Measurement of the Interaction Forces between Poly (N

Nicola C. Woodward, Martin J. Snowden, and Babur Z. Chowdhry. School of Chemical & Life Sciences, University of Greenwich, London, SE18 6PF, United ...
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Langmuir 2002, 18, 2089-2095

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Measurement of the Interaction Forces between Poly(N-isopropylacrylamide-acrylic acid) Microgel and Silica Surfaces by Colloid Probe Microscopy Nicola C. Woodward,†,| Martin J. Snowden,†,‡ and Babur Z. Chowdhry†,‡ School of Chemical & Life Sciences, University of Greenwich, London, SE18 6PF, United Kingdom, and Calorimetry Centre, Medway Sciences, University of Greenwich, Chatham Maritime, Kent, ME4 4AW, United Kingdom

Paul Jenkins*,§,⊥ and Ian Larson§,# Ian Wark Research Institute, University of South Australia, The Levels Campus, Mawson Lakes, SA 5095, Australia Received April 16, 2001. In Final Form: December 6, 2001 The interaction of an immobilized layer of copolymer microgels of poly(N-isopropylacrylamide-acrylic acid) with a silica colloid probe, separated by aqueous solutions, has been studied using atomic force microscopy. The interaction was seen to be repulsive at all separations for all pH and ionic strengths studied. Derjaguin-Landau-Verwey-Overbeek (DLVO) theory (solely electrostatic and van der Waals forces) was not able to satisfactorily describe the interaction between the silica probe and the microgel layer. The magnitude and range of the measured forces were both far greater than DLVO theory would predict. The discrepancy is ascribed to compression/deformation of the microgel and the presence of a “hairy” steric layer, composed of polyelectrolyte chains, at the microgel surface. The forces that result from these phenomena are the major components of the overall interaction observed. The range of the measured interactions varied with solution pH and ionic strength. It is believed that changes in the solution conditions give rise to variation in the dissociation and screening of the sulfate and carboxylate groups present in the microgel. In turn, this causes the microgel “core” and hairy layer to alter their conformations. Hence, electrostatic effects (although not in the DLVO sense) are important in determining the character of the silica-microgel interaction.

Introduction Microgels have been a subject of significant scientific interest as a result of their interesting physicochemical characteristics. Their properties and applications have recently been the subject of several reviews.1-4 Microgels are spongelike polymeric particles that exist in a swollen state when dispersed in a suitable solvent. The microgel particles exhibit a fully reversible conformational transition in response to changing solvent quality, which may be affected by a number of external stimuli including temperature,1-4 ionic strength,5 and pH.5 During the course of this transition, the microgel particle adopts a more compact conformation in order to minimize polymer* Corresponding author. Fax: +44 (0)151 641 1843. E-mail: [email protected]. † School of Chemical & Life Sciences, University of Greenwich. ‡ Calorimetry Centre, Medway Sciences, University of Greenwich. § Ian Wark Research Institute, University of South Australia. | Current address: Institute of Food Research, Norwich Research Park, Colney, Norwich, NR4 7LA, U.K. ⊥ Current address: Unilever Research, Port Sunlight, Quarry Road East, Bebington, Wirral, CH63 3JW, U.K. # Current address: Department of Pharmaceutics, Victorian College of Pharmacy, 381 Royal Parade, Parkville, Victoria 3052, Australia. (1) Murray, M.; Snowden, M. J. Adv. Colloid Interface Sci. 1995, 54, 73. (2) Antoinetti, M. Angew. Chem., Int. Ed. Engl. 1988, 27, 1743. (3) Saunders, B. R.; Vincent, B. Adv. Colloid Interface Sci. 1999, 80, 1. (4) Pelton, R. Adv. Colloid Interface Sci. 2000, 85, 1. (5) Snowden, M. J.; Chowdhry, B. Z.; Vincent, B.; Morris, G. E. J. Chem. Soc., Faraday Trans. 1996, 92, 5013.

