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Electrophoretic Behavior of Bare and Polymer-Coated Latices M. R. Gittings and D. A. Saville* Princeton Center for Complex Materials and the Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544 Received December 31, 1999. In Final Form: May 18, 2000 Polymer-stabilized latex dispersions find many applications but our understanding of such suspensions is limited; data on well-characterized systems can furnish needed insight. To provide this information, the behavior of bare particles and particles coated with adsorbed poly(ethylene oxide) was investigated using electrophoretic mobility and photon correlation spectroscopy measurements. Bare particles show the typical maximum in mobility as a function of salt concentration; adsorbed neutral polymer reduces the mobility and suppresses the features. Hydrodynamic size measurements indicate that the polymer layers are rather thick and almost completely occlude the diffuse charge. Nevertheless, the particles retain a significant electrophoretic mobility, indicating that the electrokinetic shear plane is located somewhere inside the polymer layer. Comparing experimental results with those from an approximate theory for fuzzy particles indicates that the layer’s principal effect is hydrodynamic.
Introduction Surface phenomena engendered by the structure of the particle-solvent interface and the behavior of nearby ions and solvent molecules clearly affect the electrokinetic behavior of colloidal particles. For example, the notion of a layer of loose polymer strands native to the particle surface1-8 has been employed to explain a mystifying maximum in the electrophoretic mobility-ionic strength relation. Surface transport, although absent in the standard theories,9,10 has been used to clarify these features.7,11-13 In addition, latices with a thin grafted polymer layer show considerable departures from smooth particle behavior in both steady and oscillatory fields.14 Alterations in particle morphology arising from heat treatment above the latex glass transition temperature influence electrokinetic behavior.5,14,15 Experiments with greatly exaggerated surface structure are a logical step forward. In this study, fuzzy particles * To whom correspondence should be addresses: Tel: 609-2584585. Fax: 609-258-0211: E-mail:
[email protected]. (1) van der Put, A. G.; Bijsterbosch, B. H. J. Colloid Interface Sci. 1983, 92, 499. (2) Hidalgo Alvarez, R.; de las Nieves, F. J.; van der Linde, A. J.; Bijsterbosch, B. H. Colloids Surf. 1986, 21, 259. (3) van den Hoven, Th. J. J.; Bijsterbosch, B. H. Colloids Surf. 1987, 22, 187. (4) Midmore, B. R.; Hunter, R. J. J. Colloid Interface Sci. 1988, 122, 521. (5) Chow, R. S.; Takamura, K. J. Colloid Interface Sci. 1988, 125, 226. (6) Bastos, D.; Santos, R.; Forcada, J.; Hidalgo-Alverez, R.; de las Nieves, F. J. Colloids Surf. 1994, 92, 137. (7) Verdegan, B. M.; Anderson, M. A. J. Colloid Interface Sci. 1993, 158, 372. (8) Seebergh, J. E.; Berg, J. C. Colloids Surf. 1995, 100, 139. (9) O’Brien, R. W.; White, L. R. J. Chem. Soc. Faraday Trans. 2 1978, 74, 1607. (10) Russel, W. B.; Saville, D. A.; Schowalter, W. R. Colloidal Dispersions; Cambridge University Press: Cambridge, 1992. (11) Zukoski, C. F.; Saville, D. A. J. Colloid Interface Sci. 1986, 114, 32, 45. (12) Elimelech, M.; O’Melia, C. R. Colloids Surf. 1990, 44, 165. (13) Hidalgo-Alvarez, R.; Moleon, J. A.; de las Nieves, F. J.; Bijsterbosch, B. H. J. Colloid Interface Sci. 1992, 149, 23. (14) Rosen, L. A.; Saville, D. A. J. Colloid Interface Sci. 1992, 149, 542. (15) Rosen, L. A.; Saville, D. A. J. Colloid Interface Sci. 1992, 140, 82.
