Colloidal Stabilization of Nanoparticles in Concentrated Suspensions

This remarkably high concentration facilitates the fabrication of a wide range of products and intermediates in materials technology, cosmetics, pharm...
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Langmuir 2007, 23, 1081-1090

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Colloidal Stabilization of Nanoparticles in Concentrated Suspensions Andre´ R. Studart,* Esther Amstad, and Ludwig J. Gauckler Department of Materials, ETH-Zurich, Zurich, CH 8093, Switzerland ReceiVed July 14, 2006. In Final Form: NoVember 6, 2006 The stabilization of nanoparticles in concentrated aqueous suspensions is required in many manufacturing technologies and industrial products. Nanoparticles are commonly stabilized through the adsorption of a dispersant layer around the particle surface. The formation of a dispersant layer (adlayer) of appropriate thickness is crucial for the stabilization of suspensions containing high nanoparticle concentrations. Thick adlayers result in an excessive excluded volume around the particles, whereas thin adlayers lead to particle agglomeration. Both effects reduce the maximum concentration of nanoparticles in the suspension. However, conventional dispersants do not allow for a systematic control of the adlayer thickness on the particle surface. In this study, we synthesized dispersants with a molecular architecture that enables better control over the particle adlayer thickness. By tailoring the chemistry and length of these novel dispersants, we were able to prepare fluid suspensions (viscosity < 1 Pa‚s at 100 s-1) with more than 40 vol % of 65-nm alumina particles in water, as opposed to the 30 vol % achieved with a state-of-the-art dispersing agent. This remarkably high concentration facilitates the fabrication of a wide range of products and intermediates in materials technology, cosmetics, pharmacy, and in all other areas where concentrated nanoparticle suspensions are required. On the basis of the proposed molecular architecture, one can also envisage other similar molecules that could be successfully applied for the functionalization of surfaces for biosensing, chromatography, medical imaging, drug delivery, and aqueous lubrication, among others.

1. Introduction The special optical, electronic, and catalytic properties of nanoparticles are expected to revolutionize a number of areas in engineering and life sciences in the near future.1,2 In engineeringrelated areas, the advantageous properties of nanosized particles are currently being considered for the preparation of fuel cells,3 energy storage media,4 sensors,5,6 as well as catalytic3,7 and composite8 materials. The progress achieved so far in these fields is in part related to the increasing availability of nanoparticles with various chemical compositions produced via wet or dry methods.9-13 * Corresponding author. Telephone number: +41-44-632-3718. Fax number: +41-44-632-1132. E-mail: [email protected]. (1) Lewin, M.; Carlesso, N.; Tung, C. H.; Tang, X. W.; Cory, D.; Scadden, D. T.; Weissleder, R. Tat peptide-derivatized magnetic nanoparticles allow in vivo tracking and recovery of progenitor cells. Nat. Biotechnol. 2000, 18 (4), 410-414. (2) Rabin, O.; Perez, J. M.; Grimm, J.; Wojtkiewicz, G.; Weissleder, R. An X-ray computed tomography imaging agent based on long-circulating bismuth sulphide nanoparticles. Nat. Mater. 2006, 5 (2), 118-122. (3) Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Ordered nanoporous arrays of carbon supporting high dispersions of platinum nanoparticles. Nature 2001, 412 (6843), 169-172. (4) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Storage of hydrogen in single-walled carbon nanotubes. Nature 1997, 386 (6623), 377-379. (5) Shipway, A. N.; Katz, E.; Willner, I. Nanoparticle arrays on surfaces for electronic, optical, and sensor applications. ChemPhysChem 2000, 1 (1), 18-52. (6) Pinna, N.; Neri, G.; Antonietti, M.; Niederberger, M. Nonaqueous synthesis of nanocrystalline semiconducting metal oxides for gas sensing. Angew. Chem., Int. Ed. 2004, 43 (33), 4345-4349. (7) Mastalir, A.; Frank, B.; Szizybalski, A.; Soerijanto, H.; Deshpande, A.; Niederberger, M.; Schomacker, R.; Schlogl, R.; Ressler, T. Steam reforming of methanol over Cu/ZrO2/CeO2 catalysts: A kinetic study. J. Catal. 2005, 230 (2), 464-475. (8) Komarneni, S. Nanocomposites. J. Mater. Chem. 1992, 2 (12), 12191230. (9) Messing, G. L.; Zhang, S. C.; Jayanthi, G. V. Ceramic powder synthesis by spray-pyrolysis. J. Am. Ceram. Soc. 1993, 76 (11), 2707-2726. (10) Pratsinis, S. E. Flame aerosol synthesis of ceramic powders. Prog. Energy Combust. Sci. 1998, 24 (3), 197-219. (11) Niederberger, M.; Bard, M. H.; Stucky, G. D. Benzyl alcohol and transition metal chlorides as a versatile reaction system for the nonaqueous and lowtemperature synthesis of crystalline nano-objects with controlled dimensionality. J. Am. Chem. Soc. 2002, 124 (46), 13642-13643.

The production of nonaggregated nanoparticles (particle radius, a < 50 nm) is, however, just the first step toward their final application. After production, nanoparticles are often dispersed in a liquid phase to render suspensions that can be processed into useful components and devices. At this stage, molecules are used to adsorb on the particle surface in order to keep them dispersed in the liquid phase. Similarly, nanoparticles investigated for drug delivery, medical diagnostics, and imaging purposes require a protective organic layer to prevent coagulation and add specific functionalities to the particle surface.14,15 The organic layer formed around the nanoparticles (adlayer) has to be sufficiently thick to provide a steric barrier that counterbalances the attractive van der Waals forces responsible for particle agglomeration. The formation of thick adlayers around nanoparticles is an efficient approach to prevent agglomeration in the diluted systems typically used in biomedical applications.16-18 However, thick adlayers eventually limit the maximum concentration of nanoparticles that can be dispersed into a liquid to form a concentrated fluid suspension. Suspensions with a high concentration of nanoparticles (>40 vol %) can be very advantageous, if not a primary requirement, in many current and future technologies for device fabrication.19,20 In ceramic (12) Masala, O.; Seshadri, R. Synthesis routes for large volumes of nanoparticles. Annu. ReV. Mater. Res. 2004, 34, 41-81. (13) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. D. A general strategy for nanocrystal synthesis. Nature 2005, 437 (7055), 121-124. (14) Kohler, N.; Sun, C.; Wang, J.; Zhang, M. Q. Methotrexate-modified superparamagnetic nanoparticles and their intracellular uptake into human cancer cells. Langmuir 2005, 21 (19), 8858-8864. (15) Bertorelle, F.; Wilhelm, C.; Roger, J.; Gazeau, F.; Menager, C.; Cabuil, V. Fluorescence-modified superparamagnetic nanoparticles: Intracellular uptake and use in cellular imaging. Langmuir 2006, 22 (12), 5385-5391. (16) Colfen, H. Double-hydrophilic block copolymers: Synthesis and application as novel surfactants and crystal growth modifiers. Macromol. Rapid Commun. 2001, 22 (4), 219-252. (17) Pastoriza-Santos, I.; Liz-Marzan, L. M. Formation of PVP-protected metal nanoparticles in DMF. Langmuir 2002, 18 (7), 2888-2894. (18) Sehgal, A.; Lalatonne, Y.; Berrett, J. F.; Morvan, M. Precipitationredispersion of cerium oxide nanoparticles with poly(acrylic acid): Toward stable dispersions. Langmuir 2005, 21 (20), 9359-9364. (19) Smay, J. E.; Cesarano, J.; Lewis, J. A. Colloidal inks for directed assembly of 3-D periodic structures. Langmuir 2002, 18 (14), 5429-5437.

