Unraveling agglomeration and deagglomeration in aqueous colloidal dispersions of very small tin dioxide nanoparticles

Understanding deagglomeration, agglomerate formation and structure for very small nanoparticles (NPs), due to their more facile agglomeration, is critical for processing or tailoring agglomerates for nanostructured materials. We propose that by controlling and fine-tuning the interplay of agglomeration (colloidal interaction) and deagglomeration (hydrodynamic forces), the design of agglomerate size, microstructure and morphology is possible even for very small NPs.

Here, we investigate very small SnO₂ NPs (10 nm) generated in the gas phase as model system. Small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS) are used to study dispersions in aqueous media across the entire pH range (2-12) at various NaCl concentrations treated with ultrasound. Parallel to size and size distribution, agglomerate morphology and microstructure are analyzed by means of the mass fractal dimension, d_m and modeled with ab initio shape simulations. The critical coagulation concentration (CCC) is determined for the alkaline region where the SnO₂ NPs are highly charged.

Quantitative analysis of SAXS and DLS data reveals that size and size distribution of the agglomerates depend similarly on the electrostatic interaction influenced by pH and salinity as observed by the zeta potential. In contrast d_m is mainly influenced by the salt concentration. Ab initio shape simulations support these experimental findings.