Chemistry Reference
In-Depth Information
Zhou and co-workers
69
developed a force field for ionic liquids contain-
ing a tetrabutylphosphonium cation ([P(C
4
)
4
]) paired with the following
amino acid-derived anions: glycine ([Gly]-), alanine ([Ala]-), serine ([Ser]-),
lysine ([Lys]-), leucine ([Leu]-), isoleucine ([Ile]-), phenylalanine ([Phe]-), pro-
line ([Pro]-), methionine ([Met]-), aspartic acid ([Asp]-), glutamic acid ([Glu]-),
glutamine ([Gln]-), and taurine ([Tau]-). These ionic liquids had been synthe-
sized by their group and evaluated for potential use in CO
2
capture applica-
tions. The force field parameters were developed using the same basic
techniques as used by other groups for different ionic liquids. Isothermal-
isobaric MD simulations were then conducted using the MDynaMix code.
Each pure liquid simulation was on 192 ion pairs with 1-ns production
runs. Liquid densities, heat capacities, and liquid microstructure were
computed and compared to experiment. Liquid densities were generally accu-
rate to within a few percent. Heat capacities were computed by difference
from simulations at adjacent temperatures using the following expression:
ð
þ
Þ
ð
Þ
H
T
T
H
t
C
p
ð
T
;
P
Þ
½
6
T
P
where
H
is the enthalpy and
T
is some small temperature differential over
which the heat capacity is assumed constant. The computed heat capacities
agreed less well with experiment than did the densities, differing anywhere
from 1 to 43% with no identifiable trend. We suspect this may not necessarily
be an indication of a poor force field, but rather that Eq. [6] should not be used
to compute heat capacities. Each harmonic bonded term in the classical
potential (Eq. [5]) will add
Nk
B
to the heat capacity, which is generally a gross
overestimation. This is a widely known limitation of classical force fields hav-
ing the form of Eq. [5].
Our group has computed heat capacities from simulations using a differ-
ent approach.
64
Instead of using Eq. [6], one can split the heat capacity into
ideal gas and excess terms:
P
þ
q
h
H
ig
H
ex
q
h
i
i
C
i
p
ð
C
e
p
¼
C
p
¼
T
;
P
Þþ
½
7
q
T
q
T
P
where
H
ig
contains intramolecular contributions to the heat capacity and
H
ex
contains all intermolecular nonbonded terms. The angle brackets indicate an
ensemble average. The ideal gas contribution to the heat capacity can be com-
puted accurately from a quantum calculation of the ions in the gas phase. The
excess portion can be computed via finite difference (Eq. [6]) from classical
condensed-phase simulations, with
H
ex
replacing the total enthalpy. We found
this method typically yields more consistent and more accurate heat capacities;
we suspect that if the heat capacities for the [P(C
4
)
4
] amino acid liquids were
