Collaborative Research: Ion Mobility in Aqueous Acids, Bases, and Salts

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With support from the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) Program in the Division of Chemistry, a research team led by Professors Steven Corcelli at the University of Notre Dame and Paul Cremer at Pennsylvania State University is investigating the mobility of ions in water with a fully integrated experimental and theoretical modeling approach. Solutions containing salt, including acids and bases, are essential in many chemical, biological, and environmental processes, and the rates of ion transport are especially relevant to emerging battery technologies employing aqueous electrolytes. However, the detailed molecular mechanism for ion transport, including the role of counterions and ion-pairing, is not fully understood. Thus, the research objective of this project is to explore ionic mobility in water at the molecular level to achieve a unified understanding of the role of counterions and ion-pairing on the structure, dynamics, and mechanisms of ion transport in aqueous acids, bases, and salts. In addition to the broader scientific impacts of the work, the project also includes advanced training opportunities for graduate and undergraduate students, as well as the development of a related laboratory experiment for high school students. Preliminary findings that counterions substantially impact hydroxide mobility suggest that ion-pairing effects are central to acid, base, and salt mobility. This project involves a systematic study of Hofmeister series ions in acid, base, and salt solutions as a function of concentration. Experimentally, diffusion constants are measured by electrochemical impedance spectroscopy, and a combination of Raman and infrared spectroscopy elucidate information about ion structure, hydration, and proton delocalization. Complementary nuclear magnetic resonance spectroscopy measurements yield proton transfer rates and information about the mobility of individual ions. Theoretically, the acids and bases are simulated with ab initio molecular dynamics and the salts with molecular dynamics. Calculations based on the simulations provide the free energy of association between respective ion pairs and aid the interpretation of the experimental Raman and infrared spectra. The spectroscopic and simulation studies synergistically combine to provide a molecular-level understanding of diffusion constant measurements. The combined studies elucidate the impact of counterions on the Grotthuss mechanism of proton hopping and the particular roles of solvent-shared versus contact ion-pairing in aqueous electrolyte solutions. The broader impacts of the work include the importance of better understanding ion mobility for many technological applications, advanced student training opportunities, and development and implementation of a high school laboratory experiment that explores ion-pairing at hydrophobic interfaces.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.