were created by adsorption of neutral poly(ethylene oxide) (PEO) on polystyrene latices. Three molecular weights were employed to give layer thicknesses of 12%, 21%, and 30% of the bare particle radius (in 1 mM NaCl). Photon correlation spectroscopy and electrophoretic mobility measurements were used to study particle behavior. Experimental Techniques and Data Analysis Materials. Polystyrene latices (Interfacial Dynamics Corp. (IDC), Portland, OR) stabilized by negatively charged sulfate groups on the particle surface were used in this study (Table 1). To clean the stock suspension, it was diluted 10-fold using deionized, doubly distilled water and repeatedly centrifuged with a Beckman model L5-65 ultracentrifuge with a type 21 rotor operated at 2800g for 9 h. The supernatant was decanted between centrifugations until its conductivity was within 1% of the suspending medium. A final centrifugation/decantation was used to concentrate the suspension, which showed neither bubble stability nor styrene odor. The suspension was stored at 6 °C. Sodium chloride was chosen as the electrolyte since its tendency to form complexes with PEO is low.16 Analytical grade NaCl (Fluka) was recrystallized and used to make the electrolyte solutions (10-5-10-2 M). Detailed descriptions of the procedures used to purify the suspension, water, and salt are given elsewhere.17,18 Three molecular weights of high-purity (>99%), monodisperse, PEOswith molecular weights (MWs) of 23 500, 56 000, and 93 750swere obtained from Polymer Laboratories, Inc. of Amherst, MA. Table 2 lists batch numbers and polydispersity indices. All PEO solutions and suspensions were stored in a dark refrigerator at 6 °C to minimize photodegradation and bacterial degradation. Preparation of Fuzzy Latex Suspensions. Polymer-coated latex suspensions were prepared by adding 81.6 mL of a 0.694% (by volume) bare latex suspension to 407.9 mL of a 720 ppm PEO solution to yield 489.5 mL of suspension. PEO adsorbs reversibly to the latex surface, and a certain bulk concentration of PEO is necessary to prevent desorption. In this work, PEO bulk concentrations of approximately 600 ppm were used to ensure (16) Braun, D. B. Poly(ethylene oxide). In Handbook of Water-Soluble Gums and Resins; Davidson, R. L., Ed.; McGraw-Hill: New York, 1980. (17) Gittings, M. R. Electrokinetics of Colloidal Particles with Adsorbed Neutral Polymer. Ph.D. Thesis, Princeton University, Princeton, NJ, 1998. (18) Gittings, M. R.; Saville, D. A. Colloids Surf. 1998, 141, 111.
10.1021/la991692b CCC: $19.00 © 2000 American Chemical Society Published on Web 07/11/2000
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Table 1. Particle Suspension Properties from IDC batch number
2-179-4
diameter, transmission electron microscopy surface charge density, conductometric titration polydispersity index particle concentration in the IDC stock suspension particle density at 25 °C
156 nm -1.08 µC/cm2 1.042 8.4% (w/w) 1.057 g/cm3
Table 2. Information on the PEO Purchased from Polymer Labs assigned MW
batch no.
