Abstract
With a goal of determining an absolute free energy scale for ion hydration, quasi-chemical theory and ab initio quantum mechanical simulations are employed to obtain an accurate value for the bulk hydration free energy of the Na+ ion. The free energy is partitioned into three parts: 1) the inner-shell or chemical contribution that includes direct interactions of the ion with nearby waters, 2) the packing free energy that is the work to produce a cavity of size λ in water, and 3) the long-range contribution that involves all interactions outside the inner shell. The interfacial potential contribution to the free energy resides in the long-range term. By averaging cation and anion data for that contribution, cumulant terms of all odd orders in the electrostatic potential are removed. The computed total is then the bulk hydration free energy. Comparison with the experimentally derived real hydration free energy produces an effective surface potential of water in the range −0.4 to −0.5 V. The result is consistent with a variety of experiments concerning acid–base chemistry, ion distributions near hydrophobic interfaces, and electric fields near the surface of water droplets.
Original language | English |
---|---|
Pages (from-to) | 30151-30158 |
Number of pages | 8 |
Journal | Proceedings of the National Academy of Sciences of the United States of America |
Volume | 117 |
Issue number | 48 |
DOIs | |
State | Published - Dec 1 2020 |
Externally published | Yes |
Funding
ACKNOWLEDGMENTS. We thank Lawrence Pratt, Susan Rempe, Christopher Mundy, Timothy Duignan, Dilip Asthagiri, Travis Pollard, and Paolo Car-loni for helpful discussions. This material is based upon work supported by the National Science Foundation under Grants CHE-1565632 and CHE-1955161. The computations were performed at the Ohio Supercomputer Center. Y.S. acknowledges the support of the College of Arts and Sciences at the University of Cincinnati.
Keywords
- Single-ion thermodynamics | hydration free energy | DFT | ab initio quantum molecular dynamics | surface potential