solvent interactions, only returning to its original conformation when solvent conditions become more favorable. Microgels have been investigated for a variety of applications such as controlled uptake and release of heavy metal ions7 and polymers.8 Their potential as enhanced oil recovery systems9 and for use as controlled heteroflocculation agents1 has also been examined. Moreover, they have been proposed as additives within the paint industry, where it has been reported that their presence has resulted in improved orientation of metal pigments, film appearance, and rheological and mechanical properties.1,3,10 The physicochemical properties of microgel dispersions have been extensively examined by a number of techniques including dynamic light scattering,11,12 rheology,13 and differential scanning calorimetry.5,14 The internal structure of microgel particles has been examined using fluorescence probe,15 nuclear magnetic resonance,16,17 and small-angle neutron scattering techniques.18,19 While these (6) Snowden, M. J.; Vincent, B. J. Chem. Soc., Chem. Commun. 1992, 16, 1103. (7) Snowden, M. J.; Thomas, D.; Vincent, B. Analyst 1993, 118, 1367. (8) Snowden, M. J. J. Chem. Soc., Chem. Commun. 1992, 803. (9) Snowden, M. J.; Chowdhry, B. Z. Chem. Br. 1995, December. (10) Ishii, Y. Colloids Surf., A 1999, 153, 591. (11) Woodward, N. C.; Chowdhry, B. Z.; Leharne, S.; Snowden, M. J. Eur. Polym. J. 2000, 36, 1355. (12) Crowther, H. M.; Vincent, B. Colloid Polym. Sci. 1998, 276, 46. (13) O ¨ le-Kiminta, D. M.; Luckham, P. F.; Lenon, S. Polymer 1995, 36, 4827. (14) Murray, M.; Rana, F.; Haq, I.; Cook, J.; Chowdhry, B. Z.; Snowden, M. J. J. Chem. Soc., Chem. Commun. 1994, 1803. (15) Pankasem, S.; Thomas, J. K.; Snowden, M. J.; Vincent, B. Langmuir 1994, 10, 3023. (16) Wu, C.; Zhou, S.; Au-yeung, S. C. F.; Jiang, S. Angew. Makromol. Chem. 1996, 240, 123.

10.1021/la0105580 CCC: $22.00 © 2002 American Chemical Society Published on Web 02/12/2002

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methods provide valuable insight into the behavior of the bulk microgel dispersion, they provide limited information with respect to the magnitude or range of forces existing between individual microgel particles. Such information would be useful when considering the potential applications of microgels. The stability of a colloidal dispersion is often predicted using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory20,21 that takes into account the interaction forces between two surfaces in an electrolyte medium. The overall interaction energy can be considered to arise from two contributions, the attractive van der Waals forces and repulsive electrostatic effects. The validity of the DLVO theory for many systems has been confirmed following measurement of surface forces using a range of techniques developed in recent years. Use of the surface force apparatus (SFA), developed by Israelachvili,22 has shown good agreement between forces determined experimentally and those predicted by DLVO theory. SFA has also confirmed the presence of other non-DLVO contributions such as steric effects in systems with adsorbed polymer layers.23,24 The atomic force microscope (AFM) developed by Binnig et al.25 enabled measurement of surface forces between a cantilever tip and a flat surface. The development of colloid probe microscopy,26 involving the attachment of a colloidal particle to a cantilever tip, has increased the versatility of the AFM. It has allowed a wider range of materials and systems to be analyzed in comparison with techniques such as the SFA. More recently, total internal reflectance microscopy (TIRM) has been used to measure surface forces between colloidal particles and a flat surface.27,28 TIRM enables detection of weaker repulsive forces such as those associated with depletion; however, its use is limited to systems without short-range attractive forces.28 Colloid probe microscopy has allowed interaction forces between a number of substrates, including mica,29 titania,30-32 and silica,29,32 to be studied. The versatility of the technique has allowed the effect of adsorption of a range of species to be investigated. Short-range steric barriers have been detected for smaller adsorbed molecules such as ions32,33 and surfactants.34 The effect of adsorbed polymer layers has been investigated and shown to correspond well to polymer scaling theory.35 Studies have

investigated the effect of changing solvent conditions35 and differing molecular weights on the steric range of adsorbed polymer layers.36 While the AFM is an effective tool in detecting the presence of steric effects, the measured range of an interaction may be subject to error. Any compressibility of the adsorbed polymer layer will prevent setting of a true constant compliance region, since this procedure is dependent upon hard contact being made between two nondeformable surfaces.35-37 Although there are difficulties inherent in analyzing interactions between deformable surfaces, investigations have been successfully carried out with respect to the interaction between the surface of an air bubble with both a glass sphere38 and a silica particle.39,40 The authors noted difficulties in assigning a zero separation due to deformation at the bubble surface. To address this problem, Hartley et al.,41 when investigating the interaction of a silica probe with a decane droplet, assigned a relative separation starting at the point at which a force of 0.01 mN m-1 was first detected. Colloid probe microscopy has previously been used to investigate the interaction between other deformable colloidal particles, for example, polystyrene latices,42 but to our knowledge a study with swellable microgels has not been reported in the literature. The versatility of the colloid probe technique makes it an appropriate method by which to study the interaction between a silica colloid probe and a surface composed of close-packed microgel particles. The microgel used in this study is a poly(N-isopropylacrylamide-acrylic acid), abbreviated to poly(NIPAM-AA), copolymer microgel (5% acrylic acid) that is sensitive to changes in ionic strength and pH. Measurements of the silica probe-microgel interactions as a function of both these parameters are reported. The primary objective of the current study was to establish whether DLVO theory would successfully describe the measured interactions between microgel and silica surfaces separated by aqueous media. A secondary objective was to determine the nature, magnitude, and cause of any non-DLVO forces that were present. This information is important, not only in understanding the stability of microgels in dispersions but also when considering their potential applications either as additives within the coatings industry or drug delivery systems.

