Surfactant-Free Microemulsion as Template for Stöber Silica Nanoparticle Synthesis

Abstract

Surfactant-free microemulsions (SFMEs) have emerged as promising structured reaction media for the synthesis of inorganic nanoparticles. Recent reports suggested that the ternary water (H2O)/ethanol (EtOH)/tetraethyl orthosilicate (TEOS) system forms SFME structures capable of directing silica particle formation through a self-templating mechanism. Here, we systematically investigate this hypothesis and evaluate the influence of SFME structuring on Stöber silica nanoparticle synthesis. Ternary and pseudoternary phase diagrams of H2O/EtOH/TEOS and H2O/EtOH/(oil + TEOS) systems were established and characterized by dynamic light scattering (DLS), small- and wide-angle X-ray scattering (SWAXS), and transmission electron microscopy (TEM). The H2O/EtOH/TEOS system was found to remain molecularly dispersed throughout the investigated monophasic region and showed no evidence of SFME aggregate formation. Consequently, silica particle formation in this system follows conventional Stöber-type kinetics rather than a self-templating mechanism. Nevertheless, synthesis at high water contents and low ammonia concentrations produced highly monodisperse silica nanoparticles in the low-nanometer size range and enabled controlled particle growth through regrowth steps. In contrast, genuine SFME systems containing toluene or 1-octanol exhibited pronounced mesoscale structuring in the pre-Ouzo and bicontinuous regions, which strongly affected nucleation and growth behavior. Time-resolved DLS measurements revealed induction periods and growth regimes characteristic of compartmentalized reaction environments, resulting in particle sizes ranging from a few nanometers to several hundred nanometers, depending on microstructure and oil composition. The results demonstrate that SFME structures do not template particle formation in the H2O/EtOH/TEOS system but can substantially modify Stöber kinetics when a genuine mesoscale organization is present. A mechanistic model is proposed in which diffusion of hydrolyzed silica species across the SFME interphase governs nucleation and growth, establishing interfacial mass transport as a key parameter for controlling silica nanoparticle formation in structured solvents.