polydispersity
23 500 56 000 93 750
20831-6 20833-6 20835-4
1.03 1.05 1.04
Table 3. Properties of the Concentrated (Stock) Fuzzy Particle Suspensions PEO MW
equilibrium bulk PEO concn (ppm)
surface coverage (mg/m2)
23 500 56 000 93 750
572 570 576
0.67 0.73 0.59
operation in the “plateau” region.19-21 When polymer is adsorbed onto the particles, it is important to add the suspension to the polymer solution drop by drop so each particle is surrounded by a sea of dissolved polymer.22 Gentle stirring ensures that the particles are well-separated during adsorption to minimize bridging flocculation. Following a recommendation by Kato et al.,19 the suspension was stored at 6 °C for 2 days to reach adsorption equilibrium. Afterward, the dilute suspension was concentrated by centrifugation. The supernatant was saved to determine the amount of adsorbed polymer (described below), and the sediments were combined and stored at 6 °C as a stock solution. For simplicity, particles with adsorbed polymer will be called fuzzy particles and their behavior compared to that of their bare counterparts. The photon correlation spectroscopy and electrophoretic mobility measurements were made on diluted suspensions having ion concentrations between 1.5 × 10-6 M H2CO3 (from dissolved atmospheric carbon dioxide) and 10-2 M NaCl. Determination of the Amount of Adsorbed Polymer. The amount of adsorbed PEO was determined (by difference) using a modified version of the Attia-Rubio method.23 Equilibrium PEO concentrations and the surface coverages for each stock latex suspension are listed in Table 3. A surface coverage of 0.60.7 mg/m2 is commensurate with those obtained by others on similar systems.22 No trend in the surface coverage with molecular weight was apparent. Photon Correlation Spectroscopy Measurements. Photon correlation spectroscopy (PCS) experiments employed a Brookhaven Instruments Corp. (Holtsville, NY) BI-2005 goniometer (version 2.0), a BI-2030 digital correlator, and a Lexel argon ion laser (model 85). All data were gathered at 25 °C, a scattering angle of 90°, and a wavelength of 514.5 nm. Volume fractions of 4.7 × 10-6 (bare particles) and 4.5 × 10-6 (fuzzy particles) provided a reasonable signal-to-noise ratio. Volume fractions were kept constant between samples because a small decrease in hydrodynamic size with increasing volume fraction was noted.17,18 The hydrodynamic sizes are ∼3% smaller than the extrapolated values at zero concentration. PCS data were analyzed using the method of cumulants (the Brookhaven BI-2030 program) to determine the hydrodynamic size of the particle via the Stokes-Einstein equation: (19) Kato, T.; Nakamura, K.; Kawaguchi, M.; Takahashi, A. Polym. J. 1981, 113, 1037. (20) Cosgrove, T.; Crowley, T. L.; Ryan, K.; Webster, J. R. P. Colloids Surf. 1990, 51, 255. (21) Cosgrove, T.; Crowley, T. L.; Vincent, B. In Adsorption from Solution; Ottewill, R. H., Rochester, C. H., Smith, A. L., Eds.; Academic Press: New York, 1983; p 287. (22) Pelssers, E. G. M.; Cohen Stuart, M. A.; Fleer, G. J. Colloids Surf. 1989, 38, 15. (23) Attia, Y. A.; Rubio, J. Br. Polym. J. 1975, 7, 135.
Do ) kBT/6πηah
(1)
Here, Do is the measured particle diffusivity, ah is the particle’s hydrodynamic radius, kB is Boltzmann’s constant (1.381 × 10-23 J/K), T is the absolute temperature, and η is the viscosity of the suspending medium. All the data represent an average of at least five measurements, where the difference between measured and calculated baselines was less than 0.1%. Electrophoretic Mobility Measurements. Steady-field mobilities were determined at various ionic strengths with a Coulter DELSA440 at an electric field of 4.0 V/cm. A series of mobility measurements were made at 11 positions spanning the cell, and data were fit to a parabola to describe the particle velocity field across the cell. R2 values showing the “goodness-of-fit” were better than 0.98. From the parabola, the electrophoretic mobility of the particles was determined. Reproducibility was often within 2% and usually better than 5%.
