Structure-function study of tertiary amines as switchable polarity solvents

Article abstract of DOI:10.1039/c3ra47724j

A series of tertiary amines have been screened for their function as switchable polarity solvents (SPS). The relative ratios of tertiary amine and carbonate species as well as maximum possible concentration were determined through quantitative ¹H and ¹³C NMR spectroscopy. The viscosities of the polar SPS solutions were measured and ranged from near water in dilute systems through to gel formation at high concentrations. The van't Hoff indices for SPS solutions were measured through freezing point depression studies as a proxy for osmotic pressures. A new form of SPS with an amine:carbonate ratio significantly greater than unity has been identified. Tertiary amines that function as SPS at ambient pressures appear to be limited to molecules with fewer than 12 carbons. The N,N-dimethyl-n-alkylamine structure has been identified as important to the function of an SPS.

Full text of DOI:10.1039/c3ra47724j

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Structure–function study of tertiary amines as
switchable polarity solvents
Cite this: RSC Adv., 2014, 4, 11039
*
Aaron D. Wilson and Frederick F. Stewart
A series of tertiary amines have been screened for their function as switchable polarity solvents (SPS). The
relative ratios of tertiary amine and carbonate species as well as maximum possible concentration were
determined through quantitative 1H and 13C NMR spectroscopy. The viscosities of the polar SPS solutions
were measured and ranged from near water in dilute systems through to gel formation at high
concentrations. The van't Hoff indices for SPS solutions were measured through freezing point
depression studies as a proxy for osmotic pressures. A new form of SPS with an amine : carbonate ratio
significantly greater than unity has been identified. Tertiary amines that function as SPS at ambient
pressures appear to be limited to molecules with fewer than 12 carbons. The N,N-dimethyl-n-alkylamine
structure has been identified as important to the function of an SPS.
Received 17th December 2013
Accepted 31st January 2014
DOI: 10.1039/c3ra47724j
www.rsc.org/advances
material precipitation at relatively low water concentrations and
thus are ignored in this study.9 Known water-compatible single-
Introduction
component SPSs include highly functionalized amidines and
guanidines,4 tertiary amines,5 and pH sensitive ionic liquids;10
the scope of research was further focused to tertiary amines
based on their potential cost effectiveness when produced at
large scale.
Developing safe, energy efficient, and environmentally friendly
alternatives to traditional solvent systems is a primary goal of
“green” and sustainable chemistry research.1 These goals can be
achieved by reducing solvent volumes, changing processes to
function with more benign solvents, or developing alternative
solvents with new behaviors that allow better process lifecycle
performance. Advances in alternative solvents are expected to
improve conventional solvent process such as oil extraction
from biomass, as well as non-standard uses of solvents. Novel
“solvent” materials will allow the development of new processes
in areas such as CO2 separations, water purication, and energy
storage. Among the most well-known and promising alternative
solvents are the various room-temperature ionic liquids,
supercritical uids, and switchable polarity solvents (SPSs)
explored in this study.
SPSs can be divided into various subcategories based on
composition and behavior.2–5 This study targeted water-
compatible single-component SPSs which are immiscible with
water in their basic form but when they are reacted with
carbonic acid, derived from exposure to ꢀ1 atm carbon dioxide,
the water miscible acid form [H+(base) HCO3ꢁ] of the SPS is
produced, reaction (1). Similar behavior can be obtained from
dual component SPSs involving a nitrogen base (amidines and
guanidines) and an alcohol or primary amine; but such systems
require balanced stoichiometry to function correctly and tend to
be water sensitive which makes them unattractive for many
applications.2,6–8 Some single component SPS, such as
secondary amines, also suffer water sensitivity in the form of
+
ꢁ
NR3(org) + CO2(gas) + H2O # HNR3
+ HCO3
(1)
(aq)
Our laboratory became interested in SPS for their use as
thermolytic draw solutes11 (versus more conventional non-ther-
molytic draw solutes12,13) in osmotically driven membrane
processes (ODMPs). As thermolytic solutes, SPS can be used in
water purication through forward osmosis (FO),14 solution
concentration through direct osmotic concentration,15 and for
osmotic heat engines through pressure retarded osmosis.16
Since its introduction in 2006, the ammonia–CO2 system has
been considered one of the more viable next generation draw
solute for FO.17 SPS draw solutes have a number of advantages
over the ammonia–CO2 system; including negating the need to
handle and store gaseous ammonia, lower permeability to
properly selected membranes, lower energy requirements, and
facile removal of SPS from water through liquid phase
separation.
Tertiary amines, such as those screened in this publication,
long have been considered unlikely candidates for carbon
capture or natural gas sweetening. Primary and secondary
amines react chemically with carbon dioxide rapidly to form
carbamates.18 Tertiary amines, on the other hand, follow a
second route in which carbon dioxide forms carbonic acid and
then reacts via an acid–base reaction with the amine.18 The
carbonic acid pathway is generally slower due to the rate of
Idaho National Laboratory, P.O. Box 1625 MS 2208, Idaho Falls, ID 83415-2208, USA.
