Chul Oh

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.

O/W/O Multiple Emulsions via One‐Step Emulsification Process

Abstract The preparation of O/W/O multiple emulsions by a one‐step emulsification process was studied using an optical microscope equipped with a camera. The O/W/O multiple emulsions could be prepared by the introduction of two surfactants and two polymers. But when one of two polymers or water‐soluble surfactant was not added, W/O emulsions with various size distributions were formed. Tween 20 added in water destabilizes W/O interfacial films formed by Span 80. At a result, the diffusion of 1‐octanol into water droplets occurred more easily. Using a polymer in the continuous phase, the swelling of W/O interfaces was retarded but the use of two polymers in the continuous and discontinuous phases assisted the micelles mixed with oil molecules to diffuse into the core of aqueous droplets and remain. Moreover, the interaction between Tween 20 and PEG promoted coalescence of oil‐swollen micelles in the water droplets. Depending upon the choice of water‐soluble nonionic surfactants and polymers, three types of emulsion droplets were observed. Variation in the shapes of the droplets was due to differences in solubilization power of the surfactants. Also, the O/W/O multiple emulsions were stable for 8 h but most of the emulsions were at least partially broken after 24 h.

Synthesis of mesoporous silica particles prepared by using multiple emulsion

Abstract The spherical silica particles with meso- and macropores at the surface and inside of particles were prepared in the hexane/water/n-decyl alcohol multiple-emulsion. Also micrometer-sized hollow silica particles could be prepared by controlling the viscosity of the aqueous phase in W/O emulsion with polyethylene glycol (PEG). The morphology of silica particles was influenced by the concentration of PEG, HPC polymer and the external oil phase (O2).