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

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.