Experimental Results Photon Correlation Spectroscopy. PCS measurements on bare and fuzzy latices (Figure 1) show that the size of the bare particles decreases with increasing ionic strength, approaching the value measured by transmission electron microscopy (TEM). Part of the difference between PCS and TEM sizes is attributable to particle shrinkage or collapse of surface asperities during drying for a TEM measurement. A portion may also stem from drag associated with the ion cloud (especially below 10-4 M NaCl). A discussion of the bare particle behavior is given elsewhere.18 Although the bare particle size changes systematically with salt strength, the fuzzy particle sizes change very little. Fuzzy particles with MW 23 500, 56 000, and 93 750 polymer show deviations of roughly 2% from mean diameters of 179, 190, and 203 nm, respectively. Figure 1 indicates that the adsorbed layer thickness increases with PEO molecular weight. To establish that the adsorbed amounts are consistent with the work of others, the relationship between the logarithm of the adsorbed layer thickness (δh) and the MW of the adsorbing neutral polymer was investigated. Accordingly, data at different ionic strengths were used to compute parameters in the formula
log δh ) b log MW + C
(2)
For example, our b value for 1 mM NaCl/600 ppm PEO is 0.66. This is comparable to scaling-law predictions for polymers adsorbed in train, loop, and tail configurations.24 The b value depends on the ionic strength and varies from 0.40 to 0.66 over the experimental concentration range. Electrophoretic Mobility. Figure 2 shows electrophoretic mobilities at different ionic strengths. Bare particle behavior, ζ-potentials, and surface charge densities were discussed in an earlier report.18 Similar trends have been encountered by other investigators.1-8,11,12,14,25-33 (24) de Gennes, P. G. Adv. Colloid and Interface Sci. 1987, 27, 189. (25) Goff, F. R.; Luner, P. J. Colloid Interface Sci. 1984, 99, 469. (26) Zukoski, C. F.; Saville, D. A. J. Colloid Interface Sci. 1985, 107, 322. (27) Bonekamp, B. C.; Hidalgo Alverez, R.; de las Nieves, F. J.; Bijsterbosch, B. H. J. Colloid Interface Sci. 1987, 118, 366. (28) Voegtli, L. P.; Zukoski, C. F. J. Colloid Interface Sci. 1991, 141, 92. (29) de las Nieves, F. J.; Daniels, E. S.; El-Aasser, M. S. Colloids Surf. 1991, 60, 107. (30) Dunstan, D. E.; Saville, D. A. J. Chem. Soc., Faraday Trans. 1992, 88, 2031. (31) Bastos, D.; de las Nieves, F. J. Colloid Polym. Sci. 1993, 271, 860. (32) Ferna´ndez Barbero, A.; Martı´nez Garcı´a, R.; Cabrerizo Vı´lchez, M. A.; Hidalgo-Alverez, R. Colloids Surf. 1994, 92, 121.
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Figure 3. Effect of the bulk PEO concentration on the mobility of MW 56 000 fuzzy latices in 1 mM NaCl. Figure 1. Hydrodynamic size of bare and fuzzy latices as a function of ionic strength: (O) bare; (b) MW 23 500 fuzzy; (9) MW 56 000 fuzzy; (2) MW 93 750 fuzzy. The solid lines represent the average sizes for the fuzzy particles. The line at 156 nm represents the hard core diameter of the bare particle as determined from transmission electron microscopy.
Figure 2. Electrophoretic mobilities of bare and fuzzy latices as a function of ionic strength: (O) bare; (b) MW 23 500 fuzzy; (9) MW 56 000 fuzzy; (2) MW 93 750 fuzzy.
With the fuzzy particles, two trends are clear. First, the mobilities of the fuzzy particles are lower and decrease with increasing adsorbed layer thickness (except at the lowest ionic strength). Part of this decrease is due to the increase in drag arising from the larger “effective” particle size. However, the decrease in mobility is larger than can be explained purely on the basis of an increased Stokes drag on a larger particle with the same charge. Other processes must slow particle movement. Figure 2 also illustrates differences between the ionic strength dependence of bare and fuzzy particles. It is clear that the ionic strength has an important effect. The fuzzy particle mobilities display a less pronounced maximum that shifts to lower ionic strengths with increasing adlayer thickness. With the two thinner layers, vestiges of the bare particle behavior remain; for the thickest layer they are almost imperceptible. The shift in the maximum is particularly evident when one compares the degree to which the adsorbed layer affects particle movement at a given ionic strength. If the mobility were simply reduced due to increased drag by the polymer, the bare particle profile would have shifted (in the vertical direction) in proportion to the layer thickness. However, data at 10-5 M NaCl show an adlayer thickness effect smaller than at 10-3 M NaCl. Although this is clearly evident in the data, we have no explanation for the behavior at present. A more comprehensive model would help address this issue. (33) Midmore, B. R.; Pratt, G. V.; Herrington, T. M. J. Colloid Interface Sci. 1996, 170.