(17) Griffiths, P. C.; Stilbs, P.; Chowdhry, B. Z.; Snowden, M. J. Colloid Polym. Sci. 1995, 273, 405. (18) Mears, S. J.; Deng, Y.; Cosgrove, T.; Pelton, R. Langmuir 1997, 13, 1901. (19) Crowther, H. M.; Saunders, B. R.; Mears, S. J.; Cosgrove, T.; Vincent, B.; King, S. M.; Yu, G.-E. Colloids Surf., A 1999, 152, 327. (20) Derjaguin, B.; Landau, L. Acta Physiochem. 1941, 14, 633. (21) Verwey, E. G. W.; Overbeek, J. Th. G. Theory of Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948. (22) Israelachvili, J. N. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975. (23) Klein, J. Adv. Colloid Interface Sci. 1982, 16, 101. (24) Luckham, P. F.; Klein, J. J. Chem. Soc., Faraday Trans. 1 1984, 80, 865. (25) Binnig, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. 1986, 56, 930. (26) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239. (27) Prieve, D.; Frej, N. Langmuir 1990, 6, 396. (28) Prieve, D. Adv. Colloid Interface Sci. 1999, 82, 93. (29) Hartley, P. G.; Larson, I.; Scales, P. J. Langmuir 1997, 13, 2207. (30) Prica, M.; Biggs, S.; Grieser, F.; Healy, T. W. Colloids Surf., A 1996, 119, 205. (31) Larson, I.; Drummond, C. J.; Chan, D. Y. C.; Greiser, F. J. Am. Chem. Soc. 1993, 115, 11885. (32) Feiler, A.; Jenkins, P.; Ralston, J. Phys. Chem. Chem. Phys. 2000, 2, 5678. (33) Biggs, S.; Scales, P. J.; Leong, Y. K.; Healy, T. W. J. Chem. Soc., Faraday Trans. 1995, 91, 2921. (34) Rutland, M. W.; Senden, T. J. Langmuir 1993, 9, 412. (35) Biggs, S. Langmuir 1995, 11, 156.

Materials. Microgel Preparation. Poly(NIPAM-AA) copolymer microgels were prepared by a free radical emulsion polymerization reaction in the absence of surfactant.43 Five grams of the monomer N-isopropylacrylamide (NIPAM, Aldrich) and 0.25 g of the comonomer, acrylic acid (AA, Aldrich) were reacted with the cross-linking agent N,N-methylenebisacrylamide (0.5 g, Aldrich) in the presence of a potassium persulfate initiator. The reaction was carried out in a three-necked vessel at 70 °C in a liter of deionized water (acidified to pH 1) for 8 h. On cooling, the microgel was dialyzed repeatedly with deionized water until the conductivity of the dialysate was less than 1 µS. Microgel Wafer Preparation. Silica wafers, obtained from Aurel GMBH (Germany), were cut to 1 cm2 squares, and the surface was coated with a thin layer of Shell Epikote 1004 resin. The

Experimental Section

(36) Braithwaite, G. J. C.; Luckham, P. F. Langmuir 1996, 12, 4224. (37) Braithwaite, G. J. C.; Howe, A.; Luckham, P. F. J. Chem. Soc., Faraday Trans. 1997, 93, 1409. (38) Butt, H. J. J. Colloid Interface Sci. 1994, 166, 109. (39) Ducker, W. A.; Zu, X.; Israelachvili, J. N. Langmuir 1994, 10, 3279. (40) Fielden, M. L.; Hayes, R. A.; Ralston, J. Langmuir 1996, 12, 3721. (41) Hartley, P. G.; Grieser, F.; Mulvaney, P.; Stevens, G. W. Langmuir 1999, 15, 7282. (42) Li, Y. Q.; Tao, N. J.; Pan, J.; Garcia, A. A.; Lindsay, S. M. Langmuir 1993, 9, 637. (43) Pelton, R. H.; Chibante, P. Colloids Surf. 1986, 120, 247.

