Chemical Kinetics of Nanoparticles in the Emulsion State during Phase-Transfer Synthesis

The creation of waterborne nanoparticles (NPs) is one of the main topics in environmental science as an area of ecofriendly research. Phase-transfer synthesis, an emulsion/solvent evaporation method, facilitates the generation of the NPs using immiscible oil and water phases in a biphasic combination. Chemical interactions of the two phases by synthesis conditions or parameters determine different chemical intermediate states of the emulsions, resulting in various sizes or shapes of the NPs. However, a molecular-level understanding and chemical origin of the emulsion have not been elucidated in detail. In this work, we study the chemical kinetics of the nanoparticles in the emulsion state during synthesis as a function of surfactant concentration. Surfactant-covered gold nanoparticles (AuNPs) with ca. 20 nm size are transferred at a medium surfactant concentration, enough to cover the AuNP and stabilize it in the water phase. However, sub-2 nm-sized AuNPs are produced at a high surfactant concentration beyond the critical micelle concentration of the surfactant. AuNPs surrounded by CTAB micelles with a hemisphere contact are not formed, facilitating chemical interactions between solvent molecules of the oil phase and the water phase to crack the AuNPs. Based on our time-dependent UV–vis absorption and confocal microscope experiments and real-time in situ X-ray scattering analyses, we observed that the intermediate states in the chemical and structural states determine the colloidal conformation. This work provides insight into the chemical mechanism with a structural variation of two components in water and oil phases. Therefore, the current finding can promote the phase-transfer synthesis, which would motivate the community to create ideal NPs.