Design, synthesis, and solution behaviour of small polyamines as switchable water additives

Article abstract of DOI:10.1039/c2gc16240g

The practice of adding salt to water to induce salting out of contaminants or to break emulsions and suspensions is generally avoided industrially because of the expense of the necessary treatment of the salty water afterwards. However, the use of switchable water, an aqueous solvent with switchable ionic strength, allows for reversible generation and elimination of salts in aqueous solution, through the introduction and removal of CO₂. In the effort to improve the efficiency of these switchable salts, a physical study modeling their reactivity and solution behaviour has been performed, resulting in a set of design principles for future switchable water additives. The resulting polyamines synthesized using this template show the highest efficiency recorded for a switchable water additive. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2gc16240g

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PAPER
Design, synthesis, and solution behaviour of small polyamines as switchable
water additives†
Sean M. Mercer,a Tobias Robert,a Daniel V. Dixon,a Chien-Shun Chen,a Zahra Ghoshouni,a Jitendra
R. Harjani,a Soran Jahangiri,b Gilles H. Peslherbeb and Philip G. Jessop*a
Received 6th October 2011, Accepted 21st December 2011
DOI: 10.1039/c2gc16240g
The practice of adding salt to water to induce salting out of contaminants or to break emulsions and
suspensions is generally avoided industrially because of the expense of the necessary treatment of the
salty water afterwards. However, the use of switchable water, an aqueous solvent with switchable ionic
strength, allows for reversible generation and elimination of salts in aqueous solution, through the
introduction and removal of CO2. In the effort to improve the efficiency of these switchable salts, a
physical study modeling their reactivity and solution behaviour has been performed, resulting in a set of
design principles for future switchable water additives. The resulting polyamines synthesized using this
template show the highest efficiency recorded for a switchable water additive.
preliminary screening that the amines that are most effective for
switchable water are different from those that are most effective
Introduction
Addition of salt to water has many practical applications, from
the breaking of emulsions, foams and suspensions to the
removal of organic contaminants from water by “salting out”.
However, the addition of salt to water is generally avoided
industrially because it is energetically costly to remove the salt
from the water afterwards and the water can be neither recycled
nor discarded while salty. We, in our previous communication,1
described an aqueous solution of switchable salinity. The sol-
ution, which we call “switchable water” or “SW”, consists of an
amine or polyamine dissolved in water. In the absence of CO2,
the ionic strength of the solution is very low (it would be zero
but for the small dissociation of the amine in water as well as
water itself). In the presence of CO2, however, the additive is
converted to the bicarbonate salt, resulting in a dramatic and
reversible rise in the ionic strength. If the amine or polyamine (B
in eqn (1) and Fig. 1) is assumed to have n protonatable sites,
and if the usual general equation for the ionic strength of an elec-
trolyte solution is assumed (eqn (2)), then the ionic strength of
the bicarbonate solution is governed by eqn (3) (where mB is the
nominal molality of the SW additive and where we neglect the
ionic strength due to the dissociation of water). While the chem-
istry is quite similar to that used in CO2 capture, the utility of the
solution is quite different; not surprisingly, then, we found in our
for CO2 capture.
B þ nH2O þ nCO2 O ½BHnꢀnþ þ n½HCO3ꢀꢁ
ð1Þ
ð2Þ
1
I ¼ Σ mZ 2
2
1
I ¼ mBðn2 þ nÞ
ð3Þ
2
As evident from eqn (3), the ionic strength, I, of the solution
is strongly dependent on the number of protonatable sites on the
additive, but only if eqn (2) applies. Because ionic strength is
not easily measured directly, we have used the salting out of
THF from a SW solution as both an indirect measure of the
ionic strength of the solution and a direct measure of the effec-
tiveness of the SW additive for the salting out application. While
salting out is certainly not the only application envisioned for
aDepartment of Chemistry, Queen’s University, 90 Bader Lane,
Kingston, Ontario, Canada. E-mail: jessop@chem.queensu.ca; Fax: +1
613 533 6669; Tel: +1 613 533 3212
bDepartment of Chemistry and Biochemistry, Concordia University,
7141 Sherbrooke St. W., Montréal, Quebec, Canada
†Electronic supplementary information (ESI) available: General pro-
cedures, experimental details, crystallography data. CCDC reference
number 860228. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2gc16240g
Fig. 1 Switchable water solutions are transformed from low ionic
strength (left) to high ionic strength (right) by the addition of CO2. The
process is readily reversible.
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Table 1 The ability of several tertiary amine, amidine, and inorganic
salt additives to separate THF from 1 : 1 w/w solutions of H2O and THF
and the resulting retention of the additive in the aqueous phase when
reacted with CO2
Scheme 1 Nitrogenous bases with multiple basic sites tested as switch-
able water additives.
