Langmuir 2001, 17, 5225-5231
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Aggregation of Colloidal Particles in the Presence of Oppositely Charged Polyelectrolytes: Effect of Surface Charge Heterogeneities Frederic Bouyer, Andrea Robben, Wei Li Yu, and Michal Borkovec* Center for Advanced Materials Processing and Department of Chemistry, Clarkson University, Box 5814, Potsdam, New York 13699 Received April 14, 2001. In Final Form: June 13, 2001 We study the early stages of flocculation in suspensions of charged colloidal particles in the presence of oppositely charged polyelectrolytes. Absolute aggregation rate constants are determined by time-resolved static and dynamic light scattering. The aggregation rates of amidine and sulfate polystyrene latex particles are measured in the presence of various polyelectrolytes as a function of the polymer dose and ionic strength. The suspension stability sensitively depends on both parameters and is mainly controlled by electrostatic forces. The polymer dose determines the overall particle charge, as revealed by electrophoretic mobility measurements. Near the isoelectric point, one observes fast aggregation, while slow aggregation dominates farther away from this point. The ionic strength strongly influences the suspension stability through screening of electrostatic interactions. With increasing ionic strength, the width of the fast aggregation regime increases, and in the slow regime the sensitivity of the rate constant on the polymer dose decreases. In the fast regime, the decrease of the ionic strength leads to a substantial enhancement of the aggregation rate constant. This effect is characteristic for charge patch flocculation and results from the presence of surface charge heterogeneities.
Introduction Interactions between particles and polymers represent a major theme in modern colloid science. While polymermediated forces dominate uncharged systems,1,2 the most important forces in charged systems are of electrostatic origin. For example, this observation applies to depletiontype interactions occurring in mixtures of particles and polyelectrolytes with equal sign of charge,3-6 as well as to aggregation phenomena in suspensions of charged particles induced by polyelectrolytes of opposite charge.7-10 In the present paper we discuss the latter case in substantial detail and demonstrate the major relevance of electrostatic interactions in the early stages of the aggregation process of colloidal particles in the presence of oppositely charged polyelectrolytes. We shall not only point out the important analogies to charge-stabilized systems but also discuss the differences originating from attractive interactions due to charge heterogeneities.7,11 The subject has a long history, and there is much interest in this topic due to various engineering applications such as wastewater treatment, mineral processing, ceramics manufacture, and papermaking.12-16 Nevertheless, it is * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Russel W. B.; Saville D. A.; Schowalter W. R. Colloidal Dispersions; Cambridge University Press: Cambridge, 1989. (2) Napper D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: London, 1984. (3) Ferreira, P. G.; Dymitrowska, M.; Belloni, L. J. Chem. Phys. 2000, 113, 9849-9862. (4) Piech, M.; Walz, J. Y. J. Colloid Interface Sci. 2000, 232, 86-101. (5) Milling, A. J.; Kendall, K. Langmuir 2000, 16, 5106-5115. (6) Sharma, A.; Tan, S. N.; Walz, J. Y. J. Colloid Interface Sci. 1997, 191, 236-246. (7) Gregory J. J. Colloid Interface Sci. 1973, 42, 448-456. (8) Ferretti R.; Zhang J.; Buffle J. Colloids Surf., A 1997, 121, 203215. (9) Ashmore, M.; Hearn, J. Langmuir 2000, 16, 4906-4911. (10) Walter, H. W.; Grant S. B. Colloids Surf., A 1996, 119, 229-239. (11) Miclavic, S. J.; Chan, D. Y. C.; White, L. R.; Healy, T. W. Langmuir 1994, 98, 902-9032.
