1. Dynamic Acid/Base Equilibrium in Single Component Switchable
Ionic Liquids and Consequences on Viscosity
David C. Cantu,†
Juntaek Lee,‡
Mal-Soon Lee,†
David J. Heldebrant,‡
Phillip K. Koech,‡
Charles J. Freeman,‡
Roger Rousseau,†
and Vassiliki-Alexandra Glezakou*,†
†
Physical Sciences Division, ‡
Energy Processes and Materials Division, Pacific Northwest National Laboratory, Richland, Washington
99352, United States
*S Supporting Information
ABSTRACT: The deployment of transformational nonaqueous CO2-capture solvent
systems is encumbered by high viscosities even at intermediate uptakes. Using single-
molecule CO2 binding organic liquids as a prototypical example, we present key molecular
features that control bulk viscosity. Fast CO2-uptake kinetics arise from close proximity of
the alcohol and amine sites involved in CO2 binding in a concerted fashion, resulting in a
Zwitterion containing both an alkyl-carbonate and a protonated amine. The population of
internal hydrogen bonds between the two functional groups determines the solution
viscosity. Unlike the ion pair interactions in ionic liquids, these observations are novel and
specific to a hydrogen-bonding network that can be controlled by chemically tuning single
molecule CO2 capture solvents. We present a molecular design strategy to reduce viscosity
by shifting the proton transfer equilibrium toward a neutral acid/amine species, as opposed
to the ubiquitously accepted zwitterionic state. The molecular design concepts proposed
here are readily extensible to other CO2 capture technologies.
Aqueous amine solvents, such as monoethanolamine, are
the industrial benchmark solvent systems being consid-
ered for CO2 capture from flue gas streams.1,2
However, their
regeneration energy requirements are cost prohibitive for most
flue gas applications under consideration,2−4
largely as a result
of the water content carried in these solvents. A number of
advanced solvents are currently being developed to adsorb and
release CO2 without the need for an aqueous carrier. Carbon
dioxide binding organic liquids (CO2BOLs) constitutes a
promising class of water-free or water-lean solvents due to their
ability to switch between the ionic and nonionic forms as they
bind or release CO2.5,6
Their functionality stems from their
ability to bind CO2 onto an aliphatic alcohol group and form a
zwitterionic species upon CO2-uptake (Figure 1).
Despite the projected lower regeneration energy penalties of
CO2BOLs compared to aqueous amines, high viscosities after
binding CO2 renders them impractical for postcombustion flue
gas applications.1,2
Other nonaqueous alternative solvent
classes under development are also limited by high viscosity
behavior upon loading with CO2. Task-specific ionic liquids7
are two-component systems that turn into carbamate-
ammonium or carbamate-imidazolium salts after carbon
binding.8
Similarly, single component siloxylated amines are
switchable and turn into a two-component carbamate-
ammonium ionic liquid after reaction with CO2.9
Ionic liquids
with aprotic heterocyclic anions10
have the advantage that their
CO2 binding energies can be tuned with chemical modifications
and that their viscosities do not increase much upon reaction
with CO2;11
however, they exhibit intrinsic high viscosities
before reacting with CO2.12
These challenges inspired the work reported here, in which
the conceptual framework that relates the molecular-level
properties to the bulk physical properties of CO2BOL capture
solvent systems was developed. Our approach relies on two
main accomplishments: (i) a computational protocol that can
provide reliable estimates of the viscosity for a variable
composition system, since the composition of the liquid
changes with CO2 loading, and (ii) the mechanism and
quantification of the free-energetics of CO2-binding in water-
free solvents, as well as the acid−base properties and
Received: February 19, 2016
Accepted: March 28, 2016
Figure 1. Structure of neutral IPADM-2-BOL not binding CO2 and
zwitterionic binding CO2.
Letter
pubs.acs.org/JPCL
© XXXX American Chemical Society 1646 DOI: 10.1021/acs.jpclett.6b00395
J. Phys. Chem. Lett. 2016, 7, 1646−1652

2. equilibrium behavior in its CO2-bound state. The single
component alkanolguanidine, a member of the CO2BOL
material family, was used as the initial solvent class of study,
with solvent viscosity reduction being the primary focus.