Interaction between Poly(NIPAM-AA) and Silica wafer was then immersed in a 0.05% (w/w) microgel dispersion and allowed to dry. It was placed on a hot stage for several minutes to allow the microgel particles to set into the resin and then left to cool. The wafers were imaged with a silicon nitride cantilever (200 µm wide-legged) to ensure that they were uniformly coated. AFM images of the microgel layers showed the particles to be monodisperse and uniformly distributed on the silicon wafer. Silica Colloid Probe Preparation. Gold-coated silicon nitride cantilevers were supplied in wafer form from Nanoprobe (Park Scientific, Mountain View). Cantilevers were prepared for force measurements by attaching silica spheres (Geltech, Orlando, FL), of roughness 0.8 nm with a maximum peak height of 6 nm over an area of 0.5 µm2, to the tip of the cantilever (200 µm wide-legged). An etched tungsten wire, mounted on a three-way micromanipulation arm attached to the stage of a light microscope (Olympus BH2), was used for the addition of resin and attachment of spheres. Care was taken to ensure that the amount of glue used was small enough so that the particle did not become immersed in it. The cantilever with attached sphere will subsequently be referred to as the colloid probe. Imaging using a CCD camera (Sony) connected to a high-resolution video monitor showed the radius of the spheres to be 4 µm. Prior to force measurements, the colloid probes were washed with deionized water and ethanol before drying in a stream of purified nitrogen. Finally, they were plasma cleaned (Harrick Plasmer Cleaner PDC-32) for 1 min. The spring constant of the cantilevers was determined by the method of Cleveland et al.44 This method involves attaching tungsten spheres of different masses to the cantilever and measuring the shifts in resonant frequency. The cantilevers used in this study were determined to have a spring constant of 0.1 ((0.01) N m-1. Reagents. Solutions were prepared having a background electrolyte of potassium nitrate. pH adjustment was made using potassium hydroxide solution or nitric acid. All reagents were obtained from BDH Chemicals and were analytical grade. AFM Measurements. Both imaging and force measurements were performed using a Nanoscope III (Digital Instruments, Santa Barbara, CA). Measurements were carried out at pH 4, 6, 8, and 10, in a background electrolyte of concentration 10-2, 10-3, and 10-4 M KNO3. Solutions were introduced to the fluid cell via a syringe and Teflon tubing and allowed to equilibrate for 15 min before measurement. The experiments were performed using standard measurement procedures described by Feiler et al.32 The typical scan range and rate used were 1 µm and 0.5-1 Hz, respectively. Data for cantilever deflection against scanner (z-piezo) position were obtained. The data were converted to force (F) as a function of scanner position. Further details of the analysis protocol adopted are given in Results and Discussion. Phase Analysis Light Scattering (PALS). Microgel dispersions were prepared at a concentration of 0.01% (w/w) over the pH range 3-10 in the presence of a background electrolyte (10-2, 10-3, and 10-4 M KNO3). Electrophoretic mobilities were obtained from PALS measurements.45 Studies carried out at the University of Greenwich46 and, more recently, by Rasmusson et al.47 and Daly et al.48 have compared the electrophoretic mobilities obtained experimentally with those calculated using the Ohshima model for soft particles.49 Good agreement was found between the mobilities obtained experimentally and those predicted using the Ohshima model. Electrophoretic mobilities obtained from PALS were converted to ζ potentials using the Smoluchowski equation.50 Due to the “soft” nature of microgel particles, use of the Smoluchowski equation is not entirely suitable for conversion of electrophoretic (44) Cleveland, J. P.; Manne, S.; Bocek, D.; Hansma, P. K. Rev. Sci. Instrum. 1993, 64, 403. (45) Miller, J. F.; Scha¨tzel, K.; Vincent, B. J. Colloid Interface Sci. 1991, 143, 532. (46) Islam, A. Unpublished results, University of Greenwich, 2001. (47) Rasmusson, M.; Vincent, B.; Marston, N. Colloid Polym. Sci. 2000, 278, 253. (48) Daly, E.; Saunders, B. Phys. Chem. Chem. Phys. 2000, 2, 3187. (49) Ohshima, H.; Makino, K.; Kato, T.; Fujimoto, K.; Kondo, T.; Kawaguchi, H. J. Colloid Interface Sci. 1993, 159, 152. (50) von Smoluchowski, M. Z. Phys. Chem. 1918, 92, 129.

