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A molecular-scale investigation of water’s response to interfaces: towards rational design of water-mediated interactions


Understanding and manipulating the behavior of water is a crucial component of designing materials at interfaces (i.e. for use in membranes, filters, chromatographic columns, etc.) as well as materials based on self-assembling molecules (i.e. micelles, hydrogels, etc.). It has long been known that water plays a significant role in mediating surface adsorption as well as interactions between biomolecules, but effective strategies to modify this behavior based on atomistic physical intuition has been lacking. Many past efforts have focused on unrealistic, toy systems in order to develop an extensive theory of water behavior in idealized inhomogeneous environments. Here I put this theory to the test in realistic yet simple and well-controlled molecular simulations, beginning the process of identifying practical extensions to inform the design of real materials.

To isolate and understand key water physics, I investigate ever-important hydrophobic interactions and explore the thermodynamic information contained within water’s structural response to hydrophobes. Specifically, consideration of the behavior of three-body angle distributions, which relate to the tetrahedral structure of water, allows for the construction of simple thermodynamic models for adsorption and solvation. I show that, in general, the success of such thermodynamic models may be attributed to fundamental connections between perturbations to water structure and the relative entropy of a solvation process, which includes indirect thermodynamic contributions associated with solvent restructuring.

In proteins and other biological settings, hydrophobic regions do not occur in isolation, but instead are part of heterogeneous interfaces involving a variety of chemical groups. To study the role of chemical patterning in inducing unique water behavior at such surfaces, I present methods coupling a genetic algorithm to molecular dynamics simulations to optimize arbitrary surface properties through the reorganization of functional groups. Water mobility near an interface is optimized in this way, ultimately revealing novel relationships between water dynamics, structure, and thermodynamics. I also use this algorithm to discover interfacial chemical patterns that repel or attract a diverse set of small-molecule solutes, as might be necessary to prevent fouling during water purification processes or selectively capture desirable products during chromatography. Overall, I extend our current knowledge of water’s molecular-scale response to both solutes and macroscopic interfaces, effectively expanding the design space for interfacial materials in terms of these atomistic-level insights.

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