Preparation and characterization of fine silver powder with colloidal emulsion aphrons

Colloidal emulsion aphrons (CEAs) can be considered as the micrometer-sized water-in-oil emulsion cores encapsulated by a “soapy shell” consisting of multi-layer surfactant molecules. The emulsion core sizes of CEA dispersion are roughly in 10–100 μm, whereas the sizes of inner phase are in 1–5 μm. CEAs can be used as a microreactor to synthesize fine powder materials. Silver powders with various sizes and shapes were prepared with di-(2-ethylhexyl) phosphoric acid (D2EHPA) as the mobile carrier in membrane phase, ascorbic acid (Vc) as reducing agent in inner phase and cyclohexane as the membrane of CEAs. The linear silver particles were formed under low mobile carrier concentration in membrane phase or low ascorbic acid concentration in inner phase, whereas the granular silver particles with comparatively small sizes were obtained under the high mobile carrier concentration or high ascorbic acid concentration, and also silver powder of “nano-porous microsphere” was prepared with CEAs by adjusting the D2EHPA concentration in membrane phase, ascorbic acid concentration in inner phase and the molar ratio of silver ion to ascorbic acid (image). Compared with emulsion liquid membrane (ELM) method, the silver powder prepared with CEAs was usually in granular form with smaller sizes because of the higher transfer rate, but for that prepared with ELMs was only in linear form.