Effects of Bulk PEO Concentration on Mobility. The influence of PEO on the electrokinetic behavior of the particles was investigated by varying the concentration of MW 56 000 PEO from 400 to 800 ppm in 1 mM NaCl. The results (Figure 3) show that the mobility decreases by ∼15% as the bulk PEO concentration doubles from 400 to 800 ppm. These changes are larger than can be explained simply by an increase in viscosity due to PEO.34 Although the presence of bulk PEO alters the electrophoretic mobility of the suspending solution, it is unlikely that bulk PEO is responsible for the differences between bare and fuzzy particle mobilities. Discussion The presence of an adsorbed layer of polymer clearly alters the behavior of suspended particles, and the change depends, to one degree or another, on how the layer impedes the motion of solvent and ions. Judging from the PCS measurements, the diffuse double layer appears to be almost completely occluded by the polymer layer. If this is the case, the effective charge of the particle and the electrophoretic mobility would both be very small. Clearly the presence of polymer alters the position of the shear surface; however, the hydrodynamic shear surface (as determined by PCS) may not be coincident with the electrokinetic shear surface. Several questions arise: To what extent does the adsorbed polymer impede the motion of solvent and ions? Where is the effective electrokinetic shear surface located? Do polarization and relaxation processes play important roles in electrophoretic motion with fuzzy particles? A Simple Interpretation. In the standard electrokinetic model, the ζ-potential is the potential at the hydrodynamic shear surface (defined by PCS), and charge behind the shear surface is immobile. With bare particles, the hydrodynamic shear surface is near the geometric surface of the particle; with fuzzy particles it is nearer the edge of the polymer layer. Although one can use the standard electrokinetic model to compute the ζ-potential or charge from the mobility measured for bare particles, the situation is more involved for a particle with adsorbed polymer. Given that the adsorbed polymer is uncharged and the particle’s surface charge is not altered by the addition of polymer, the potential at the apparent shear surface of a fuzzy particle (the ζ-potential) should be lower than that of its bare counterpart. In this picture, the effective particle charge corresponds to the potential at the shear surface. We explored this scenario with the following methodology. (34) For example, using a radius of gyration of 4.6 nm for MW 56 000 PEO (interpolated from the data of Kato et al.19) gives a volume fraction of 0.004 for an 800 ppm PEO solution. On the basis of the Einstein viscosity formula, the effect of PEO “particles” should be insignificant.
Electrophoretic Behavior of Latices
Figure 4. A comparison of measured mobilities for the fuzzy (MW 56 000) latices with mobilities calculated in different ways: mobilities calculated for charge densities estimated using the lower (3) and upper (4) branches of the mobility-ζ-potential relation (see the text); (9) experimental values.
From a bare particle electrophoretic mobility measurement at a given ionic strength, ζ-potentials are computed on the “lower” and “upper” branches of the mobility-ζpotential relation.9 These define the charge on the latex surface of the particle. Then, from the equilibrium GouyChapman model, potentials were computed at positions corresponding to the shear surface of a fuzzy particle for each surface charge using White’s asymptotic formula35 for the potential surrounding a spherical particle.36 The effective ζ-potentials calculated in this way decrease with increasing salt concentration and approach values close to zero. At the highest salt strengths, the ζ-potentials are low (