Langmuir, Vol. 18, No. 6, 2002 2091 mobilities to ζ potentials. In the current study, the use of the Smoluchowski equation serves merely to provide an estimate for the ζ potential of the microgel particles. This can then be used as an initial value for the diffuse layer potential of the microgel, a parameter in the standard AFM data fitting procedure. The diffuse layer potential value chosen will reflect how a hard particle with the same mobility would behave; any discrepancies between theory and experiment will be ascribable to the softness of the microgel particles. Light Scattering. Microgel dispersions of 0.05% (w/w) were prepared in the presence of 10-2, 10-3, and 10-4 M KNO3 with the pH adjusted over the range 3-10. Hydrodynamic diameters of the microgel particles were measured using a Zetasizer 3000 (Malvern Instruments, Malvern, U.K.) equipped with a neonhelium laser operating at λ ) 634 nm.

Results and Discussion A number of issues were addressed in order to analyze the raw force against separation data. The standard AFM analysis is complicated when one of the surfaces is comprised of deformable microgel particles as opposed to a nondeformable solid. Throughout Results and Discussion, the reader will find outlines of the problems we encountered and descriptions of our solutions. We have included these as they are necessary in order to appreciate the interpretation of results that is presented. Agreement with DLVO Theory. This section highlights the inability of DLVO theory to satisfactorily describe the measured interactions between silica and microgel surfaces under all the solution conditions that were studied. The Derjaguin approximation51 can be used to relate the force, F, between a sphere of radius R and a flat surface to the interaction energy per unit area between parallel plates (Epp):

F/R ) 2πEpp

(1)

For the purpose of this work, the geometry of the microgel layer has been approximated to that of a flat plate. This approximation is supported by both the close-packed nature of the microgel film and the large difference between the diameter of the silica colloid probe and the individual microgel particles. Suresh and Walz,52 when investigating the effect of surface roughness on the interaction between a sphere and a flat plate, modeled the roughness as hemispherical asperities. They found that the total interaction energy was decreased only at very small separations. It is the force behavior at larger separations that is primarily of interest in the current study. DLVO theory separates the total interaction energy per unit area between two macroscopic bodies into an attractive van der Waals interaction free energy per unit area (Evdw) and a repulsive electrical double-layer interaction free energy per unit area between two parallel flat plates (Eedl):

Epp ) Evdw + Eedl

(2)

For simplicity, the effect of retardation on the van der Waals force has been ignored. The double layer interaction, Eedl, has been calculated for the constant potential and constant charge limits of the nonlinearized PoissonBoltzmann equation. The constant charge limit gives an upper bound on the electrical double-layer interaction, while the constant potential condition yields the lower bound.53 (51) Derjaguin, B. V. Kolloid-Z. 1934, 69, 155. (52) Suresh, L.; Walz, J. Y. J. Colloid Interface Sci. 1997, 196, 177.

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Figure 1. Normalized force-separation curve taken between a silica sphere (R ) 4 µm) and a poly(NIPAM-AA) microgelcovered wafer separated by a 10-4 M KNO3 solution at pH 8. The diamonds ([) correspond to the experimental data. The solid line corresponds to the data derived from the theoretical model for constant charge. The diffuse layer potentials used were ψSiO2 ) -50 mV and ψNIPAM-AA ) -20 mV for the silica and microgel surfaces, respectively. The Hamaker constant used was 2 × 10-20 J.

Figure 2. Measured electrophoretic mobilities and calculated ζ potentials obtained for poly(NIPAM-AA) microgel particles (0.01% w/w) dispersed in aqueous potassium nitrate solutions: ([) 10-2 M, (9) 10-3 M, and (2) 10-4 M KNO3.

A normalized force-separation curve, taken between a silica colloid probe and the microgel layer at pH 8.0 in a background electrolyte concentration of 10-4 M KNO3, is shown in Figure 1. The data have been plotted in the regular manner; that is, the onset of constant compliance, the point at which the cantilever deflection becomes linear with respect to the motion of the piezo-controlled surface, has been taken to be zero separation. The theoretical plot derived from the constant charge limit of the DLVO theory is also shown. This was calculated using a diffuse layer potential of -20 and -50 mV for the microgel and the silica surfaces, respectively. The potential used for the microgel was obtained from our own electrophoresis results (see Figure 2). That for silica was taken from the work of Feiler et al.32 These authors measured the symmetric interaction, under the same solution conditions as our study, between two silica particles (from the same batch from which we obtained our silica particles). The value for the Hamaker constant of 2 × 10-20 J used is typical for silica surfaces interacting across an aqueous medium.32 The ζ potential (measured in this study) of the microgel and the fitted diffuse layer potential of the silica from Feiler et al. (obtained under identical solution conditions to our own AFM data) have been used through(53) Wiese, G. R.; Healy, T. W. Trans. Faraday Soc. 1970, 66, 490.

