Controlled emulsion droplet solvent evaporation for particle production

Abstract

In this work, we are motivated by the need to produce particles of well-controlled size, shape and morphology for general application in catalysis, environmental remediation, nanomedicine, pharmaceuticals, the development of new materials, and other fields. Moreover, our approaches are guided by the desire for continuous and scalable production, in contrast to the batch-wise processes typically used. We employ the emulsion droplet solvent evaporation method, which is extremely versatile, to create, for example, magnetic nanoparticles, polymeric Janus beads, and crystalline particles. The emulsion droplets act as confined spaces, or templates, within which the particles can form. Upon removal of the solvent, primary magnetite nanoparticles pack into dense magnetic clusters, polymers precipitate as beads, or small molecules crystallize out of the solution to form spherical particulates. The thesis is comprised of experimental, theoretical and computational work that discusses the control of polymeric Janus bead morphology; demonstrates the potential of various operations for integration into large-scale manufacturing systems for monodisperse particle production; and offers insight into solvent and particle diffusion during the solvent evaporation process. The formation of Janus beads by solvent evaporation-induced phase separation of polymer blends is studied using a model system of polystyrene (PS), poly(propylene carbonate) (PPC) and chloroform. The phase separation of the polymer solutions in the bulk is analyzed and a phase diagram is constructed. PS/PPC Janus beads of varying composition are synthesized and we demonstrate the ability to tune the morphology by varying the type and concentration of the surfactant. Thermodynamic models that describe the particle morphologies as functions of the interfacial tensions are discussed. The remainder of the thesis focuses on the development and characterization of continuous, high-throughput synthesis methods for functional particles based on solvent evaporation techniques. We introduce membrane emulsification and pervaporation as operations that have the potential to be integrated into such a process. We develop a population balance model to describe the transport of solvent from nanocrystal- or polymer-laden droplets in an emulsion as it flows through a pervaporation unit. The solvent transport is simulated using a high-resolution finite volume algorithm, which affords a smooth solution with second-order accuracy. The simulations provide information regarding the evolution of the particle size distributions and the diffusional behavior of the droplets. Furthermore, the required fiber length to remove the solvent completely from an emulsion can be determined in terms of natural dimensionless constants that arise from the structure of the model equations, making the model useful as a design tool. For systems with a high Biot number, we show that a lumped capacitance assumption, which greatly simplifies the model and reduces the computational requirement, is valid. Finally, we investigate the evaporative crystallization of glycine and alanine, and the clustering of magnetite nanocrystals, in emulsion films flowing down an inclined plane. The temperature and the solvent evaporation configuration are shown to have a significant effect on the transport behavior of the solvent and droplets. The potential of the inclined plane system in particle production is established, and the flow of emulsion droplets of different sizes is studied, using an experimental test apparatus.