Chemistry Reference
In-Depth Information
The partial charges used in Eq. [3] were derived from ab initio calculations of
a single gas-phase ion pair at the restricted Hartree-Fock (RHF)/6-31G* level
of theory followed by fitting to the electrostatic potential of the minimized
structure using the CHarges from ELectrostatic Potential Grid (CHELPG)
method.
40
A united-atom model was adopted, with all hydrogens subsumed
into their neighboring carbon atoms, and the PF
6
anion was treated as a
sphere. The Lennard-Jones and dihedral parameters for the cation were taken
directly from the OPLS force field,
41
using similar compounds as surrogates.
The anion Lennard-Jones potential was scaled using SF
6
as a model. Most
researchers now use the OPLS or CHARMM
42
force field functional form
in ionic liquid simulations.
The calculations done by Shah, Brennecke and Maginn
39
demonstrated
that, by using a simple force field like that in Eq. [3] along with parameters
obtained from a mix of quantum calculations and literature sources, properties
such as liquid densities could be reproduced to within 3-5% over a very wide
temperature range, and that derivative properties such as expansivities and
compressibilities could also be obtained to reasonable accuracy. This study
39
was also the first to estimate an enthalpy of vaporization for an ionic liquid.
Although many researchers had stated previously that ionic liquids were ''non-
volatile'' and had ''no detectable vapor pressure,''
43
this work predicted that
the enthalpy of vaporization was large but not infinite, suggesting that ionic
liquids should indeed have a vapor pressure. It has now been shown conclu-
sively that ionic liquids
do
have a measurable vapor pressure, and it is most
encouraging that recent measurements of the enthalpy of vaporization
44-46
agree very well with these early modeling predictions.
Later that same year, several other studies appeared in which force fields
having the basic functional form as that in Eq. [3] were developed for imida-
zolium-based ionic liquids. Margulis, Stern and Berne
47
conducted MD
simulations of [C
4
mim][PF
6
] using an explicit atom model that included fully
flexible bond lengths and bond angles, in addition to a dihedral potential. Par-
tial charges were fit to the electrostatic potential obtained from Hartree-Fock
calculations, while all other cation parameters were taken from the all-atom
OPLS potential.
41
The parameters for PF
6
were taken from previous work
of Kaminsky and Jorgensen.
48
Margulis, Stern and Berne
47
conducted a num-
ber of MD simulations in which they used 200 ps of equilibration time and
50 ps of production time. Computed liquid-phase densities were in very
good agreement with experimental values. They also examined the nature of
the dynamics of the system and were the first group to note anomalous and
complex dynamical behavior, indicative of a supercooled liquid. This anoma-
lous dynamical behavior turns out to have been an extremely important find-
ing as related to our ability to compute dynamical properties of ionic liquids,
as we shall see later. They also made the first estimate of the conductivity of an
ionic liquid from a simulation, using the apparent self-diffisivities and the
Nernst-Einstein model.
