Spherical-to-cylindrical transformation of reverse micelles and their templating effect on the growth of nanostructures

Abstract

We discuss a complete mechanistic study on the anisotropic growth of zinc oxalate nanostructures within reverse micelles. We have employed small-angle X-ray scattering (SAXS), dynamic light scattering (DLS), and transmission electron microscopy (TEM) to understand the detailed growth of the nanostructures. We have been able to observe the generation of nuclei and their aggregation to a critical size beyond which they form nanostructures of higher dimensions in self-assembled templates. One of our aims was to find a correlation between size and shape of microemulsion droplets (MDs) and that of the resulting nanostructures of zinc oxalate (ZO) which grow within the MDs. Combination of SAXS and DLS show in situ growth of nanoparticles in the individual droplets which consume the water-insoluble product formed and undergo exchange coalescence with other droplets. The structural transition of the MDs is captured by observing the change in shape anisotropy, together with a detailed structural analysis of micelles in which the nanostructures grow as a function of time. Importantly, once the reaction is triggered, the nucleation of the droplets start instantly, and a very short period is noticed where MDs become cylindrical with approximate aspect ratio of 4:1 in which nanostructures grow anisotropically and achieve an average critical size of 55 nm (elongated nanoparticles) signifying the existence of short nucleation-dominant particle growth period, beyond which a transition from elongated nanostructures to small rods is observed. The critical size for the elongated droplets is 80 nm in length and 18 nm in diameter, and these critical dimensions at the point of transition are a new finding about an asymmetric particle before the rods begin to start self-assembling. Once the shape of microemulsions turns cylindrical, the dynamical exchange with other microemulsions is very fast at both ends, resulting in the formation of nanorods of zinc oxalate and an increase in the aspect ratio of these rods. This growth process can be viewed as a morphologically templated nucleation process, and the droplets act as shaping vesicles for the formation of ZO nanorods. This study is significant since it attempts to correlate the size and shape of the reverse micellar (microemulsion) droplets with the newborn product nanoparticles inside the droplets and the subsequent growth of the nanoparticles within the droplets.