COMBINED PROCESS OF ELECTRO-SPRAYING AND FREEZE-DRYING FOR PRODUCTION OF NANOPARTICLE ASSEMBLIES WITH VIEW OF REDISPERSION

The main objective of this paper is to analyse the effect of combined processing method of electro-spray and freeze-drying of colloidal systems on size and structure of produced nanoparticle assemblies as well as on their redispersion behaviour. In this context, the charged colloidal droplets of latex, titania, and silica are sprayed into liquid nitrogen for further freeze-drying to produce nanoparticle assemblies. The nanoparticle assemblies are then redispersed in an aqueous system. The size analysis of colloidal droplets produced by the electro-spraying is carried out by using high-speed digital video recording and image analysis. The results show that with increasing electric field strength (i.e. voltage) the size of droplets decreases. However, there is a voltage range in which the size of droplets reduces significantly to a minimum value. Furthermore, in this range, size distribution of droplets is bimodal (large droplets and small satellites) and a transition occurs from dripping mode to jet mode. Outside the upper part of this range at high voltages, a narrow size distribution is observed. Non-destructive structural analysis of the anoparticle assemblies is carried out using SEM and X-ray micro-tomography. The analysis reveals different structures of nanoparticle assemblies epending on type of the nanoparticles. The stability of redispersed nanoparticles in water and their size distribution are characterised using centrifugal lighttransmission spectroscopy and nanosizer, respectively. The analysis reveals different stability conditions of the redispersed nanoparticles which are affected by theirphysicochemical properties and the process parameters. In terms of titania system, it is shown that with an ppropriate formulation of PEG (as binder of nanoparticles) and pH in the initial TiO2 solution, it is possible to produce nanoparticle assemblies having adequate strength and good re-dispersion properties.

Keywords:

electro-spray , nanoparticle assemblies , colloids , redispersion