10.1021/la062042s CCC: $37.00 © 2007 American Chemical Society Published on Web 12/20/2006

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manufacturing, for instance, crack-free inorganic films and structures produced by tape-casting, screen-printing, or direct assembly techniques can hardly be produced using suspensions with low solids content. Therefore, a number of studies have recently addressed the rheological behavior and colloidal stability of concentrated suspensions containing nanoparticles.21-25 However, the concentration of nanoparticles in fluid suspensions (viscosity < 1 Pa‚s at 100 s-1) is typically lower than 35-40 vol %.22,23,25-29 The limited concentration of suspensions containing nanoparticles is attributed to the excluded volume formed by the stabilizing adlayer, which increases the “effective” volume fraction of solids in the suspension (φeff). The effective volume fraction φeff is given by the sum of the “real” volume fraction of solids (φ) and the excluded volume of the stabilizing adlayer (φexcl) as follows:30

[ (δa)]

φeff ) φ + φexcl ) φ 1 +

3

(1)

where a is the particle radius and δ is the adlayer thickness. The “effective” volume fraction of solids (φeff) is limited by the maximum packing density of particles (∼0.64 for random packing). Consequently, the high δ/a ratio and the excluded volume (φexcl) of nanoparticles coated with thick adlayers lead to a decrease in the suspension’s “real” solids content (φ) (eq 1). In order to increase the concentration of nanoparticles in fluid suspensions, one can, in principle, simply reduce the adlayer thickness (δ) around the particle by providing short-chain dispersants for particle stabilization. However, an excessive decrease in the adlayer thickness leads to particle agglomeration, decreasing the maximum concentration of nanoparticles that can be achieved. Therefore, an optimum adlayer has to be provided in such concentrated systems that is thin enough to avoid an excessive excluded volume around the particle and is at the same time sufficiently thick to keep particles stable against agglomeration.30 (20) Smay, J. E.; Gratson, G. M.; Shepherd, R. F.; Cesarano, J.; Lewis, J. A. Directed colloidal assembly of 3D periodic structures. AdV. Mater. 2002, 14 (18), 1279-1283. (21) Zaman, A. A.; Singh, P.; Moudgil, B. M. Impact of self-assembled surfactant structures on rheology of concentrated nanoparticle dispersions. J. Colloid Interface Sci. 2002, 251 (2), 381-387. (22) Shen, Z. G.; Chen, J. F.; Zhou, H. K.; Yun, J. Rheology of colloidal nanosized BaTiO3 suspension with ammonium salt of polyacrylic acid as a dispersant. Colloids Surf., A 2004, 244 (1-3), 61-66. (23) Bell, N. S.; Schendel, M. E.; Piech, M. Rheological properties of nanopowder alumina coated with adsorbed fatty acids. J. Colloid Interface Sci. 2005, 287 (1), 94-106. (24) Rao, R. B.; Kobelev, V. L.; Li, Q.; Lewis, J. A.; Schweizer, K. S. Nonlinear elasticity and yielding of nanoparticle glasses. Langmuir 2006, 22 (6), 24412443. (25) Tseng, W. J.; Tzeng, F. Effect of ammonium polyacrylate on dispersion and rheology of aqueous ITO nanoparticle colloids. Colloids Surf., A 2006, 276 (1-3), 34-39. (26) Schilling, C. H.; Sikora, M.; Tomasik, P.; Li, C. P.; Garcia, V. Rheology of alumina-nanoparticle suspensions: Effects of lower saccharides and sugar alcohols. J. Eur. Ceram. Soc. 2002, 22 (6), 917-921. (27) Liu, Y. Q.; Gao, L. Effect of 2-phosphonobutane-1,2,4-tricarboxylic acid adsorption on the stability and rheological properties of aqueous nanosized 3mol%yttria-stabilized tetragonal-zirconia polycrystal suspensions. J. Am. Ceram. Soc. 2003, 86 (7), 1106-1113. (28) Kirby, G. H.; Harris, D. J.; Li, Q.; Lewis, J. A. Poly(acrylic acid)poly(ethylene oxide) comb polymer effects on BaTiO3 nanoparticle suspension stability. J. Am. Ceram. Soc. 2004, 87 (2), 181-186. (29) Li, C. P.; Akinc, M.; Wiench, J.; Pruski, M.; Schilling, C. H. Relationship between water mobility and viscosity of nanometric alumina suspensions. J. Am. Ceram. Soc. 2005, 88 (10), 2762-2768. (30) Sigmund, W. M.; Bell, N. S.; Bergstrom, L. Novel powder-processing methods for advanced ceramics. J. Am. Ceram. Soc. 2000, 83 (7), 1557-1574.

In a previous article using model alumina nanoparticles,31 we estimated that an adlayer thickness between 3 and 4 nm should be optimum for the preparation of fluid suspensions with up to about 45 vol % of 65-nm particles. In this previous study, the dispersants applied were not sufficiently long (δ < 2.6 nm) to provide the optimum adlayer thickness, and thus the solids content of the investigated nonaqueous suspensions were limited to approximately 30 vol %. The aim of this study is to deliberately design and evaluate dispersant molecules that can provide the optimum adlayer thickness required for maximizing the concentration of nanoparticles in colloidal aqueous suspensions. For that purpose, we investigated the same model nanoparticles used in our previous study, for which an optimum adlayer thickness between 3 and 4 nm has been estimated. It is important to note that, even though this study is focused on concentrated suspensions, the chemistry and architecture of the synthesized molecules are also suitable for the dispersion of nanoparticles in diluted systems of relevance for biomedical applications.

