Formation of Nanoparticles of Water-Soluble Molecules: Experiments and Mechanism

An aqueous solution of CaCl 2 as a model system has been used to develop the mechanism of how a highly water-soluble molecule can be synthesized as a solid nanoparticle with controlled diameter. This was achieved by first dissolving CaCl 2 in nanometer-sized water drops of a water-in-oil (w/o) microemulsion system and later precipitating it as CaCl 2 nanoparticles during the evaporation of the microemulsion. Nanoparticles were found by transmission electron microscopy (TEM) and atomic force microscopy (AFM) to increase in mean diameter from 25 to 88 nm as the diameter of a templating microemulsion droplet increased from 1.6 to 4.5 nm. The droplet size was varied by increasing the water to surfactant molar ratio ( R ) from 2 to 10. Variation of the absolute surfactant concentration, in itself, did not alter the particle size. From these findings, we propose that the CaCl 2 nucleus forms during evaporation by heterogeneous nucleation on the surfactant headgroups of some of the drops. The nucleus then grows by the remaining dissolved CaCl 2 molecules in the drop. This is followed by the addition of CaCl 2 molecules contributed by those surrounding drops, which have no nucleus. The diameter of the final nanoparticle therefore exceeds that of the initial water drop. The number of such contributing drops (within a hypothetical contributing region around a drop containing a nucleus) was estimated by equating the timescales of CaCl 2 nucleation with that of the Brownian diffusion of drops in the oil medium. Calculations based on this new concept quantitatively explained our experimental data on increasing the mean nanoparticle diameter with drop diameter. The final solid nanoparticle product is coated with surfactant, so it can be easily resuspended in any other solvent and used as and when required.