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Altering Surface Charge Nonuniformity on Individual Colloidal Particles Jason D. Feick,† Nkiru Chukwumah,‡ Alexandra E. Noel,§ and Darrell Velegol* The Pennsylvania State University, Department of Chemical Engineering, University Park, Pennsylvania 16802 Received August 22, 2003. In Final Form: December 19, 2003 Charge nonuniformity (σζ) was altered on individual polystyrene latex particles and measured using the novel experimental technique of rotational electrophoresis. It has recently been shown that unaltered sulfated latices often have significant charge nonuniformity (σζ ) 100 mV) on individual particles. Here it is shown that anionic polyelectrolytes and surfactants reduce the native charge nonuniformity on negatively charged particles by 80% (σζ ) 20 mV), even while leaving the average surface charge density almost unchanged. Reduction of charge uniformity occurs as large domains of nonuniformity are minimized, giving a more random distribution of charge on individual particle surfaces. Targeted reduction of charge nonuniformity opens new opportunities for the dispersion of nanoparticles and the oriented assembly of particles.
Introduction Particles and cells become charged when immersed in an aqueous medium. This happens, for example, as surface groups dissociate or specific ions adsorb to the particle surface.1 As a result of these charge groups, an electrostatic surface potential arises, usually causing a repulsive force between suspended particles that reduces aggregation in suspension. Over the past 60 years, various models have described these electrostatic forces between surfaces, including the well-known Derjaguin-Landau-VerweyOverbeek (DLVO) model.2 But these models assume a uniform surface charge density on the particle surfaces, and recent theory3,4 shows that assuming a different boundary condition, a nonuniform surface charge distribution, often significantly reduces the repulsive forces predicted by the classical models. Thus, charge nonuniformity might explain the unexpected colloidal instability often seen experimentally for Brownian particles.5-7 Figure 1 shows a schematic of a randomly charged spherical particle. Although measurements of charge nonuniformity have been done on flat surfaces8,9 and artificially created nonuniform surfaces,10 experimental difficulties have impeded measurements of charge nonuniformity on suspended micron size particles.11 With * To whom correspondence should be addressed. E-mail: velegol@ psu.edu. † Current address: E Ink Corp., 733 Concord Avenue, Cambridge, MA 02138. ‡ Current address: Amgen, 40 Technology Way, West Greenwich, RI 02817. § Current address: ExxonMobile Research and Engineering Co., 3225 Gallows Rd, Fairfax, VA 22037. (1) Russel, W. B.; Saville, D. A.; Schowalter, W. R. Colloidal Dispersions; Cambridge University Press: New York, 1989, with corrections 1991; examples on p 277. (2) Israelachvili, J. N. Intermolecular & Surface Forces, 2nd ed.; Academic Press: New York, 1992. (3) Miklavic, S. J.; Chan, D. Y. C.; White, L. R.; Healy, T. W. J. Phys. Chem. 1994, 98, 9022. (4) Velegol, D.; Thwar, P. K. Langmuir 2001, 17, 7687. (5) Parker, J. L.; Claesson, P. M.; Attard, P. J. Phys. Chem. 1994, 98, 8468. (6) Suresh, L.; Walz, J. Y. J. Colloid Interface Sci. 1996, 183, 199. (7) Behrens, S. H.; Christl, D. I.; Emmerzael, R.; Schurtenberger, P.; Borkovec, M. Langmuir 2000, 16, 2566. (8) Butt, H.-J. Biophys. J. 1992, 63, 578. (9) Rotsch, C.; Radmacher, M. Langmuir 1997, 13, 2825. (10) Chen, J. Y.; Klemic, J. F.; Elimelech, M. Nano Lett. 2002, 2, 393.
Figure 1. Schematic of a randomly charged particle (radius R). (a) The dots symbolize 64 randomly placed charges, which in turn give rise to a surface potential. Imposed gridlines simply guide the eye for local area averaging. (b) A local area average taken with a length scale R/8. This local averaging enables us to define an average and standard deviation of surface charge density or surface potential. The dark regions have a high local charge density. For square regions with edge R/8, there are N ) 804 regions (53 shown here). (c) A local area average taken with a length scale R/4 (N ) 201 regions). When observing larger regions, variations appear to “gray out”.
atomic force microscopy, only the top of the particle can be examined. Furthermore, the effective size of the tip can often be too large to obtain the required spatial resolution. The various spectroscopies either lack the required spatial resolution (e.g., X-ray photoelectron spectroscopy (XPS), attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy) or must be done in a vacuum (e.g., Auger, time-of-flight secondary ion mass spectrometry (ToF-SIMS)) and therefore do not allow nanoscale measurement of charge nonuniformity on particles in aqueous media. Nor does translational electrophoretic mobility have the sensitivity to reveal charge nonuniformity on individual particles.19 The essential physics that now enables us to measure the charge nonuniformity is that while uniformly charged particles with thin electrical double layers do not rotate by electrophoresis (e.g., doublet b in Figure 2), regardless of their shape,12 nonuniformly charged particles (e.g., doublet a in Figure 2) do rotate in an applied electric field (E∞).13 Since shape does not affect the physics giving our measurement12 but does enable us to visualize rotations using optical microscopy, we examine colloidal homodou(11) Schellenberger, K.; Logan, B. E. Environ. Sci. Technol. 2002, 36, 184. (12) Morrison, F. A., Jr. J. Colloid Interface Sci. 1970, 34, 210. (13) Fair, M. C.; Anderson, J. L. Langmuir 1992, 8, 2850.
