Synthesis Of Nanoparticles In W/O Microemulsions: A Monte Carlo Simulation

In W/O microemulsion, the droplets provide perfect media as microreactors forpreparation of nanoparticles, and due to the compartmentalized droplets, themonodispersed nanoparticles size can be easily controlled in such media. Particles cannucleate, grow and aggregate in the water pool under the confinement of the water-oilinterface, so the flexibility of the interface, the size of the water pool and also the ratioof reactants can affect the characteristics of the resultant particles.In this paper the formation process of nanoparticles in W/O microemulsion wassimulated by Monte Carlo method . The influencing mechanisms of such factors asthe ratio of reactants, the size of the water pool and film's flexibility on the particlessize distribution were obtained from the simulation results. The simulation resultsshow that the growth process finished rapidly under the condition of low reactantsconcentration, and whether the aggregating process can take place depends on theflexibility of the film. If the film is rigid, there is no plenty of time for particles toaggregate, so the smaller size particles can be produced and the size distribution isrelatively monodisperse. On the contrary, if the film is flexible, the particles canaggregate sufficiently, so the ratio of reactants became the determinant factor for theparticle size and its distribution. At low reactants concentration, the quantity ofproduced particles is not enough for the formation of large particles, so the particlesize is unimodal distribution regardless of the flexibility of the film, even though theprepared particles under the condition of flexible film demonstrated morepolydispersed size distribution in comparison with that under the condition of rigidfilm. When the concentration of reactants is high, the flexibility of the film has moreinfluence on the particle size: the particle size tends to be more monodisperse for rigidfilm compared with flexible film which is accordance with the result under lowconcentration. Under high reactant concentration, the particle size can demonstrateunimodal or bimodal distribution which depends on the competition of growth andaggregation process. For rigid film, the growth process is more important, and smallerparticles are in the inferior position in the competition for new produced particle withlager particles, which made the size of smaller particles almost keep constant, whilethe lager particles continue to grow in the growth process. Thus, it come to thebimodality of the particle size distribution. When the film is more flexile, the 5aggregation process can carry out smoothly, and smaller particles can not keep theirsize constant any longer, it just coagulate onto the lager ones immediately. So thenumber of small particles decrease and that of the lager increase. As a result, theunimodal distributed particle can be obtained in the end.As for the mechanism of particles formation, the rapid coagulation theory ofSmoluchowski was evaluated in this paper. The simulation results indicate that theSmoluchowski mechanism need modification when apply to the preparation ofnanoparticles in the microemulsions.