E-mail: aaron.wilson@inl.gov; Fax: +(208)-526-8511; Tel: +(208)-526-1103
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carbonic acid formation. However, tertiary amines and
mixtures which include tertiary amines have recently been
reported for carbon capture in conjunction with phase change
processes.19–37 Amines included in our study have also been
investigated by Zhang as carbon capture agents where he
Results and discussion
Due to the fact that many SPS applications would benet from
higher SPS concentrations in the polar form, the maximum
concentration was characterized for a series of tertiary amines,
1–26. This series was selected not because each amine was
expected to be a top performer but rather they are representative
of many small tertiary amines with features such as rings
systems, different length carbon chains relative to the nitrogen,
and alkyl chain branching. These representative amines were
selected not only to set limits on the maximum and minimum
number of carbons in a functional SPS but also to determine
how structural features affect that function. The set of amines
included the re-visitation of 12 tertiary amines that had been
previously studied for SPS behavior, conrming that 8 amines
transition from a water immiscible state to a water miscible
state with the introduction of CO2 and thus “function” as SPS.5
In addition, 14 previously unreported amines were investigated
for SPS behavior and, of these, 8 additional amines have been
identied to function as SPS. The structures of the amines that
functioned as SPS are illustrated in Fig. 1 and those that did not
measurably function as SPS under our experimental conditions
are illustrated in Fig. 2.
While a variety of properties were recorded including
viscosity, density, and freezing point depression; it was the
NMR spectroscopic studies that were of primary importance
where it was used to measure both concentration and compo-
sition of SPS solutions. The procedure to identify the maximum
concentration involved combining known quantities of water
and tertiary amine and purging the solution with carbon
dioxide at ambient pressure. The volume of the amine that did
not react to form the polar water soluble SPS was measured and
a tentative maximum solution concentration was calculated
based on the initial masses and unreacted volume. This volume
derived concentration measurement was used to corroborate
the concentrations found through quantitative NMR studies of
the polar SPS solutions.
refers to them as biphasic or lipophilic amine solvents.19–22
A
phase change carbon capture system, DMX™, has been
developed by IFP Energies nouvelles; however the chemical
composition of their formulation was not available to us.23,24
Heldebrant is exploring performance of single-component
CO2-binding organic liquids (CO2BOLs).25–28 Hu of 3H Company
has reported a two phase acid capture system involving a
polarity switching amine.29–33 Eckert has worked with
switchable polarity ionic liquid.34–37
a
There have been various publications addressing the use of
SPS for processing, extraction, and separation. This includes
plastic recycling,5 extraction of oils from biomass and
microbes,38–42 activation of recalcitrant biomass,43 and the use
of SPS as a chemical synthesis solvent.2,44 The use of SPS in these
applications has similarities to distillable room temperature
ionic liquids (RTIL) which are oen comprised of amines and
carboxylic acids.45–48 There is also the well-established use of
ammonium carbamate and carbonate salts as polyurethane
polymerization catalysts.49
Each of the potential applications can benet from achieving
+
a higher concentration of the SPS polar form, [HNR3 HCO3ꢁ];
however, how the concentration is considered best depends on
the application. In the case of FO, the osmotic strength of the
draw solute is the thermodynamic driving force for the water
transport process and is best measured by molality.50 In solvent
extraction, the osmotic pressure is less important than the
volume of non-aqueous amine contained within the SPS polar
+
form. A solution with a high weight percent (wt%) of [HNR3
]
allows the solution volumes used in the solvent extraction
process to be minimized. In a carbon capture system, the SPS
carbon capture agent would ideally have an extremely large
capacity for carbon dioxide and the ideal unit is wt% of CO2.
The most serious drawback of high concentration solutions is
increased viscosity which may be problematic for many
applications.
Theoretical treatments of high concentration solutions are
usually modeled through activity, as. Activity (as ¼ ysxs) is a
product of the mole fraction, (xs, moles solute per total moles
solvent and solute), and a dimensionless empirically based
activity coefficient, ys. Thus, the mole fraction, xs, was also
considered when looking for concentration trends.
Previous to this study, the information concerning the
maximum concentration of amine based SPS in their polar form
was limited to N,N-dimethylcyclohexylamine (5) and N,N,N0-
tributylpentanamidine.4,5 Tertiary amines are among the most
attractive SPS reported so far due to their simplicity and low
cost. These advantages motivated the screening of tertiary
amines 1–26 for a variety of physical properties similar to the
study recently published by Eckert.36 This screening has iden-
tied a new form of SPS, as well as structural features and
limitations of tertiary amines that are fundamental to their
performance as SPS.
Fig. 1 Structures of tertiary amines that functioned as SPS.
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The quantitative 13C and 1H NMR spectra were conducted as
neat solutions with a coaxial insert containing C6D6 as a refer-
ence. As examples, Fig. 3 and 4 feature the spectra for solution 50.
