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Polyelectrolyte-Mediated Interaction between Similarly Charged Surfaces: Role of Divalent Counter Ions in Tuning Surface Forces T. Abraham,† A. Kumpulainen,‡ Z. Xu,† M. Rutland,‡ P. M. Claesson,‡ and J. Masliyah*,† Department of Chemical & Materials Engineering, University of Alberta, Edmonton, Canada T6G 2G6 and Department of Chemistry, Royal Institute of Technology, SE-10044, Stockholm, Sweden, and Institute of Surface Chemistry, SE-11486 Stockholm, Sweden Received July 9, 2001. In Final Form: October 5, 2001 The effects of divalent salts (CaCl2, MgCl2 and BaCl2) in promoting the adsorption of weakly charged polyelectrolyte (polyacrylic acid), PAA, Mw ∼ 250000 g/mol) on mica surfaces and their role in tuning the nature of interactions between such adsorbed polyelectrolyte layers were studied using the interferometric surface forces apparatus. With mica surfaces in 3 mM MgCl2 solutions at pH ∼8.0-9.0, the addition of 10 ppm PAA resulted in a long-range attractive bridging force and a short-range repulsive steric force. This force profile indicates a low surface coverage and weak adsorption. The range of the force can be related to the characteristic length scale RG of polyelectrolyte chains using a scaling description. An increase of the PAA concentration to 50 ppm changed the attractive force profile to a monotonic, long-range repulsive interaction extending up to 600 Å due to the increased surface coverage of polyelectrolyte chains on the mica surfaces. Comparison of the measured forces with a scaling mean field model suggests that the adsorbed polyelectrolyte chains are stretched, which eventually give rise to the polyelectrolyte brush like structure. When the mica surfaces were preincubated in 3 mM CaCl2 at pH ∼8.0-9.0, in contrast to the case of 3 mM MgCl2, the addition of 10 ppm PAA resulted in a more complex force profile: long-range repulsive forces extending up to 800 Å followed by an attractive force regime and a second repulsive force regime at shorter separations. The long-range electrosteric forces can be attributed to strong adsorption of polyelectrolyte chains on mica surfaces (high surface coverage) which is facilitated by the presence of Ca2+ ions, while the intermediate range attractive forces can be ascribed to Ca2+ assisted bridging between adsorbed polyelectrolyte chains. Also interesting is to note various relaxation processes present in this system. In contrast to both MgCl2 and CaCl2 systems, with mica surfaces in 3 mM BaCl2 solution at pH ∼8.0-9.0, the addition of 10 ppm PAA resulted in precipitation of polyelectrolyte chains on mica surfaces, resulting in an extremely long-range monotonic repulsive force profile. In summary, our study showed that divalent counterions (Mg2+, Ca2+, and Ba2+) exhibit significantly different behavior in promoting PAA adsorption on mica surfaces, modifying and controlling various surface interactions.
Introduction Polyelectrolyte mediated interactions are of fundamental importance and have many practical implications. In industrial applications, polyelectrolyte mediated interactions are important in many flocculation and stabilization processes, for instance, polyelectrolyte assisted dewatering,1 flocculation of fine oil sand tails,2 flocculation of coal tailings,3 colloidal stabilization and destabilization,4 etc. In such situations, the ionic macromolecules are used to tune surface interactions, which are strongly dependent upon various physicochemical parameters, such as adsorbed layer conformation and surface coverage. For instance, low surface coverage with loops and tails extending far from the surfaces can give rise to long range attractive bridging force which promotes the flocculation * To whom correspondence should be addressed. † Department of Chemical & Materials Engineering. ‡ Department of Chemistry. (1) Farinato, R. S.; Huang, S. Y.; Hawkins, P. In Colloid-Polymer Interactions; Farinato, R. S., Dubin, P. L., Eds; Wiley-Interscience Publications: New York, 1999; p 3. (2) Xu, Y.; Cymerman, G. In Polymers in Mineral Processing; Laskowski, J. S., Ed.; MET SOC: British Columbia, 2000; p 591. (3) Choung, J.; Liu, J.; Xu, Z.; Szymaski, J. In Polymers in Mineral Processing; Laskowski, J. S., Ed.; MET SOC: British Columbia, 2000; p 439. (4) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: London, 1983.
