biology of the Cell, 3rd ed.; Garland Publishing: New York, 1994. (47) Hanson, S. R.; Harker, L. A. Blood Coagulation and Bloods Materials Interactions. In Biomaterials Science: An Introduction to Materials in Medicine; Ratner, B. D., Hoffman, A. S., Schoen, F. J., Lemons, J. E., Eds.; Academic Press: San Diego, 1996; p 194.
