Separating nanoparticles from microemulsions.

Water-in-oil microemulsions (w/o µEs) stabilized by the cationic surfactant cetyltrimethylammonium chloride (CTACl) have been used as reaction media to generate Au nanoparticles (Au-NPs). In addition the pure µEs have been used as media to disperse Au and Pd-NPs, which have been pre-synthesised in aqueous phases and stabilized by sodium 2-mercaptoethanesulfonate (MES) ligands, and also commercially available SiO(2)-NPs. A general method for recovery and separation of the nanoparticles from these mixed NP-µE systems has been demonstrated by tuning phase behavior of the background microemulsions. Addition of appropriate aliquots of water drives a clean liquid-liquid phase transition, resulting in two macroscopic layers, the NPs preferentially partition into an upper oil-rich phase and are separated from excess surfactant which resides in a lower aqueous portion. UV-vis and (1)H NMR spectroscopy have been used to follow these separation processes and quantify the recovery and recycle efficiencies for the different NPs. For example, ∼90% of the microemulsion-prepared Au-NPs can be recovered; with even greater separation efficiencies attainable for pre-synthesised MES-stabilized Au-MES-NPs (∼98%) and Pd-MES-NPs (92%). For the silica NP-µE dispersions gravimetry indicates ∼84% recovery of the NPs. TEM images of all systems showed that NP shapes and size distributions were generally preserved after these phase transfer processes. This low-energy and cost-effective purification route appears to be a quite general approach for processing different inorganic NPs, having advantages of being isothermal, using only commercially available inexpensive components and requiring no additional organic solvents.

Polymer-induced recovery of nanoparticles from microemulsions.

A new isothermal approach to the recovery of inorganic nanoparticles (NPs) is demonstrated. The NPs can be incorporated into a background microemulsion (ME) supporting fluid, and they can be recovered by addition of non-adsorbing polymer. A clean liquid-liquid (L-L) phase transition can be readily induced by addition of polymer to the MEs. Furthermore, the L-L transitions are also observed in the presence of added NPs, but now the nanoparticles concentrate in the lower co-existing ME phases. Once recovered, the NPs can be redispersed by adding extra ME as a solvent.

Recycling functional colloids and nanoparticles.

The stability and separation of colloids and nanoparticles has been addressed in numerous studies. Most of the work reported to date requires high cost, energy intensive approaches such as ultracentrifugation and solvent evaporation to recover the particles. At this point of time, when green science is beginning to make a real impact, it is vital to achieve efficient and effective separation and recovery of colloids to provide environmental and economic benefits. This article explores recent advances in strategies for recycling and reusing functional nanomaterials, which indicate new directions in lean engineering of high-value nanoparticles, such as Au and Pd.

Recycling nanocatalysts by tuning solvent quality.

Catalytic surfactant stabilized gold-containing nanoparticles have been recovered by a new isothermal low-energy approach, by controlled and reversible changes in colloid stability based on fine-tuning of solvent quality. Once recovered, the nanoparticles can be re-dispersed in the solvent, or indeed dispersed into a different solvent. The morphology of the nanoparticles is not significantly affected by the recovery process and they can be used and reused as oxidation catalysts.

Recovery of nanoparticles made easy.

Here is demonstrated a novel approach to reversible control over nanoparticle (NP) stability, permitting facile recovery for the reuse of inorganic NPs. For the first time, the separation of NPs is achieved by suspending the nanostructures in a background-supporting colloidal fluid, which itself shows a liquid-liquid critical-type phase transition at a temperature T(c) instead of using a normal molecular solvent.

Tri-chain hydrocarbon surfactants as designed micellar modifiers for supercritical CO2.

Getting their feet wet: Low-cost hydrocarbon surfactants act as fluid modifiers for supercritical carbon dioxide (scCO(2)). Increased terminal branching of the surfactant chains aids micelle formation (see middle picture: CO(2) green), and more chains allows water to be incorporated (right, blue).

Control over microemulsions with solvent blends.

The effect of solvent mixtures on the phase behavior of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) stabilized water-in-oil microemulsions has been studied by using heptane/dodecane, decane/dodecane, octane/dodecane, and nonane/undecane blends. Small-angle neutron scattering was employed to explore the effect of changing the solvent composition on the microemulsion properties, especially near the cloud point (T(cloud)) and the liquid-liquid critical separation (T(crit)). It is shown that droplet interactions can be strongly influenced by changing the solvent blend compostion, which has implications for the locations of observed phase boundaries. Of particular interest is the use of carefully selected solvent blends, which have the effect of lowering T(crit) by up to 6 degrees C from the value found in pure decane.

Low energy methods of phase separation in colloidal dispersions and microemulsions.

The majority of work on phase separation of colloidal systems has been concerned with the energy intensive approaches such as ultracentrifugation, solvent evaporation, changes of temperature and pressure etc. However, in modern nanotechnology it is desirable to minimize environmental impact in order to achieve separation and recovery of colloidal products. In this review recent research on phase separation methods, requiring relatively lower energy consumption are summarized. These include polymer-, solvent- and photo-induced approaches to phase separation.