process. Conversely, high surface coverage and osmotically stretched adsorbed layers can induce long-range electrosteric repulsive force leading to stabilization of systems. Several surface force measurements established the effects of both low and high polyelectrolyte adsorption densities on surface interactions.5,6 Polyelectrolyte adsorption in general is driven by either electrostatic attraction between the polyelectrolyte segments and the surface carrying opposite charges, specific/ chemical interactions between polyelectrolytes and surface sites, or poor solvent environment induced by screening of the polyelectrolytes charges due to added salts. The polyelectrolyte adsorption is favored by multisegment adsorption, release of counterions from both surface and polyelectrolyte chains, and low diffusion of chains near the surfaces. The adsorption process leads, however, to loss of entropy due to confinement of the chains on the surfaces. The adsorbed layer structure of polyelectrolytes and the resultant surface interactions have been explored theoretically in great detail. The main theoretical tools include analytical mean field models,5,7-18 and molecular (5) Borukhov, I.; Andelman, D.; Orland, H. J. Phys. Chem. B. 1999, 103, 5042 and references therein. (6) Claesson, P. M. In Colloid-Polymer Interactions; Farinato, R. S., Dubin, P. L., Eds.; Wiley-Interscience Publications: New York, 1999; p 287 and references therein. (7) Pincus, P. Macromolecules 1991, 24, 2912.
10.1021/la011037f CCC: $20.00 © 2001 American Chemical Society Published on Web 12/01/2001
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simulations.19-22 Studies with these models showed that the layer structure and resulting surface interactions are a strong function of polyelectrolyte charge density, surface coverage, surface potentials, concentration of added salts, and other relevant system characteristics such as quality of solvent and solution pH. There has been a considerable experimental effort to understand the structure of adsorbed polyelectrolyte layers and the nature of interactions with and without added monovalent salts.22-28 To our knowledge, the adsorbed polyelectrolyte layers and the nature of interactions in the presence of multivalent salts have not been studied in more than a few cases.29,30 In particular, Berg et al.29 investigated interactions between mica surfaces in low molecular weight sodium polyacrylate (NaPAA) solutions containing divalent salt, calcium chloride. They found that the presence of calcium ions initiates NaPAA adsorption on mica surfaces that eventually results in short-range attractive forces (∼50-80 Å) between the mica surfaces. They attributed such forces to Ca2+ assisted bridging between the polymer carboxylate groups. Until now, the systematic studies on investigating the role of divalent counterion size in polyelectrolyte systems at interface have not been carried out. There is, however, an experimental work emphasizing the effect of divalent counterion size on conformation of polyelectrolyte chains in solutions using light scattering methods.31 In general, these studies demonstrated that larger divalent counterion induces a larger contraction (conformational change) of polyelectrolyte coils than smaller divalent counterions. The main goal of the present investigation is to compare the effect of different divalent counterions, Mg2+, Ca2+, and Ba2+, on promoting the adsorption of negatively charged, high molecular weight PAA to negatively charged surfaces, and the role of the ions in the subsequent surface interactions in such a system. This endeavor aims to improve our current understanding of the role of divalent salts in polyelectrolyte systems, specifically, their effects on conformational and interaction properties at interfaces. (8) Ross, R. S.; Pincus, P. Macromolecules 1992, 25, 2177. (9) Misra S.; Tirrell, M.; Mattice, W. Macromolecules 1996, 29, 6056. (10) Podgornik, R. J. Phys. Chem. 1991, 95, 5249. (11) Podgornik, R. J. Phys. Chem. 1992, 96, 