Formation of Gold Nanoparticles in Water-in-Oil Microemulsions: Experiment, Mechanism, and Simulation

Self-assembled water-in-oil (W/O) microemulsions have been reported as a suitable route for synthesis of size-controlled nanoparticles. However, the mechanism of formation of nanoparticles in microemulsions is still not completely understood. In this work, gold nanoparticles (GNPs) were synthesized via the W/O microemulsion route. As the molar ratio of water and dioctyl sodium sulphosuccinate (AOT) (R) increased from 2.5 to 5.0 to 7.5, the corresponding water drop diameter increased from 2.7 to 5.0 to 7.3 nm. In parallel, the mean hydrodynamic diameter of GNPs increased from 6.5 to 11.3 to 15.6 nm for corresponding R values of 2.5, 5.0, and 7.5. Therefore, although there is a monotonically increasing trend of the mean diameter of GNPs with the initial drop diameter, for all values of R, the mean diameter of GNPs was significantly higher than the initial drop diameter. Consequently, previously known simulation vastly underpredicts the experimental GNP diameter. However, only on redefining the particle-particle coagulation event (during coalescence of microemulsion drops containing particles) does the current kinetic Monte Carlo (kMC) simulation agree well with the experimental results. In addition, we also find that the coagulation efficiency of solid nanoparticles (β_p) increases with R, and β_p is lesser than the coalescence efficiency of liquid drops (β_d) over the range of R values concerned. Hence, a combined simulation and experimental study enumerates the dynamics of size evolution of nanoparticles and the events involved in their formation in a W/O microemulsion system.