Solvent effects on the growth and steric stabilization of copper metallic nanoparticles in AOT reverse micelle systems

Organic and inorganic reactions within the aqueous cores of water-in-oil AOT reverse micelle systems are viable methods for the production of nanomaterials of controllable composition and geometry while maintaining narrow size distributions. Considerable research has been done in order to better understand the governing features which comprise the AOT reverse micellar system, particularly stability and the intermicellar exchange of the contents within the aqueous core. The intermicellar exchange rate is affected by the bulk solvent type, the contents dissolved within the core, and the size of the reverse micelle or the water content, referred to as the W value, where W is the molar ratio of the water to AOT surfactant concentrations. Synthesis of nanomaterials within the AOT reverse micelle system is a strong function of the intermicellar exchange process and the factors mentioned previously. This study examines the effects of varying the bulk liquid solvent and the W value on the growth rate and ultimate particle size of copper nanoparticles produced via reduction of CuAOT(2) within the micelle core. Particle growth is measured in-situ using time-resolved UV-Vis absorbance spectroscopy, and the particle size is determined by both UV-Vis measurements and TEM analysis. A total interaction energy model is implemented to represent the attractive van der Waals forces acting between the metallic particles and the repulsive osmotic and elastic forces which result from the surfactant tail-tail and solvent-tail interactions responsible for the steric stabilization of the metallic particles within the microemulsion. The model is able to predict the ultimate particle sizes obtained experimentally for copper and silver nanoparticles synthesized using a variety of bulk liquid solvents including isooctane, cyclohexane, and n-alkanes ranging from pentane to dodecane.