2. Approach In order to achieve a significant coverage of the nanoparticle surface and to allow for deliberate control of the adlayer thickness, we designed dispersant molecules that exhibit a head-tail architecture similar to that of amphiphilic surfactants (GallolPEG, Figure 1a). The molecule head consists of a 1,2,3trihydroxybenzene group (pyrogallol), which is known to adsorb very efficiently on amphoteric oxide surfaces via ligand exchange reactions.32 The tail of the molecule comprises a water-soluble poly(ethylene glycol) (PEG) chain with tailor-made length. The new dispersant molecule is expected to efficiently adsorb on the alumina surface with the pyrogallol head group and to extend the water-soluble PEG chain toward the aqueous medium. Assuming such conformation on the particle surface, the length of the PEG tail was deliberately chosen to provide the optimum adlayer thickness required for maximizing the suspension’s solids content. Since PEG moieties can prevent the nonspecific adsorption of proteins on surfaces,33 the Gallol-PEG molecules synthesized here can also be applied on planar surfaces or nanoparticles used in biosensing, drug delivery, and imaging applications. The strong chelating properties of the pyrogallol head group should also be favorable in such applications due to their efficient anchoring on metal oxide surfaces. It is important to note that the dispersants synthesized in this work are not amphiphilic molecules like conventional surfactants and thus are readily soluble in water at quite high concentrations. This is a crucial requirement for the surface modification of a large concentration of high-surface-area nanoparticles in water, since a high amount of soluble dispersant is needed in the aqueous phase to reach a significant particle surface coverage. To our knowledge, small dispersant molecules displaying the features highlighted above have never been previously exploited for the colloidal stabilization of particles. In this work, the effect of these novel dispersant molecules on the colloidal stability and rheological behavior of concentrated nanoparticle suspensions was compared to that achieved using (31) Studart, A. R.; Amstad, E.; Antoni, M.; Gauckler, L. J. Rheology of concentrated suspensions containing weakly attractive nanoparticles. J. Am. Ceram. Soc. 2006, 89 (8), 2418-2425. (32) Hidber, P. C.; Graule, T. J.; Gauckler, L. J. Influence of the dispersant structure on properties of electrostatically stabilized aqueous alumina suspensions. J. Eur. Ceram. Soc. 1997, 17 (2-3), 239-249. (33) Jeon, S. I.; Lee, J. H.; Andrade, J. D.; Degennes, P. G. Protein surface interactions in the presence of polyethylene oxide. 1. Simplified theory. J. Colloid Interface Sci. 1991, 142 (1), 149-158.

Colloidal Stabilization of Nanoparticles

Figure 1. Stabilization mechanisms investigated in this work to disperse the nanoparticles in concentrated suspensions: (a) steric layer formed by Gallol-PEG molecules with tailored head-tail architecture for the deliberate control of the adlayer thickness (δ); (b) steric layer provided by a state-of-the-art comb-like copolymer (PMA-PEG); and (c) electric double layer formed by counter- and co-ions around the surface of charged particles. For clarity, electrical charges were omitted in panels a and b.

a state-of-the-art comb-like copolymer (Figure 1b) and a simple electrostatic approach for particle stabilization (Figure 1c). 3. Materials and Methods 3.1. Materials. Spherical alumina particles with an average diameter of 65 nm (d90 ) 130, d10 ) 33 nm) were used as model nanoparticles in this work (Nanophase Technologies Co., Romeoville, IL). The powder consisted of a mixture of δ- and γ-Al2O3 (∼70 and 30%, respectively) with an overall density of 3.6 g/cm3. Particles displayed a specific surface area of 38 m2/g and were used as received from the supplier. The tailor-made Gallol-PEG molecules depicted in Figure 1a were synthesized through the esterification reaction between gallic acid (molar mass 170 g/mol, Fluka AG, Buchs, Switzerland) and PEG monomethyl ether (PEG-Me, Fluka AG, Buchs, Switzerland). Two grades of PEG-Me (average molar mass of 350 g/mol and 550 g/mol) were chosen in order to provide an adlayer thickness close to the

Langmuir, Vol. 23, No. 3, 2007 1083 3-4 nm required for maximizing the concentration of alumina nanoparticles in the suspension.31 The esterification reaction to obtain the Gallol-PEG molecules was performed at 130 °C for 5 h using a PEG-Me-to-gallic acid mass ratio of approximately 10. An excess of PEG-Me was used to ensure total conversion of the gallic acid molecules into the ester and to avoid condensation reactions among the gallic acid molecules. A concentration of 4 wt % (based on gallic acid) of fuming sulfuric acid (oleum) was also used to protonate the carboxylic acid group of the gallate molecules and thus favor the esterification reaction. The reaction flask was initially flushed with nitrogen gas and subsequently kept under vacuum (50 mbar) for water extraction during esterification. The formation of the ester was followed by thin layer chromatography and confirmed by H NMR spectroscopy (Bruker BioSpin NMR, 500 MHz, Bruker AG, Fa¨llander, Switzerland). Complete esterification was achieved using the abovementioned conditions, resulting in a mixture of Gallol-PEG ester (10 wt %) and PEG-Me (90 wt %) as the final product. Preliminary rheological experiments revealed that pure PEG-Me behaves as an inert molecule in the aqueous suspension and does not exhibit any dispersing effect on the evaluated nanoparticles. Additionally, no depletion effect is expected from these molecules due to their small size (on average, 18 MΩ‚cm, Nanopure water system, Barnstead, Dubuque, IA), KNO3 salt, as well as KOH and HNO3 1M aqueous solutions (Titrisol, Fluka AG, Buchs, Switzerland) were the other chemicals used for suspension preparation. 3.2. Adsorption Measurements. The adsorption of Gallol-PEG molecules on the surface of alumina particles was measured with the help of UV-visible spectroscopy (UV/vis Spectrometer, Lambda 2, Perkin-Elmer GmbH, U ¨ berlingen, Germany). Gallol-PEG molecules exhibit a peak in UV absorption at a wavelength of 274 nm. This absorbance peak was used to determine the concentration of dispersant in supernatant solutions obtained after centrifugation of 1 vol % suspensions (5417R, Eppendorf, Hamburg, Germany). The final amount of dispersant adsorbed on the particle surface was obtained by subtracting the concentration in the supernatant from the concentration initially added to the suspensions. Suspensions containing different initial concentrations of GallolPEG were prepared by adding the alumina powder to an aqueous solution containing 0.01 mol/L KNO3 salt as the inert electrolyte. The suspensions were ultrasonicated for 5 min with pulsed signals at a frequency of 0.5 Hz (Vibracell, 600 W, model VCX600, Sonics and Materials, Inc., Newtown, CT) and afterward equilibrated for 1 h under magnetic stirring. All suspensions exhibited a pH of 5.5 after ultrasonication.

