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 mu m thick and a support layer 405 +/- 5 mu m in thickness. The mean pore diameters of the skin and support layers were 0.67 and 4.7 mu m, 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. (c) 2007 Elsevier B.V. All rights reserved.