becoming clear that the use of modern instrumentation and well-defined model systems represents the key to further progress. Despite the availability of reliable and noninvasive techniques for probing particle aggregation,21-26 such phenomena are still often being studied by sedimentation or spectrophotometric techniques. Such results are notoriously difficult to interpret, and any precise information on particle aggregation cannot be extracted with confidence. In the present study we use time-resolved simultaneous static and dynamic light scattering (SSDLS)24-26 to measure the absolute aggregation rate constants of the aggregation of charged submicrometer particles in the presence of oppositely charged polyelectrolytes. Welldefined negatively and positively charged polystyrene latex particles in conjunction with different types of polyelectrolytes are used to demonstrate that the aggregation follows qualitatively the same pattern in all systems considered. Adsorption of polyelectrolytes to oppositely charged surfaces usually leads to charge overcompensation and charge reversal. A colloidal suspension is therefore unstable near its isoelectric point (IEP) and is stable farther away. This stability pattern closely resembles the stability of charged amphoteric particles and represents a manifestation of the aggregation mechanism discussed within the theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO).27,28 Several studies have confirmed the same pattern in the presence of polyelec(12) Horn, D. In Polymeric Amines and Ammonium Salts; Goethals, E. J., Ed.; Pergamon Press: Oxford, 1980. (13) Gregory, J. In Industrial Water Soluble Polymers; Fitch, C. A., Ed.; The Royal Society of Chemistry: London, 1996. (14) Purchas, D. B. In Handbook of Water Purification, 2nd ed.; Lorch, W., Ed.; John Wiley: New York, 1987. (15) Bunker, D. Q., Jr.; Edzwald, J. K.; Dahlquist, J.; Gillberg, L. Water Sci. Technol. 1995, 31, 63-71. (16) Pugh, R. J. In Surface and Colloid Chemistry in Advanced Ceramics Processing; Pugh, R. J., Bergstrom, L., Eds.; Marcel Dekker: New York, 1994.
10.1021/la010548z CCC: $20.00 © 2001 American Chemical Society Published on Web 07/27/2001
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trolytes and demonstrated that the IEP typically lies within the region of fast aggregation.7-10 If the interactions are indeed dominated by electrostatics, the stability of these systems must be sensitive to the ionic strength as well. The present study describes several characteristic trends with the ionic strength and provides a qualitative explanation based on analogies with chargestabilized systems. Probably the most interesting finding from the present study is the observation of a substantial enhancement of the aggregation rate constant in the presence of polyelectrolytes in the fast regime. This enhancement is highly specific to polyelectrolyte-induced flocculation and represents evidence of the attractive forces due to surface charge heterogeneities, as originally suggested by Gregory.7 Experimental Section Materials. Surfactant-free amidine and sulfate latex particles were purchased from Interfacial Dynamics Corporation (Portland, OR). The sulfate particles have a diameter of 170 nm, a coefficient of variation of 2.3%, and a surface charge density of 16 m Cm-2. The amidine particles have a diameter of 190 nm, a coefficient of variation of 3.0%, and a surface charge density of 42 m Cm-2. The particles were dialyzed for several weeks in deionized water from a Barnstead Nanopure apparatus (UV/UF, Dubuque, IA) until the conductivity did no longer appreciably differ from that of pure water. Several polyelectrolytes have been studied. Cationic branched polyethylene imine (PEI) was used in the form of the commercial product Lupasol P. This polymer is highly branched and has a broad molar mass distribution with an average molecular mass of about 6 × 105 as determined by light scattering.17 We have also studied the high-molecular weight fraction of PEI of average molecular weight around 5 × 106 