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

Abstract

An aqueous solution of CaCl2 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 CaCl2 in nanometer-sized water drops of a water-in-oil (w/o) microemulsion system and later precipitating it as CaCl2 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 CaCl2 nucleus forms during evaporation by heterogeneous nucleation on the surfactant headgroups of some of the drops. The nucleus then grows by the remaining dissolved CaCl2 molecules in the drop. This is followed by the addition of CaCl2 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 CaCl2 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.

Synthesis of NaX (X = F, Cl, Br, I) Nanoparticles

“…All the NP diameters are in the 1.5 to 2.5 nm range. These NPs are significantly smaller than those previously reported for soluble binary compounds [25]. Solubility product values (K sp M 2 ) increase going from fluoride to iodide.…”

“…Bandyopadhyaya and coworkers (RSB) have developed a model to describe this process. They supported their model with experiments conducted on the CaCl 2 system [25]. Ions in the incipient saturated solution precipitated on the surfactant head groups during the evaporation process.…”

“…In these cases the corresponding salts were dissolved and prepared with AOT analogously to the NaX NPs. However, SAED shows that the product NPs are all consistent only with [25] and ZnSO 4 [26]. The only monovalent cation salts previously reported were with Na + and so this exchange chemistry would not have been evident in the presence of an Na + -containing surfactant.…”

“…This resulted in precipitation of the soluble salt from solution during solvent evaporation. Using this method, NPs of Cu(NO 3 ) 2 , Na 2 HPO 4 [24], CaCl 2 [25], Na 2 S and ZnSO 4 [26] have been achieved. The resulting NPs were capped with the surfactant that allows redispersion in dry organic solvents.…”

Metal‐halide Nanoparticle Formation: Electrolytic and Chemical Synthesis of Mercury(I) Chloride Nanoparticles

“…[7][8][9] Other methods include the controlled precipitation of metal halides in aqueous media, for example, AgBr NP suspensions stabilised in solution by materials such as gelatine [10] and cucurbit[n]uril macrocycles. [11] Although the area has seen significant development, there are few examples for the formation of particles of sizes 20-100 nm without complex multi-step chemical processes and stabilising organic coatings.…”