Growth processes for nanometer-scale au-particles in water-in-oil microemulsions

Growth processes of gold particles prepared with the aid of microemulsions are studied focussing on the relationship between the produced particles and the structure of the microemulsion. The microemulsion was composed of sodium dioleylphosphate (SDOLP) or pentaethylene-glycol dodecylether (C12E5). It is found from the size and distribution of the produced particles that there are several retardation processes for Au-particle growth in the SDOLP system. The first retardation appears near the diameter d_m of the original microemulsion droplets, followed by successive retardation in lower ranges of water-to-surfactant ratio Wo. The second retardation occurs around the diameter d_nc of the natural curvature of the microemulsion droplets. Whereas, for larger Wo-values, the first and the second retardations occur around d_nc and a diameter d_ms much larger than d_nc. The size distribution of the produced particles is rather broad due to the wide gaps between d_m, d_nc and d_ms. In the C12E5-system, the size distribution was much broader than that in the SDOLP-system. This arises from the absence of the second retardation which restrains the particle growth of the maximum size. SDOLP can work as a stabilizer of the produced particles. This is caused by the peculiar nature of the microemulsion composed of SDOLP. Taking into account the above retardation mechanism, the size-distribution of Au-particles can be controlled, and Au-particles with sharper size-distribution can be prepared.