1
The H NMR spectrum contains chemical shis, d, which have
been assigned to the exchangeable protons of water (H2O),
carbonates (HCO3ꢁ and H2CO3), and ammonium (H+NR3) ions,
Fig. 4. The analysis of this data can be simplied by ascribing two
protons tocarbonicacid anditssalts. Based on 1H NMRspectrum
integration, the ratio of water and carbonic acid to amine can be
calculated. This ratio combined with the amine to carbonic acid
ratio derived from the quantitative 13C NMR, Fig. 3, allow for
the calculation of the relative mole ratio of amine : carbonic
acid : water. With the molecular mass and solution density it is
possible to calculate mole fractions, molarities, molalities, and
weight percent (Table 1), all of which were considered in looking
for trends associated with physical properties (Table 2).
Fig. 2 Structures of tertiary amines that did not function as SPS.
The concentrations calculated from quantitative 13C and 1H
NMR spectra studies allow the calculation of van't Hoff indices
based on either the sum of tertiary amine/ammonium molality
or the total species molality, which includes tertiary amines,
tertiary ammonium, and carbonate species, Table 2. The total
species molality is more informative as it removes the variation
in the relative carbonate concentration and is more directly
related to the ion dissociation. The degree of dissociation
cannot be perfectly gauged because these indices are compos-
ites of various forms of “ion pairing”, which reduce the van't
Hoff index, and the role of “bound” waters of hydration, which
raise the indices.50,51 Freezing point depression studies were
conducted to obtain experimental van't Hoff indices which
allow estimation of osmotic pressure, Table 2.50 The van't Hoff
indices were based on values measured generally between 0.10
and 2.0 Osm per kg, such as Fig. 5.
The correlation over the 0.10 Osm per kg to 2.0 Osm per kg
range, as measured by freezing point depression, was generally
linear where total species van't Hoff indices ranged between
0.73 and 0.94 (Table 2) for osmotic SPS, indicating high degrees
of dissociation for nearly all the solutions measured. One
example that did not follow this trend is solution 70, which
deviated negatively from linearity when the amine
The NMR studies also were used to identify composition
characteristics of the polar SPS solutions. This compositional
data indicated that the assumption featured in eqn (1) that all
SPS form in a ratio of one tertiary amine to one carbonic acid is
incorrect. There are two types of SPS whose primary composi-
tional difference is in the amine : H2CO3 ratio.
Osmotic SPS
An SPS that can produce an osmotic driven ux across a semi-
permeable membrane can be considered “osmotic”. Osmotic
SPS amines are characterized by a maximum concentration of
their polar SPS form, aer which additional amine is rejected by
the aqueous phase and remains separated in a nonpolar phase,
even in the presence of excess ambient pressure carbon dioxide.
Osmotic SPS systems remain fully liquid under all the experi-
mental conditions and no precipitate is observed; although
clouding is common during the switching process. Osmotic SPS
can be diluted with water and display predictable decreases in
osmotic pressure. Correlation between osmolality and molality
produces representative van't Hoff indices. To accurately
measure the maximum concentration of these SPS, quantitative
NMR spectroscopy was conducted.
Fig. 3 The quantitative 13C NMR of dimethylcyclohexylamine bicar-
bonate solution 50.
Fig. 4 The quantitative 1H NMR of dimethylcyclohexylamine bicar-
bonate solution 50.
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Table 1 The relative integration of quantitative NMR select properties and concentrations of tertiary amine bicarbonate solutions
Paper
Molecular
mass
Number (amine)
Mole
fraction Molal
(amine + H2CO3) (amine) (amine) (amine)
Density
Amine :
(H2O + H2CO3) : wt%
Molarity
Amine
(solution) H2CO3 (13C) amine (1H)
Dimethylbutylamine
Triethylamine
1-Ethylpiperidine
Methyldipropylamine
Dimethylcyclohexylamine
Dimethylhexylamine
1-Butylpyrrolidine
10
101.2
101.2
113.2
115.2
127.2
129.2
127.2
129.2
135.2
143.3
149.2
157.3
155.3
157.2
171.3
185.4
1.05
1.05
1.09
1.01
1.10
0.98
0.99
1.02
1.05
1.00
1.03
0.92
1.04
1.00
0.88
—
1.06
1.05
1.05
1.07
1.05
1.23
1.26
1.09
1.14
1.14
2.42
1.91
1.02
1.05
2.70
2.87
5.13
9.46
4.93
67.9
51.1
70.6
27.9
77.0
64.1
63.4
36.1
31.9
11.1
70.6
65.9
34.8
2.3
0.163
0.096
0.169
0.037
0.199
0.135
0.134
0.050
0.041
0.011
0.184
0.143
0.041
0.002
0.149
0.117
13.3
6.52
14.0
2.23
18.0
9.94
9.84
3.04
2.47
0.63
13.8
10.2
2.47
0.11
10.4
7.72
4.47
3.35
4.47
1.63
4.55
3.50
3.56
1.98
1.77
0.56
4.16
3.20
1.67
0.11
3.03
2.97
20
30
40
25.8
50
4.03
6.40
6.44
19.2
23.4
88.8
4.45
5.97
23.5
508
5.73
7.54
60
70
Diethylbutylamine
80
Dimethylbenzylamine
Methyldibutylamine
Dimethylphenethylamine
Dimethyloctylamine
Diethylcyclohexylamine
90
100
110
120
130
Dimethyl-2-ethylhexylamine 140
Dimethylnonylamine
Dimethyldecylamine
150
160
66.8
61.5
Table 2 The properties of tertiary amines bicarbonate solutions
wt%
Amine : van't Hoff van't Hoff Max.
