Effect of surfactant type on microbubble formation behavior using Shirasu porous glass (SPG) membranes

We have recently proposed a new method for generating monodispersed microbubbles from Shirasu porous glass (SPG) membranes with a narrow pore size distribution. In this study, to investigate the effects of surfactant type on microbubble formation behavior from SPG membranes, the microbubble formation experiments were performed using differently charged surfactants. Sodium n-dodecylbenzenesulfonate (SDBS), polyoxyethylene (20) sorbitan monolaurate (Tween 20) and cetyltrimethylammonium bromide (CTMA) were used as anionic, nonionic and cationic surfactants, respectively. Air was pressurized into a surfactant solution of 2.0 mol m−3 through an SPG membrane with a mean pore diameter of 5.1 μm at a transmembrane/bubble-point pressure ratio of 1.1. In systems containing SDBS and Tween 20, monodispersed microbubbles with mean bubble diameters of 35.6 and 43.0 μm were generated, respectively, from the membrane. The CTMA-containing system resulted in polydispersed bubble formation, due to the adsorption of CTMA molecules (cations) onto the negatively charged membrane surface, which lowered the hydrophilicity of the membrane surface. The microbubbles generated were smaller for SDBS than for Tween 20. This is probably because for the case of SDBS, the microbubbles detached from the pores as soon as they were formed, due to a strong electrostatic repulsion between the negatively charged bubble surface and the negatively charged membrane surface, which assists in microbubble detachment from the pore openings. The gaseous-phase flux was about 11–15 times larger for Tween 20 than for SDBS, but much smaller for CTMA, which is a consequence of the fact that the proportion of active pores is significantly lower for SDBS than for both Tween 20 and CTMA.