Optimization of flow conditions and investigation of crystallization kinetics from a supersaturated solution in a two-phase microfluidic system

The effects of thermodynamic conditions on the kinetics of nucleation from supersaturated solutions have been of great interest in controlling the synthesis of nano/micro particles with advanced physical, chemical, and optical properties. Microfluidic systems offer a medium, where the nucleation phenomena can be better controlled and examined. This study is aimed at optimization of two-phase flow conditions for producing uniform picoreactors in a microfluidic system and investigation of the thermal conditions for nucleation from a supersaturated solution. The microfluidic system consists of an x-channel for the formation of droplets followed by a serpentine channel integrated with a temperature controlled transparent heater. The solution contains zirconium chloride (ZrCl4) and 1,4-benzenedicarboxylic acid (BDC) dissolved in N,N-dimethylformamide (DMF), i.e. a precursor solution for the synthesis of UiO-66 crystals with a metal organic framework (MOF) structure. The precursor solution, as the dispersed phase, and silicone oil, as the continuous phase, were used to form monodispersed spherical picoliter reactors that travelled with equal interdroplet distances in the microfluidic system. The desired size of the droplets was optimized by varying the ratio of the flow rates of the dispersed and the continuous phases, Qd/Qc. The reduction in Qd/Qc caused droplets to be smaller and more spherical. At the optimized condition, where Qd/Qc was 0.05, the average diameter of the droplets was 105.23±5 µm reducing to 70±3 µm just before they entered the serpentine, losing about 30 % of the size. Shrinkage of the droplets, due to the dissolution of the solvent into the oil causing supersaturation of the precursor solution, continued in the hot region of the device until nucleation started. Crystallization experiments were carried out at 50, 80 and 100 °C to determine the initial point and the rate of nucleation based on temperature and picoreactor (droplet) size. Initial point of nucleation gave the residence time of the droplet in the heated serpentine region until nucleation took place in the droplet. The rate of nucleation was determined by counting the number of droplets with nuclei over time. The size of the droplets affected the nucleation time as smaller droplets had nucleation earlier at the same temperature. For the case of 80 °C, the 70 µm droplets nucleated at 30 ±5 seconds, while the 90 ±5 and 110 ±5 µm droplets nucleated at 109 ±3 and 112 ±5 seconds, respectively. The generation of supersaturation in the droplets due to shrinkage drove the nucleation earlier at higher temperatures resulting with more droplets nucleated. This scenario followed Lamer's model of instantaneous nucleation. Kinetic rate constants for nucleation, k𝑁, were calculated using the classical nucleation theory as 0.0003175 s-1, 0.0156 s-1, and 0.1174 s-1 for 50, 80, and 100 °C respectively. The activation energy for nucleation was found as 119 kJmol-1. XRD analyses of the synthesized particles showed a positive influence of higher temperatures within the range applied confirming the potential of the microfluidic system to carry out crystallization from supersaturated solutions.