1084 Langmuir, Vol. 23, No. 3, 2007 The centrifugation procedure was conducted in two steps. First, the equilibrated suspension was loaded into 1500 µL plastic tubes and centrifuged for 30 min at a speed of 14 000 rpm. The supernatant obtained (1000 µL/tube) was then centrifuged again for 30 min at 14 000 rpm to ensure complete removal of the colloidal particles. Using this procedure, a clear supernatant volume of approximately 1.5 mL was obtained for each initial dispersant concentration investigated. The concentration of Gallol-PEG molecules in the supernatant solutions was determined with the help of a UV absorbance calibration curve obtained from solutions containing known amounts of dispersant. 3.3. Zeta Potential Analysis. Zeta potential measurements (DT1200, Dispersion Technology, Inc., Mount Kisco, NY) were performed on 1 vol % alumina suspensions containing 0.01 mol/L KNO3 salt as the inert electrolyte. The suspensions were prepared as described above for the adsorption experiments. Measurements at different pHs were carried out by adjusting the initial pH to 3 and titrating the suspensions with a 1M KOH aqueous solution up to a pH of approximately 11. 3.4. Dynamic Light Scattering. Dynamic light scattering experiments were performed to investigate the agglomeration of nanoparticles in 2 vol % suspensions containing different electrolyte concentrations (Malvern Zetasizer Nano ZS, Malvern Instruments Ltd., Worcestershire, UK). The suspensions were ultrasonicated for 5 min, as described above, and equilibrated for at least 12 h before the analysis. Dynamic light scattering measurements were also carried out on dilute dispersant solutions (1 g/L) in order to determine the radius of gyration of the dispersant molecules. In this case, the concentration of KNO3 in solution was varied between 0.01 and 1 mol/L. Results obtained from the light scattering analyses correspond to an average value from at least 10 separate measurements. 3.5. Preparation of Concentrated Suspensions. Nanoparticle suspensions were prepared by first adding the alumina powder, dispersant, salt solution, and double-deionized water into a 20 mL zirconia milling jar together with a 20 mm zirconia milling ball. Deagglomeration was subsequently accomplished in a shaking mill at 25 Hz for 5 min (Retsch MM200, Retsch GmbH, Haan, Germany). Preliminary tests showed that these milling conditions are sufficient to completely deagglomerate the powder. 3.6. Rheological Evaluation. The rheological behavior of the suspensions was evaluated in a stress-controlled rheometer using a cone-plate configuration (model CS-50, Bohlin Instruments, Cirencester, U.K). Steady-shear measurements were performed by first applying a preshearing rate of 80 s-1 for 60 s, followed by a resting period of an additional 60 s and, finally, a stepwise stress increase until a shear rate of about 400 s-1 was reached. Oscillatory experiments were conducted using the same setup by applying a sinusoidal stress of increasing amplitude at a frequency of 0.1 Hz. A solvent trapping system was employed to avoid water evaporation during the experiment. Preliminary comparative tests with profiled plates and the vane tool showed that no slipping effect occurred when using the cone-plate configuration. All measurements were performed right after the deagglomeration process at a fixed temperature of 25 °C.

4. Results and Discussion 4.1. Dispersant Characteristics. Dynamic light scattering analyses were carried out in dispersant solutions in order to estimate the radius of gyration of the dispersant molecules in aqueous medium, which should be indicative of the adlayer thickness formed by the molecule around the particles. An average radius of gyration of 4.8 nm was obtained for the PMA-PEG copolymer in a 0.01 M KNO3 aqueous solution. However, in the case of the Gallol-PEG molecules, the radius of gyration could not be reproducibly detected, most probably because their molecular length is lower than the detection limit of the instrument. The adlayer thickness (δ) of the Gallol-PEG molecules were thus estimated based on the dispersant molecular architecture. The dispersant length was calculated taking into account the

Studart et al.

Figure 2. Molecular length distribution of the Gallol-PEG molecules synthesized in this study. The upper x-axis indicates the adlayer thicknesses expected upon the adsorption of the molecules on the particle, assuming a stretched conformation from the surface.

length of the atomic bonds in the dispersant, assuming that the molecules display a stretched conformation on the surface with the head group attached to the particle and the PEG tail completely extended toward the aqueous phase (Figure 1a). Each of the PEG-Me reactants used for the synthesis of the Gallol-PEG molecules consisted of a mixture of molecules of various lengths. As a result, the Gallol-PEG molecules also exhibited a molecular length distribution, which is shown in Figure 2 in terms of the number of repeating units n in the PEG tail (Figure 1a). These data were derived from the mass spectroscopy analysis conducted after synthesis. Figure 2 indicates that the synthesized Gallol-PEG 350 and Gallol-PEG 550 molecules display PEG chains of various lengths, but with predominantly 6 and 9-10 repeating units (n) in the tail, respectively. On the basis of these data, we estimated the adlayer thickness (δ) that the Gallol-PEG molecules can provide upon adsorption on the particle surface, as shown in Figure 2. In these calculations, the adsorption of the molecules on the particles was assumed to occur through two adjacent hydroxyl groups of the head group (Figure 1a),32 which results in a tilt angle of 35° of the molecule with respect to the perpendicular direction from the surface. Even though the extended conformation is favored by the good solubility of PEG molecules in water, we are aware of the fact that deviations from this idealized configuration are very likely. Therefore, we use the calculated adlayer thicknesses on a comparative basis, assuming that the real thickness is directly proportional to the value estimated here. Figure 2 shows that an adlayer thickness (δ) of approximately 2.5 nm is expected from the Gallol-PEG 350 molecules if the most predominant tail length (n ) 6) is considered. However, it must be noted that the upper fraction of Gallol-PEG 350 molecules (nearly 20%) exhibit tail lengths within the optimum δ range of 3-4 nm.31 On the other hand, the majority of Gallol-PEG 550 molecules exhibit chain lengths that can lead to the targeted adlayer thickness between 3 and 4 nm (Figure 2). In this case, however, one-third of the molecules are longer (4.2 < δ < 5.9 nm) than the desired adlayer thickness. 4.2. Adsorption Behavior. The adsorption of Gallol-PEG molecules with tailored length on the surface of alumina particles takes place via a ligand exchange reaction between the hydroxyl groups on the alumina surface (-OH or -OH2+) and the hydroxyl groups of the 1,2,3-trihydroxybenzene head group (-OH or -O-).32 The reaction is favored by providing good leaving groups at the oxide surface (e.g., -OH2+) and strong nucleophilic groups

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molecules in the liquid medium, and k1 and k2 are adsorption max constants. The fitting shown in Figure 3b led to Cmax ads,1, k1, Cads,2, 2 2 and k2 values of 1.28 µmol/m , 642 L/mol, 0.79 µmol/m , and 70 573 L/mol, respectively. The two-surface Langmuir model is typically used to describe the adsorption behavior of species that adsorb strongly on the surface at small concentrations and reach a saturated condition at higher concentrations.34 Even though the model does not provide information on the adsorption mechanisms involved, we can reasonably attribute the pronounced adsorption at low added concentrations (e1 µmol/m2) to the electrostatic attraction between the negatively charged adsorbing molecule and the positively charged surface (10 < ζ > 40 mV, see section 4.3 below). At higher concentrations added (g2 µmol/m2), the affinity of the adsorbing species toward the surface decreases due to the lower electrical potential on the particle surface (ζ < 10 mV, see below) and the steric hindrance caused by the already adsorbed species. The free energy of adsorption of the Gallol-PEG molecules on the alumina surface (∆Gads) was calculated for these two adsorption regimes, using the following equation:31,36

