Sustainable silica microcapsules

Abstract

Microencapsulation is a critical technology for a wide range of commercial applications, including drug delivery, home and personal care products, fragrance release, agrochemicals, food manufacture, energy storage and self-healing materials. In many cases, highly crosslinked polymer microcapsules are utilized, which are now regarded as microplastic pollutants. Herein we report a new route to sustainable micrometer-sized silica microcapsules based on the judicious use of a binary mixture of chitosan and hydroxypropyl cellulose. This synergistic emulsifier system enables the preparation of oil-in-water emulsions with a mean droplet diameter of approximately 5–10 μm. Chitosan adsorption at the oil–water interface confers cationic surface charge, which directs the surface deposition of environmentally benign silica from the aqueous continuous phase when employing TMOS as a silica precursor. However, the addition of TEOS to the oil phase prior to high-shear homogenization is also required to produce the most robust silica microcapsules. Interestingly, the two biopolymers located within the silica shells confer mechanical flexibility, which may offer an advantage for controlled release applications. The mean silica shell thickness can be varied from 50 to 175 nm, and thermogravimetry analysis of the dried silica microcapsules indicated a mean biopolymer content of around 29% to 38% by mass. Preliminary experiments indicate that substantial release of a model payload (dimethyl phthalate) occurs within 6 h at 20 °C. Thus, these microcapsules are highly porous towards sparingly water-soluble small molecules. On the other hand, they retain a model water-insoluble dye for at least sixteen weeks when stored at 20 °C.