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Figure 3. A schematic drawing to show how the stiffness of a surface affects setting the constant compliance region. In the case of the nondeformable surface, hard contact is made when the probe makes contact with the particle surface (d0). In the case of a deformable surface, soft contact occurs at the point at which the probe reaches the surface (d0) while hard contact occurs once the surface has been fully compressed by a further distance (dc).

out the paper in order to predict the theoretical microgelsilica interaction forces. The constant charge DLVO plot calculated using appropriate values of diffuse layer potential does not agree with the experimental data. In particular, the magnitude of the interaction is dramatically underestimated at all separations. In fact, to obtain the correct magnitude for the interaction, it is necessary to increase the product of the fitted diffuse layer potentials by more than an order of magnitude. For example, if the diffuse layer potential of the silica were maintained at -50 mV, then that for the microgel would need to be increased to in excess of -200 mV. Our choice of Hamaker constant is also worthy of discussion in light of comments by Saunders,3 who suggested that a microgel would have a Hamaker constant similar to that of the solvent medium. It is possible that we have used a Hamaker constant that is too large. However, this will exaggerate only the shortrange attractive forces. This is the opposite of what we observe in Figure 1; only the theoretical fit shows an attractive “turnover” at small separations. It appears that within the framework of DLVO theory it is not possible to fit the experimental data while still maintaining realistic values for the Hamaker constant and the diffuse layer potentials of both surfaces. The experimental results indicate that additional forces, not considered by DLVO theory, also contribute to the overall interaction force measured between the silica and microgel surfaces. It is proposed that these non-DLVO forces arise due to the inherent deformability of the microgel particle and steric interactions arising from “hairy polyelectrolyte layers”47 present at the microgel surface. We shall consider both of these effects in more detail later. Relative Separations. The deformability of microgel particles is well recorded in the literature with a number of studies having been carried out using rheology13,54 and viscometry.55 For deformable surfaces, the absolute position of the constant compliance region will depend on the degree of deformability of the surface. The less deformable the interface between two surfaces, the closer the point of constant compliance will be to the position of the unperturbed surface. This is illustrated diagrammatically in Figure 3. In their measurements between an oil droplet and a colloid probe, Hartley et al.41 circumvented this problem by defining a zero “relative separation” at the position for which deflection of the cantilever was first clearly measured. A normalized force of 0.01 mN m-1 was (54) Wolfe, M. S.; Scopazzi, C. J. Colloid Interface Sci. 1989, 133, 265. (55) Buscall, R. Colloids Surf., A 1994, 83, 33.

Interaction between Poly(NIPAM-AA) and Silica

Figure 4. Normalized force against relative separation for the interaction of a silica sphere (R ) 4 µm) and a poly(NIPAMAA) microgel layer in 10-4 M KNO3 at ([) pH 4, (2) pH 6, (b) pH 8, and (9) pH 10. Also shown are the point of zero separation used in conventional analysis and the theoretical constant charge DLVO plot from Figure 1.

Figure 5. Normalized force against relative separation for the interaction of a silica sphere (R ) 4 µm) and a poly(NIPAMAA) microgel layer in 10-3 M KNO3 at ([) pH 4, (2) pH 6, (b) pH 8, and (9) pH 10.

chosen to treat their data; that is, zero relative separation corresponded to the position at which a force of 0.01 mN m-1 was first measured. In the current study, values for the normalized force, to which the positional data were scaled, were chosen depending upon the level of noise present in each set of experiments. For each of the raw force data sets analyzed, the “limiting” slope of the constant compliance region, used for the distance calibration of the cantilever deflection, was compared to slopes obtained between two nondeformable silica surfaces. The slopes obtained were found to be the same for all measurements and could, therefore, be used to accurately convert raw deflection data to force. Figure 4 shows the normalized force against relative separation results for the interaction between silica and microgel surfaces separated by 10-4 M KNO3 solutions at various pH values. All the data were analyzed following the protocol of Hartley et al.41 Also shown is the theoretical constant charge model (at pH 8) and the position at which zero was set when the data were analyzed in the conventional manner. Figures 5 and 6 show the normalized force against relative separation results collected at 10-3 and 10-2 M KNO3, respectively. Figure 7 presents the normalized force against relative separation separated by aqueous potassium nitrate solutions of varying concentration at pH 8. From the data presented in Figures 4-7, it can be seen that the overall interaction is repulsive under all the pH and ionic strength conditions examined. No evidence of attractive turnovers (indicative of van der Waals forces) at small separations was detected. The

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Figure 6. Normalized force against relative separation for the interaction of a silica sphere (R ) 4 µm) and a poly(NIPAMAA) microgel layer in 10-2 M KNO3 at ([) pH 4, (2) pH 6, (b) pH 8, and (9) pH 10.

