Preparation and Formation Mechanism of Silica Microcapsules (Hollow Sphere) by Water/Oil/Water Interfacial Reaction

Silica microcapsules (hollow spheres) were readily prepared by an interfacial reaction using a water/oil/water (W/O/W) emulsion system. A W/O emulsion consisting of an aqueous solution of sodium silicate (WP-1) and an n-hexane solution of Tween 80 and Span 80 (OP) was added to another aqueous solution of a precipitant (WP-2). The reaction of sodium silicate with a precipitant to form silica on this W/O/W emulsion system (WP-1/OP/WP-2) forms the hollow structure spontaneously. Therefore, no core material often utilized in silica hollow sphere fabrication was required in this process. The formation and the particle size of the silica microcapsule depended on the precipitant employed. When NH4HCO3 was used as precipitant, the particle size of the silica microcapsule was successfully controlled by the volume ratio of WP-1/OP/WP-2, the rotation number of the homogenizer, and the concentration of sodium silicate in WP-1. However, this control of the particle size was not achieved when other precipitants such as NH4Cl were used. In the case of NH4HCO3, silica formation takes place at the outer interface between OP and WP-2 on the W/O/W emulsion system. On the other hand, when NH4Cl is utilized, silica is yielded at the inner interface between WP-1 and OP. These differences of reaction mechanisms of sodium silicate among precipitants were important factors in the preparation and properties of microcapsules.