Producion of Nanoparticle Assemblies by Electro-Spraying and Freeze-Drying of Colloids: A New Method to Resolve Handling Problem of Nanoparticles

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To resolve handling problem of nanoparticles, due to their small size, a new methodology of electro-spraying and freeze-drying was developed for colloidal nanoparticles of silica and titania to transform them to solid macro-scale nanoparticle assemblies.  The assemblies were then redispersed in an aqueous system to investigate the effect of formulation of original solutions and the process parameters on reversibility of the system to a stabilised colloidal condition.  The electro-spraying was employed to control the size of droplets and consequently the size of nanoparticle assemblies in the freeze-drying. High speed digital video recording of the spray process revealed that within a narrow range of voltage, the size of droplets reduced sharply to a minimum value, where a narrow size distribution was obtained. Non-destructive structural analysis of the freeze-dried nanoparticle assemblies using X-ray micro-tomography represented different structures of the nanoparticle assemblies depending on type of nanoparticles.  The stability analysis of redispersed nanoparticles in water (using centrifugal stability analyser) and their size distribution (obtained by nano-sizer) showed different stability conditions. These conditions were affected by physicochemical properties of nanoparticle assemblies and process parameters. In terms of titania, it was found that with an appropriate formulation of PEG solution (as binder of assemblies) and optimum size of the nanoparticle assemblies it was possible to produce assemblies having adequate strength and good re-dispersion properties.