separated by ultrafiltration and a linear poly(vinylamine) (PVA) with a molecular weight of 4 × 105. All these polymers were supplied by the BASF Corp. (Ludwigshafen, Germany). Fifth generation poly(propylene imine) dendrimers (PPID) with 1,4- diaminobutate as core molecule and a molecular mass of 7.2 × 103 were synthesized by the DSM Corp. (Geleen, The Netherlands). The only anionic polyelectrolyte used in this study was sodium polystyrene sulfonate (PSS) with a molecular mass of 3.1 × 105 purchased from Polymer Standard Corp. (Mentor, OH). All studies were performed in KCl at pH ) 4, adjusted by HCl. In this way one can achieve a moderate buffer capacity without the use of interfering buffers. Under these conditions all polyamines are fully charged,18,19 with the exception of PEI, which has a degree of ionization of about 80% at this pH value.20 Being a strong polyelectrolyte, PSS is fully charged at pH ) 4. Light Scattering. Particle aggregation was studied by timeresolved light scattering. Absolute aggregation rate constants were measured by the recently developed SSDLS technique.24-26 The measurements were carried out on a multiangle light scattering instrument (ALV/CGS-8, Langen, Germany). The instrument has eight independent fiber optic detectors on a (17) Alince, B.; van de Ven, T. G. M. J. Colloid Interface Sci. 1993, 155, 465-470. (18) van Duijvenbode, R.; Borkovec, M.; Koper G. J. M. Polymer 1997, 39, 2657-2664. (19) Katchalsky, A.; Mazur, J.; Spitnik, P. J. Polym. Sci. 1957, 23, 513-521. (20) Borkovec, M.; Koper, G. J. M. Macromolecules 1997, 30, 21512159. (21) Lips, A.; Willis, E. Trans. Faraday Soc. 1973, 69, 1226-1236. (22) van Zanten, J. H.; Elimelech, M. J. Colloid Interface Sci. 1992, 154, 1-7. (23) Lichtenbelt, J. W.; Ras, H. J. M.; Wiersema, P. H. J. Colloid Interface Sci. 1974, 49, 281-285. (24) Holthoff, H.; Egelhaaf, S. U.; Borkovec, M.; Schurtenberger, P.; Sticher, H. Langmuir 1996, 12, 5541-5549. (25) Holthoff, H.; Schmitt, A.; Ferna´ndez-Barbero, A.; Borkovec, M.; Cabrerızo-Vılchez, M. A.; Schurtenberger, P.; Hidalgo-A Ä lvarez, R. J. Colloid Interface Sci. 1997, 192, 463-470. (26) Holthoff, H.; Borkovec, M.; Schurtenberger, P. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 1997, 56, 69456953.
Bouyer et al. rotating goniometer and uses a krypton ion laser operating at a wavelength of 647 nm (Innova 301, Coherent, CA). Static and dynamic light scattering can be thus performed at eight independent angles in a simultaneous fashion whereby the angular resolution can be increased by repositioning the goniometer. For time-resolved dynamic light scattering measurements, scattered light was detected at 90° with a pseudo-crosscorrelation detection. The temperature of the cell was 25 °C. All glassware was cleaned either in concentrated HCl or in a mixture of hot H2O2 and H2SO4. The latex suspension and polymer solution were prepared at appropriate concentrations. The ionic strength was adjusted with KCl and adjusted to pH ) 4 with HCl, and all solutions were equilibrated at 25 °C prior to use. The aggregation process was initiated by adding the required amount of the polymer solution (0.2-1 mL) to the latex suspension (3-4 mL) in the light scattering cell. The loss of the polyelectrolyte due to adsorption on the glass surface of the vial was found to be negligible. After the cell was gently shaken, the aggregation was monitored by time-resolved light scattering. For experiments in the fast aggregation regime, the particle concentration was chosen to be around 1.5 × 1014 m-3 (∼0.4 mg/L) and the signal was accumulated every 12 s. For slower aggregation, the particle concentration was progressively increased and the accumulation time was extended to 20 s. Absolute aggregation rate constants were measured by SSDLS.24-26 By analyzing the relative rate of change of the static light scattering intensity, I(q,t), one can show that the aggregation rate constant k is given by21
|
1 dI(q,t) I(q,0) dt
tf0
(
) kN0
I2(q)
2I1(q)
)
-1
(1)
