| Summary: | Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, 2016. === Cataloged from PDF version of thesis. === Includes bibliographical references (pages 189-203). === The heterogeneous molecular interactions that operate at material interfaces control the efficiency of chemical engineering processes as diverse as adsorption, emulsification, heat exchange, and froth flotation. In particular, the process of colloidal self-assembly harnesses the rich tapestry of interactions that operate at several length scales, including van der Waals and electrostatic interactions, the hydrophobic effect, and entropic considerations, to drive the autonomous aggregation of simple building blocks into intricate architectures. This bottom-up approach has increasingly become the mainstay of the colloids community in its quest to design and fabricate increasingly complex soft-matter assemblies for pharmaceutical, catalytic, optical, or environmental applications. Accurately modeling and manipulating interfacial interactions across many different length scales is vital to optimizing the self-assembly and stability of colloidal suspensions. With the above background in mind, in this thesis, I illustrate the modeling of interfacial phenomena at a range of length scales, with a particular focus on utilizing a combination of computer simulations and molecular-thermodynamic theories to evaluate the free energies associated with the formation and reconfiguration of revolutionary colloidal systems, including dynamically-responsive colloids and two-dimensional nanomaterial suspensions. First, I examine the interplay between interfacial tensions during the one-step fabrication, and stimuli-responsive dynamic reconfiguration, of three-phase and four-phase complex emulsions. This fabrication makes use of the temperature-sensitive miscibility of hydrocarbon, silicone, and fluorocarbon liquids and is applied to both microfluidic and scalable batch production of complex droplets. I demonstrate that droplet geometries can be alternated between encapsulated and Janus configurations by judicious variations in interfacial tensions, as controlled via conventional hydrocarbon and fluorinated surfactants, as well as by stimuli-responsive and cleavable surfactants. Subsequently, I examine the molecular origins of the ability of surfactants to modulate the interfacial tensions at fluid-fluid interfaces, including developing a computer simulation-aided molecular- thermodynamic framework to predict the adsorption isotherms of non-ionic surfactants at the air-water interface. The use of computer simulations to evaluate free-energy changes is implemented to model a surfactant molecule possessing tumor-selective cytotoxicity. Utilizing potential of mean force calculations, I shed light on the preference of this anti-cancer drug for certain types of lipid bilayers, including advancing a hypothesis for the mechanism through which this drug induces apoptosis. I then utilize potential of mean force calculations to evaluate the formation of colloidal suspensions of two novel two-dimensional materials: phosphorene and molybdenum disulfide (MoS2). I focus on the correlations between the structural features of commonly-used solvents and: (1) their ability to intercalate between nanomaterial sheets and induce exfoliation, and (2) their effect on the energy barrier hindering the aggregation of the phosphorene and MoS2 sheets. The combination of simulation-based computation of the potential of mean force (PMF) between pairs of nanomaterial sheets, as well as the application of theories of colloid aggregation, offers a detailed picture of the mechanics underlying the liquid-phase exfoliation and the subsequent colloidal stability of phosphorene and MOS2 sheets in the commonly-used solvents considered. The agreement between the predicted and the experimentally-observed solvent efficacies provides a molecular context to rationalize the currently prevailing solubility-parameter-based theories, and for deriving design principles to identify effective nanomaterial exfoliation media. === by Vishnu Sresht. === Ph. D.
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