Emulsions stabilized by carbon nanotube-silica nanohybrids.

Emulsions have been stabilized by carbon nanotube-silica nanohybrids. The as-prepared nanotube-silica particles generate water-in-oil (w/o) emulsions, regardless of the water/oil volume ratio used. The emulsion volume fraction was much higher than that obtained with nanotubes only, and it was found to increase with water/oil ratio due to an increasing amount of water retained in the emulsion droplets. However, beyond a certain water/oil ratio, the emulsion fraction rapidly decreased. This point of collapse is a strong function of the amount of carbon nanotube-silica particles in the system. Oxidation of the nanohybrids by nitric acid can effectively modify the particle surfaces and change the resulting emulsion properties. The treatment in nitric acid increases the density of functional groups on the carbon nanotubes. Depending on the extent of functionalization, the effect of oxidation can vary from increasing the volume of emulsion stabilized to reversing the type of emulsion to oil-in-water (o/w). The emulsions are remarkably stable against coalescence and sedimentation and can be easily separated by filtration or centrifugation, which make them suitable for applications in interfacial catalytic processes in which the catalyst can be easily recovered after reaction.