Analysis of a technical-grade w/o-microemulsion and its application for the precipitation of calcium carbonate nanoparticles

For a directed synthesis of inorganic nanoparticles in water-in-oil (w/o)-microemulsions the detailed investigation of the microemulsion system is of particular importance. This study illustrates that the phase behavior of the ternary as well as of the quaternary mixture (with reactants) has to be analyzed to identify microemulsion regions suitable for particle synthesis. Besides detailed phase prism investigations the microemulsion, consisting of water/reactant, cyclohexane and Marlipal O13/40, was further characterized. Dynamic light scattering studies have been conducted for determining the droplet size and viscosity measurements have been undertaken to clarify the internal structure. A time scale analysis of the involved processes during particle formation (e.g., droplet exchange, reaction, nucleation, growth and mixing) points out, that the utilization of w/o-microemulsions for precipitation reactions is an effective way to overcome mixing problems, which usually are present in bulk precipitation. This is demonstrated for the precipitation of CaCO3 from two microemulsions containing dissolved Na2CO3 and CaCl2, respectively. The influence of microemulsion composition, initial reactant concentration and holding time on the final particle size and shape was investigated. The resulting CaCO3 nanoparticles have been characterized by means of a transmission electron microscope. Depending on the applied reaction conditions it is shown that different particle sizes and morphologies can be obtained.