Sustainable silica microcapsules for agrochemical applications

Polymer microcapsules have been widely used to deliver agrochemical actives for crop protection but are now regarded as microplastic pollutants. In this Thesis, alternative inorganic microcapsules have been developed to address this environmental problem. Silica was identified as a suitable microcapsule material owing to its low cost and benign environmental profile. First, silica deposition onto micron-sized latex particles using the well-known Stöber silica process was optimised with the aid of an adsorbed layer of chitosan. This model system was chosen to understand the problem of secondary nucleation, but the ethanol-rich nature of the reaction milieu made it unsuitable for microcapsule syntheses. Next, chitosan and hydroxypropyl cellulose (HPC) were used as dual emulsifiers to stabilise a series of oil-in-water emulsions, which were characterised in terms of their mean droplet diameter and electrophoretic behaviour. Various parameters (emulsifier concentration, stirring rate, chitosan/HPC mass ratio, solution pH etc.) were systematically optimised to obtain relatively stable emulsions comprising oil droplets of 5-10 µm diameter. Such oil droplets were coated with a thin overlayer of silica using two soluble silica precursors: tetraethyl orthosilicate (TEOS) and tetramethyl orthosilicate (TMOS). More specifically, TEOS was dissolved in the oil phase prior to emulsification, and TMOS was added to the aqueous phase immediately after emulsification. Various parameters were optimised to control silica deposition onto the oil droplets while varying the target silica shell thickness from 50 to 200 nm. The resulting silica microcapsules were characterised by optical microscopy, laser diffraction, scanning electron microscopy, aqueous electrophoresis, fluorescence microscopy and thermogravimetry. A UV spectroscopy assay was performed to assess the retention of sparingly water-soluble molecules within the microcapsules, revealing that these species were released within a few hours at 20 °C. The retention of the highly hydrophobic Nile Red dye was subsequently monitored by fluorescence microscopy, showing no signs of release over a period of months.