Comparative Study of The Solubility of DMF in Silicone Oil for Use in a Droplet-Based Microfluidic System Towards Synthesis of MOF Nanoparticles

Abstract

Synthesis of nanoparticles with enhanced adsorption capacities based on a complex metal organic framework (MOF) structure requires the employment of supersaturated solutions prepared with solvents that possess multiple properties at elevated temperatures. N,n-dimethylformamide (DMF), represented as HCON(CH3)2, is the widely used solvent with a high solvating and coordinating ability for metal salts and organic ligands, thermal stability at high temperatures, and high dispersing capacity in the synthesis of nanoparticles with MOF structures. Synthesis of MOF particles is usually performed using hydrothermal and/or solvothermal methods at high temperatures in conventional batch reactors using conventional heating systems. Single- or two-phase microfluidic systems that enable the use of smaller amounts of solution or microwave systems that provide rapid heating of the solution as it flows have become promising methods yielding more uniform smaller particles and reduced synthesis times. Solubility of the solvent is a critical factor when the solution is used in two-phase suspensions or two-phase microfluidic systems involving an oil as the carrier phase. Silicone oil (Polydimethylsiloxane, [-Si(CH3)2O-]n) is a nonpolar hydrophobic substance that is almost immiscible with DMF and is frequently used as the carrier fluid in two-phase droplet microfluidics. The promotion of the dissolution of the solvent into the carrier oil phase due to surface tension or convective forces in suspended systems, which is further enhanced in flowing systems and at the micro scale, creates a serious problem. Any mass transfer that may occur between the droplet phase and the continuous phase changes the concentration of the precursor solution in the droplets and, accordingly, the quality of the final product. This is an undesirable situation that must be managed by preventing the mass transfer between the two phases or by compensating for the possible mass loss in the initial precursor solution. In this thesis, we conducted a comparative study to investigate the solubility of DMF in silicon oils with different viscosities, 100 cSt, 500 cSt, and 1000 cSt, at both micro and macro scales with and without flow effects at various temperatures, 25 ℃, 50 ℃, 75 ℃, and 100 ℃. Any possible diffusion between the two phases can affect the synthesis time and the quality of the final products in two-phase microfluidic systems, where suspended uniform droplets flowing with the oil phase are used as picoliter vessels. Such a problem has not been pronounced until now in the studies of MOF syntheses employing microfluidic systems, where DMF and silicone oil are used together, because the microfluidic channels consisted of opaque capillary tubes. Such systems do not allow for the observation of the whole process starting from the formation of the droplets to the formation of particles on an optical microscope. The transparent microfluidic devices used in this study allowed for the observation of the shrinkage of droplets on an inverted microscope. To identify the effects of the droplet-based microfluidic system on the mass transport into the oil, we compared the solubility of DMF in silicone oil at macro and micro scales and under stationary and flow conditions using three different systems: 1. A stationary macro-scale system, 2. a stationary micro-scale system, and 3. a microfluidic system. We discussed the effects of size and convective mass diffusion by comparing the results obtained at the macro and micro scales using stationary and flow conditions and showed that solubility is enhanced under flow conditions at smaller scales.