% THF
% Amine
retaineda
Entry Base or salt
Loading
separateda
1
Dimethyl-
ethanolamine
DBUc
0.80 molal 70 0.6%b
98.0 0.2%b
generate bolaform electrolytes in solution, should be more effec-
tive than monoamines, according to eqn (3). A bolaform electro-
lyte is a polyvalent electrolyte species where the sites of charge
are separated from each other by a series of atoms.3 We have pre-
viously reported a diamine (compound 2, Scheme 1) that was
more effective than monoamines at salting out of THF from 1 : 1
w/w solutions of THF and H2O (Table 1). However, these salts
are still not as capable of salting out organics as the inorganic
salts shown in Table 1, so further improvement and development
of larger polyamines is necessary.
For effective switching, the amine sites on an additive must be
neither too basic nor insufficiently basic (similar to the findings
for ionic liquids used for CO2 capture);4 in our experience, a
pKaH (the pKa value of the protonated species) range of 8 to 10.5
is appropriate. If the amine is insufficiently basic, then CO2 at
1 atm will be insufficient to protonate the amine with high con-
version. If the amine is too basic, then the reversion will require
severe conditions, take too long, or be impractical to achieve.
Symmetry arguments predict that symmetric diprotic acids, a
diprotonated diamine for example, with chemically equivalent
functional groups will possess two dissociation constant values
with a ratio of four (K1/K2 = 4) if the ionization events are com-
pletely independent of one another.5 A deviation resulting in a
ratio greater than four would represent some sort of dependence
of the second ionization event upon the first ionization event.
One possible intra-molecular based explanation of this deviation6
is that when the two functional groups are too close, a single ion-
ization event at one group affects the electron density and there-
fore acidity or basicity of the second functional group.
Therefore, we must ensure that protonation events at one amine
functional group on a polyamine chain do not greatly decrease
the capacity of subsequent amine groups to be protonated (i.e.
pKaH(1) − pKaH(2) should be close to 0.6).
2
3
4
5
6
7
8
9
0.80 molal 59 0.7%
0.80 molal 75 2.3%
0.80 molal 82 0.6%b
0.80 molal 78 0.9%b
0.80 molal 88 1.6%
96.5 1.7%
98.1 2.2%
99.2 0.4%b
99.3 0.4%b
n.a.
n.a.
n.a.
1c
2
3d
NaCl
NaCl
(NH4)2SO4
(NH4)2SO4
variede
98 0.5%
0.80 molal 92 1.2%
variede
99 0.6%
n.a.
a Determined by 1H NMR spectroscopy. b From Ref. 1. c This additive
was able to salt out organics upon reaction with CO2, but CO2 could not
be released under working conditions to allow for recycling. d This
additive, a tetramine, could only be protonated at two sites by carbonic
acid, as determined by 1H NMR spectroscopy. e Salt was added at
various amounts until a consistent volume of THF separated from the
aqueous solution was observed (see ESI†).
switchable water additives, it has the benefit of being easily and
rapidly tested.
In general, the desired performance characteristics of a SW
additive used for salting out are rapid switching, high level of
THF expulsion from the aqueous phase, and high retention of
the additive in the aqueous phase. Optimally, these additives
should be commercially available or be easily synthesized from
readily available and inexpensive starting materials. Our initial
communication showed that several diamines have a greater
ability to salt out THF from water than an equimolal amount of
monoamines in the presence of CO2. Here, we describe the
results of our efforts to further delineate the structural design par-
ameters that control the effectiveness of SW additives.
Results and discussion
a) Monoamines, diamines and amidines
For linear ditertiary diamines Me2N(CH2)xNMe2, the first and
second pKaH values show that four carbons are necessary to
insulate the one basic site electronically from the other and thus
to ensure that both pKaH values fall within the range that is
appropriate for facile switching (Fig. 2). N,N,N′,N′-Tetramethyl-
1,4-diaminobutane, 2, is the smallest ditertiary diamine that has
both pKaH values within our working range. We conclude that
polyamines should have a minimum of four carbons between
each amine group to maintain appropriate basicity of each amine
after the protonation of a neighbouring group.
How large an increase in the ionic strength could one expect
when applying CO2 to a solution of 0.1 molal of 2? We can cal-
culate an approximate answer by assuming solution ideality and
by using molarity rather than molality. A 0.1 M solution of 2
would have a pH of 11.6 and the relevant ions would be [2·2H+]
(0.006 mM), [2·H+] (4 mM), [OH−] (4 mM) and a negligible
concentration of H+, giving a total ionic strength of ∼4 mM.