Figure 7. Normalized force against relative separation for the interaction of a silica sphere (R ) 4 µm) with a poly(NIPAMAA) microgel layer in aqueous potassium nitrate solutions at pH 8: (9) 10-2 M, (2) 10-3 M, and ([) 10-4 M KNO3.

measured interaction forces are consistent with the welldocumented stability of microgel dispersions. It has been reported6 that even in the presence of relatively high electrolyte concentrations, microgels remain dispersed below their transition temperatures. The measured force-separation curves reveal two main trends: At a fixed pH, the relative separation range, over which the silica-microgel interaction is detectable, increases with decreasing ionic strength. At a fixed ionic strength, the smallest range of interaction is obtained below the pH at which acrylic acid dissociates. At pH values above the pKa of the carboxylate group in poly(acrylic acid), the relative separation range increases as the pH is lowered. Effect of Ionic Strength. In this section, we will consider the likely reasons behind the observed decrease in interaction range as the ionic strength is increased. The data, shown in Figure 7, reveal that the interaction ranges are approximately 120, 55, and 18 nm at 10-4, 10-3, and 10-2 M KNO3, respectively. This interaction range is far greater than would be expected if the forces between the surfaces were governed by solely DLVO-type electrostatics. Inspection of the light scattering data in Figure 8 shows that the hydrodynamic diameter of the microgel particles decreases with increasing ionic strength. The hydrodynamic diameters at pH 8 in 10-4, 10-3, and 10-2 M KNO3 are around 1050, 960, and 850 nm, respectively. Table 1 shows the decrease at the higher electrolyte concentrations, relative to the value at pH 8 and 10-4 M KNO3, of both the hydrodynamic radii of the microgel particles and the interaction range. There is a

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Figure 8. Hydrodynamic diameters measured by dynamic light scattering for poly(NIPAM-AA) microgel particles (0.01% w/w) dispersed in an aqueous potassium nitrate solution: ([) 10-2 M, (2) 10-3 M, and (b) 10-4 M KNO3. Table 1. Measured Decrease in Both the Hydrodynamic Radius of the Microgel Particles and the Range of the Silica-Microgel Interaction as a Function of the Concentration of the Potassium Nitrate Solution

KNO3 (M)

decrease in hydrodynamic radius (nm)

decrease in interaction range (nm)

10-4 10-3 10-2

n/a 45 100

n/a 37 102

very close agreement between the two quantities, suggesting that the change in the interaction range with ionic strength is related to the size of the microgel particles. Let us now consider how the size of the microgel changes with ionic strength. The poly(NIPAM-AA) microgels possess two distinct types of ionizable groups: sulfate (pKa at pH 2) and carboxylate (pKa at pH 4.2) arising from the initiator and acrylic acid monomer, respectively. With increasing ionic strength, greater shielding of these ionized groups within the microgel “core” decreases the osmotic imbalance between the microgel interior and exterior. This results in water moving out of the microgel particle causing the core to deswell.3 Furthermore, the ionized groups also exist on the surface of the microgel as part of “hairy” polyelectrolyte chains. Greater shielding of these charged (“surface”) groups, due to the increased ionic strength, causes the chains to adopt a more compact conformation. As a result, the hairy layer will extend less far into the bulk solution. Both the deswelling of the core and the contraction of the hairy layer will lead to a reduction in the hydrodynamic diameter of the microgel particles. As a consequence, the deformability of the microgel core will decrease, while the range at which the hairy layer interacts with an approaching surface will be reduced. These two effects will both act to decrease the range of the measured interaction force at higher ionic strengths. Quantification of the exact contribution of each component is outside the scope of the present work, but it could be investigated by varying the degree of cross-linking (and hence deformability) present in the microgel. Although electrostatic effects (in the true DLVO sense) are not the major component of the force behavior observed, they do control the range of the dominant steric and deformation forces. Effect of pH. This final section will discuss the experimentally observed changes in the range of the interaction forces between silica and microgel surfaces when the solution pH is varied. An explanation will be given that is consistent with the data presented. Figures 4, 5, and 6 show the normalized force against separation data, at various pHs, for the silica-microgel