Carbon : Density (amine + Density
H2CO3
(
index
(amine)
index
(S ions)
osmotic
pressure (atm) (cP)
Viscosity
Amine
Number nitrogen (amine) H2CO3)
(solution)
13C)
Dimethylbutylamine
Triethylamine
1-Ethylpiperidine
Methyldipropylamine
Dimethylcyclohexylamine
Dimethylhexylamine
1-Butylpyrrolidine
10
6
6
7
7
8
8
8
8
9
0.721
0.726
0.824
0.734
0.849
0.744
0.814
0.748
0.900
0.745
0.89
0.765
0.845
0.768
0.773
0.778
67.9
51.1
70.6
27.9
77.0
64.1
63.4
36.1
31.9
11.1
70.6
65.9
34.8
2.3
1.05
1.05
1.09
1.01
1.10
0.98
0.99
1.02
1.05
1.00
1.03
0.92
1.04
1.00
0.88
—
1.06
1.05
1.05
1.07
1.05
1.23
1.26
1.09
1.14
1.14
2.42
1.91
1.02
1.05
2.70
2.87
1.81
1.73
1.72
1.69
1.73
1.37
1.36a
1.79
1.37
1.77
—
—
1.82
—
—
—
0.93
0.88
0.88
0.87
0.88
0.76
0.76a
0.93
0.73
0.94
—
—
0.92
—
—
—
616
288
641
25.0
10.6
71
2.8
108
20
30
40
92.9
50
835
328
325a
135
87.1
27.3
—
—
114
—
60
25.5
29
70
Diethylbutylamine
80
11.3
2.8
1.6
15.0
58
4.8
1.1
86
Dimethylbenzylamine
Methyldibutylamine
Dimethylphenethylamine
Dimethyloctylamine
Diethylcyclohexylamine
90
100
110
120
130
9
10
10
10
10
11
12
Dimethyl-2-ethylhexylamine 140
Dimethylnonylamine
Dimethyldecylamine
150
160
66.8
61.5
—
—
Gel
a
Based on amine concentration <1.05 mol kgꢁ1
.
concentration exceeded 1 mol kgꢁ1, Fig. 6, suggesting higher
order “ion pairing” equilibrium processes. No other solutions
deviated from linearity, but solution 80 would not freeze cleanly
at or above 1.2 mol kgꢁ1 of amine, which may be a result
solution out gassing or exceeding a eutectic point.
It was previously reported that 5 forms a solution in a 1 : 1
(v/v) ratio with water which our lab reported as a viable ODMP
draw solute.11 The maximum concentration of 50 was revisited
and was found to have a maximum concentration of 77 wt%,
which corresponds to 18 molal and 4.6 M by amine. This
concentration is considerably higher than the value previously
reported. The osmotic pressure of solution 50 has been esti-
mated at 836 atm based on previously described methods,50
which is considerable for an FO draw solute.
Non-osmotic SPS
Osmotic SPS were not the only SPS form encountered in this
study; there were also “non-osmotic” SPS which differed in both
composition and behavior. These SPS are coined “non-osmotic”
because they did not produce the expected osmotic driven ux
across a semi-permeable membrane when used as a FO draw
solution in their “polar” form. The van 't Hoff indices of non-
osmotic SPS could not be measured because the solutions do
not dilute homogenously when water is added. All measured
freezing points were much lower than expected given the
concentration. For these reasons Table 2 does not contain a
maximum osmotic pressure for non-osmotic SPS. This is an
unexpected result for a solution made from the addition of
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Fig. 5 The van't Hoff plot for diethylcyclohexylamine, 130. The dia-
monds are based on molality of tertiary amine and tertiary ammonium
ions. The squares are based on the total species molality including
tertiary amines, tertiary ammonium ions, and carbonate species.
Fig. 7 The NR3 : H2CO3 ratio represents the ratio between all forms of
amine protonated and unprotonated and the sum of carbonate,
bicarbonate, and carbonic acid; diamonds represent “osmotic” SPS,
crosses represent “non-osmotic” SPS.
Modeling the maximum concentration equilibrium
Due to the discovery of non-osmotic SPS, the model SPS
maximum concentration must be revisited. Since the organic
amine is an immiscible material, its concentration does not
change with the equilibria and thus can be taken as unity.
Likewise the solvent, in this case water, is usually in a large
excess such that its effective concentration does not change
with the equilibrium and usually can also be treated as unity.