10.1021/la0355545 CCC: $27.50 © 2004 American Chemical Society Published on Web 03/11/2004
Altering Surface Charge Nonuniformity
Figure 2. Light microscope images of two “uniformly charged” homodoublets in 10 mM KCl solution. The applied electric field is 10 V/cm, and the average ζ potential of the singlets (4.3 µm spheres) in suspension is -123 mV. Doublet a rotated clockwise in the electric field by rotational electrophoresis, revealing its charge nonuniformity. Doublet b did not rotate, indicating that it was uniformly charged. Since doublets a and b are in the same shear plane, doublet b also verifies that the rotations are not due to fluid shear rotation (ref 16), dielectrophoretic rotation (E∞2 effects) (ref 17), or Brownian motion.
blets. Orientation changes of spheres have been visualized in the literature (e.g., ref 14, where the sphere was tagged, or ref 15, which describes the classic experiments of Perrin), but the use of doublets requires no visualization marker (e.g., a fluorophor). Recently our lab group has developed the experimental technique of “rotational electrophoresis” and used it to show that individual 4.3 µm polystyrene latex (PSL) particles have a significant amount of charge nonuniformity (50-75% of the average ζ potential, or σζ ) 0.500.75ζ0).18,19 This large observed charge nonuniformity prompted two questions: (1) Do the doublets have a random distribution of charge, or do large domains of charge nonuniformity exist on the particles (in the limit, the doublets would be simply composed of two different but uniform spheres)? (2) Can polyelectrolytes or surfactants reduce the charge nonuniformity on the doublets? In this paper, we describe how we have used rotational electrophoresis to show that the adsorption of molecular additives (polyelectrolytes and surfactants) to bare PSL particles can reduce the native charge nonuniformity on sulfated latices by as much as 80%, while in some cases not changing the average zeta potential (ζ0) to any significant extent. The reduction of charge nonuniformity occurs as large domains of nonuniformity are minimized, giving a more random distribution of charge on individual particle surfaces. Methods and Materials The rotational electrophoresis of doublets occurs until the net dipole moment (D) on the doublet is parallel to E∞ (Figure 3). Since the vector orientation (e) of colloidal doublets is readily visible in a light microscope, we observe the angular velocity (Ω) of individual polystyrene latex doublets (diameter of each sphere ) 4.3 µm) in E∞ ≈ 10 V/cm. The particles used in the rotational electrophoresis experiments were 4.3 µm sulfated polystyrene (PS) latex particles (Interfacial Dynamics Corp., Portland, OR, batch 1314,1). The particles were cleaned using sedimentationdecantation.20 MilliQ deionized (DI) water (1 µS/cm) was used for all solutions. Glassware was cleaned using sonication with VWR Aquasonic cleaning solution followed by scrubbing with Alconox cleaner. In lieu of scrubbing with Alconox, the electrophoresis cell was sonicated and soaked in 15.8 N nitric acid overnight. (14) Lettinga, M. P.; van Zandvoort, M. A. M. J.; van Kats, C. M.; Philipse, A. P. Langmuir 2000, 16, 6156. (15) Perrin, J. Les Atomes, 11th ed.; Translation: Atoms, by Hammick, D. L. L.; Ox Bow Press: Woodbridge, CT, 1990. (16) Nir, A.; Acrivos, A. J. Fluid Mech. 1973, 59, 209. (17) Baloch, K. M.; van de Ven, T. G. M. J. Colloid Interface Sci. 1989, 129, 91. (18) Feick, J. D.; Velegol, D. Langmuir 2002, 18, 3454. (19) Feick, J. D.; Velegol, D. Langmuir 2000, 16, 10315. (20) Zukoski, C. F., IV; Saville, D. A. J. Colloid Interface Sci. 1985, 107, 322.
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Figure 3. Rotational electrophoresis of a colloidal doublet. In this schematic, the ζ potential dipole (D) of the doublet is not parallel with the center-to-center vector (e). Dilute colloidal suspensions (∼0.05%V) were made by mixing the particles into an unbuffered (pH of 4-6) 10 mM KCl solution along with one of the surface additives. The surface additives used were the anionic polyelectrolyte sodium polystyrene sulfonate (NaPSS, Aldrich 24,305-1), the cationic polyelectrolyte polyethylenimine (PEI, Sigma P-3143), the anionic surfactant sodium dodecylbenzene sulfonate (SDBS, Aldrich 28,995-7), the cationic surfactant cetyltrimethylammonium bromide (CTAB, Aldrich 85,582-0), and the nonionic surfactant Triton X-100 (TX-100, EM Science TX 1568-1). NaPSS and PEI had molecular weights of 70 000 and 750 000, respectively. Three experimental trials were run for the bare particle systems, while two were run for each of the additive systems. For the bare particle system, trials 1 and 2 were allowed to aggregate for 14 h, while trial 3 was for 8 h. The cationic CTAB and PEI systems were allowed to aggregate for 2 h, while the NaPSS and TX-100 systems aggregated for 24 h, and SDBS for 50 h. The SDBS system was allowed to aggregate for 50 h because very few doublets were found at 24 h (Smoluchowski rapid flocculation time for the bare particle system ≈ 3.5 h). The rotational electrophoresis experiments were done in a custom microelectrophoresis apparatus consisting of three main parts: (1) the microelectrophoresis cell and blackened electrodes, (2) a Keithley 2410 current source, and (3) a Nikon TE 300 video microscope. The cell consisted of two Pyrex glass cylinders connected by a borosilicate capillary tube (0.2 mm thick i.d. × 2 mm wide i.d. × 5 cm long, Vitro Com, Mountain Lakes, NJ), all fastened to a microscope slide for mechanical stability. To minimize settling effects, the video microscope was placed on its back using an aluminum support system. The average ζ potentials of the particles were measured using a ZetaPALS. All experiments were conducted at 20-25 °C, with variations of