886. (12) Borisov, O. V.; Zhulina, E. B.; Birshtein, T. M. Macromolecules 1994, 27, 4795. (13) Bo¨hmer, M. R.; Evers, O. A.; Scheutjens, J. M. H. M. Macromolecules 1990, 23, 2288. (14) Zhulina, E. B.; Borisov, O. V. J. Chem. Phys. 1997, 107, 5952. (15) Zhulina, E. B.; Borisov, O. V.; Birshtein, T. M. Macromolecules 1999, 32, 8189. (16) Miklavic, S. J.; Marcelja, S. Macromolecules 1988, 21, 6718. (17) Joanny, J. F.; Chatellier, X. J. Phys. II (France) 1996, 1667. (18) Linse, P. Macromolecules 1996, 29, 326. (19) Dickinson, E.; Eriksson, L. Adv. Colloid Interface Sci. 1991, 34, 1. (20) Dickinson, E.; Euston, S. R. J. Chem. Soc., Faraday Trans. 1991, 87, 2193. (21) Muthukumar, M. J. Chem. Phys. 1987, 86, 7230. (22) Dahlgren, M. A. G.; Waltermo, A.; Blomberg, E.; Claesson, P. M.; Sjostrom, L.; Akkesson, T.; Jonsson, B. J. Phys. Chem. 1993, 97, 11769. (23) Dahlgren, M. A. G. Langmuir 1994, 10, 1580. (24) Lowack, K.; Helm, C. Macromolecules 1998, 31, 823. (25) Abraham, T.; Giasson, S.; Gohy, J. F.; Jerome, R. Langmuir 2000, 16, 4246. (26) Biggs, S. Langmuir 1995, 11, 156. (27) Kelley, T. W.; Schorr, P. A.; Kristin, D. J.; Tirrell, M.; Frisbie, C. D. Macromolecules 1998, 31, 4297. (28) Hariharan, R.; Biver, C.; Russel, W. B. Macromolecules 1998, 31, 7514. (29) Berg, J. M.; Claesson, P. M.; Neuman, R. D. J. Colloid Interface Sci. 1993, 161, 182. (30) Malmsten, M.; Claesson, P. M.; Siegel, G. Langmuir 1994, 10, 1274. (31) Ikeda, Y.; Beer, M.; Schmidt, M.; Huber, K. Macromolecules 1998, 31, 728.
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Experimental Section Materials. A high molecular weight polyacrylic acid (PAA, Mw∼250,000 g/mol) and calcium chloride (CaCl2‚4H2O) of Sigma ultrapure grade were purchased from Sigma-Aldrich Canada. Magnesium chloride (MgCl2‚6H2O) of Suprapur grade and sodium hydroxide (NaOH) of 99.9+ purity were obtained from Merck. The mica used in the experiments was supplied by Mica Supplies Ltd., Dorset, England. Method. Force-Measuring Technique. The interaction forces F(D) between mica surfaces as a function of separation distance D were measured using the Mark IV surface forces apparatus.32,33 The specific details are well described in the literature.32,33 With this instrument, the resolution in distance determination is about 2 Å and the detection limit of force is about 10-7 N. The mica sheets are glued onto cylindrical silica disks and mounted at a cross cylindrical configuration. This arrangement is used to avoid alignment problems, which would be encountered with parallel surfaces. The top surface is rigidly mounted while the bottom surface is mounted on a flexible leaf spring of known stiffness. The two mica surfaces are brought together in a gas or liquid medium using a differential displacement mechanism. The separation distance between the two surfaces is measured using a multiple beam interferometry.34 The force F(D) between the two surfaces is obtained by multiplying the deflection of the spring by its stiffness. The deflection of the spring is measured as the difference between the distances moved by the driving shaft supporting the lower surface and the resulting change in separation between the surfaces. When the gradient of the force ∂F/∂D exceeds the measuring spring stiffness, the system becomes mechanically unstable and the surfaces jump instantaneously to a closer stable region. The measured force F(D) is generally normalized by the local geometric mean radius R of the interacting surfaces. In this manner the effect of different curvatures for different experiments is eliminated and the magnitude of the force F/R can be compared from one experiment to another. The radius R is determined from the shape of the FECO (fringes of equal chromatic order) at each measuring position. Based on the Derjaguin approximation for D , R, the normalized interaction force F/R can be related to the interaction free energy per unit area between two flat surfaces, W(D) by
F(D) ) 2πW(D) R
(1)
