We compute individual ion activity
coefficients (IIACs) in aqueous
NaCl, KCl, NaF, and KF solutions from explicit-water molecular dynamics
simulations. Free energy changes are obtained from insertion of single
ionsaccompanied by uniform neutralizing backgroundsinto
solution by gradually turning on first Lennard-Jones interactions,
followed by Coulombic interactions using Ewald electrostatics. Simulations
are performed at multiple system sizes, and all results are extrapolated
to the thermodynamic limit, thus eliminating any possible artifacts
from the neutralizing backgrounds. Because of controversies associated
with measurements of IIACs from electrochemical cells with ion-selective
electrodes, the reported experimental data are not widely accepted;
thus there remains a knowledge gap with respect to the contributions
of individual ions to solution nonidealities. Our results are in good
qualitative agreement with these reported measurements, though significantly
larger in magnitude. In particular, the relative positioning for the
activity coefficients of anions and cations matches the experimental
ordering for all four systems. This work establishes a robust thermodynamic
framework, without a need to invoke extra hypotheses, that sheds light
on the behavior of individual ions and their contributions to nonidealities
of aqueous electrolyte solutions.
We obtain activity coefficients in NaCl and KCl solutions from implicit-water molecular dynamics simulations, at 298.15 K and 1 bar, using two distinct approaches. In the first approach, we consider ions in a continuum with constant relative permittivity (ɛr) equal to that of pure water; in the other approach, we take into account the concentration-dependence of ɛr, as obtained from explicit-water simulations. Individual ion activity coefficients (IIACs) are calculated using gradual insertion of single ions with uniform neutralizing backgrounds to ensure electroneutrality. Mean ionic activity coefficients (MIACs) obtained from the corresponding IIACs in simulations with constant ɛr show reasonable agreement with experimental data for both salts. Surprisingly, large systematic negative deviations are observed for both IIACs and MIACs in simulations with concentration-dependent ɛr. Our results suggest that the absence of hydration structure in implicit-water simulations cannot be compensated by correcting for the concentration-dependence of the relative permittivity ɛr. Moreover, even in simulations with constant ɛr for which the calculated MIACs are reasonable, the relative positioning of IIACs of anions and cations is incorrect for NaCl. We conclude that there are severe inherent limitations associated with implicit-water simulations in providing accurate activities of aqueous electrolytes, a finding with direct relevance to the development of electrolyte theories and to the use and interpretation of implicit-solvent simulations.
We
obtain activity coefficients and solubilities of NaCl in water–methanol
solutions at 298.15 K and 1 bar from molecular dynamics (MD) simulations
with the Joung–Cheatham, SPC/E, and OPLS-AA force fields for
NaCl, water, and methanol, respectively. The Lorentz–Berthelot
combining rules were adopted for the unlike-pair interactions of Na+, Cl–, and the oxygen site in SPC/E water,
and geometric combining rules were utilized for the remainder of the
cross interactions. We found that the selection of appropriate combining
rules is important in obtaining physically realistic solubilities.
The solvent compositions studied range from pure water to pure methanol.
Several salt concentrations were investigated at each solvent composition,
from the lowest concentrations permitted by the system size used up
to the experimental solubilities. We first calculated individual ion
activity coefficients (IIACs) for Na+ and Cl– from the free energy change due to the gradual insertion of a single
cation or anion into the solution, accompanied by a neutralizing background.
We obtained the salt solubilities by comparing the chemical potentials
in solution with solid NaCl chemical potentials calculated previously
using the Einstein crystal method. Mean ionic activity coefficients
obtained from the IIACs are in reasonable agreement with experimental
data, with deviations increasing for solutions of higher methanol
content. Predictions for the salt solubility are in surprisingly good
agreement with experimental data, despite well-known challenges in
the simultaneous calculation of activity coefficients and solubilities
with classical MD simulations. The present study demonstrates that
good predictions for these two important phase equilibrium properties
can be obtained for mixed-solvent electrolyte solutions using existing
nonpolarizable models and further suggests that the previously proposed
single ion insertion technique can be extended to complex mixed-solvent
solutions as well.
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