Understanding the effect of surfactants’ hydrophobicity on the growth of lanthanum sulfide nanospheres in water-in-oil microemulsions: a detailed dynamic light scattering, small angle X-ray scattering, and microscopy study

Abstract

In this study, for the first time, an effort is made to examine the effect of surfactant hydrophobicity on the synthesis of lanthanide series nanoparticles inside the cores of reverse micelles. Tween 20 and Tween 80, two different types of nonionic surfactants, are employed to develop the microemulsions wherein lanthanum sulfide nanoparticles are synthesised. Growth of the nanostructures is examined by employing dynamic light scattering (DLS), small angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and field emission electron microscopy (FESEM). Nanospheres are produced by both the microemulsion media at any stage of the reaction, as supported by SAXS analysis and TEM images. There are generally two stages in the synthesis of nanospheres for both types of microemulsions: the nucleation phase and growth phase. The reverse micellar medium based on Tween 20 offers faster nucleation and growth of lanthanum sulfide nanoparticles with larger sizes than the reverse micellar medium based on Tween 80. Tween 80 contains a double bond in its hydrophobic tail and participates in π–π interaction. This enhances the surface film rigidity and produces a well-defined phase boundary, which results in the reduction of intermicellar exchange. The end outcome is the formation of slow transient dimers at the nucleation step and this leads to the aforementioned results.