(445a) The Microstructure and Rheology of Gels Consisting of Heteroaggregated Nanoparticles

Fumed metal oxide nanoparticles are widely used in the paints and coatings industry as abrasives, fillers, reinforcing agents, and to improve flow behavior. When added to polar solvents, oppositely-charged, fumed metal oxide nanoparticles will heteroaggregate into a volume-spanning gel network. The bulk rheology of the particle dispersion can be controlled and tuned by changing the pH, particle ratio, particle concentration, and ionic strength. Additionally, the gels exhibit a yield stress, resist sedimentation, and are stable over a wide temperature range. Phase behavior studies and corresponding rheological characterization has demonstrated that the nanoparticle network only exists when the solution pH is in the range where the different nanoparticle species have opposite surface charges. Here, ultra-small angle neutron scattering (USANS) and light scattering is used to characterize the structure of the primary hetero-aggregates in binary mixtures of fumed silica (SiO₂) and fumed alumina (Al₂O₃). Specifically focusing on how pH and particle ratio affect the microstructure and size of the heteroaggregates. We find that at low particle volume fractions or extreme particle ratios, the nanoparticlesy aggregate into secondary ‘hetero-particles’ consisting of clusters with hydrodynamic radii on the order of 3-4 nanoparticle radii. As the number of attractive particles is increased, these larger ‘hetero-particles’ link into a fractal gel network and generate the observed bulk rheological behavior.