Local Effects in the X-ray Absorption Spectrum of CaCl2, MgCl2, and NaCl Solutions Page: 4 of 12
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structure of the first solvation shell water molecules are
perturbed by the cation. In addition to the computa-
tional first principles molecular dynamics approaches,
the experimental technique of core level X-ray absorp-
tion spectroscopy (XAS) has been shown to be capable
of identifying cation-specific effects in salt solutions[14].
In particular, experimental results have shown that fea-
tures of the X-ray absorption spectra of monovalent salt
solutions are independent of the cation's identity, while
there is some species dependence for the divalent cations
in salt solutions[14]. The X-ray absorption spectra of
these salt solutions show specific enhancements in the
pre-edge, main-edge, and post-main-edge peaks with re-
spect to bulk water which, by necessity, must somehow
describe the effect that these ions have on the local en-
vironment of the water molecules. Theoretical deter-
mination of molecular dynamics trajectories and X-ray
absorption spectra for these systems can provide a use-
ful link between the features of the spectrum and lo-
cal properties that include dipoles, ion-water tilt angles,
and hydrogen bonding configuration.
In the following, we calculate theoretical X-ray ab-
sorption spectra based upon snapshots taken from equi-
librated, first principles molecular dynamics on salt so-
lutions containing the cations Ca2+, Mg2+, and Na+ as
well as the counterion Cl-. We will provide classifica-
tion of spectral features based upon both tilt angle and
hydrogen bonding network for the three cations that
helps to explain differences in the experimental spec-
tra for these three cations. We also estimate trends
in the spectra with concentration by reweighting our
theoretical results. We will show that electronic struc-
ture methods are consistent with current experimental
results and also provide insight into the differences be-
tween the spectra of differing cations.
II. METHODS
Both first principles Born Oppenheimer molecular dy-
namics (BOMD) and static calculations for determina-
tion of X-ray absorption characteristics were carried out
with the PBE generalized gradient approximation [15].
The cubic simulation cell contains either one divalent
cation, Ca2+ or Mg2+, or one monovalent cation, Na+
(details for Na+ in parentheses) and two (one) Cl- an-
ions as counterions as well as 110(52) water molecules,
which yields a 0.5 M concentration in a cell that is
14.91(11.74) A on each side. The valence electronic
wavefunctions were described in a plane wave basis that
was truncated at 85 Ry, a cutoff commensurate with
our use of norm-conserving, nonlocal pseudopotentials
of the Hamann-type [16] to represent core electrons.
The semicore states, 3p for Ca or 2p for Na and Mg,
are also treated as valence electrons. The dynamics
simulations, which were carried out using the QBOX
code[17], were performed with a 'rigid water model' for
Ca and Mg (water in the NaCl simulation was kept
flexible) in which the intramolecular geometry of thewater molecules is kept fixed[18]. The elimination of
high-frequency ionic motion provides a number of ben-
efits, the most notable of which is allowing for a longer
time step of 0.97 fs, while the flexible NaCl simulations
were carried out with a 0.48 fs timestep. The produc-
tion runs for the CaCl2 and MgCl2 cases were both
about 28ps in length, and they were collected in the
NVE ensemble following an initial equilibration at 340
K using velocity resealing for about 3 ps. The produc-
tion runs for the NaCl simulations were about 17 ps in
length, and they were collected in the NVT ensemble
at 400 K. Elevated temperatures were used, particu-
larly for the flexible water simulations, in response to
recent work in the literature that indicates flexible wa-
ter may be overstructured at room temperature (300
K)[19, 20]. Both CaCl2 and MgCl2 simulations were
based upon an initial 1 ns classical molecular dynam-
ics run for NaCl solution with TIP5P water using the
Tinker code[21, 22]. The NaCl simulations, which were
carried out later, started from configurations generated
in previous Na+ in water simulations[11]. In order to
examine the influence of the cations on the electronic
structure of water, a localized molecular orbital analysis
was performed by obtaining maximally localized Wan-
nier functions (MLWFs) [23] from 15 snapshots for each
cation, 5 of which coincide with those used in X-ray
absorption calculations (below).
X-ray absorption spectra were calculated for MgCl2,
CaCl2, and NaCl salt solutions with an approach imple-
mented in the QUANTUM-ESPRESSO code[24]. Results
were obtained from five equally spaced snapshots along
the collected molecular dynamics trajectory for each
cation. Both first solvation shell waters and randomly
selected bulk waters were sampled from each snapshot.
In order to model the spectrum of bulk water, six wa-
ter molecules were randomly selected from each snap-
shot of each cation where first solvation shell waters
were also sampled. A total sample size of 30 bulk water
molecules for each different cation simulation is consis-
tent with the sample size chosen in previous work[25].
Following the recent implementation of Prendergast and
Galli[25], the final state of the electronic system is cal-
culated in the presence of the core hole, as modeled by a
pseudopotential derived from an oxygen atom with one
electron removed from the is level, which results from
the X-ray excitation. The excited electron resides in the
first unoccupied band, and the self-consistent potential
is used to generate unoccupied levels which are higher
in energy than the first excitation. Each excited oxygen
atom corresponding to a water molecule in the first sol-
vation shell in the snapshot was calculated following this
recipe, as were the oxygen atoms in six additional ran-
domly distributed bulk water molecules in the snapshot.
In order to obtain an averaged X-ray absorption spec-
trum for the total system, these individual contributions
were weighted by their relative concentration in the unit
cell. We extrapolate to higher concentrations of salt
solution, the number of first shell water molecules was
reweighted with respect to bulk water using results from
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Kulik, H J; Correa Tedesco, A A; Schwegler, E; Prendergast, D & Galli, G. Local Effects in the X-ray Absorption Spectrum of CaCl2, MgCl2, and NaCl Solutions, article, April 12, 2010; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc829624/m1/4/: accessed April 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.