∆Gads ) -kBT ln Figure 3. Adsorption behavior of Gallol-PEG 350 molecules on the surface of alumina nanoparticles. Graph a indicates the amount of adsorbed molecules as a function of the concentration of dispersant initially added to the suspension (normalized to the powder surface area). Graph b shows the adsorption isotherm and the Langmuir fitting achieved. All measurements were obtained from 1 vol % suspensions at pH 5.5.

on the adsorbing molecule (e.g., -O-). These conditions prevail around the pKa values of the adsorbing molecule,32 which, in the case of Gallol-PEG molecules, is of approximately 6.5 and 10 as determined by titration experiments. Adsorption is presumably favored by the formation of a chelate complex between the -OH neighboring groups of the adsorbing molecule and the Al3+ atom on the alumina surface.32 Figure 3 shows the adsorption behavior of Gallol-PEG 350 molecules on the surface of alumina nanoparticles. An adsorption maximum of approximately 1.7 µmol/m2 was obtained for initial dispersant concentrations higher than 4 µmol/m2. This plateau value is quite similar to the maximum adsorption of 1.8 µmol/m2 observed for alkyl gallate (or Gallol-PE) molecules in nonaqueous media.31 For a maximum adsorption in the range of 1.7 - 1.8 µmol/m2, the surface area occupied by each adsorbing molecule is 0.9 - 1.0 nm2, which is in good agreement with results obtained for other small adsorbing molecules at maximum coverage.34 In addition to the considerable surface coverage imparted, the Gallol-PEG molecules also exhibit a high affinity for the alumina surface, as evidenced by the pronounced initial slope of the adsorption isotherm shown in Figure 3b. This adsorption behavior can be described using a two-surface Langmuir model,34,35 where the amount of surface-adsorbed molecules Cads is given by

k1Cmax k2Cmax ads,1Ceq ads,2Ceq + Cads ) 1 + k1Ceq 1 + k2Ceq

(2)

max where Cmax ads,1 and Cads,2 are the maximum amount of surfaceadsorbed molecules, Ceq is the equilibrium concentration of

(34) Hidber, P. C.; Graule, T. J.; Gauckler, L. J. Citric acid - A dispersant for aqueous alumina suspensions. J. Am. Ceram. Soc. 1996, 79 (7), 1857-1867. (35) Sposito, G. On the use of the Langmuir equation in the interpretation of adsorption phenomena. 2. The 2-surface Langmuir equation. Soil Sci. Soc. Am. J. 1982, 46 (6), 1147-1152.

( ) k VLM

(3)

where k is the adsorption constant, kB is the Boltzmann constant, T is the temperature, and VLM is the molar volume of the liquid medium (0.018 L/mol for water). On the basis of the k1 and k2 constants estimated from the two-surface Langmuir model (eq 2), we obtained adsorption free energies of -15.2 and -10.5 kBT for low and high concentrations of added dispersant, respectively. These values are noticeably higher than the free energy of -7.9 kBT estimated for the adsorption of other alkyl gallates (GallolPE) on alumina surfaces in nonaqueous media.31 This can be explained by the electrostatic attraction between the oxide surface and the adsorbing molecules in water, which ultimately increases the affinity of the gallol anchoring groups toward the alumina surface. 4.3. Zeta Potential. Zeta potential (ζ) measurements revealed that the surface adsorption of Gallol-PEG 350 molecules shifts the alumina isoelectric point (IEP) from 9.2 to a minimum value of 6.8 (Figure 4a). Similar results were obtained for the GallolPEG 550 molecules. Since the PEG tail does not carry any electric charge, the IEP shift is caused solely by the pyrogallol head group. The minimum IEP obtained here for the Gallol-PEG molecules is in good agreement with the IEP of 6.2 obtained by Hidber et al.32 for the adsorption of pyrogallol on alumina surfaces. The effect of the PMA-PEG copolymer on the zeta potential of alumina particles is shown in Figure 4b. In this case, the IEP is shifted to a minimum value as low as 3.7, owing to the high concentration of negative charges on the deprotonated PMA backbone. Deprotonation occurs at pH values higher than the pKa of the functional group, which, in the case of methacrylate groups, is close to 4.5. It is interesting to note that the dispersant concentration required to shift the alumina IEP is 1 order of magnitude higher in the case of the Gallol-PEG molecules (1 µmol/m2) in comparison to that of the PMA-PEG copolymer (0.10 µmol/m2). This suggests that the packing density of Gallol-PEG molecules on the alumina surface is substantially higher than that in the case of the comb-like copolymer. (36) Lyklema, J. Fundamentals of Interface and Colloid Science; Academic Press: London, 1991; Vol. 1.

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Figure 6. Apparent viscosity as a function of shear rate for 30 vol % suspensions containing alumina nanoparticles dispersed with an electrostatic, a Gallol-PEG (grades 350 and 550), or a PMA-PEG copolymer layer. Suspensions with Gallol-PEG 350, Gallol-PEG 550, and PMA-PEG were prepared using initial dispersant concentrations of 0.3, 0.2, and 0.04 µmol/m2, respectively. No salt was added to the suspensions. Figure 4. Zeta potential of alumina particles as a function of pH for different initial concentrations of (a) Gallol-PEG 350 molecules and (b) PMA-PEG comb-like copolymer.

Figure 5. Shear stress vs shear rate curves for 30 vol % suspensions containing alumina nanoparticles dispersed with an electrostatic, a Gallol-PEG (grades 350 and 550), or a PMA-PEG copolymer layer. The fitting lines were obtained using the Casson model for shearthinning fluids. Suspensions with Gallol-PEG 350, Gallol-PEG 550, and PMA-PEG were prepared using initial dispersant concentrations of 0.3, 0.2, and 0.04 µmol/m2, respectively. No salt was added to the suspensions.

