Novel route to nanoparticle dispersion using supercritical carbon dioxide

An experimental study was carried out to determine the effects of supercritical carbon dioxide (scCO2) on the break up of untreated and treated alumina agglomerates in polystyrene. Alumina nanoparticles were treated with fluoroalkanes to achieve a better affinity with scCO2. All agglomerates were found inside pores (cells) that were created by scCO2 expansion during depressurization, and the observed cell size increased with increasing alumina agglomerate size. When untreated alumina was used, the agglomerates were not affected by scCO2 expansion. On the other hand, large agglomerates (L ≥ 15 μm) of the treated alumina showed signs of catastrophic fragmentation, whereas small agglomerates of the treated alumina were intact upon scCO2 expansion. This was attributed to the amount of scCO2 saturation around and within small agglomerates, which does not provide enough energy to break up the agglomerate. This conclusion was also supported by the fact that cell sizes were proportional to agglomerate size. Large agglomerates can absorb more scCO2 than smaller ones, and therefore, have an adequately larger bursting pressure to overcome interactions between individual alumina nanoparticles that make up the agglomerate.