Assuming the solvent as unity is not strictly proper for the SPS
equilibria where the water concentration drops below 0.8 mole
fraction and may also be involved in ionic hydration. Thus, the
water concentration is included in this equilibrium. Because
these are non-aqueous solutions of variable consistency, there
are no known Henry's law constants for these systems. Thus, the
CO2 concentration is not calculated based on the partial pres-
sure. In addition, because CO2 partial pressure is the indepen-
dent variable, it is preferred for the calculation of an
equilibrium constant.52 These assumptions yield the equilib-
rium expression, eqn (3), from the reaction described by eqn (2).
Fig. 6 The van't Hoff plot for diethylcyclohexylamine, 70. The dia-
monds are based on molality of tertiary amine and tertiary ammonium
ions. The squares are based on the total species molality including
tertiary amines, tertiary ammonium ions, and carbonate species. Only
the filled-marker data was used to calculate the trend lines. The higher
concentration open-marker data feature a negative deviation from
linearity.
carbon dioxide and tertiary amine to water. When water is
added to a concentrated non-osmotic SPS, a portion of the
solution dilutes, and is reected in freezing point osmometry
measurements, but another portion phase separates as the
nonpolar tertiary amine. To understand this dilution phenom-
enon, the relative concentrations of the species in solution must
be known. The solutions are comprised of amines species, both
protonated and unprotonated, and carbonate species, which is
mostly bicarbonate with equilibrium quantities of carbonate
and carbonic acid. Because of the complexity involved with
tracking the equilibrium concentration, it is useful to consider
this acid–base system in its non-ionized form to compare the
“amine” to “carbonic acid” ratios. The ratio of tertiary amine to
carbonic acid for non-osmotic SPS varies between 1.82 and 2.87
based on the integration of the quantitative 13C NMR spectra.
These values for non-osmotic SPS are signicantly higher than
polar SPS with all values listed in Tables 1 and 2 and plotted
in Fig. 7.
+
ꢁ
NR3(org) + CO2(pressure) + H2O # HNR3
+ HCO3
(2)
(3)
(aq)
Â
à Â
ꢁÃ
þ
HNR3
HCO3
aq
K1 ¼
pCO2½H2Oꢂ
NR3(org) # NR3(aq)
(4)
(5)
+
ꢁ
+ HCO3
NR3(aq) + CO2(pressure) + H2O # HNR3
(aq)
+
a[HNR3
+ NR3(aq)] + NR3(org) # NR3(aq) + bH2O (6)
(aq)
Â
à Â
ꢁÃ
b
þ
HNR3
pCO2½H2Oꢂ HNR3þ þ NR3
HCO3 ½NR3ꢂ ½H2Oꢂ
aq
aq
Â
Ã
K
¼
ðeqn ð4Þꢁð6ÞÞ
a
(7)
aq
Non-osmotic SPS concentrations are not as simple as they
can be modeled multiple ways. Because there are appreciable
amounts of aqueous tertiary amine in non-osmotic SPS, the
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model needs to consider the conversion of organic tertiary
amine to aqueous tertiary amine (eqn (4)). When aqueous
tertiary amine is considered directly, eqn (2) is converted into
eqn (5). As the aqueous tertiary amine and aqueous tertiary
ammonium bicarbonate concentrations increase, the concen-
tration of dissolved organic materials increases. This results in
a solution polarity decrease, which shis the solution towards
something more like a water immiscible organic solvent. This
shi in polarity allows the solution to accept more tertiary
+
amine (eqn (6)). In this model, we assume HNR3 and NR3
contribute equally to the polarity shi for the sake of simplicity.
Presumably, the increase in aqueous amine when the products
are favored in eqn (4) allows the further conversion of carbon
dioxide into bicarbonate and protonated tertiary amine (eqn (5))
in a positive feedback loop. This feedback loop does two things:
(1) increases the carbonate concentration relative to the osmotic
Fig. 8 Maximum acquired concentration as a function of the C : N
ratio.
+
SPS and (2) increases the absolute concentration of HNR3 (aq)
and HCO3ꢁ. The relative concentration of NR3(aq) also increases
and the solution moves further away from a composition that is
strictly aqueous. If water is added to a concentrated non-
osmotic SPS solution, a portion of the NR3(aq) phase separates as
the SPS solution polarity is driven to a more polar form. The
ability of water to shi the polarity of the solution is featured in
its role as a product dependent on the value “b” in eqn (6). The
role of water is complex and it may be necessary to identify
portions as “free” or “bound” in the SPS solution, but the
treatment in eqn (5) and (6) is sufficient to model the current
information yielding an equilibrium expression, eqn (7).
Limits of SPS function at ambient pressure and concerns with
measurements
The number of carbons or the carbon : nitrogen (C : N) ratio of
a tertiary amine is a useful proxy for both the mass and polarity
of the tertiary amine; more carbons in the amine result in
higher molecular mass, and thus lower overall polarity. Hansen
solubility parameters and its components were explored to
describe tertiary amine polarity without success; the calculated
Fig. 9 Maximum acquired concentration as a function of the C : N at 1
atm CO2 for SPSs featuring the N,N-dimethyl-n-alkylamine structure
plotted in molarity for both the amine (solid diamond) and carbonate
components (open diamond).
parameters varied little and were less intuitive than the simple the carbonate concentration. For example, focusing on the
C : N ratio. N,N-dimethyl-n-alkylamine series, Fig. 9, it is clear that the
In this study, no tertiary amines with a C : N ratio of less than carbonate concentration steadily decreases from C : N 6 to 11.
