Kinetics of the formation of nano-sized platinum particles in water-in-oilmicroemulsions

The effect of surfactant type and temperature on the kinetics of the format ion of platinum nanoparticles in water-in-oil microemulsions by chemical re duction of ptCl(6)(2-) were examined with time-resolved UV-vis absorption s pectroscopy. The surfactants used were poly(ethylene glycol)monododecyl eth ers (C12E4, C12E5, C12E6), sodium bis(2-ethylhexyl)sulphosuccinate (AOT), a nd mixtures of the alcohol ethoxylates and AOT. The oil domain was n-heptan e. The microemulsion droplet size was measured by a dynamic light scatterin g technique (photon correlation spectroscopy) and the final platinum partic le size was determined by transmission electron microscopy. The reaction ra te for platinum particle formation was approximately the same in microemuls ions based on either of the alcohol ethoxylates but considerably lower for microemulsions based on AOT. In microemulsions based on mixtures of an alco hol ethoxylate and AOT the reaction rate was similar to that obtained when alcohol ethoxylate was the sole surfactant. The reaction was observed to be particularly rapid in microemulsions based on combinations of AOT and C12E 5 or C12E6, and the rate was relatively independent of the ratio of the non ionic and anionic surfactants. The reaction was found to be of first order for platinum nanoparticles formed in alcohol ethoxylate-, AOT-C12E5-, and A OT-C12E6-based microemulsions, whereas in microemulsions with AOT and AOT-C 12E4 the reaction rate seemed to be of higher reaction order. The platinum particles were found to be less than 5 nm in average diameter, which was co nsistent with the microemulsion droplet size. (C) 2001 Academic Press.