PNNL

Abstract

Recent advances in the use of molecular simulations and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy to understand solvated ions in aqueous solutions are described. We report and discuss the results of the EXAFS spectra, Debye-Waller factors and the related properties governing solvation processes of different ions in water, as well as in different solvents (methanol). Molecular dynamics (MD) trajectories are coupled to electron scattering simulations to generate the EXAFS spectra, which are found to be in very good agreement with the corresponding experimental measurements. From these spectra, both the ion-oxygen and the ion-hydrogen distances for the first hydration shell are predicted to be within 0.1-0.2 Å. The ionic species studied range from monovalent to divalent, positive and negative: K+, Ca2+ and Cl-. This work demonstrates that the combination of MD-EXAFS and the corresponding experiment measurement provides a powerful tool in the analysis of the solvation structure of aqueous ionic solutions. We also investigate the value of electronic structure analysis of small aqueous clusters as a benchmark to the empirical potentials. In a novel computational approach, we compute the Debye-Waller factors combining a harmonic analysis of data obtained from electronic structure calculations on finite ionwater clusters, and we present a direct comparison with results from a harmonic classical statistical mechanical analysis of an empirical potential. Work was supported by the Office of Science, Office of Basic Energy Sciences, Chemical Sciences Division of the U.S. Department of Energy (DOE). The Pacific Northwest National Laboratory is operated by Battelle for DOE.