Since D is in the order of 10-6 m or less and R is 1 or 2 cm, the condition D , R is satisfied. Forces are measured both in compression and in decompression and the rate of separation and compression are the same at 4 nm/s. All the force profiles in this communication are the average of at least six reproducible experiments. For clear presentation, only one profile for each measurement condition is shown in the figures. Experimental Procedure. All solutions were prepared with Millipore water of resistivity 18.2 MΩ cm. A stock solution of PAA at a concentration of ∼2000 ppm was prepared a week prior to the experiment. The pH of all the solutions was maintained or adjusted to 8.0-9.0 using NaOH. The salt solutions were prepared at least a day before the experiments. Mica surfaces were incubated for at least 2 h in the aqueous electrolyte solutions and overnight (∼12 h) after PAA addition, unless otherwise stated. It is important to note that a 3 mM MgCl2 solution in the presence of 10 or 50 ppm PAA was visibly clear. On the other hand, 3 mM CaCl2 solution with 10 ppm PAA was slightly turbid, indicating formation of aggregates. For BaCl2, 3 mM solution with 10 ppm PAA was turbid. The turbidity is, however, not sufficiently high to affect interferometric measurements. Polyacrylic Acid. Relevant Characteristics. Polyacrylic acid is a polyelectrolyte, whose charge density depends on the pH of the solution. Such polyelectrolytes are referred to as weakly charged or annealed polyelectrolytes.35,15 The dimension of the chain is a function of monomer size, number of monomer units, (32) Isarelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1992. (33) Parker, J. L.; Christenson, H. K.; Ninham, B. W. Rev. Sci. Instrum. 1989, 60, 3135. (34) Israelachvili, J. N. J. Colloids Interface Sci. 1973, 44, 259.
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pH and added salt concentration. At pH >7 (dissociation constant >0.7), the chain is expected to be stretched, whereas at pH Ca2+ > Mg2+), Ba2+ is the least hydrated one and therefore it has the highest tendency to interact with the oppositely charged counter species, such as PAA and mica surfaces. Sabbagh and Delsanti39 classified different divalent counterions according to their power to precipitate the PAA as: Mg2+ < Ca2+ < Ba2+. The visual inspection of the solutions reveals the similar trend. PAA in 3 mM MgCl2 aqueous solution appears as a clear solution whereas PAA in 3 mM CaCl2 aqueous solution results in a slightly turbid appearance. An even stronger attraction between PAA chains is induced by the addition of BaCl2, which makes the solution more turbid, indicating a more extensive aggregation than in the 3 mM CaCl2 solution. The same trend is observed for the adsorption of PAA on mica. A less than PAA monolayer coverage was formed on mica surface from aqueous solution containing 3 mM MgCl2 and 10 ppm PAA. Multilayer of PAA with weakly adsorbed PAA outer layers were inferred when the adsorption occurred from 3 mM CaCl2 solution containing 10 ppm PAA whereas a thick PAA surface precipitate formed when 3 mM BaCl2 was present in the 10 ppm PAA solution. Conclusions
steric forces caused by polyelectrolyte layers adsorbed on the surface. The short-range attractive force may be associated with the Ca2+-assisted surface bridging or ionion correlation effects. We note that the depletion of loosely adsorbed layers from the interface during compression could explain an inward jump due to a reduction of the repulsive force. However, such a mechanism cannot give rise to the strong adhesion observed on separation. Clearly, this attraction has to arise from the interactions between the chains remaining on the surfaces. To understand the observed slow relaxation, the system was left overnight (∼12 h.) and the forces were measured again. The force profile (denoted by C in Figure 5) then looks rather similar to the one observed during first approach. Subsequent force measurements resulted in force profiles that were similar to the ones obtained at the second and third approaches during day 1 (B in Figure 5). The same procedure can be repeated again with the same