Clean water supplies are required for industry and general life. However, water shortages dueto pollution and human activity are increasingly common, and new, more efficient, materials need to be made to increase clean water supplies. To do this, fundamental information on the interaction of water with ions at the atomic level is required, but there is a knowledge gap in the literature on this topic. As experiments can have limited resolution on this scale, Molecular Dynamics (MD) simulations are used to examine the behavior of various probe molecules and self-assembled monolayers (SAMs) on inorganic surfaces and in electrolytic solution. This thesis describes three projects in which MD simulations have been used in combination with experiments by collabora- tors to provide new insights concerning the interaction of ions and molecules at aqueous interfaces. In the first project we consider ions interacting with acetone in water, using infrared spec- troscopy in combination with theory to study interactions of the carbonyl with metal ions. The Stark Model allows for comparison between experiment (IR spectra) and MD simulations using probe molecules. In this work, acetone was utilized as a probe molecule due to it’s distinct IR spec- tra and strong dipole. Using 1, 3, and 5 molal concentrations of NaCl, LiCl, and ZnCl2, we find that polarizable forcefields better match experimental results compared to nonpolarizable force- fields. Sodium cations frequently bind and diffuse away from the oxygen of the acetones, while lithium has a stronger bond to that oxygen but infrequently finds itself in the first solvation shell of the acetone. Both sodium and lithium cations shift the electric fields the oxygen feels, but the loss of hydrogen bonds partially cancels out the effect of the ions. Zinc cations do not closely interact with the oxygen. Further work should be done to capture what effects simulations are missing to better match experimental results. In the second project we consider the interaction of metal ions with small polar molecules at aqueous interfaces to determine how the ions get distributed in a model for materials used in water purification. Zwitterionic cysteine SAMs have been simulated on a corundum surface with 1 - 1000 mM concentrations of Pb(NO3)2, Cu(NO3)2, and Ca(NO3)2 in aqueous solution. Using MD simulations to compare to X-Ray Refraction results, the population of ions was investigated at different locations in the system. We find that Pb2+ cations strongly prefer to stay at the interface of aqueous solution and SAM and show the largest population in this region. Cu2+ and Ca2+ do interact with the surface to a lesser extent, but at high surface density of the SAM, no ions diffuse into the brush at any concentration observed in simulation. The surface density of the SAM in experiment likely varies and molecules are not packed closely together. To further understand the effect of ions on probe molecules at interfaces, simulations of 3-mercapto-2-butanone at high and low surface densities on gold were designed. Using 1, 3, and 5 molal concentrations of CsI, CsCl, LiI, and LiCl, the behavior and effect of these ions on the oxygen of the 3-mercapto-2-butanone was investigated. We find significant differences from pre- vious similar work with acetone, indicating the gold surface has a large effect on system dynamics and ion binding. At lower ionic concentrations, we observe red-shifts in the electric fields, while at higher ionic concentrations, this trend seems to reverse and we mostly observe blue-shifts in the electric fields experienced by the oxygen. At high surface density, 3-mercapto-2-butanone molecules interact with each other, providing significant steric hindrance to the interaction of car- bonyl to ion. At low surface densities, 3-mercapto-2-butanone molecules position with the car- bonyl parallel to the surface, providing cations and water molecules limited access to the oxygen of the carbonyl.

Creator

• Felts, Alanna

DOI

• https://doi.org/10.21985/n2-hv6m-1t19

Subject

• Chemistry
• Computational chemistry

Language

• en

Alternate Identifier

• etdadmin_upload_1010659
• http://dissertations.umi.com/northwestern:16704

Date created

• 2023-01-01

Resource type

• Dissertation

Rights statement

• In Copyright