Adding CO2 at 1 bar to that solution lowers the pH to 6.8,
giving a solution containing [2·2H+] (99 mM), [2·H+] (1 mM),
Before attempting to find any more new amine additives, we
sought to establish the amount of THF salted out by common
inorganic salts (NaCl and (NH4)2SO4) at our standard test molal-
ity of 0.8 molal (Table 1, entries 6 and 8), as well as a ceiling
amount of THF that could be removed from water by such inor-
ganic salts at higher concentrations (Table 1, entries 7 and 9).
The slightly larger amount of THF that is removed via these salts
compared to the amine additives at equimolal loading (Table 1,
compare entries 1–6 and 8) shows that improvement of switch-
able water additives is necessary to mimic the performance of
typical non-switchable salts used in salting out processes.
We attempted to use amidine bases, such as DBU and com-
pound 1 (Scheme 1), as switchable water additives but were not
successful because the reversion from the amidinium bicarbonate
species was too difficult at moderate temperatures. In addition,
amidines are more expensive and suffer from hydrolytic instabil-
ity at alkaline conditions.2
To mimic the salting out capabilities of the inorganic salts
studied, diamines and larger polyamines, which when protonated
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Fig. 2 pKaH values of Me2N(CH2)xNMe2 (● = pKaH(1), ■ = pKaH(2)
)
relative to the number of carbons found between the two amine
groups.7–9 The shaded region highlights the desired range of pKaH
values for a switchable water additive.
Fig. 4 Comparison of pKaH values of linear α,ω-diaminoalkane hydro-
chlorides versus the number of carbons, x, separating the two amine
groups.10–12 ■ pKaH(1), ▲ pKaH(2), ● pKaH(1) − pKaH(2)
.
As shown, the amount of THF salted out is nearly indepen-
dent of x up to a value of 6, but the amount of THF salted out
quickly decreases with the 1,7-diaminoheptane dihydrochloride
salt and ultimately to zero salting out when the eight-, nine-, and
10-carbon spaced salts are used. A similar trend was observed
with 1 : 1 w/w solutions of acetonitrile and water (Fig. 3). With
either solvent, there appears to be a weak maximum at x = 4. On
a practical level, it is clear that a four-carbon spacer is best, but
the reason for the poor performance at x > 6 was not immedi-
ately clear. We then sought to elucidate the cause of the dimin-
ished salting out effect of diamine dihydrochloride salts with
longer spacers.
Fig. 3 The amount of organic solvent salted out from 1 : 1 w/w mix-
tures of ●: THF/H2O or ■: acetonitrile/H2O with a 0.50 molal loading
of α,ω-diaminoalkane dihydrochloride salts, [H3N(CH2)xNH3]Cl2,
where x is the number of carbons between the ammonium groups.
c) Accounting for the diminishing salting out effect
[HCO3−] (199 mM) and negligible concentrations of CO32−, H+
and OH−, giving a total ionic strength of ∼300 mM. Therefore,
in this idealized case, the ionic strength would jump two orders
of magnitude upon addition of CO2.
In the attempt to rationalize these observations we explored four
possible hypotheses.
Hypothesis #1. Our first hypothesis was that intra-molecular
(through-bond) communication between the two ammonium
groups diminishes or is lost when the carbon spacer is larger
than six carbons, so that the two charge centres are functioning
essentially as two monocations as far as eqn (2) is concerned. To
determine the spacer length at which electronic communication
is lost, we first looked into the dependence of basicity on spacer
length. A survey of pKaH values of primary α,ω-diaminoalkane
dihydrochlorides from literature10–12 (Fig. 4) shows that the ratio
of the first and second KaH dissociation constants levels off at a
value of roughly 4 (or pKaH(1) − pKaH(2) ∼ 0.6) as the number of
carbons between the two functional groups exceeds six. This
suggests that simple diamines with carbon spacers of greater
than six, when protonated, have two ammonium groups that are
electronically independent of each other.
b) Long-linker bolaform electrolytes
Once the minimum number of carbons between amine nitrogen
atoms had been found to be four, we wondered whether there
was either an optimum number or an upper limit beyond which
salting out ability would be lost. We hypothesized that if the
carbon spacer were excessively long, then the dication would
begin to function more like a pair of singly charged cations and
the Z term in eqn (2) would thus fall to 1 instead of 2.
To observe this threshold value of spacer length, we carried
out a series of simple salting out experiments using 1 : 1 w/w
solutions of THF and H2O. Using a series of nonswitchable α,ω-
diaminoalkane dihydrochloride salts [H3N(CH2)xNH3]Cl2 with a
varying number of carbons between the ammonium groups, we
observed the amount of THF expelled from each aqueous sol-
ution upon dissolution of the salts in the aqueous solution
(Fig. 3).