result. It is not very surprising that the relaxation time is long. Rather this has been observed whenever the interaction between the adsorbing polyelectrolyte and the surface is strong.23,43,44 In fact, in some systems the relaxation is so slow that equilibrium is not reached within reasonable experimental time scales (weeks).23,43 Analogously, a strong binding of the PAA to mica surface in the presence of Ca2+ may have contributed to the slow relaxation in the present case. Another contribution to the slow relaxation may arise from a slow build-up of a weakly bound outer PAA layer again facilitated by the presence of Ca2+ ions. Forces between Mica Surfaces in 3 mM BaCl2/ 10 ppm PAA. Surface Precipitation. A typical force profile obtained across the 3 mM BaCl2 solution in the presence of 10 ppm PAA at pH ∼8.0-9.0 is shown in Figure 6. A long-range repulsive force commencing at about ∼1500 Å is observed. Surfaces can be pressed to a separation distance of ∼ 600 Å. The long-range nature of the force is simply a consequence of the formation of a precipitate. Barium ions are clearly more effective in promoting (43) Dedinaite, A.; Claesson, P. M. Langmuir 1999, 15, 5376. (44) Guldberg-Pedersen, H.; Bergstro¨m, L. J. Am. Ceramic Soc. 1999, 82, 1137.
Divalent counterions (Mg2+, Ca2+, and Ba2+) exhibit significantly different behavior in promoting PAA adsorption, modifying surface interactions between two negatively charged mica surfaces. Depending upon their hydrated size, the divalent salts give very different adsorbed polyelectrolyte structures (conformations) and therefore different types of surface interactions. With 3 mM MgCl2 and 10 ppm PAA at pH∼8.0-9.0, the highly hydrated Mg2+ induces a weak PAA adsorption on the surface, whereby leading to a low surface coverage. This gives rise to a long-range bridging attraction between the surfaces. The scaling description of the bridging force indicates the conformation of adsorbed chains and allows one to predict the range of surface interactions and the magnitude of the forces relative to the characteristic length scale RG of the chains. A five time increase of PAA concentration (i.e., 50 ppm PAA) transformed the surface interaction into a purely repulsive one. In this case, the comparison of the measured repulsive force with the mean field model suggests that the adsorbed layers are indeed in the stretched states compared to the unperturbed chain dimension. The proposed modeling effort allows us to quantify the range and magnitude of the forces based on the added salt concentration and overall chain dimension. In contrast to similar MgCl2 system, with 3 mM CaCl2, the less hydrated Ca2+ induces a much stronger polyelectrolyte adsorption from 10 ppm PAA solution leading to a significantly high surface coverage and hence a longrange repulsive force between the surfaces. The least hydrated Ba2+, on the other hand, causes the precipitation of PAA chains on the surfaces under similar conditions, which eventually gives rise to a very long range and strong repulsive force between the surfaces. In general we have shown that polyelectrolyte mediated surface interactions are strong functions of divalent ion size and their relative concentration to polyelectrolyte. The difference in ion size and therefore degree of hydration causes the divalent counterions to interact with the charged solid-liquid interface and polyelectrolyte molecules in very different manner. The significance of this study is to provide a scientific basis for tuning surface forces and hence controlling the stability of colloidal
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systems by carefully choosing divalent salt, polyelectrolyte, and their concentrations. Acknowledgment. T.A. greatly appreciates the hospitality of Royal Institute of Technology and Institute of
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Surface Chemistry, Stockholm, Sweden, during his stay at Stockholm. Financial support from NSERC-Syncrude Industrial Research in Oil Sands is greatly appreciated. LA011037F