where N0 is the initial particle concentration per unit volume, q is the magnitude of the scattering vector, and I1(q) and I2(q) are the scattering intensities of the monomers and dimers, respectively. For dynamic light scattering, one measures the apparent hydrodynamic radius, rh(q,t), which is again related to the rate constant by24
(
|
1 drh(q,t) rh(q,0) dt
) kN0 1 -
tf0
)
rh,1 I2(q) rh,2 2I1(q)
(2)
where rh,1 and rh,2 are the hydrodynamic radii of the monomers and dimers, respectively. By combining eqs 1 and 2 one obtains24
| (
1 dI(q,t) I(q,0) dt
) 1-
tf0
)
rh,1 rh,2
-1
|
1 drh(q,t) rh(q,0) dt
tf0
- kN0 (3)
A plot of the relative rate of change of the scattering intensity as a function of the relative rate of change of the hydrodynamic radius should be linear. The intercept yields the absolute rate constant k, and the slope yields the relative hydrodynamic radius of the dimer rh,2/rh,1. The advantage of this method is that there are no assumptions being made about the optical and hydrodynamic properties of the aggregates. Once the constancy of these quantities has been established, the aggregation rate constant can be more easily measured by time-resolved dynamic light scattering together with eq 2 by choosing a suitable reference system. It is customary to report the stability ratio
W)
kfast k
(4)
where kfast is the reference aggregation rate constant. The latter is chosen in the fast aggregation regime in the presence of a sufficient amount of salt. Conventional dynamic light scattering was further used to measure the thickness of adsorbed polymer layers. The thickness was obtained from the difference between the hydrodynamic radii, with and without added polymer. To avoid particle aggregation, the particle concentration had to be decreased to 1.5 × 1013 m-3 (∼0.04 mg/L) and the signal was accumulated up to 20 min. In most cases, the presence of an adsorbed layer could not be
Aggregation of Colloidal Particles
Langmuir, Vol. 17, No. 17, 2001 5227 Table 1. Simultaneous Static and Dynamic Light Scattering Results in the Fast Aggregation Regime ionic strengtha (mM)
polymer dose mg/g of latex
aggregation rate constantb k (m3 s-1)
10 0.1 200
2.0 2.0 0.0
3.2 × 1018 4.8 × 1018 3.2 × 1018
b
Figure 1. SSDLS data in an aggregating sulfate latex suspension in the presence of branched PEI. The relative rates of changes of the scattering intensity and of the hydrodynamic radius are plotted. The intercept yields the aggregation rate, and the slope yields the relative hydrodynamic dimer radius. Ionic strength is adjusted with KCl, (a) 10 and (b) 0.1 mM. established with confidence or one could barely detect its presence (layer thickness < 4 nm). The polymer layer could be clearly detected by dynamic light scattering in the case of PEI, where layers up to 10 nm could be observed. The formation of the layer manifests itself in an initial transient around 5-10 min in the apparent hydrodynamic radius. This transient has to be taken into account when extracting the aggregation rate for the timeresolved light scattering data. Electrophoretic Mobility. Electrokinetic experiments were carried out on a laser Doppler velocimetry setup (Delsa 440C, Coulter-Beckmann Instruments) operating at 30 V/cm and at a modulating frequency of 1 Hz. The solutions were prepared in the same fashion as for the aggregation experiments. The final particle concentrations were about 4 × 1016 m-3 (∼100 mg/L).
Results and Discussion Absolute Aggregation Rate Constants. SSDLS was used for the measurements of the aggregation rate constants. The relative rates of change of the scattered intensity and of the hydrodynamic radius were determined. The results for the sulfate latex particles in the presence of branched PEI are shown in Figure 1. The polymer concentration was 2 mg/g of latex and two ionic strengths were studied. Figure 1a shows the data for 10 mM, while Figure 1b refers to 0.1 mM. From the linear plots one can extract the relative hydrodynamic dimer
stability ratio W SSDLS DLS 0.99 0.67 1.00
0.95 0.65 1.00
a Ionic strength was controlled by addition of KCl at pH ) 4. The error bars are
