Preparation of hollow silica microspheres in W/O emulsions with polymers.

Micrometer-sized hollow silica particles were synthesized by sol-gel reaction in water-in-oil emulsion. To obtain hollow structures in silica particles, the viscosity of water droplets in W/O emulsion was controlled with polyethylene glycol (PEG) or polyvinylpyrrolidone (PVP). To stabilize the emulsion structure, hydroxypropyl cellulose (HPC) was added to the oil phase. Without HPC, the particles have an irregular shape and hardly have a particulate form. As the concentration of HPC increased from 0.8 to 1.4 wt%, the size of silica particles decreased from 10 to 1 microm. But above 1.4 wt%, the solution was very viscous, so that it was difficult to handle. Especially, the role of PEG or PVP in the water phase was very important, not only because it stabilized the W/O emulsion structure, but also because it influenced the formation of hollow structure. Interestingly, the hollow silica particles were formed when the molar ratio of water to TEOS (Rw) was 4 and the concentrations of PEG and HPC were 6 and 1.4 wt%, respectively. Also, when PEG was replaced with polyvinylpyrrolidone (PVP), hollow silica particles ranging from 3 to 7 microm were formed.

Preparation of Silica Particles Encapsulating Retinol Using O/W/O Multiple Emulsions.

Retinol, a cosmetic ingredient, was entrapped within inorganic microspheres obtained from sol-gel reaction of TEOS in o/w/o multiple emulsions as microreactors. In o/w/o multiple emulsions, the retinol was emulsified as an internal oil phase in a aqueous solution of 3.0 wt% Tween 20 prior to emulsification into an external oil phase. The multiple emulsions appeared to be stable enough in the presence of HPC polymer in the external oil phase. In sol-gel reaction, the hydrolysis and condensation rate of TEOS were greatly dependent upon the catalyst and the molar ratio of H(2)O to TEOS (R(W)). In this study, sphere-like microspheres entrapping retinol were best formed with the addition of NH(4)OH as a catalyst when the concentration of TEOS was at the R(W) value of 4. Microspheres obtained under these conditions were 15-40 &mgr;m with very dense surfaces containing a few globules 1.17-2.35 &mgr;m. Also, they showed the slower release of retinol into the external ethanol phase and higher loading and encapsulation efficiency. Copyright 2001 Academic Press.