However, many of our findings can be transferred to other
solvent classes. To the best of the authors’ knowledge, this
work contains the first ab initio simulations of CO2 capture by a
single component switchable ionic liquid, as well as identifying
and quantifying the molecular level interactions, such as
internal hydrogen bonding and acid−base equilibria, that
ultimately control bulk viscosity. This study is also the first to
propose that single-component CO2BOLs in their CO2-bound
state do not necessarily exist as purely zwitterionic species but
rather exhibit a dynamic acid−zwitterion equilibrium that could
be exploited to further reduce viscosity.
The CO2 binding free energy is one of the deciding criteria in
the design of gas separation solvents. Herein, a concerted
mechanism for CO2 binding is proposed based on Blue Moon
ensemble13
simulations, where a CO2 molecule was placed in a
simulation box with 34 IPADM-2-BOL molecules, and a series
of ab initio molecular dynamics (AIMD) simulations are
performed at fixed values of the distance between the CO2
carbon atom and the alcohol oxygen atom of a IPADM-2-BOL
molecule, rC−O. Details on the computational approach, free
energy profile calculations, and error estimates appear in the
Computational Methods section and Supporting Information
(SI). We note that CO2 capture involves both solvation and
binding; herein we focus on the binding event. Further
discussion about overall capture appears in the SI.
Figure 2 shows the free energy profile of CO2 addition
reaction as a function of rC−O, where 2A corresponds to the
CO2-bound system, 2B is the transition state, and 2C
corresponds to the solvated CO2 in the vicinity of the alcohol.
For rC−O distances less than 2.00 Å (Figure 2A), CO2 is bound
in the form of an alkylcarbonate, whereas the H atom that
originally belonged to the OH group remains on the guanidine
N. For distances greater than 2.20 Å (Figure 2C), IPADM-2-
BOL remains in its alcohol form, and CO2 is mostly linear with
the ∠OCO angle averaging ∼175°. The angle decreases to
∼165° for rC−O distances between 2.0 and 2.2 Å. CO2 binding
happens in an effectively concerted mechanism: at rC−O ∼ 2.00
Å, the ∠OCO angle becomes ∼150° with the simultaneous H
transfer to the nitrogen of the guanidine base. The CO2
structure is consistent with a partial charge transfer to form a
CO2
δ‑14,15
and subsequent formation of a CO3
−
moiety in
IPADM-2-BOL (Figure 1, Figure 2B).
From the free energy profile, we determine that CO2 binding
by IPADM-2-BOL proceeds with a barrier of 16.5 ± 1.2 kJ/mol
and a binding free energy of −5.8 ± 1.6 kJ/mol. The binding
free energy is consistent with the experimentally obtained
values of diazabicyclo[5.4.0]-undec-7-ene (DBU) containing
dual-component CO2BOLs that range between −5.7 to −9.7
kJ/mol.6
This implies that at 40 °C, there is an equilibrium
between solvated and bound CO2, in accord with NMR
measurements reported by Heldebrant et al.6
The free energy
barrier of 16.5 kJ/mol and the activation energy of 9.8 kJ/mol
(Figure 2) are compatible with the experimental observation
that this process readily occurs at 40 °C.
For monoethanolamine (MEA) the energy barrier is more
than twice that of CO2BOLs. In dry MEA, density functional
methods give a barrier of 35.5 kJ/mol.16
For wet MEA, the
energy barrier estimated by density functional calculations to be
50.0 kJ/mol,17
in good agreement with the activation energy of
46.7 kJ/mol computed with the Arrhenius relation from
experimental data.18
However, these large barriers are likely
associated with high-energy intermediate states involving
protonation of primary alcohols and carbamate formation. On
the other hand, Ozturk et al. measured lower activation
energies for the dual-component CO2BOL systems 1,1,3,3
tetramethylguanidine (TMG)/1-hexanol (9.7 kJ/mol)19
and
DBU/1-hexanol (13.7 kJ/mol),20
while proposing similar
intermediates as for MEA. On the basis of our mechanistic
results, we believe that a direct carbonate formation is possible,
and the only requirement for the low activation barrier is acid/
base proximity.
