In situ formation of ZnO nanoparticles in PEG 400 under microwave solvothermal conditions using a design of experiments approach

Abstract Zinc oxide (ZnO) nanoparticles are widely used in cosmetic and pharmaceutical formulations; however, their dispersion in viscous organic media is often limited by aggregation and poor compatibility. In this work, the in situ formation of ZnO nanoparticles directly within polyethylene glycol (PEG 400) is investigated using a microwave-assisted solvothermal approach. A design of experiments (DoE) framework was applied to evaluate the influence of heating ramp, reaction temperature, and holding time on particle morphology and crystallinity, as well as on liquid-phase properties. Electron microscopy indicates that higher temperatures and longer reaction times tend to promote the formation of smaller and more anisotropic particles. X-ray diffraction shows the formation of crystalline ZnO with a wurtzite structure under all investigated conditions. The statistical analysis suggests relationships between synthesis parameters and particle aspect ratio, although with limitations inherent to the experimental design. The PEG phase exhibited progressive yellowing under more severe conditions, suggesting possible oxidative processes, whereas Raman spectroscopy did not reveal significant changes in the polymer’s main molecular features within the technique’s sensitivity. Overall, the results provide insight into nanoparticle formation within viscous oligomeric media and indicate that in situ synthesis in PEG may represent a viable route to improve compatibility between inorganic nanoparticles and organic matrices, potentially reducing the need for post-synthesis dispersion steps.