4.4. Rheological Behavior. Preliminary rheological measurements were performed in 30 vol % suspensions containing varying initial dispersant concentrations. A substantial decrease in the suspension viscosity was observed for Gallol-PEG 350 and 550 concentrations higher than 0.2 µmol/m2. This indicates that a surface coverage of only 10% of the maximum coverage possible (1.7 µmol/m2, Figure 3) is already sufficient to promote the dispersion of particles with the Gallol-PEG molecules. For the PMA-PEG copolymer, minimum viscosity was achieved at concentrations between 0.035 and 0.060 µmol/m2. On the basis of these results, optimum dispersant amounts of 0.3, 0.2, and 0.04 µmol/m2 were used for the dispersion of all the suspensions prepared with Gallol-PEG 350, Gallol-PEG 550 and PMAPEG, respectively. These optimum contents correspond to 0.51, 0.47, and 3.34 wt % of dispersant relative to the mass of powder. Attempts to adjust the pH of concentrated nanoparticle suspensions (>15 vol %) with acidic and alkaline solutions resulted in a remarkable increase in the liquid ionic strength, owing to the pronounced buffering effect of the alumina surface and the deprotonated dispersant species present in the aqueous phase. Therefore, no pH adjustments were carried out prior to the rheological characterization.

Under such conditions, all suspensions stabilized electrostatically (Figure 1c) or with Gallol-PEG molecules (Figure 1a) displayed a pH of approximately 5.5, whereas those prepared with the PMA-PEG copolymer exhibited a pH between 6.2 and 8.0 after deagglomeration. At these pH values, the alumina particles exhibit a zeta potential of +22 mV and in the range of -16 and -25 mV when coated with optimum concentrations of Gallol-PEG and PMA-PEG, respectively. This low zeta potential level is not sufficient to provide an electrical double layer that prevents particle agglomeration. Therefore, particles coated with such dispersants are predominantly stabilized by the steric layer formed by the surface-adsorbed molecules. However, in the absence of dispersants the alumina particles display a zeta potential of 45-50 mV (at pH 5.5), which is high enough to allow for the electrostatic stabilization of the suspension (Figure 1c). The rheological behavior of 30 vol % suspensions containing alumina nanoparticles dispersed using the stabilization mechanisms investigated here is illustrated in Figures 5 and 6 for the pH conditions and dispersant concentrations mentioned above. In order to evaluate the yield stress of the suspensions, the shear stress (τ) versus shear rate (γ˘ ) curves shown in Figure 5 were fitted with the Casson equation for shear-thinning fluids:

τ1/2 ) τy1/2 + (ηcγ˘ )1/2

(4)

where τy and ηc are the Casson yield stress and viscosity, respectively. Concentrated suspensions (30 vol %) containing the tailormade Gallol-PEG molecules exhibited a nearly Newtonian behavior, with a remarkably low yield stress (∼1 Pa) and viscosity levels as low as 0.13-0.18 Pa‚s at a shear rate of 100 s-1. On the other hand, suspensions stabilized electrostatically or with a PMA-PEG steric layer displayed a noticeable yield stress of 50 Pa and viscosities close to 1 Pa‚s at 100 s-1 (Figures 5 and 6). Even though PMA-PEG and electrical double layers are often efficiently applied for the dispersion of submicron-sized particles, the results obtained here show that the adlayer thicknesses imparted by such stabilizing layers might not be necessarily suitable for the dispersion of concentrated nanoparticle suspensions. This clearly illustrates the importance of deliberate control of the adlayer thickness when stabilizing nanoparticles in concentrated suspensions. 4.5. Effect of Ionic Strength. The adlayer thickness provided by the stabilizing layers evaluated here can be modified by varying

Colloidal Stabilization of Nanoparticles

Figure 7. Apparent viscosity and yield stress of 30 vol % suspensions as a function of the inert electrolyte concentration after particle stabilization with (a) a Gallol-PEG 350, (b) a PMA-PEG, and (c) an electrostatic layer. Suspensions with Gallol-PEG 350 and PMAPEG were prepared using initial dispersant concentrations of 0.3 and 0.04 µmol/m2, respectively. The yield stress τy was obtained by fitting the rheological data with the Casson equation.

the ionic strength of the suspension liquid medium. In the case of the electrostatic layer, an increase in ionic strength leads to an enhanced screening of the particle surface charge, reducing the thickness of the Debye layer around the particles (Figure 1c). For the tailor-made Gallol-PEG molecules, high salt concentrations are expected to decrease the solubility of the PEG tail37 and thereby reduce the adlayer thickness. In the case of the comblike copolymer, an increase in ionic strength favors a more coiled conformation of the PMA-PEG chains as a result of the lower repulsion between the deprotonated methacrylate groups of the molecule as well as the lower solubility of the PEO segments in water. However, in this case, a more coiled conformation might not necessarily lead to a thinner adlayer since the PMA backbone would tend to form more loops on the particle surface, changing the molecule conformation away from the idealized scheme depicted in Figure 1b. The effect of the ionic strength on the rheological behavior of the nanoparticle suspensions was investigated using different concentrations of KNO3 as the inert electrolyte, as indicated in Figure 7. Figure 7a shows that the viscosity and yield stress of suspensions dispersed with Gallol-PEG 350 is nearly independent of the ionic strength for salt concentrations up to approximately 0.1 M KNO3. Further salt addition leads, however, to an increase in suspension viscosity and yield stress, probably due to a decrease in the adlayer thickness. Similar results were obtained for the Gallol-PEG 550 molecules. Dynamic light scattering experiments in diluted suspensions indicated that very limited particle agglomeration occurs for salt concentrations of up to 0.1 M (37) Florin, E.; Kjellander, R.; Eriksson, J. C. Salt effects on the cloud point of the poly(ethylene oxide) + water-system. J. Chem. Soc., Faraday Trans. 1 1984, 80, 2889-2910.

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Figure 8. Size distribution of particles/agglomerates formed in suspensions containing (a) Gallol-PEG 350 molecules, (b) PMAPEG copolymer, and (c) no dispersant. The size of the agglomerates is indicated in terms of the initial particle size d for suspensions prepared with different salt concentrations. Suspensions containing PMA-PEG copolymer and 0.10 M salt exhibited large agglomerates that readily settled in the measuring cell and thus could not be analyzed in these light scattering experiments.

KNO3, with most of the particles remaining as singlet or doublet units (Figure 8a). In the case of the PMA-PEG copolymer (Figure 7b), the viscosity and yield stress achieved are orders of magnitude higher than that obtained with the Gallol-PEG molecules. The light scattering results shown in Figure 8b reveal that aggregates are formed in these suspensions for all salt concentrations investigated (0-0.1 M KNO3). The average radius of gyration of 4.8 nm obtained for this dispersant (section 4.1) suggests that the adlayer thickness provided by these molecules around the particles is sufficiently thick to prevent particle agglomeration. Therefore, the particle agglomeration observed in these suspensions (Figure 8b) probably results from depletion and bridging flocculation. Depletion-induced attraction between colloidal particles occurs when nonadsorbed molecules are larger or comparable in size to the average distance (S) between particles in the suspension.38 An interparticle distance of 7.9 nm was estimated for 30 vol % suspensions dispersed with PMA-PEG, which is in fact comparable to the radius of gyration of the PMA-PEG molecules (see Supporting Information). This supports the idea that depletion flocculation might have favored the agglomeration of particles in suspensions containing the PMA-PEG copolymer. In addition to the depletion flocculation, the bridging of particles through the simultaneous adsorption of dispersant in two neighboring surfaces is very likely to occur for such small interparticle distances. This bridging effect is favored in the case of the PMAPEG copolymer due to the presence of negatively charged carboxylate anchoring groups throughout the entire molecule (Figure 1b). Therefore, even if the length of PMA-PEG (38) Asakura, S.; Oosawa, F. On interaction between 2 bodies immersed in a solution of macromolecules. J. Chem. Phys. 1954, 22 (7), 1255-1256.