Our estimate of free energy barrier is only ∼7 kJ/mol higher
than the activation energy, which is indicative of a small
entropic contribution at the transition state, owing to the
proximity of the alcohol/amine moieties in the single
component systems: unlike dual component systems, solvent
reorganization at the transition state is not required. The
relatively low barrier then suggests that capture in CO2BOLs is
likely to be diffusion limited. Because the solvent viscosity
increases exponentially with CO2 loading,21
the capture rate
will decrease as more CO2 is added. These phenomena were
observed when the CO2 absorption rates of single and dual-
component CO2BOL solvents were measured with wetted-wall
experiments.22
Motivated by the conventional picture of acid/base
equilibrium, where an organic acid is in dynamic equilibrium
with its conjugate base, we made the connection with the CO2-
bound zwitterionic alkylcarbonate species, which acts as both
an acid and a base, and asked the question whether nonionic
CO2-binding species are feasible. Given that the rheological
properties (e.g., high viscosity) of CO2BOLs are intimately
coupled to the population of the charged species, tuning the
equilibrium between charged zwitterionic and noncharged acid
species could alter their fluid properties.
To test this hypothesis, we conducted a series of AIMD
simulations, with the metadynamics23
protocol to accelerate the
proton transfer between the carboxylate and guanidium groups,
on three target molecules that cover a range of relative acid/
base strengths. All simulations started with a carbon loaded
CO2BOL molecule in its zwitterionic form placed in a
Figure 2. Free energy (red) and energy (blue) profiles of CO2 binding
by IPADM-2-BOL at 40 °C obtained with Blue Moon ensemble
simulations as a function of the CO2 carbon to IPADM-2-BOL alcohol
oxygen distance; see SI. In the images, dark gray is C, white is H, red is
O, and blue is N. The H atom that moves between alcohol to
guanidium base is highlighted in turquoise. The energy diagram on the
right summarizes the whole capture and binding process.
The Journal of Physical Chemistry Letters Letter
DOI: 10.1021/acs.jpclett.6b00395
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3. simulation box of the capture solvent. The free energy profile of
the proton transfer process was calculated with respect to an
appropriately tailored collective variable that describes both
zwitterionic (charged) and acid (noncharged) CO2-bound
states; see Computational Methods and SI for details.
The IPADM-2-BOL frame was modified in two different
ways: (i) by introducing an oxime group to reduce the acidity
of alkanol moiety (EODM-2-BOL), and (ii) by fluorination of
the guanidium core to decrease the basicity at the N site
(IPATFMM-2-BOL). Figure 3 shows the resulting free energy
landscape for proton shuttling between the charged (zwitter-
ion) and noncharged (acid) forms for IPADM-2-BOL (blue),
EODM-2-BOL (green) and IPATFMM-2-BOL (red). Table S1
in SI summarizes the results. An error analysis of the
metadynamics simulations is also presented in SI, Figure S3.
Based on the computed free energy landscapes, equilibrium
constants and relative populations of the acid and zwitterion
were calculated. The relative acid:zwitterion populations thus
determined, 1:4000 for IPADM-2-BOL, 3:1 for EODM-2-BOL
and 8:1 for IPATFMM-2-BOL, clearly demonstrate how simple
molecular modifications can influence the ratio of charged and
noncharged species that ultimately provide an appropriate
description of the liquid phase.
In silylamines,24
increased CO2 capture capacity was
attributed to stabilization of carbamic acid by the carbamate
and ammonium species of the ionic liquid. Similarly for
monoethanolamine, a carbamic acid−carbamate equilibrium
has also been proposed, but has not been verified.25
Although,
the concept of such acid/zwitterionic equilibrium has been
discussed in the context of enhancing the CO2 adsorption
capacity, the present study is the first to consider it as a
molecular design strategy for the reduction of viscosity.
