Production of monodispersed emulsions using shirasu porous glass membranes

Membrane emulsification (ME) is a new technology for making monodisperse emulsions over a wide spectrum of mean droplet sizes, ranging from cca. 0.5 μm to several tens of μm. ME involves production of droplets individually (drop-by-drop) by extrusion of pure disperse phase through a porous membrane into moving continuous phase (direct ME) or the passage of previously prepared coarse emulsion (premix) through the membrane (premix ME). The membrane must not be wetted with the continuous phase, i.e. O/W emulsions are prepared using a hydrophilic membrane and W/O emulsions using a hydrophobic membrane. If the membrane wall is wetted with the continuous phase, droplet disruption can be followed by phase inversion, i.e. a fine W/O emulsion can be produced from the O/W coarse emulsion and vice versa. In order to force a disperse phase or premix through the membrane, a pressure gradient across the membrane must be imposed, usually by pressurizing the disperse phase with compressed gas in a pressure vessel. The most suitable membrane for emulsification is the so-called Shirasu porous glass (SPG) membrane. This membrane can be fabricated with a wide range of mean pore sizes (0.05-20 μm) and with a wall porosity between 50 and 60 %. Hydrophobic modification of SPG membrane can be carried out by surface coating with silicone resin. The membrane resistance was unchanged after surface treatment with silicone resin, which means that the pores were not plugged by the resin, even in the submicron range of mean pore sizes.