Membrane dispersion systems as a platform for nanoparticle & microparticle manufacturing

<p dir="ltr">There is a great need for both organic and inorganic particles in various industries, such as the food, chemical, pharmaceutical, etc. Particle synthesis via conventional chemical methods lack control over nucleation, mixing, growth and stability of encapsulated active species/ingredients, thus affecting the final particle size, uniformity, yield, structural properties, etc. Sufficient mixing, mass transfer and reaction volume can improve physical and chemical properties of particles. The use of emulsion methods to synthesise particles has been previously explored and shows significant potential for controlling physical & chemical properties of particles. The work presented here highlights the versatility of membrane dispersion systems, showcasing their ability to produce highly uniform nano/microparticles across various applications, such as encapsulation of food ingredients within a gelatine & gum arabic microcapsule and adsorption of ions for drinking water using ZIF-8 and silica particles. These nano/microparticles particles were produced using a batch (LDC-1; Micropore Technologies Ltd.) and a continuous system (CXF-1; Micropore Technologies Ltd). The LDC-1 was used for preliminary testing and screening, due to its small volume (ca. 100 ml), and where possible the synthesis process was scaled-up using the CXF-1. The findings showed a superior control over particle size, uniformity and morphology offered by membrane dispersion systems compared to conventional methods. Despite challenges such as the amorphization of ZIF-8 particles synthesised within the microreactors (i.e. the emulsion droplets) or inability to control the particle size of silica using the continuous system, these systems demonstrated promising capabilities for improving encapsulation efficiency and overall product quality, as it was the case with gelatine & gum arabic microcapsules or the synthesis of ZIF-8 without the need to confine the reaction within the microreactors. Furthermore, although the continuous manufacturing of mesoporous silica microparticles could not be demonstrated here, the batch membrane dispersion system showed the potential to move towards a more sustainable manufacturing process.</p><p dir="ltr">The findings are separated into four experimental chapters. In the first experimental chapter, or the first part of the work, dry coacervate microcapsules of different size and shell thickness were manufactured combining a continuous single pass crossflow membrane emulsification system, the CXF-1, and spray drying to obtain a dry powder. Complex coacervation is a phase separation process in which two oppositely charged polymers or macromolecules in a solution come together at the liquid-liquid interface to form a dense coacervate shell. The process can be employed to encapsulate oil droplets, creating stable microcapsules that protect and control the release of oil while achieving a high encapsulation efficiency of active ingredients. A single-pass crossflow membrane emulsification system with a single cylindrical 10 x 100 mm membrane module with 10 μm pore produced emulsion droplets between 71 μm and 114 μm with a dispersed phase (oil content) in the final emulsion between 3.3 and 6.2 vol/vol% and a total emulsion output mass rate between 25.68 kg h^-1 - 49.68 kg h^-1. Emulsions manufactured by membrane emulsification were nearly monodispersed with the highest span not exceeding 0.68. Addition of maltodextrin to the emulsion prior to spray drying increased the viscosity and prevented the capsules breakage. Microcapsules up to a mean droplet diameter of 113.19 ± 0.81 μm preserved the shell and had a yield up to 78.43 ± 0.97 wt.%, a surface oil as low as 9.35 ± 0.88 wt.% and an encapsulation efficiency of 71.09 ± 0.87 wt.%.</p><p dir="ltr">In the second part of the work mesoporous silica microparticles were synthesised by a room temperature CO₂-induced gelation route of sodium silicate. The sodium silicate solution was confined within the microdroplets produced by membrane emulsification using the LDC-1 coupled with a hydrophilic and hydrophobic circular membrane with a diameter of 23 mm and a 20 µm average pore diameter. By confining the reaction mixture inside the droplet volume, the final particle size & uniformity could be controlled to achieve a wide range of particle diameters <i>D</i>₅₀ = 61 - 153 µm, achieving uniformity values as low as <i>Span </i>= 0.41 and a surface area as high as 427 m² g^-1.</p><p dir="ltr">In the third part of the work zeolitic imidazolate framework-8 (ZIF-8) was synthesised by a room temperature precipitation route from solutions containing 50 % less organic solvent and a 1:3 metal precursor to organic linker molar ratio. The reaction mixture was confined within microdroplets produced by membrane emulsification using a the LDC-1 coupled with a circular membrane with a diameter of 23 mm and a 20 µm average pore diameter. By confining the reaction mixture inside a microdroplet the particle size, uniformity and morphology can be carefully controlled to achieve a wide range of particle diameters between <i>D</i>₅₀ = 113.0 - 1551.0 nm and morphologies while achieving uniformity values as low as <i>Span</i> = 0.312.</p><p dir="ltr">In the final part of the work ZIF-8 was synthesised via membrane micromixing using the LDC-1 coupled with a 23 mm size membrane with a 20 µm average pore diameter, and CXF-1 containing a cylindrical 10 x 100 mm membrane with a 10 µm average pore diameter. By controlling the flowrate of either the organic linker or the metal precursor through the membrane (i.e. adjusting the ratio of organic linker to metal precursor over time), it was possible to control the nucleation and growth of ZIF-8 to achieve a wide range of sizes, improve uniformity and product quality. Synthesised ZIF-8 nanoparticles were then tested as an ion-exchange material for the removal of Cu²⁺ from aqueous solutions. These showed high adsorption capacity (ca. 673 mg g^-1) of Cu²⁺ and high removal efficiency of 98.77 - 99.79 % from an initial Cu²⁺ concentration of 0.01 - 1.0 g L^-1.</p>