For all tested CO2BOL compounds, the free energy barriers
to switch between the CO2-bound states are low (∼23 kJ/mol
for IPADM-2-BOL zwitterion to acid, the rest below 16 kJ/mol,
Figure 3, SI Table S1), indicating that at carbon capture
conditions, an equilibrium between the charged and non-
charged species will be facile. Proton transfer kinetics will not
determine equilibrium populations, only their relative free
energies. To determine if a correlation between solvent
dielectric and the acid−zwitterion equilibrium exists, we
computed ΔG values between the different CO2-bound states
for the three species (Figure 3) with electronic structure
calculations (see Computational Methods) using a continuum
dielectric model for a range of values (SI Table S2). For all
compounds, as the dielectric constant of the solvent decreases,
the equilibrium shifts toward the acid state. The ΔG values
obtained with AIMD agree within ∼6 kJ/mol with implicit
Figure 3. Free energy profiles, obtained from ab initio molecular
dynamics simulations with the metadynamics technique to accelerate
proton transfer, of the acid−zwitterion equilibrium for IPADM-2-
BOL, the oxime derivative (EODM-2-BOL), and the fluoro-
substituted derivative (IPATFMM-2-BOL). See abbreviations for
compound definition.
Figure 4. (A) IPADM-2-BOL viscosities. Experimental data points (yellow), calculated with all CO2-loaded molecules zwitterionic (orange) and
CO2-loaded molecules in 1:1 acid:Zwitterion (green). (B) Snapshots from classical MD simulations show that as CO2 loading increases, the
extended solvent structure becomes a highly heterogeneous mixture of CO2-bound molecules (red) and solvent molecules (blue).
The Journal of Physical Chemistry Letters Letter
DOI: 10.1021/acs.jpclett.6b00395
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4. solvent electronic structure calculations for dielectric constants
ranging from 12 to 6.8.
We note that the relative CO2 capture (CO2 solvation and
binding) energies (see SI) of EODM-2-BOL and IPATFMM-2-
BOL are within ∼15 kJ/mol of that for IPADM-2-BOL using
gas phase calculations indicating possible similar CO2 capture
capacities. Given the fact that CO2 solvation energies using a
continuum dielectric model for single molecule calculations are
not reliable for IPADM-2-BOL (see SI), we cannot properly
assess if the modified compounds will have enhanced or
lessened CO2 capture capacity. The single molecule calculation
performed with a continuum dielectric model (mentioned in
the previous paragraph to determine the relative energies of
acid/zwitterion states) all have CO2 bound and do not need to
consider CO2 solvation. EODM-2-BOL and IPATFMM-2-BOL
were chosen for AIMD simulations to demonstrate that the
equilibrium could be shifted toward the acid state significantly.
We are currently studying full CO2 capture (solvation and
binding) with AIMD in CO2BOLs that will appear in an
upcoming publication.
Because CO2BOLs are switchable ionic liquids,5
the
extended solvent dielectric will vary as the polarity of individual
molecules change with CO2 capture. We estimated solvent
dielectric constants with from the dipole moments based on the
Debye−Onsager model (SI Tables S3, S4) and from classical
molecular dynamics (MD) simulations (Computational
Methods) for IPADM-2-BOL at different CO2 loadings (SI
Table S5). The MD calculated viscosities range from 4 to ∼12,
showing an inverse dependency with CO2 loading, suggesting
that at higher loadings (lower dielectrics), the acid state of
CO2-bound molecules will be more prevalent. An increase in
dielectric constant with CO2 loading would be expected given
that more polar molecules (zwitterions) are present at higher
loading. However, the dielectric constant of the solvent
decreases because they tend to form clusters that could result
in smaller net dipoles. The extended solvent structure obtained
from classical MD simulations (Figure 4B) is highly
heterogeneous, with different polar (CO2-bound charged
zwitterion) and nonpolar (unbound neutral alcohol) regions.
This finding is supported by the fact that heterogeneous
domains in the solvent structure were detected, with ultraviolet
visible and infrared spectroscopic experiments, of dual
component CO2BOLs.26,27
We note that simple models such
as the Debye−Onsager equation do not predict the correct
trends in changes of static dielectric constants, since their
estimation using single molecule dipole moments does not take
into account the spatial arrangement between neighboring
molecular species; see SI for more details.
The impact of acid/base equilibrium on the viscosity is
illustrated by two independent sets of classical MD simulations
on the IPADM-2-BOL system at varying CO2 loading: (i) one
with all CO2-bound molecules in zwitterionic form and (ii)
another with 1:1 acid:zwitterion populations. The first system
represents the liquid state consistent with an equilibrium
shifted mainly toward the zwitterionic form, as discussed above.