6 were explored. Tertiary amines with low C : N ratio have many Solution 160 is excluded from this analysis because it forms a gel
undesirable characteristics including low boiling points, high distinct from the other liquid solutions. A trend line can be
vapor pressures, higher water solubility, and a more difficult tted to the carbonate concentrations of the N,N-dimethyl-n-
switch between the nonpolar and polar phases. Release of CO2 alkylamine series, which includes osmotic SPS (10 and 60) and
for these amines generally requires substantially higher non-osmotic SPS (120 and 150) that indicates increasing the C : N
temperatures or greater volumes of purge gas, followed by ratio results in a decline in the carbonate concentration.
cryogenic amine capture, which complicates their utility. These Because osmotic and non-osmotic systems are linear when the
factors serve to limit the use of these amines as SPS and moti- C : N ratio is plotted against carbonate molarity, this is conve-
vates the efforts to dene the proper upper thresholds of the nient trend for comparing all SPS systems. The linearity of the
C : N ratio for tertiary amines that function as SPS.
trend also could be taken to suggest that concentration
When the total weight percent of both osmotic SPS (C : N phenomena inuence the maximum concentrations of both
6–10) and non-osmotic SPS (C : N 10–12) are plotted against osmotic and non-osmotic SPS in a similar way, rendering the
C : N their ratio, high weight percentages are found until C : N previous equilibrium analysis (eqn (4)–(7)) unnecessary. Such
12 aer which no tertiary amines were found to form SPS, Fig. 8. a conclusion does acknowledge that while the C : N ratio is a
The loss of SPS formation above C : N ¼ 12 may be explained useful proxy for polarity and molecular mass, it is not a
when the concentration is broken into the tertiary amine and fundamental physical property commonly used to compare
11044 | RSC Adv., 2014, 4, 11039–11049
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equilibrium states and thus, in many ways, an arbitrary unit,
making the resulting trend similarly arbitrary.
The observed trend in Fig. 9 is a product of known theoret-
ical and experimental inuences. The slope of the regression
might be steeper or t a different mathematical/concentration
model if not for three phenomena. First, as discussed earlier,
there is a positive feedback loop associated with non-osmotic
SPS and their elevated concentration of aqueous amine. This
raises the concentration of carbonate in solution for 120 and 150
(and 160) which dene the low end of the trend. The second
feature is the home-built experimental apparatus for this study
pushed carbon dioxide through a column of solution which was
then exhausted through a condenser open to the atmosphere
through a needle, as shown in the experimental section. For
lower viscosity amines, the CO2 pressure rapidly equilibrated
with ambient pressure; however, the amines with viscosities
greater than 50 cP (Table 2), namely 30, 50, 120, and 150 (and 160)
provide back pressure on the carbon dioxide ow slightly
elevating the CO2 pressures directed at the solution.
The third phenomenon that affects the trend observed in
Fig. 9 is the stability of the solutions. Not all solutions are stable
for signicant periods of time. Solutions 110 and 150 (and 160)
are prone to venting carbon dioxide when mild pressure or
vacuum is applied or even mixing in the absence of a saturated
carbon dioxide atmosphere. When conducting NMR experi-
ments, approximately 20% of solutions 110 and 150 (and 160)
phase separated into the nonpolar amine form, suggesting they
may be metastable supersaturated states.
Fig. 10 The amines which deviate from the N,N-dimethyl-n-alkyl-
amine core structure with b, g, and d carbons as well as a rings systems
labeled and separated into groups according to whether their
carbonate concentration are greater than (>), equal to (¼), or less than
(<) the trend line formed from the carbonate concentration of the N,N-
dimethyl-n-alkylamine solutions (Fig. 9).
contribution model similar to the Hansen system, but dedi-
cated to the tertiary amine SPS concentration model, is
proposed below.
At the core of the functional group contribution treatment is
the linear relationship between C : N ratio and the maximum
Each of these three phenomena tend to inate the observed
concentration at high C : N ratios. Because two of these
phenomena are related to the experimental process and design,
the conclusions and performance trends regarding high C : N
ratios and the upper threshold for tertiary amine function as
SPS may be generous.
ꢁ
+
molarity of the HCO3 and HNR3 concentrations in the N,N-
dimethyl-n-alkylamine series, which holds for the alkyl ¼ butyl
(10), hexyl (60), octyl (120), nonyl (150) series. Amines which
deviate from the N,N-dimethyl-n-alkylamine skeletal structure
can be grouped into two overlapping groups. The rst group
of amine structures all contain carbons extending the core
Structural features of SPS
Within the range of C : N ratios between 6 and 12 there are
structural features that inuences SPS performance. Better
performing SPS are those that can reach the highest concen-
trations. Of the better performing SPS, it was noted that every
example featured the core structure of N,N-dimethyl-R-amine
(10, 50, 90, 110, and 150), or 1-alkylpiperidine (30). The N,N-
dimethyl-R-amine grouping can be broken down into N,N-
dimethyl-n-alkylamines (10 and 150), N,N-dimethylcyclohexyl-
amine (50), and N,N-dimethylphenealkylamines (90 and 110).
