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2009, 113, 8458–8461 Published on Web 06/01/2009
Attractive Electrostatic Forces between Identical Colloidal Particles Induced by Adsorbed Polyelectrolytes Ionel Popa, Graeme Gillies,† Georg Papastavrou, and Michal Borkovec* Department of Inorganic, Analytical, and Applied Chemistry, UniVersity of GeneVa, Sciences II, 30, Quai Ernest-Ansermet, CH-1211 GeneVa 4, Switzerland ReceiVed: May 1, 2009; ReVised Manuscript ReceiVed: May 27, 2009
Polyelectrolytes adsorb strongly at oppositely charged surfaces, thereby dramatically influencing the corresponding interaction forces. In this letter, we report on direct force measurements with the atomic force microscope (AFM) between two individual particles in an aqueous colloidal suspension in the presence of polyelectrolytes near the isoelectric point. From systematic variations of the molecular mass, the ionic strength, and analysis of adhesion events, we conclude that the observed attractive forces are mainly due to electrostatic patch-charge interactions. The same type of attractive forces is equally influencing interactions between proteins as well as hydrophobic or mineral surfaces. Polyelectrolytes are manufactured nearly in megaton-per-year quantities, and they are principally used as flocculants in water purification,1,2 retention-aids in papermaking,3 or as stabilizing agents in food, cosmetic, or pharmaceutical products.4,5 More recently, polyelectrolytes became equally popular as building blocks of self-assembling nanomaterials,6-9 particularly in the layer-by-layer deposition process leading to functional thin films or coatings on planar substrates,8 or capsules of tunable permeability.9 Many of these applications rely on the strong affinity of polyelectrolytes to oppositely charged surfaces.1,2,10-14 Several authors have followed the corresponding adsorption process and charge reversal by electrophoresis.2,10,11 Figure 1 presents a typical example. The electrophoretic surface potential (i.e., ζ-potential) of amidine-terminated poly(styrene) latex particles is shown as a function of the dose of negatively charged poly(styrene sulfonate) (PSS). At low PSS dose, the surface potential is positive due to the positive charge of the bare latex particles. With increasing dose, the surface potential decreases due to the adsorption of the negatively charged PSS. At the isoelectric point (IEP), the neutralization is stoichiometric meaning that about one PSS charge is necessary to neutralize one surface charge. With increasing dose, the surface potential becomes increasingly negative indicating further adsorption of PSS.10,13,14 The adsorption is driven by electrostatic attraction between the polyelectrolyte and the substrate and hydrophobic interactions between the poly(styrene) motifs in the polymer and the latex. The lateral charge heterogeneities are induced by electrostatic repulsion between the polyelectrolyte chains. Similar electrophoresis studies show that systems with negatively charged particles and cationic polyelectrolytes behave in an analogous fashion, indicating the importance of electrostatic interactions.2,10,11,13 In some cases, super stoichiometric * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: 0041 22 379 6405. † Current address: Adolphe Merkle Institute, University of Fribourg, CH1723 Marly 1, Switzerland.
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Figure 1. Surface potentials of amidine latex particles of about 3 µm in diameter as a function of the dose of poly(styrene sulfonate) (PSS) as determined from electrophoresis experiments and direct force measurements in 1 mM KCl and pH 4. With increasing dose, the particle surface reverses its charge at the isoelectric point (IEP) and reaches maximum adsorption (saturation plateau). The insets present AFM phase images of a bare particle (left), a particle at the IEP (middle) and at the saturation plateau (right). The solid line serves to guide the eye only.