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Figure 9. Interaction potential energy (UT) as a function of the separation distance (D) in water between 65-nm alumina particles displaying (a) an electrostatic and (b) a Gallol-PEG 350 steric layer at the surface. The graphs illustrate the effect of the concentration of a 1:1 inert electrolyte (e.g., KNO3) on the interaction potential energy. The inset in graph b shows the energetic barrier always present in the case of particles coated with Gallol-PEG molecules. The following parameters were used for the calculation of the interaction potential energy (see text): a ) 32.5 nm,  ) 7.08 × 10-10 C2/Nm2, A ) 3.67 × 10-20 J,45 T ) 298 K, δ ) 2.5 nm, F ) 1.1 g/cm3, Mw ) 449 g/mol, ν ) 0.03 nm2, χ ) 0.42, φp ) 0.28, and ζ ) 40 and 20 mV for the electrostatic and the Gallol-PEG layers, respectively.

copolymers were deliberately chosen to provide the optimum adlayer thickness around particles, bridging flocculation would still occur due to the multiple anchoring groups present in the dispersant molecule. On the other hand, this bridging phenomenon cannot occur in the case of the Gallol-PEG dispersants, since these molecules exhibit only one anchoring group per adsorbing species (Figure 1a). Interestingly, the addition of salt to the electrostatically stabilized suspensions resulted in a remarkable decrease in viscosity and yield stress, reaching levels as low as that achieved with the Gallol-PEG molecules (Figure 7c). However, the minimum viscosity and yield stress is only obtained within a narrow salt concentration range between 0.01 and 0.05 mol/L. Figure 8c shows that no particle aggregation occurs in suspensions containing KNO3 concentrations of up to 0.05 mol/L. However, further salt addition leads to significant agglomeration (Figure 8c), which explains the abrupt increase in the viscosity and yield stress of the concentrated suspensions (Figure 7c). These results can be explained with the help of the interaction potential energy curves shown in Figure 9. The interaction potential energy curves describe the variation in free energy (UT) resulting from the interaction of two colloidal particles separated by a distance D in a given liquid medium (see Supporting Information). Figure 9a shows that the electrostatically stabilized particles exhibit an energetic barrier in the interaction potential curves that is gradually reduced by increasing the salt concentration in the liquid medium. Salt additions within the range 0.01-0.05 M favorably decrease the Debye length around the particles (δ), keeping an energetic barrier against particle agglomeration. This allows for the substantial reduction in viscosity and yield stress depicted in Figure 7 for low salt concentrations (0.01-0.05 M KNO3). However, at salt concentrations higher than 0.05 M, the energetic barrier vanishes, leading to particle agglomeration

Studart et al.

Figure 10. Yield stress as a function of the solids volume fraction of alumina suspensions stabilized with an electrostatic, a GallolPEG (grades 350 and 550), and a PMA-PEG layer on the particle surface. Suspensions stabilized electrostatically, with the PMAPEG copolymer, and with Gallol-PEG 550 contained 0.05, 0.20, and 0.05 M KNO3, respectively. The yield stress τy was obtained by fitting the rheological data with the Casson equation.

(Figure 8) and to a significant increase in the suspension viscosity and yield stress (Figure 7). On the other hand, particles coated with the Gallol-PEG molecules display a strong energetic barrier against agglomeration for all salt concentrations in the range of 0.01-0.10 M (Figure 9b). This is attributed to the fact that the repulsive energy potential in this case is imparted predominantly by the steric effect of the surface-adsorbed molecules (Supporting Information, term UstR in eqs 4 and 5), which is not affected by variations in ionic strength as much as the electrostatic repulsion between particles (Supporting Information, term UelR in eqs 4 and 5). Therefore, suspensions stabilized with Gallol-PEG molecules display low viscosity and yield stress within a broader range of salt concentrations (0.01-0.10 M, Figure 7). This can be highly advantageous for the stabilization of particles that extensively dissolve in water or in the case of suspensions containing high concentrations of other ionic species that contribute to an increased ionic strength (e.g., binders, gelling agents, surfactants). Nevertheless, an excessive increase in ionic strength (>0.1 M KNO3) leads eventually to suspensions with high viscosity and yield stress (Figure 7). This is probably related to the lower solubility and thus coiled conformation of PEG molecules under high salt concentrations, which ultimately results in a lower adlayer thickness around the particles. 4.6. Effect of Solids Content. The effect of solids content on the suspension rheological behavior was investigated for suspensions containing the optimum salt concentrations depicted in Figure 7. Under these conditions, an optimum adlayer thickness around the nanoparticles is expected. Suspensions stabilized electrostatically and with the PMAPEG copolymer were prepared with 0.05 and 0.20 M KNO3, respectively. Even though no remarkable effect of the salt concentration was observed for 30 vol % suspensions dispersed with Gallol-PEG (Figure 7), salt addition was observed to change the viscosity of these suspensions at solids content approaching 40 vol %. At a solids fraction of 40 vol %, suspensions containing Gallol-PEG 350 displayed minimum viscosity in the absence of salt, whereas suspensions prepared with Gallol-PEG 550 exhibited minimum viscosity at a salt concentration of 0.05 M KNO3. These conditions were thus used to investigate the rheology of concentrated suspensions dispersed with Gallol-PEG molecules. The influence of the solids volume fraction on the yield stress of suspensions prepared at these optimum conditions is shown in Figure 10. The yield stress was determined by fitting the experimental flow curves with the Casson equation (eq 4).

Colloidal Stabilization of Nanoparticles

Figure 11. Shear stress dependence of the storage (G′) and loss (G′′) moduli of 30 vol % suspensions containing alumina nanoparticles dispersed with an electrostatic layer (0.05 M KNO3). The data were obtained under oscillatory shear conditions at a frequency of 0.1 Hz.