Because the pKas of the studied tertiary amines are largely
equivalent, it is assumed that an aspect of either or both polarity
and intramolecular steric interaction are dictating the differ-
ences in the observed SPS performance.
There are many methods to model steric and polar interac-
tions. Tolman cone angles have been used extensively to model
the sterics inuences of tertiary phosphines on their interac-
tions with Lewis acid metal centers.53–55 While Tolman cone
angles have not been used to describe amines, the phosphine
values were used to conduct an evaluation that, while internally
Fig. 11 Maximum acquired concentration for SPSs featuring additional
carbons functionality in addition to the N,N-dimethyl-n-alkylamine
core structure. Trend line based on the N,N-dimethyl-n-alkylamine
series from Fig. 9 included here for a reference. The conditions and
consistent, was ultimately unsuccessful. A functional group labeling are the same as Fig. 9.
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and 110), piperidine (30), and pyrrolidine (70). The structure of
ring containing systems are also included in Fig. 10 and are
labeled as (<, ¼, and >) because their concentrations vary
compared to what is predicted by the N,N-dimethyl-n-alkyl-
amine trend. The maximum concentrations of ring containing
systems in relation to N,N-dimethyl-n-alkylamine trend line are
plotted in Fig. 12.
Alkyl substituents in the absence of a ring system reduce a
tertiary amine's effectiveness as an SPS. These carbons can be
described as b, g, and d carbons, each with a different ability to
inuence steric crowding at nitrogen, Fig. 13. The b, g, and d
carbons have the potential to sterically disrupt the space around
the nitrogen's lone pair to differing degrees. Such steric
hindrance does not inherently prevent the coordination of a
proton, due to its small size. Most of the amines in this study
readily form highly concentrated protic ionic liquids with
strong acids. Carbonic acid, derived form carbon dioxide, is
neither a strong acid nor a concentrated acid under ambient
conditions. The steric hindrance around the nitrogen lone pair
likely prevents formation of extended solvent and counter ion
(bicarbonate) network necessary to stabilize the polar form of
the SPS in the aqueous phase.
Fig. 12 Maximum acquired concentration for SPSs featuring ring
systems. Trend line based on the N,N-dimethyl-n-alkylamine series
from Fig. 9 included here for a reference. The conditions and labeling
are the same as Fig. 9.
The potential for maximum steric interaction increases with
the carbon's proximity from the nitrogen, d > g > b. In contrast
rotational degrees of freedom have the opposite effect based on
as the carbon's potential to relax away from the amine which
also increases according to the carbon's proximity from the
nitrogen, d > g > b.
The coefficients used in eqn (8) were produced by empirically
adjusting the values to produce a one to one linear relationship
between the experimental and calculated molar concentrations,
Table 3 and Fig. 14. Based on the structures and concentrations
observed in this study, the steric effect on the carbonate
concentration of a g (1.1) carbon is approximately double the
effect of a b (0.55) or d (0.5) carbon. It is expected that more
distant carbons would have little effect on the nitrogen.
Fig. 13 Position of b, g, and d carbons with respect to the nitrogen.
N,N-dimethyl-n-alkylamine structure but with no ring systems.
For example, these carbons would include both carbons of
dimethyl-2-ethylhexylamine’s, 140, ethyl group or 1 carbon from
each of the ethyl groups in diethylbutylamine, 80, Fig. 10. The
structures of these are amines and their pertinent carbons are
labeled as (<) in Fig. 10 because their concentration are lower
than predicted by the N,N-dimethyl-n-alkylamine trend.
The concentrations of ring free systems in relation to N,N-
dimethyl-n-alkylamine trend line are also plotted in Fig. 11. The
second group is tertiary amines whose structures include a ring
system such as a cyclohexyl group (50 and 130), phenyl group (90
SPS(H2CO3(M)) ¼ 7.86 ꢁ 0.62(S carbon) ꢁ 0.55b
ꢁ 1.1g ꢁ 0.5d + 1.2(a ring)
(8)
Table 3 Properties of tertiary amines that functioned as SPS
Number
of total carbon
a ring
systems
Molar
(exp.)
Molar
(calc.)