charge neutralization accompanied by coadsorption of salt ions is observed.11 In spite of these minor differences, analogous behavior is observed in systems involving oppositely charged polyelectrolytes and particles. Qualitatively different behavior is only reported for polyelectrolytes with exceptionally low line charge densities, which resemble neutral well-soluble polymers.15,16 Colloidal stability2,10,11,16 and rheology17 of particle suspensions are closely related to their electrophoretic mobility. In particular, the stability of amidine latex particles in the presence of PSS is in line with electrophoresis data.10 This system is similar to the one studied here, with the exception that the particles were smaller and that their surface charge density was lower. Away from the IEP, one observes slow aggregation, which is consistent with repulsive forces due to double layer 2009 American Chemical Society
Letters overlap. At the IEP, the particles aggregate more rapidly than at high salt concentrations, confirming the existence of additional attractive forces besides van der Waals interactions. Analogous conclusions were reached based on stability and rheological studies of comparable systems, again reiterating the generic nature of these phenomena.2,10,11,17 The origin of these additional attractive forces continues to be debated.1,12,15,17,18 In analogy to neutral polymer systems, bridging polyelectrolyte chains were suggested to be responsible for this attractive force.1,3,12,15,18 However, it was equally suggested that attractive electrostatic forces could be induced by interactions between patches of the nonuniformly charged surfaces.1,11,17 To clarify the origin of these interactions, direct force measurements were carried out with the surface forces apparatus (SFA), 19,20 colloidal probe, 15,21,22 or similar techniques.18,23 These studies confirmed the strongly repulsive forces between surfaces at high polyelectrolyte dose, where the adsorbed layer is in its saturated state. These measurements suggest that well above the IEP, the governing interactions comprise electrostatic double layer forces at larger distances and steric forces due to the overlap of adsorbed polymer layers at smaller ones. The nature of the attraction, however, remained unclear and only a handful of authors were even able to pinpoint their existence near the IEP.18,19 Under these conditions, attractive forces exceeding van der Waals interactions were reported. However, this attraction occurred only in a transient-like fashion during the formation process of the adsorbed layer, which precluded the clarification of its nature. In this letter, we report on direct measurements of such forces between two individual colloidal particles in aqueous suspensions with the atomic force microscope (AFM). Our new technique permits to tune the amount of adsorbed polyelectrolyte precisely, enabling us to perform accurate force measurements near the IEP. We use the AFM-based colloidal probe technique15,21,22,24 adapted to the symmetrical system of two identical particles (Figure 2). One of the amidine latex particles is attached to a tip-less cantilever, while the other is immobilized together with many other particles at a planar substrate. The new aspects of this approach are 2-fold. First, the particles are directly immobilized in situ on silanized surfaces in degassed water without drying, thus excluding effects of microscopic bridging bubbles.25 Second, forces are measured in a colloidal suspension with an internal surface area exceeding several square meters. The PSS dose can be therefore controlled precisely, enabling us to make reproducible measurements at identical conditions as in the electrophoresis experiment, particularly close to the IEP (Figure 1). The experimental details can be found in the Supporting Information. Previous studies relied on surface areas orders of magnitude smaller, precluding a precise control of the adsorbed amount and comparison with other techniques.18,19 Representative force profiles are shown in Figure 2. At low and high PSS doses, the forces are repulsive and show weak unspecific adhesion upon retraction. At the IEP, forces are attractive upon approach, and one observes a pronounced unspecific adhesion peak upon retraction. In rare cases, pulling events indicating bridging of individual polymer chains can be identified.24,26 They mostly include plateau events, resulting from peeling or sliding of polyelectrolytes chains and their subsequent desorption. In some cases, however, stretching of individual polymer chains can be observed as well. Figure 3 shows the approach forces F normalized to the effective particle radius Reff ) R1R2/(R1 + R2) where R1 and R2
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Figure 2. Direct force measurements with the AFM between individual colloidal particles (top left). Proposed attraction mechanisms based on electrostatic patch-charge or bridging forces as probed by AFM (top right). Typical approach-retraction force profiles where arrows indicate the spring instabilities. Repulsive forces between (a) bare particles and (b) between particles with adsorbed polyelectrolyte near saturation. (c) Near the IEP, one observes attractive forces with an unspecific adhesion peak. Rarely, one observes pulling events due to bridging polymer chains, involving (d) peeling, (e) pulling, and (f) multiple chain events. (g) Histograms of the adhesion energies per unit area in the presence and absence of pulling events.