system in 10-4, 10-3, and 10-2 M potassium nitrate solutions, respectively. At fixed ionic strength, the observed range of the interaction was in the following order: pH 6 > pH 10 ≈ pH 8 > pH 4. This was true for all three ionic strengths investigated. The smallest interaction range observed occurred at pH 4. The pKa of the carboxylate group in the acrylic acid component of the microgel is at pH 4.2 (note that the pKa of the sulfate group occurs at pH 2; it will be dissociated under all pH conditions studied in this work). Below pH 4.2, the carboxylate groups are not dissociated. Biggs35 demonstrated that poly(acrylic acid) collapses at pH values below the pKa. This observation was supported by fluorescence probe data that indicated that the homopolymer adopts a highly globular conformation when uncharged. The consequence of this for the poly(NIPAMAA) particles used in the current study are twofold. First, it would be expected that the microgel core will deswell due to the decreased amount of ionized charge it contains. Second, the hairy polyelectrolyte layers will also adopt a less extended conformation to minimize polymer-solvent interactions. This is evident upon consideration of the hydrodynamic diameter data in Figure 8: the diameter at pH 4 is considerably less than that at the higher pH values. As a consequence, the microgel will be less deformable and possess a thinner hairy layer. This will lead to a reduction in range of both the deformation and steric contributions of the repulsive force. Furthermore, the potential on the silica surface is not fully developed at the lower pH and this will impact upon the minor (DLVO) electrostatic contribution to the force. We also note that pH 4 was the only pH at which adhesion between the two surfaces was observed on the retract cycle of the AFM measurements. We believe that on contact of the silica and microgel surfaces, the hairy layer of the microgel can minimize polymer-solvent interactions by attaching to the silica surface. This results in a “sticky” polymer bridge between the two surfaces. The observed interaction range is greater at pH values in excess of the pKa of the carboxylate group in acrylic acid than at pH 4. Increasing the pH of the electrolyte solution results in ionization of the carboxylate groups that in turn causes the charge on the microgel particle to become increasingly negative. Indeed, the electrophoretic mobility and ζ potential results in Figure 2 indicate that the negative microgel charge increases with pH, reaching plateau values at a pH of around 6. This increase in charge results in a swelling of the microgel core and an extension of the hairy layer into solution. The microgel becomes more deformable, while the thickness of the steric layer is also increased. The hydrodynamic diameter data shown in Figure 8 support this suggestion. The outcome is an increase in the range of the deformation and steric contributions of the overall force. There is also a small increase in the (DLVO electrostatic) repulsive force as a result of the increased surface potential on the silica surface as the pH is increased from 4 to higher values. The largest interaction range occurred at pH 6 and not pH 8 or 10. The difference between the ranges is quite marked. This is a puzzling observation since the measured hydrodynamic diameters and electrophoretic mobilities of the microgel particles at all three pH values were similar. The result, however, was reproducible, and we believe it is a real effect and not an experimental artifact. We suggest that the explanation is linked to the possibility that at pH 6 a proportion of the charge groups in the microgel may not be ionized. These nonionized groups may be predominantly located in the microgel core, while the groups in the hairy layer at the surface may be almost

Interaction between Poly(NIPAM-AA) and Silica

fully ionized. This situation would result in the core of the microgel being more compressible at pH 6 than at the higher pH values. However, the microgel would maintain a similar ζ potential at all these pHs (since the fully ionized groups in the hairy layer would be expected to be the main contributors to the ζ potential). Furthermore, the consistent level of ionized charge groups at the surface of the microgel would ensure that the extension of the hairy layers would also be comparable at all the pH values. This would result in a hydrodynamic diameter that would not vary significantly with pH. However, a full explanation of this observation requires far more extensive investigation and is outside the scope of this study. Conclusions Colloid probe microcopy has been used to show that under conditions of changing ionic strength and pH, the N-isopropylacrylamide-acrylic acid microgel-silica interaction is repulsive at all separations. The interaction forces cannot be adequately described by the DLVO theory alone. It appears that both deformation of the whole microgel

Langmuir, Vol. 18, No. 6, 2002 2095

particle and the presence of a hairy steric layer, composed of polyelectrolyte chains, around the microgel core are the major contributions to the overall interaction force. It has been shown that changing the ionic strength or pH of the solution can vary the range of both the deformation and steric components of the interaction. This is ascribed to the changes in screening of the sulfate and carboxylate groups present in the microgel with varying electrolyte concentrations and the variation in dissociation of the carboxylate groups with pH. These electrostatic effects cause changes in the conformation of the microgel core and hairy layer. Acknowledgment. The Australian Research Council is thanked for funding this work through the Small Grant Scheme. Adam Feiler (Ian Wark Research Institute) is acknowledged for his help with experimental aspects of the AFM work. Roland Keir (University of South Australia) is thanked for his assistance with the PALS measurements. LA0105580