Quantum Models of Ion Solvation Thermodynamics

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Thomas Beck of the University of Cincinnati is supported by an award from the Chemical Theory, Models and Computational Methods program in the Division of Chemistry to develop new computational methods to study ions in liquids. Ions are charged atomic or molecular species that can dissolve in solvents like water or organic liquids. Ions are important in a very broad range of natural and technological systems. Table salt is the most common everyday example, but ions of many kinds figure prominently in electrochemistry, colloids, surface chemistry, atmospheric chemistry, soaps and detergents, biological membrane ion channels, geology, and energy storage devices (batteries and supercapacitors). Not all ions are the same; in fact, the ion identity (through its size, shape, and chemistry) can dramatically affect the properties of a given system. The focus of this project is to gain a deeper understanding of what causes such large differences between ionic species. This research aids in developing more accurate models for how the ions interact with solvents such as water or organic liquids (such as ethylene carbonate, the solvent in the lithium (Li)-ion batteries used in every electronic device). Towards this end, Beck and his coworkers employ accurate models that come from the fundamental theory of nature, quantum mechanics. Their goal is to learn about basic aspects of ion solvation and then produce reliable models that can help understand problems like atmospheric chemistry and the development of longer-lasting batteries. The educational component of this project centers on continued development of a course on sustainable energy. Professor Beck is currently the chair of the State Users Group at the Ohio Supercomputer Center, with over 1000 active computational science users. This project addresses fundamental aspects of ion solvation in water and organic solvents. Ion solvation figures prominently in surface chemistry, atmospheric chemistry, soaps and detergents, biological membrane ion channels, geology, and energy storage devices (batteries and supercapacitors). Beck and his coworkers aim to establish a fundamental energy scale for single-ion solvation. This allows for a firmer groundwork for studying specific ion effects. The research also leads to the development of better classical models that can be applied to much larger systems such as sea salt aerosols and improved batteries. The behavior of ions near interfaces is an essential aspect of all of these problems. The research has four objectives: to study ions in water, in organic solvents, and in biological channels, and to further develop a new stochastic algorithm for quantum simulations on parallel computers. The study of ions in water leads to the accurate prediction of the effective surface potential of water, a long sought quantity in condensed phase chemistry. This work employs ab initio quantum simulations and quasi-chemical theory to obtain quantitative estimates of the bulk and real hydration free energies. The study of ions in organic solvents applies the same ideas to ions in ethylene carbonate, a key electrolyte in Li-ion batteries. That research improves current models for these systems. The study of ions in biological channels examines the interaction of protons and chloride ions in the bacterial CLC channel protein. The development of stochastic algorithims may lead to a new forward-looking algorithm for large-scale quantum simulations. The method employs stochastic differential equations to solve for the one-particle density matrix. Besides the scientific impacts, the research broadly impacts education through development of a course on sustainable energy and leadership of the State Users Group at the Ohio Supercomputer Center, with over 1000 active computational science users.