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February 22, 2006 Nanoparticle-Polymer Complexation: Electrostatic Self-assembly as a Route to Stable Dispersions of Hybrid NanocolloidsA. Sehgal1, Y. Lalatonne1, J.-F. Berret2, and M. Morvan1 The inherent instability of inorganic nanoparticle sols may be resolved by complexation with ion-containing polymers at the particle interface. We exploit a precipitation-redispersion (P-R) mechanism to achieve the complexation of short chain polyelectrolytes with cerium oxide nanoparticles in order to extend their stability over a wide pH range. Small-angle X-ray scattering in conjunction with static and dynamic light scattering reveals that P-R may yield hybrid nanocolloidal complexes with a single inorganic core-charged corona structure. The electrostatic self-assembly of ionic stickers anchors the chains onto the surface, resulting in a polymeric brush that provides steric and electrostatic stabilization. This simple strategy to achieve stable dispersions surmounts the critical limitation in the use of nanoparticles, allowing a facile means to translate the intrinsic properties of mineral oxide nanoparticles to a range of novel applications.
Emerging nanomaterials utilize not only the chemical composition but also the size, shape and surface-dependent properties of nanoparticles in applications with remarkable performance characteristics. As synthesized, aqueous dispersions of metal-oxide nanoparticles (e.g. oxides of cerium, iron, or zirconium) exhibit some common properties: i) the particles have crystalline structure, ii) the sols are typically synthesized in extremely acidic (or basic) conditions, and iii) the particles are stabilized by electrostatics and are extremely sensitive to perturbations in pH, ionic strength, and concentration. In order to translate the intrinsic properties of nanoparticles to industrially relevant uses there is a need for a robust means to stabilize nanoparticle dispersions in aqueous media for a variety of processing conditions. This instability of inorganic nanoparticle sols was resolved by complexation with charged ion-containing polymers. To achieve the goal, we have developed a two-step process, defined as the precipitation-redispersion (P-R) process. In this study we address the issue of the adsorption and of the complexation between cerium oxide nanoparticles and short poly(acrylic acid) (PAA) chains (molecular weight, Mw=2000 g·mol-1). We demonstrate that we may considerably extend the range of pH and concentration stability of cerium nanosols by irreversibly adsorbing weak polyelectrolytes on the surface. This process does not require mechanical stimulation.
As synthesized, cerium oxide sols at pH 1.4 consist of monodisperse cationic nanocrystalline particles with a hydrodynamic diameter DH = 10 nm. Interparticle interactions in the form of strong van der Waals attraction, electrostatic repulsion, and surface chemistry, act in concert to make the charged nanosol highly sensitive to the electrostatic environment. Any perturbation in pH(>3) results in aggregation and macroscopic precipitation of the nanoparticle suspension, severely limiting the utility of nanoceria. Figure 1 shows that when cerium oxide sols are mixed with poly(acrylic acid) solutions (with the same concentration c and pH 1.4), the solution undergoes an instantaneous and macroscopic precipitation. The mixing is characterized by XMix = VCeO2 / VPAA2K, where VCeO2 and VPAA2K are the respective volumes of the cerium and polymer solutions. Sedimentation or centrifugation gave two distinctly separated phases. Complexation of the polymer chains with several particles results in precipitation due to colloidal bridging, which is reminiscent of classical associative phase separation. As the pH is progressively increased, the suspension spontaneously redisperses into a clear solution. This evolution of the CeO2-PAA2K solutions at X=1 and c = 1 wt. % is illustrated in Figure 1a for the pH range 1.4-10. The precipitation-redispersion was followed by static and dynamic light scattering experiments. DH was found to decrease progressively from 18 nm at pH 7.5 to 12.5 ± 1 nm at pH 10 (Figure 2b). This latter value is approximately 3 nm larger than the diameter of the bare particles (DH = 9.8 nm), suggesting that the nanoceria are now coated by a PAA corona.
Figure 2 illustrates the SAXS intensity profiles performed on both bare and PAA2K-coated nanoparticles. At low
concentration (c < 0.5 wt. % and X = 1), where no structure factor is apparent, the scattered intensity is
proportional to the concentration and the q-dependence of the intensity reflects the form factor of the aggregates.
The form factors for the bare and coated CeO2-nanoparticles have been shifted vertically so as to superimpose the
scattering cross-sections at high wave-vectors (Figure 2). This result indicates that on a local scale (i.e. below 5 nm)
both particles have the same structure. A slight deviation below 0.03 Å-1 is ascribed to the PAA corona surrounding the
particles. This is also illustrated in the inset (Figure 2) as intensities in a Guinier representation. The logarithm
of the intensity decreases linearly with q2 and from the straight lines we deduce a radius of gyration RG = 3.52 ± 0.02
nm for the bare particles and RG = 4.8 ± 0.02 for the coated CeO2-PAA2K particles. As q -> 0, the intensity of the coated
particles is 1.4 times that of the bare nanoceriums. This value can be used to estimate the number of adsorbed polymers.
We assume that a coated nanoparticle is of the core-shell type, having electronic densities ΡCeO2 and ΡPAA. The excess of
intensity as q -> 0, due to the PAA shell with respect to that of the bare particles, may be estimated:
An insight into the conformation of the adsorbed polymeric corona is obtained. With 50 chains per particle and a monomer molar volume v0 = 54.8 Å3, the radius of the polymer nanoparticles would be increased by 0.4 nm. With a fully extended polyelectrolyte corona the hybrid nanocolloid would result in DH = 18 nm. With the intermediate redispersed DH ~ 13 nm, the PAA conformation may be described as multisite adsorption of -COOH moieties along the contour length with the remainder constituting a solvated polyelectrolyte brush. This conformation of adsorbed polyelectrolyte chains is illustrated schematically in Figure 3. The P-R process is a simple route to obtain single nanoparticles irreversibly coated with PAA chains. With a 2 - 3 nm polyelectrolyte brush surrounding the particles, the cerium sols are stable over a broad range of pH values and concentrations. The process described could easily be extended to other nanoparticle systems. This opens new opportunities with electrostatic self assembly as a means to dramatically improve the stability of inorganic nanosols. BEAMLINE FUNDING PUBLICATION FOR MORE INFORMATION |
The National Synchrotron Light Source at Brookhaven National Laboratory in New York, is a national user research
facility funded by the U.S. Department of Energy's Office of Basic Energy Science. The NSLS operates two electron
storage rings: an X-Ray ring and a Vacuum UltraViolet (VUV) ring which provide intense light spanning the electromagnetic
spectrum from the infrared through x-rays. Each year over 2300 scientists from universities, industries and government
labs perform research at the NSLS.
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