Figure 3. Forces between latex particles normalized to the effective radius as a function of the surface separation for different doses of PSS with a molecular mass of 2260 kg/mol as measured in 1 mM KCl and pH 4.0. The schemes on the top indicate the progressive polyelectrolyte adsorption with increasing dose. (a) PSS dose below and at the IEP and (b) dose at and above the IEP. The solid lines are fits of the force profiles with the Poisson-Boltzmann theory.
are the respective radii of the two spheres as a function of the separation distance. The averaged force profiles are shown for different PSS doses in 1.0 mM KCl and pH 4. The results for doses approaching the IEP from below are given in Figure 3a.
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Figure 4. Attractive forces between latex particles normalized to the effective radius in the presence of PSS at the IEP. The van der Waals force is indicated as black solid line, and further, the best fits to the data with added exponential attraction are shown. The observed decay length is consistent with attractive interactions due to patch-charge heterogeneities. The schemes on the top compare the adsorbed polyelectrolytes with the extent of the diffuse layer. (a) Variation with the ionic strength for PSS of 2260 kg/mol. (b) Variation with the molecular mass at an ionic strength of 0.1 mM.
The forces between the bare particles are strongly repulsive. These forces weaken and an attractive component appears at shorter distances as the dose is increased. At the IEP, forces become attractive. When the dose is increased beyond the IEP, a long-range repulsion sets in as shown in Figure 3b. The saturation of the adsorbed layer is evidenced since the repulsive forces no longer vary with the polymer dose. The attractive force is still present far beyond the IEP, but weakens close to saturation. The long-range repulsive part of the force results from the overlap of electrical double layers. These forces are consistent with Poisson-Boltzmann theory27 and the expected Debye length κ-1 = 9.5 nm. The fitted diffuse layer potentials are in agreement with the potentials obtained by electrophoresis (Figure 1) confirming that electrophoresis probes the diffuse layer potential and that the conditions in the AFM experiment correspond precisely to those in the bulk colloidal suspension. Attractive forces at the IEP are shown in Figure 4. For higher salt levels or for low molecular mass PSS, the forces are consistent with the van der Waals force with the expected Hamaker constant of 9.0 × 10-21 J for polystyrene.28 A stronger attractive force is observed for high molecular mass PSS and especially at low salt levels. These additional forces are compatible with an exponential profile F/Reff ) -Ae-qh, where q-1 is the decay length and A the magnitude of its amplitude. The amplitude decreases with increasing ionic strength and decreasing molecular mass of PSS (Table S1). The decay length decreases with decreasing molecular mass and appears independent of the ionic strength. The data for PSS of 2260 kg/mol are consistent with a constant decay length of q-1 = 2.4 nm, while its value is smaller for the PSS of lower molecular mass. The forces observed with PSS of the lowest molecular mass are indistinguishable from van der Waals forces. Further details in the analysis of the force data are given as Supporting Information. Our results strongly suggest that the observed additional attractive forces are due to electrostatic attraction between patchcharge heterogeneities. Theoretical analysis of the problem shows that two overall neutral but nonuniformly charged surfaces interact across an electrolyte solution with an exponential force law at larger distances.29 In the case of a square lattice, the decay length q-1 is given by the relation q2 ) κ2
Letters +(π/a)2, where 2a is the lattice constant. Our observations for PSS of 2260 kg/mol are consistent with a lattice constant of 2a = 15 nm, which is well comparable to the size of the surface heterogeneities due to adsorbed PSS observed with AFM (Figure 1). The amplitude can be estimated for 0.1 mM as A = 2.7 mN/m in agreement with experiment (Table S1). With decreasing molecular mass of the PSS, one expects a decrease in the lattice spacing, which is consistent with the observed decrease in decay length and amplitude. We suspect that the patch-charge attraction results from correlations between the adsorbed polyelectrolytes. A similar patch-charge attraction mechanism has been proposed to explain attractive forces between adsorbed surfactant films, where surface charge heterogeneities originate from patchy surfactant bilayers.30-32 However, in the latter case the heterogeneities seem to rearrange upon approach,32 while in the present case we suspect that they are frozen due to the