Double Emulsion Droplets as Microreactors for Synthesis of Mesoporous Hydroxyapatite
Original entry: Caspar Floryan, APPHY 225, Fall 2010
"Double Emulsion Droplets as Microreactors for Synthesis of Mesoporous Hydroxyapatite" Ho Cheung Shum, Amit Bandyopadhyay, Susmita Bose and David A. Weitz, Chemistry of Materials 21(22), 5548–5555 (2009).
emulsions, droplets, reactor, synthesis, hydroxyapatite
This article discusses the synthesis of Hydroxyapatite inside double emulsions.
This process takes inside a droplet-generating microfluidic device. An injection tube containing an aqueous solution of hydroxyapatite reactants flows into a junction where it meets an oil phase and another aqueous phase. Here the reactant phase breaks into droplets encapsulated by shells of oil. The microlfuidic setup is illustrated in figure 1 below.
Once encapsulated, the reactants are deposited in a collection tube. Here the reaction is initiated by increasing the pH, done by adding ammonium hydroxide to outside phase. Once the reaction is complete, the hydroxyapatite is removed by increasing the osmolarity of the surrounding fluid, thus swelling the droplet and breaking apart the oil shell. These process steps are shown below in figure 2:
Photos of the emulsions are shown in figure 3. Image (b) shows the reactant phase surrounded by a thick shell of oil. Image (c) shows the droplets after the external osmolarity was increased, swelling them and thinning he oil shell. Images (d) and (e) show close-ups of a swollen droplet.
This method of fabricating hydroxyapatite has several advantages. The reactants are shielded from externalities by the oil phase. Further reactants can also be added by fusing droplets together. The size and geometry of the droplets can easily be tuned and the hydroxyapatite created here has exceptionally high porosity and surface area. The surface area can be as high as 162m^2/g.
This method can be used to synthesize particles of other compounds with exceptional control over their nanoscale porosity and morphology.