Thomas Beck of the University of Cincinnati is supported by the Theory, Models and Computational Methods program in the Chemistry Division to carry out computational studies of specific ion or Hofmeister effects for anions. This work employs a novel theoretical approach for the statistical mechanics of liquids, the quasi-classical theory (QCT). The QCT partitions the excess chemical potential into three contributions by enacting a spatial partitioning with a hard-sphere constraint: 1) the cavity free energy for digging a hole in the solvent 2) the free energy for placing the solute into the cavity, and 3) the free energy to release the hard-sphere constraint, allowing direct contact between the solute and solvent. For ion solvation problems, step 2) above yields the largest contribution to the hydration free energy. It was found that a mean-field estimate of this term is quite accurate. This method permits computations of free energies at the quantum mechanical level, and in binding sites in proteins.
Specific ion or Hofmeister effects are very important in a wide array of chemical, physical, and biological systems, for example the swelling of biological membranes, solvation free energies and activities, surface tension increments, bubble interactions, ion channel transport, colloid interactions, pH measurements, buffer behavior and protein folding and stability. Professor Beck is involved in organizing a workshop on ions in chemistry and biology to be held in Telluride CO. He is also heavily involved in undergraduate and graduate curriculum development.
. The identity of the ions in water and other solvents such as ethylene carbonate can have profound effects on both macroscopic (phase behavior, transport, bubble lifetimes, etc.) and microscopic (density profiles near proteins and electrochemical interfaces) properties. We developed a range of new theoretical and computational tools to probe at a basic level why these remarkable effects occur. Those tools include new theories for partitioning ion solvation thermodynamic properties into the important physical parts and computational tools for implementing the theories. We were able to gain a clear understanding of the physical origin of wide differences in ion hydration entropies for different ions (ranging from small water-ordering ions like fluoride to large water-disordering ions like iodide). The behavior of ions near the water liquid-vapor surface was also examined; a major finding in our research is that there is a contribution to the driving force for ions near surfaces from the interfacial potential of the solvent (particularly water). Our work then moved on to explore these interfacial potential effects at a fundamental level. We were able to determine a new value for the electrochemical surface potential of water. Obtaining a reliable value for this quantity is important both for predicting ion density profiles near the surface and for determining single-ion thermodynamic quantities. These quantities are essential for gaining a quantitative, predictive theory of specific ion effects. Our project has led to fundamental results that can now be used in developing more accurate models for ions in water and other solvents. The research has impacts in a broad range of fields, including basic chemistry, electrochemistry, surfactant science, atmospheric science, nano-science, and biology (ion channels). Understanding how ions distribute near interfaces is essential for developing better batteries, supercapacitors, and other nano-scale devices. In addition to the broad scientific impact, the project has led to the training of 5 graduate students in theoretical and computational chemistry. These students are developing a wide range of skills in theoretical chemistry and high performance computing. Work near the end of the project focused on developing a new freshman seminar course at the University of Cincinnati, "Sustainable Energy and Society" which is aimed at developing energy literacy in young students. The PI is now teaching that course, and the students are heavily engaged in learning quantitative reasoning, estimation skills, and in gaining knowledge about the role of energy in our society.