Oscillatory measurements confirmed that the suspensions exhibit a gel-like behavior for solids volume fractions higher than 30 vol %, as exemplified in Figure 11 for the case of electrostatically stabilized particles. The storage modulus and the yield stress of gelled colloidal networks are strongly influenced by interparticle forces, solids content, particle size, and network microstructure.39-44 Particularly, the yield stress (τy) dependence on the solids content (φ) (Figure 10) has been described by the power law τy ∝ φγ, with the exponent γ being mainly determined by the microstructure of the colloidal particle network. Open networks are characterized by a weak dependence of τy on the solids content (φ) and thus exhibit typically low γ values. Densely packed networks, on the other hand, display yield stress strongly dependent on the solids fraction, which results in high γ values. Figure 10 reveals that suspensions dispersed electrostatically or with the Gallol-PEG 350 and 550 molecules are more strongly affected by the solids fraction (γ ) 17.29, 11.89, 12.74, respectively) compared to those stabilized with the PMA-PEG copolymer (γ ) 5.09). This suggests that the adlayer thicknesses imparted by the electrostatic (0.05 M KNO3) and Gallol-PEG layers lead to a densely packed network of particles in the suspension. The highly packed colloidal network achieved with the electrostatic and the Gallol-PEG layers allows for the preparation of fluid suspensions with up to 40 vol % solids, as opposed to the solids content of only 30 vol % achieved with the state-ofthe-art PMA-PEG copolymer for the same viscosity level (Figure 12). This remarkably high solids content suggests that the tailoredmade Gallol-PEG molecules and the slightly screened electrostatic layer (0.05 M KNO3) provide adlayer thicknesses close to the optimum δ range. However, it is important to emphasize that the (39) Derooij, R.; Potanin, A. A.; Vandenende, D.; Mellema, J. Steady shear viscosity of weakly aggregating polystyrene latex dispersions. J. Chem. Phys. 1993, 99 (11), 9213-9223. (40) Derooij, R.; Potanin, A. A.; Vandenende, D.; Mellema, J. Rheological behavior of weakly aggregating colloids - Viscosity of the structure formed in a steady shear-flow. Colloid J. 1994, 56 (4), 476-486. (41) Yanez, J. A.; Shikata, T.; Lange, F. E.; Pearson, D. S. Shear modulus and yield stress measurements of attractive alumina particle networks in aqueous slurries. J. Am. Ceram. Soc. 1996, 79 (11), 2917-2924. (42) Yanez, J. A.; Laarz, E.; Bergstrom, L. Viscoelastic properties of particle gels. J. Colloid Interface Sci. 1999, 209 (1), 162-172. (43) Channell, G. M.; Miller, K. T.; Zukoski, C. F. Effects of microstructure on the compressive yield stress. AIChE J. 2000, 46 (1), 72-78. (44) Wyss, H. M.; Tervoort, E. V.; Gauckler, L. J. Mechanics and microstructures of concentrated particle gels. J. Am. Ceram. Soc. 2005, 88 (9), 23372348. (45) Bergstrom, L. Hamaker constants of inorganic material. AdV. Colloid Interface Sci. 1997, 70, 125-169.

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Figure 12. Apparent viscosity as a function of the solids volume fraction for alumina suspensions stabilized with an electrostatic, a Gallol-PEG (grades 350 and 550), and a PMA-PEG layer on the particle surface. The short-dotted line indicates the minimum viscosity predicted in an earlier study31 for suspensions containing 65-nm alumina particles coated with an adlayer thickness of 3.75 nm. Suspensions stabilized electrostatically, with the PMA-PEG copolymer, and with Gallol-PEG 550 contained 0.05, 0.20, and 0.05 M KNO3, respectively.

concentrated nanoparticle suspensions stabilized with GallolPEG molecules are less susceptible to changes in the liquid ionic strength compared to those dispersed solely with an electrostatic layer (Figures 7 and 8). The steric layer imparted by the Gallol-PEG 550 molecules proved to be particularly suitable for the preparation of fluid concentrated suspensions. The high solids content achieved with this dispersant is quite close to the maximum solids content predicted in our earlier study on suspensions containing the same 65-nm alumina particles used in this work (Figure 12). This indicates that the deliberately designed Gallol-PEG molecules seem to provide an adlayer thickness (δ) that is indeed close to that predicted in our earlier investigation (3-4 nm) for minimizing the viscosity of the evaluated nanoparticle suspensions. The lower viscosity achieved with Gallol-PEG 550 in comparison to that measured with Gallol-PEG 350 also suggest that the adlayer thickness is controlled by the length of the most abundant molecules rather than the length of the longest molecules within the dispersant size distribution (Figure 2).

Conclusions Tailored-made dispersants with a head-tail architecture were successfully used for the preparation of fluid aqueous suspensions containing more than 40 vol % of alumina nanoparticles (average diameter, 65 nm). The novel dispersants comprise a pyrogallol head group that efficiently adsorbs onto the metal oxide surface and a water-soluble PEG tail that extends itself toward the suspension aqueous phase. The length of the PEG tails was deliberately chosen to provide a dispersant steric layer that is thick enough to prevent extensive particle agglomeration and, at the same time, is sufficiently thin to avoid an excessive excluded volume around the particles. This allowed for the preparation of fluid suspensions with solids contents close to the maximum concentration predicted in an earlier study for 65-nm alumina particles. The rheological behavior of suspensions prepared with the new dispersants is independent of the solution ionic strength for monovalent salt concentrations up to 0.1 M. This is of particular importance for the stabilization of suspensions that are prone to significant variations in ionic strength. The metal chelating ability of the pyrogallol group should also enable the use of the GallolPEG molecules synthesized here for the stabilization of nanoparticles of other metal oxides in water. The versatile nature of this head group and the possibility to customize the length and

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chemistry of the molecule’s tail should also allow for the functionalization of oxide surfaces for a variety of different purposes, as, for instance, the formation of nonfouling layers for biosensors, the lubrication of surfaces in aqueous medium, and the protection and targeting of particles for drug delivery, biological separation, and medical imaging. Acknowledgment. We thank Prof. A. Dieter Schlu¨ter, Prof. Paul Smith, Mr. Ramchandra Kandre, Dr. Christoph Kocher (ETH Zurich, Switzerland), and Dr. Ricardo L. A. Dias (University of Zurich, Switzerland) for their assistance in the synthesis of the Gallol-PEG molecules; Mr. Anatol Zingg (EMPA, Switzerland) and Mr. Joachim Pakusch (BASF, Germany) for kindly supplying

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the PMA-PEG copolymer; Mr. Bjo¨rn Schimmo¨ller and Prof. Sotiris Pratsinis (ETH Zurich, Switzerland) for their help with the dynamic light scattering experiments; Miss Claudia Strehler for conducting part of the experimental work; as well as the Swiss National Science Foundation for the financial support to this study (Grant No. 200021-100570/1). Supporting Information Available: Calculations of the interparticle distance in colloid suspensions and of the interaction potential energy between colloidal particles. This material is available free of charge via the Internet at http://pubs.acs.org. LA062042S