The second hypothetical system allows us to probe the impact
of the charged state while keeping all other factors the same;
see SI for details and force field parametrization. Figure 4 shows
very good agreement between the computed (orange) and
experimental22
(yellow circles) viscosities for IPADM-2-BOL.
However, the hypothetical 1:1 mixture (green) shows a
pronounced drop in viscosity, on the order of 30−50% for
the higher loadings. This appreciable viscosity reduction
strongly suggests potential for viable, nonionic CO2 capture
solvent systems, brought upon by simple molecular mod-
ifications once the appropriate equilibria drivers are taken into
account.
To further probe this hypothesis, we computed the
viscosities of EODM-2-BOL and IPATFMM-2-BOL with
solvent molecules in different CO2-bound states at 25% mol
loading. The 25% mol CO2 loading was chosen for this analysis
because for IPADM-2-BOL it has been projected to be the
thermodynamically optimal lean-solvent loading for postcom-
bustion CO2 capture.28
For IPADM-2-BOL, at 25% mol
loading, the 1:1 acid:zwitterion system undergoes ∼48%
viscosity reduction compared to the all-zwitterion system. At
the same loading, viscosity reductions of 34% and 61% were
observed in the 1:1 state for IPATFMM-2-BOL and EODM-2-
BOL solvents, with respect to the all-zwitterion state; see Table
1, and SI Table S7 for additional details.
As a final point, we note that the intrinsic viscosities of the
three solvents varied greatly (Table 1), regardless of the
charged state of the CO2-bound molecules (i.e., zwitterion vs
acid). After careful analysis of the extended solvent structures of
zwitterionic systems, we identified that strong H-bonds
between the carboxylate and protonated amine sites of
neighboring CO2-bound molecules in a charged (zwitterionic)
state are the decisive structural factor for controlling viscosity;
see SI Figures S4 and S5. Lower viscosities are observed when
significant populations of charged CO2-bound species maintain
internal H-bonds. We have determined that high populations of
internal H-bonding is a critical indicator of localized and
directional charge separation that is nevertheless contained
within each zwitterion, and therefore making these species less
likely to be involved in agglomeration with other charged
molecules in the liquid. As a result, significant COO−
---H+
N
population between neighboring zwitterions results in high
viscosities, while internal H-bonding leads to lower viscosities.
Because internal hydrogen bonding is not feasible in two-
component ionic liquids, it has been suggested that the strong
hydrogen bonding between COO−
and NH3
+
groups of
neighboring molecules in two-component CO2-bound alkanol-
amines is the leading factor for their increased viscosity.29
Therefore, we propose two strategies to reduce CO2BOL
viscosity in the CO2-bound state: (i) have a significant
concentration of acid (noncharged) molecules, or (ii) a high
population of zwitterions (charged) that sustain a high degree
of internal H-bonding. We also note that even the case of the
acid (noncharged species) internal hydrogen bond will also
help localize the acid−base properties and stabilize the bound
CO2.
Table 1. Viscosities of the Solvents at 25% Mol Loading, and
the Percentage of CO2-Bound Molecules, All Zwitterionic,
with an Internal Hydrogen Bonda
25% mol loading 100% zwitterion
1:1
zwitterion:acid
system
viscosity
(cP)
% internal H-
bond viscosity (cP)
EODM-2-BOL 45.5 92 17.9
IPADM-2-BOL 149.5 34 77.7
IPATFMM-2-BOL 328.5 13 214.2
a
See SI for details and for a visual representation of the internal
hydrogen bond.
The Journal of Physical Chemistry Letters Letter
DOI: 10.1021/acs.jpclett.6b00395
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5. This paper demonstrates several critical aspects of CO2
capture by water-lean solvent systems that can be controlled
by deliberate molecular modifications. We have identified two
key structural motifs that play a pivotal role in determining CO2
adsorption kinetics and bulk liquid viscosity. The first one is
ascribed to the close proximity of the amine and alcohol
moieties. This is reflected in the concerted mechanism of CO2
binding by the nucleophilic alcohol and concurrent proton
transfer to the amine. The overall effect is fast CO2 binding
kinetics associated with low entropic contribution to the free
energy barrier. Proximity also enables a higher likelihood of
CO2-bound zwitterionic species to have internal H-bonding
that reduces viscosity. The second emerging factor is a tunable
acid/base equilibrium. Enthalpically, a high acidity at the
alcohol site allows for a more efficient CO2 activation at the
transition state and an efficient proton transfer to the amine.