Absolute
difference
Amine
Number
b carbons
g carbons
d carbons
Dimethylbutylamine
Triethylamine
1-Ethylpiperidine
Methyldipropylamine
Dimethylcyclohexylamine
Dimethylhexylamine
1-Butylpyrrolidine
10
6
6
7
7
8
8
8
8
9
4.21
3.19
4.26
1.52
3.33
2.84
2.83
1.82
1.55
0.49
1.72
1.67
1.64
0.10
1.12
4.15
3.05
3.63
1.88
3.56
2.91
3.01
1.81
1.19
0.14
1.67
1.67
1.22
0.07
1.05
0.06
0.14
0.63
0.36
0.23
0.07
0.18
0.01
0.36
0.35
0.05
0.00
0.42
0.03
0.07
20
2
2
1
1
30
1
1
1
40
1
50
60
70
2
2
Diethylbutylamine
80
Dimethylbenzylamine
Methyldibutylamine
Dimethylphenethylamine
Dimethyloctylamine
Diethylcyclohexylamine
Dimethyl-2-ethylhexylamine
Dimethylnonylamine
90
1
1
100
110
120
130
140
150
9
1
3
1
1
10
10
10
10
11
1
1
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which suggested that a C : N ratio between 6 and 12 is
necessary. Furthermore, a new form of non-osmotic SPS with
an elevated amine to carbonate ratio in its polar form has
been identied and characterized. This structure–function
analysis and identication of a new form of SPS has impli-
cations on the design of SPS systems based on untested
amine systems, as well as the selection of SPS for a variety of
applications including solvent extraction systems, plastic
recycling, synthetic media, acid gas capture, and osmotically
driven membrane processes (ODMPs).
Experimental methods
General
Deionized water was used for these experiments. N,N-Dime-
thylbutylamine, triethylamine, 1-ethylpiperidine, N-methyl-
dipropylamine N,N-dimethylcyclohexylamine 99% (N(Me)2Cy),
1-butylpyrrolidine, N,N-diethylbutylamine, N,N-dimethylbenzyl-
amine, N-methyldibutylamine, N,N-dimethylphenethylamine,
N,N-dimethyloctylamine, N,N-diethylcyclohexylamine, 2-ethyl-
Fig. 14 The correlation between the observed maximum molarity of
SPS and those calculated from eqn (8).
hexylamine,
amine, N,N-dimethyaniline, N-ethyldiisopropylamine, tripropyl-
amine, triisopropylamine, 4-N,N-Trimethylaniline, N,N-
N,N-dimethylnonylamine, N,N-dimethyldecyl-
The effects of a ring system on tertiary amines SPS function
is more ambiguous than the argument presented above. All of
the systems containing carbons beyond N,N-dimethyl-n-alkyl-
amine skeleton but no ring system performed more poorly than
the N,N-dimethyl-n-alkylamine series. Of the systems that con-
tained ring systems and additional carbons some performed
better than the N,N-dimethyl-n-alkylamine series, including 30
and 50, but not all ring containing systems performed better.
Solutions 70 and 130 have concentrations that are much higher
than expected, lying on the line for the N,N-dimethyl-n-alkyl-
amine series despite each containing two additional b carbons.
Solution 110 essentially lies on N,N-dimethyl-n-alkylamine trend
line which suggests that the steric cost and benet of ring
system carbon g to the nitrogen are roughly equal or are
negligible. As for 90, denitively resolving the subtle steric and
electronic effects associated with the benzyl ring system is
beyond the current scope of this paper but it models well as g
carbon sterics with no ring benet. Based on these systems, an
“a ring system” which includes the cyclohexyl groups (50 and
130), piperidine (30), and pyrrolidine (70) provides an enhance-
ment to an amine's SPS function, which is not observed for
more distant ring systems (90 and 110). The benet of an a ring
system (1.2) was incorporated into eqn (8), Table 3 and Fig. 14.
Aniline derivatives were not observed to function as SPS. The
nonfunctionality of the aromatic aniline derivatives is not pre-
dicted by eqn (8), and is attributed to the pKa of 5–6 resulting
from an amine directly bonded to an aromatic ring which is
signicantly lower than the alkyl substituted tertiary amines
with pKa of 8–11.
dimethyl-o-toluidine, tributylamine, N,N-dicyclohexylmethyl-
amine, N,N-dimethyldodecylamine, tripentylamine were
obtained from Aldrich and used as received. All equipment was
usedinaccordance withmanufacturerspecicationunlessstated
otherwise. Freezing point depression osmometry was performed
using an Advanced Instruments Inc. Model 3250 Osmometer.
Viscosity measurements were made using the falling bob method
with a Cambridge Applied systems VL4100 viscometer.
Conclusion
In this study, a functional group contribution model has
been developed for tertiary amine SPS. Also, structural limits
Fig. 15 Carbon dioxide additional cell used to convert two phases of
for tertiary amines that function as SPS were identied, amine and water to a single phase polar SPS, 10 through 160.
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Nuclear magnetic resonance (NMR) spectra were acquired supplied by Idaho National Laboratory via the Laboratory
on a Bruker Avance III 600 MHz spectrometer with a magnetic Directed Research and Development Fund (LDRD) under
eld strength of 14.093 Tesla, corresponding to operating project 14-079.
frequencies of 600.13 MHz (1H), and 150.90 MHz (13C). All NMR
were captured with a co-axial insert containing C6D6 (Cam-
bridge Isotopes Laboratories). H NMR spectra were collected
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Energy through contract DE-AC07-05ID14517. Funding was
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