Preparation of nanoscale organosols and hydrosols via the phase transfer route

Aqueous reactions support the preparation of a wide variety of inorganic nanoparticles (NPs), starting from relatively inexpensive precursors. However, the long-term stability of hydrosols is sensitive to changes in water chemistry, especially at high NP concentrations. On the other hand, by using an appropriate stabilizer, NPs prepared in organic phases more commonly display smaller sizes, higher stability, and monodispersity. Subsequently, phase transfer of freshly prepared NPs from an aqueous medium into an organic carrier constitutes a reliable and inexpensive route for preparing highly concentrated and stable organosols. The reverse transfer serves the preparation of small-sized and highly monodispersed hydrosols. The kinetics of phase transfer and the stability of the resultant sols are key considerations and are reliant on mixing and on the hydrophilic/lipophilic balance (HLB) of the particles. This balance is in turn dependent on the surface interaction between the phase transfer agent and the particles, as well as the interactions between the phase transfer agent, the continuous phase, and other additives, whenever applicable. This article reviews different studies that examined the phase transfer of NPs between organosols and hydrosols and elucidates the governing interactions. Scale-up of this preparation route lies in readily dispersible dried coated particles, or stable highly concentrated sols. Particle-independent multi-cycle phase transfer, with a minimum effect on NP size, monodispersity, and functionality is an attractive frontier.