Here, we introduced the concept of noncharged CO2 capture
solvent systems by adjusting the acid/base properties of the
solvent molecules, so that a significant fraction of the CO2-
loaded molecules can exist in a noncharged (acid) form. This
can be achieved by either increasing the acidity of the alcohol or
by decreasing the basicity of the amine. Noncharged CO2
capture systems exhibit appreciably lower viscosities than the
analogous zwitterionic form. A quantitative structure-viscosity
relationship of CO2BOLs will be the subject of a forthcoming
publication. We should also point out that the concepts
outlined here are distinct from similar discussions in the
literature due to fact the CO2BOLs are single molecule
switchable ionic liquids, with CO2-bound molecules exhibiting
an acid−zwitterion equilibrium, as opposed to the typical two
component ionic liquids. In two component ionic liquids,
hydrogen bonding and salt bridges29
as well as molecular
stacking due to side chain length30
have been identified as
viscosity contributors. Analogous to having a larger population
of acid (noncharged) species in single molecule systems,
molecular liquids have been proposed to decrease viscosity in
typical two component ionic liquid mixtures;31
however, this
results in reduced CO2 loading capacity. This work specifically
shows that one can decrease the extended H-bond network by
enhancing internal H-bonding and decrease the overall ionic
strength by tuning the internal acid/base equilibrium. Both
factors strongly contribute to substantial viscosity reduction
without impacting CO2 loading. Finally, the chemical guidelines
outlined here for controlling CO2 uptake kinetics and viscosity
reduction can be ubiquitously applied to both carbonate/
carbonic and carbamate/carbamic solvent systems.
■ COMPUTATIONAL METHODS
All electronic structure calculations were performed using
Gaussian0932
with the M06L33
functional and the 6-31++G**
basis set. Structure optimizations were done for all individual
molecules to get force field equilibrium bond lengths and
angles, electrostatic potential (ESP) charges, and CO2 binding
energies. A polarizable continuum model34
was used for
implicit solvent calculations for a range of dielectric constant
values; see SI.
The CP2K35,36
package was used to perform spin-polarized
density functional theory based AIMD simulations using the
gradient-corrected PBE functional37
for exchange correlation.
Dispersion corrections were included through the third
generation of the empirical Grimme DFT-D3 approximation.38
For all atoms, core electrons were modeled with GTH
pseudopotentials,39
and a double-ζ quality basis set40
for
valence electrons. A 340 Ry cutoff was used for the plane wave
basis for the electrostatic energy. AIMD simulations were done
in the NVT ensemble with a 0.5 fs time step using a Nosé−
Hoover41,42
thermostat to keep the solvent box at 40 °C.
To simulate IPADM-2-BOL CO2 capture and uncover its
mechanism, the Blue Moon13
procedure was used. The distance
between the CO2 carbon and the IPADM-2-BOL oxygen
(Figure 5A, rC−O) was used as a collective variable for
thermodynamic integration. A series of 11 points was used
and the forces on the constraint were compiled during ∼10 ps
of well-equilibrated trajectory to construct the potential of the
mean force.
The metadynamics23
technique was employed to induce the
internal proton transfer in acid−zwitterion equilibrium
simulations. The direct metadynamics version43
as imple-
mented in the CP2K package was used. Gaussian hills were
added every 10 fs with a hill height of 0.00016 au (0.1 kcal/mol,
0.42 kJ/mol) and a width of 0.08 in units of CV. Simulations
were carried out until multiple CV crossing events were
observed (see SI Figure S2), typically ∼6 ps. The CV used
(Figure 5B, eq 1) is the difference between the coordination
number (CN) of the carboxylate oxygen and H atom, and the
CN of the guanidium nitrogen and H atom. CV values range
from −1 (acid where CNNH = 0 and CNOH = 1) to 1
(zwitterion where CNNH = 1 and CNOH = 0). The collective
variable (CV) is defined as the difference between the N−H
and O−H coordination numbers (CNab) and have the
following functional forms:
= −CV CN CNNH OH (1)
=
−
−
( )
( )
CN
1
1
ab
r
r
r
r
NN
ND
ab
ab
0
0 (2)
where CNab is the coordination number (CN) between atoms a
and b, rab the distance between atoms a and b, r0 the threshold
distance for bonding, and the NN and ND exponents that
determine the curve. The values for ro, NN, and ND were 2.46
(a.u.), 10, and 22, for both oxygen−hydrogen and nitrogen−
hydrogen CNs.
