Chemistry Reference
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(small-large) tend to stay solvent separated. Detailed-atomistic computer simula-
tions performed by Vlachy, Dill, and coworkers have confirmed this qualitative
picture [
65
].
3.1.1 Can Classical Force Field Models Predict Properties of Aqueous
Electrolytes Quantitatively?
In biomolecular force fields the nonbonded interactions are usually described
with Lennard-Jones and Coulomb potentials [
66
]. Several of the popular non-
polarizable force fields use (partial) electronic charges that do not vary in response
to changes of the local electric field at the positions of the interaction sites. While
such simple force field models do realistically describe hydration thermodynamic
properties of nonpolar and polar chemical compounds [
66
,
67
], it is not a priori
clear if this also holds for chemical groups that dissociate to form charged species in
solution. Molecular simulations with non-polarizable force fields have indeed
shown that short-range ion-ion attractions in water, and, as a result, solution
osmotic coefficients, depend strongly on the ion parameters and water models
used in the simulations [
68
]. Interestingly, quantum-mechanical calculations have
indicated (cf. Sect.
2
) that electronic polarization of water molecules in the first
hydration shells of mono- and divalent ions is relatively weak compared to water
self-polarization [
31
]. Therefore, non-polarizable classical models should in
principle be suitable to describe ion hydration in bulk. In principle, efforts could
be made to obtain classical non-polarizable force fields for electrolyte solutions
by coarse-graining over electronic degrees of freedom in quantum-mechanical
calculations (e.g., using data from CPMD simulations). This approach bears the
advantage of not having to rely on experimental input in the development of the
model. To this end, an inverse Monte Carlo method has been used in the past [
33
]
in which it was illustrated that a simple single-exponential form describes the short-
range lithium-water interaction in bulk water better than the Lennard-Jones inter-
action type. In view of this result [
33
], it perhaps is not surprising that recent
attempts to parameterize classical force fields based on experimental data have pointed
out limitations in applying standard mixing rules to describe the short range ion-
water interaction with Lennard-Jones potentials [
69
,
70
]. Successful parameteriza-
tion could be achieved by applying scaling factors for the ion-water Lennard-Jones
interaction, the effect of which reduces the short-range ion-water repulsion [
69
,
70
].
The scaling is particularly substantial for the smaller lithium ion [
70
]. The standard
mixing rules were shown to fail for the fluoride anion as well [
71
]. A number of
recent force field parameterization studies have used experimental salt activity
coefficients as model input [
69
-
72
]. There, it was shown that the resulting non-
polarizable force fields are transferable over a fairly broad range of salt concentra-
tions and provide quantitative accuracy. Similar observations were made by Kalcher
and Dzubiella [
73
] who investigated aqueous LiCl, NaCl, and KCl solutions with an
existing non-polarizable model. These authors found that the osmotic coefficients
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