Continuous Production of Water‐Soluble Nanocrystals through Anti‐Solvent Precipitation in a Fluidic Device

Abstract

This paper describes a simple and robust method for the continuous production of water‐soluble nanocrystals using anti‐solvent precipitation under diffusion control in a fluidic device. We use sodium chloride (NaCl) as an example to demonstrate the concept. In a typical process, aqueous NaCl and ethanol (the anti‐solvent) serve as the focused and focusing phases, respectively, for the generation of a coaxial‐flow system. Upon contact with each other, the rapid diffusion between water and ethanol leads to the formation of NaCl nanocrystals at the interface while a gradient in NaCl concentration is created along the flow direction. The nucleation and growth of NaCl nanocrystals can be readily tuned by varying the hydrodynamic parameters such as the ratio between the flow rates of the two phases and the total volumetric rate. By optimizing these parameters, we are able to produce NaCl nanocubes and nanospheres as small as 20 nm and 6 nm, respectively, while attaining a narrow distribution in size. We have also successfully generated KCl nanocrystals with similar controls, demonstrating the generality of this method for the production of water‐soluble nanocrystals.

Tunable NaBH4 Nanostructures Revealing Structure‐Dependent Hydrogen Release

“…The crystal structure of NaBH 4 is isomorphous to NaCl; therefore, the nucleation and growth of NaBH 4 can also be explained based on the studies on NaCl crystals. [ 28,29 ] Wang et al [29d] argued that the formation of cubic morphology of NaCl is due to the fast diffusivity of Na + and Cl − ions and a nondirectional growth in aqueous solutions. In the presence of glycerol, a medium of higher viscosity than water, the crystal growth conditions can be changed where Na/Cl diffusion is slower and NaCl can nucleate along rhombic decahedron, octahedron, and submicron‐cubic structures [29a] .…”

“…[ 28,29 ] Wang et al [29d] argued that the formation of cubic morphology of NaCl is due to the fast diffusivity of Na + and Cl − ions and a nondirectional growth in aqueous solutions. In the presence of glycerol, a medium of higher viscosity than water, the crystal growth conditions can be changed where Na/Cl diffusion is slower and NaCl can nucleate along rhombic decahedron, octahedron, and submicron‐cubic structures [29a] . The fact that we have observed cubic morphologies of NaBH 4 in IPA and with ODA (both with NH functionalities) could be due to the fast ionic diffusivity of Na + /BH 4 − ions at the IPA/ODA interface and this may lead to the thermodynamically favorable growth of cubes enclosed by [100] facets.…”

Monitoring the rapid nanocrystal transformation via trapped intermediates of silica encapsulation

“…On the other hand, the synthesis of water-soluble salt nanocrystals has long been a challenge [29,30]. Even the nanocrystals of relatively more stable salts would suffer from low contrast and blurry borderlines under TEM.…”

Marine-cloud brightening: an alternative system

“…Recent work by Chen et al (2019) has focused on generating salt nanoparticles of configurable dimensions, ranging from several nm to more than 50 nm in diameter. The particles are generated using anti-solvent precipitation from a concentrated 2M NaCl solution (2 mol NaCl dissolved per 1 L of water), using ethanol to precipitate the salt nanoparticles.…”

“…Given the chosen flow-rate ratio R = 40 by volume (Chen et al, 2019), the final concentration of ethanol in the ethanol-water mixture should be approximately 97.5% by volume, which is near azeotropic (96% by volume). Based on Kunnakorn et al (2013), the energy cost of dessicating azeotropic ethanol is measured at 1.25 MJ kg −1 (1.134 MJ kg −1 for the pervaporation system, and 0.1 MJ kg −1 to raise the ethanol temperature from 20 • C to the pervaporation temperature of 70 • C).…”