Classical MD simulations of CO2BOL solvents at various
CO2 loadings were done at 40 °C to calculate viscosities, self-
diffusion coefficients, and dielectric constants. CO2-bound
Figure 5. Rare event AIMD simulation variables: (A) CO2 carbon to
IPADM-2-BOL alcohol oxygen distance is the reaction coordinate for
the Blue Moon simulations and (B) difference of coordination
numbers is the collective variable for metadynamics.
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6. molecules were either all zwitterionic, 1:1 zwitterionic:acid, or
all acid to gauge how nonionic acid-state CO2-bound
CO2BOLs change solvent viscosity.
The GROMACS44
package version 4.6.6 was used for all
classical molecular dynamics (MD) simulations. Optimized
geometries and charges obtained from electronic structure
calculations were used with the OPLS-AA45
force field. Certain
dihedral angle parameters were obtained from electronic
structure calculations (see SI). Mixtures of CO2-bound and
unbound CO2BOLs were constructed at varying CO2 loadings
(mol %: 0, 10, 15, 20, 25, 30), with a total of 1,728 molecules.
Energy minimizations and high-temperature runs for proper
mixing were done, followed by NPT ensemble equilibrations at
1 bar and 40 °C until the volume and total energy display
steady-state behavior, typically occurring between 5 and 10 ns
of simulation. Equilibrated box dimensions appear in the SI.
Viscosities were calculated with three methods: (1) the
Green−Kubo approach by calculating the integral of the
pressure tensor autocorrelation function,46
(2) the non-
equilibrium method47
as implemented in GROMACS, which
is based on the fluctuation−dissipation theorem, where added
energy is consumed through viscous friction and viscosities can
be calculated from the relaxation times following an applied
acceleration, and (3) by comparing the calculated self-diffusion
coefficient with the self-diffusion coefficient and viscosity of
water.48
The recently proposed approach49
to calculate
viscosity in two-component ionic liquids, based on ion pair
lifetimes, is not compatible with single molecule solvent
systems. Additional details on the calculations and their errors
appear in Tables S6 and S7 of the SI.
■ ASSOCIATED CONTENT
*S Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpclett.6b00395.
Additional data. (PDF)
■ AUTHOR INFORMATION
Corresponding Author
*E-mail: Vanda.Glezakou@pnnl.gov.
Author Contributions
D.C.C. planned and executed the simulations, J.L. and M.-S.L.
constructed classical potentials and helped with data analysis.
D.J.H., P.K.K., and C.J.F. provided and discussed experimental
data and insights on CO2 capture systems. R.R. and V.-A.G.
planned and supervised the research. All authors contributed to
the writing of the paper.
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
The authors wish to dedicate this paper to Prof. M. Parrinello
on the occasion of this 70th birthday. May his creativity,
unstoppable innovation, and limitless enthusiasm continue to
inspire many more generations of scientists. We gratefully
acknowledge the U.S. Department of Energy’s Office of Fossil
Energy for funding through award FWP-65872. Computational
resources were provided through a NERSC User Proposal.
PNNL is proudly operated by Battelle for the U.S. Department
of Energy. The authors wish to thank Mr. T. Brouns for his
constant support of this work, as well as the editor Prof. B.
Mennucci and the reviewers for their insightful comments and
criticisms.
■ ABBREVIATIONS
AIMD: ab initio molecular dynamics
MD: molecular dynamics
DBU: diazabicyclo[5.4.0]-undec-7-ene
TMG: 1,1,3,3 tetramethylguanidine
IPADM-2-BOL: 1-((1,3-dimethylimidazolidin-2-ylidene)-
amino)-propan-2-ol
IPATFMM-2-BOL: (Z)-1-((1-methyl-3-(trifluoromethyl)-
imidazolidin-2-ylidene)amino)-propan-2-ol
EODM-2-BOL: 1,3-dimethylimidazolidin-2-one oxime
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