strong attractive forces between the polyelectrolytes and the substrate. In analogy to neutral polymers in good solvents,24,27 bridging of polyelectrolyte strands was suggested to be responsible for additional attractive forces.1,3,12,15,18 The individual retraction force curves indicate that bridging forces are unimportant. Force curves upon retraction always feature at short distances an adhesion peak. Sometimes, random pulling events of individual chains can be detected (Figure 2d-f). When surfaces are bridged with a polyelectrolyte chain at contact, these bridging chains will be pulled during retraction, and finally detach from the surface. Such pulling events represent an unmistakable signature of polymer bridging. However, such bridging events occur extremely rarely (∼2%, see Table S1). Moreover, attractive forces are observed irrespective whether such pulling events occur or not. To quantify this point, distributions of the work of adhesion in the presence and the absence of pulling events are compared in Figure 2g. This quantity is determined by all attractive forces present, including any eventual patch-charge or bridging attractions. One observes that both distributions are very similar. Indeed, no correlation between the strength of the attraction and occurrence of pulling events can be established. A further argument against bridging is that similar attractive forces near IEP are observed in the case of oppositely charged adsorbed dendrimers.33 Dendrimers are highly branched and compact macromolecules, and no bridging forces as for linear polymers can exist. Finally, bridging forces show only weak molecular mass dependence.12 We conclude that attractive forces occurring between surfaces with oppositely charged adsorbed polyelectrolytes are primarily electrostatic in nature and originate from interactions between patch-charge surface heterogeneities. This force can be substantially stronger than the expected van der Waals force. Two observations discussed above indicate that the observed attraction cannot be explained by polymer bridging. First, the observed forces are consistent with theoretical predictions of patch-charge interactions. Second, attractive forces are observed throughout, and irrespective of the rare occurrence of pulling events of individual polyelectrolyte chains. The importance of patchcharge attraction is further supported by close similarities in the colloidal stability in oppositely charged polyelectrolyteparticle systems.2,10,11 These similarities can be naturally explained by electrostatic interactions. However, we suspect that bridging forces are probably important in exceptionally weakly charged polyelectrolyte systems,15,16 in analogy to the established case of neutral polymers.24,27,34 However, the nature of this crossover remains unclear. We have demonstrated that attractive forces between charged surfaces with adsorbed polyelectrolytes of opposite charge are
Letters induced by patch-charge electrostatic interactions. Recently, it was equally shown that same mechanism equally controls interactions between proteins,35 hydrophobic surfaces,30,31,36 and mineral particles.37,38 Electrostatic patch-charge interactions are therefore emerging as one of the major mechanisms of attractive forces between surfaces across water. Acknowledgment. Financial support by Swiss National Science Foundation, University of Geneva, Swiss Federal Office for Education and Science, and COST Action D43. We thank Bo Jo¨nsson for comments on the manuscript and Benjamin Hernach for laboratory help. Supporting Information Available: Detailed information on the materials, experimental protocols, and data analysis. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Bolto, B.; Gregory, J. Water Res. 2007, 41, 2301–2324. (2) Schwarz, S.; Jaeger, W.; Paulke, B. R.; Bratskaya, S.; Smolka, N.; Bohrisch, J. J. Phys. Chem. B 2007, 111, 8649–8654. (3) Salmi, J.; Osterberg, M.; Stenius, P.; Laine, J. Nord. Pulp Paper Res. J. 2007, 22, 249–257. (4) Mezzenga, R.; Schurtenberger, P.; Burbidge, A.; Michel, M. Nat. Mater. 2005, 4, 729–740. (5) Studart, A. R.; Amstad, E.; Gauckler, L. J. Langmuir 2007, 23, 1081–1090. (6) Whitesides, G. M.; Boncheva, M Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4769–4774. (7) Ballauff, M. Prog. Polym. Sci. 2007, 32, 1135–1151. (8) Decher, G. Science 1997, 277, 1232–1237. (9) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111– 1114. (10) Gillies, G.; Lin, W.; Borkovec, M. J. Phys. Chem. B 2007, 111, 8626–8633. (11) Kleimann, J.; Gehin-Delval, C.; Auweter, H.; Borkovec, M. Langmuir 2005, 21, 3688–3698. (12) Akesson, T.; Woodward, C.; Jonsson, B. J. Chem. Phys. 1989, 91, 2461–2469. (13) Grosberg, A. Y.; Nguyen, T. T.; Shklovskii, B. I. ReV. Mod. Phys. 2002, 74, 329–345.
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