Local effects in the X-ray absorption spectrum of salt water

ORAL

Abstract

We have used first principles molecular dynamics and theoretical X-ray absorption spectroscopy (XAS) to investigate the aqueous solvation of cations in MgCl$_2$, CaCl$_2$, and NaCl solutions. We focus our discussion on the species-specific effects that Mg$^{2+}$, Ca$^{2+}$, and Na$^{+}$ have on the X-ray absorption spectrum of the respective solutions. For the divalent cations, we find that the water molecules that form a rigid first solvation shell around Mg$^{2+}$ and a more flexible solvation shell around Ca$^{2+}$ also exhibit differing hydrogen bonding characteristics. Acceptor hydrogen bonds present in the water surrounding Ca$^{2+}$ enhance a post-edge peak near 540 eV in the XAS spectrum, while the absence of such hydrogen bonding features for the first shell surrounding Mg$^{2+}$ corresponds to a diminished intensity at the post-edge peak. For Na$^+$, we find that a broad tilt angle distribution results in broadened post-edge features, despite donor-and-acceptor populations comparable to Ca$^{2+}$. We present re-averaged spectra of the MgCl$_2$, CaCl$_2$, and NaCl solutions that provide an explanation of concentration-dependent features that have been found in corresponding experimental measurements.

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Authors

  • Eric Schwegler

    LLNL, Lawrence Livermore National Laboratory, Livermore CA, Lawrence Livermore National Laboratory

  • Heather Kulik

    Stanford University

  • Nicola Marzari

    Oxford University, University of Oxford

  • Alfredo Correa

    Lawrence Livermore National Laboratory

  • David Prendergast

    Molecular Foundry, Lawrence Berkeley National Laboratory, Molecular Foundry, Lawrence Berkeley National Laboratory (LBNL), Lawrence Berkeley National Laboratory, Lawrence Berkeley National Lab

  • Giulia Galli

    University of California, Davis, Department of Chemistry and Department of Physics, University of California at Davis, Davis, California, USA, Department of Chemistry \& Department of Physics, Unversity of California, Davis, Department of Chemistry and Department of Physics, UC Davis, UC Davis, University of California-Davis, Department of Chemistry and Department of Physics, University of California, Davis, Univeristy of California, Davis, University of California Davis, Davis, CA95616, University of California, Davis, USA