Preparation and characterization of a new asymmetric type of Shirasu porous glass (SPG) membrane used for membrane emulsification

To obtain a high throughput of the dispersed-phase flux through a Shirasu porous glass (SPG) membrane in membrane emulsification, a new type of asymmetric SPG membrane was prepared from two types of primary glass in the NaO–CaO–Al2O3–B2O3–ZrO2–SiO2 system with different growth rates of phase separation. The Al2O3-rich primary glass with a slower phase separation growth rate and the B2O3-rich primary glass with a faster growth rate were laminated in two layers to be formed into a tube at 1373 K. The primary glass tube was heat-treated at 993 K for 30 h to cause the phase separation, and subsequently leached out with hydrochloric acid, resulting in an asymmetric tubular SPG membrane. The asymmetric SPG membrane consisted of an inner skin layer 25 ± 2 μm thick and a support layer 405 ± 5 μm in thickness. The mean pore diameters of the skin and support layers were 0.67 and 4.7 μm, respectively. With the asymmetric membrane, uniform-sized emulsion droplets were prepared by a membrane emulsification technique. The dispersed-phase flux through the asymmetric membrane increased by a factor of approximately 20 or more compared with that through the symmetric membrane, due to the much smaller hydrodynamic resistance of the asymmetric membrane than that of the symmetric membrane.