Preparation of highly uniform Pickering emulsions by mercaptocarboxylated gold nanoparticles

•We prepared 2-nm water-soluble spherical mercaptocarboxylated Au nanoparticles.•Highly uniform W/O Pickering emulsions stabilized with the Au nanoparticles were prepared.•The obtained emulsions were very stable for more than 12 months.•The emulsion size and its distribution depend on the rotation speed of homogenizer and NaCl conc.•It was found that the aggregated Au nanoparticle stabilize the W/O Pickering emulsions. This paper investigated preparation of Pickering emulsions stabilized with mercaptocarboxylated Au nanoparticles. We used 15-mercaptopentadecanoic acid (MPDA) as a stabilizing agent and obtained 2nm spherical Au nanoparticles by the phase transfer method. Highly uniform water-in-oil (W/O) emulsions stabilized with Au nanoparticles ranging in size from 0.5 to 1.2mm were successfully prepared using a homogenizer without any surfactants. Adding 0.1M or more of NaCl to the system formed stable monodisperse emulsions. The size and size distribution of the emulsions depended on the homogenizer rotation speed, and there was an optimal rotation speed to obtain highly monodisperse emulsions with a size distribution of less than 10%. The size of the emulsions also increased with increasing NaCl concentration while the size distribution became broader. Dynamic light scattering data of Au nanoparticle aqueous solutions with NaCl indicated that the aggregated Au nanoparticles dispersed and that the size of the dispersed aggregates increased with increasing NaCl concentration. SEM observation of dried emulsions revealed shell-like structures formed with Au nanoparticle aggregates. In the Pickering emulsion system using MPDA-Au nanoparticles, Au nanoparticles first aggregate with NaCl to form relatively hydrophobic aggregates in an aqueous solution. The aggregates then stabilize the water/toluene emulsions that are formed by the homogenizer.