To determine whether a lack of electronic communication is
the deciding factor in disallowing salting out by diammonium
salts with large carbon spacers, we performed another set of
THF salting out experiments using benzidine dihydrochloride, 7,
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Table 2 Ability of various diamine dihydrochloride salts to salt out
THF from a 1 : 1 w/w solution of H2O and THF
four-carbon spaced species, we concluded this was not the cause
of our observations.
% THF
separated
Hypothesis #3. We then hypothesized that an increase in ion-
pairing of the alkylammonium and chloride ions occurs at
greater spacer lengths, reducing the effective ionic strength gen-
erated by the α,ω-diaminoalkane dihydrochloride salts. Past lit-
erature has noted that bolaform electrolytes are prone to
extensive ion-pairing in aqueous solution.3 If this phenomenon
is dependent on the carbon spacer length between the diammo-
nium ions, this could account for the lack of salting out ability of
the longer salts due to a diminished effective concentration of
charged species and resulting ionic strength in solution.
However, a recent study of ion-pair association constants of
similar diamine dihydrohalide species has shown that the associ-
ation constant values, KA, of the diamine dihydrohalide ion pairs
decrease as the number of carbon spacers between ammonium
groups is increased.15
Compound Structure
Loading
4
0.50 molal 0%
0.50 molal 51%
0.30 molal 46%
0.50 molal 58%
0.30 molal 0%
5
5
6
7a
8a
0.30 molal 0%
9
0.50 molal 53%
0.50 molal 68%
10
11
0.50 molal 73%
Hypothesis #4. Our attention then focused upon the increasing
organic content of the salts. We proposed that the increase of
organic content in the longer bolaform electrolytes is helping to
solubilize the organic substrates in aqueous solution, thus pre-
venting a salting out effect despite the high ionic strength. Pre-
vious modeling studies by Wang and co-workers have shown
that a dicarboxylate dianion, when adequately solvated, will fold
in, bringing its two charged termini closer than expected.
Remarkably, the aqueous solvation stabilization overcomes elec-
trostatic repulsions of the two charged ends leaving the organic
backbone sticking out in the aqueous solution.16 We hypoth-
esized that a similar folding of the diammonium species may be
occurring, which could give increasingly large pockets of
organic content as the carbon spacers of the diammonium salts
are increased. An increasing size of local organic environments
in aqueous solution might stabilize the THF or MeCN in
aqueous solution via a hydrophobic effect, despite the higher
ionic strength environment in the bulk solution.
To support this hypothesis, salting out experiments were per-
formed using 1,8-diamino-3,6-dioxaoctane dihydrochloride, 9, a
1,8-diaminooctane dihydrochloride analogue with two oxygen
atoms in the backbone (pKaH(1) = 8.74, pKaH(2) = 9.68).17 This
salt provided considerable salting out of THF from aqueous sol-
ution despite having a linker length of eight atoms between the
ammonium groups. The presence of a more hydrophilic back-
bone and an observed salting out effect suggests that the carbon
content of the bolaform electrolyte must be taken into account
when assessing salting out capability. Additional diammonium
dihydrochloride salts shown in Table 2 (entries 10 and 11)
demonstrate that it is hydrophobic organic content far from the
centres of charge (in these cases in the chain between the
ammonium groups) which affects the salting out effect;
additional methyl groups on the N atom seem to make little
difference. Compound 10, despite having eight carbon atoms, is
closer in performance to compound 5 than compound 4.
a 0.30 molal was used rather than the typical 0.50 molal loading due to
low solubility of the salt in water.
a biphenyl analogue (Table 2). This species has pKaH values of
3.85 and 4.9513 resulting in KaH(1)/KaH(2) ratio much greater than
4, showing that electronic communication still exists between the
N atoms. One may then expect this salt to act similarly to the
simple diamines of six carbons or fewer. However, this salt did
not cause salting out of THF at 0.30 molal loading. The same
experiment was performed using the slightly less rigid diamine
1,4-benzenediethanamine dihydrochloride, 8, at 0.30 molal
loading, however, we obtained a similar result to the benzidine
salt in our experiments. These two diamines, despite having elec-
tronic communication between the N atoms, do not salt out THF.
In contrast, a very long chain diamine with ether linkages (com-
pound 9 in Table 2) did indeed salt out THF. We conclude that
the presence or absence of electronic communication throughout
the backbone linking the ammonium groups is not the deciding
factor that determines the salting out abilities of these diammo-
nium species.
Hypothesis #2. In our second hypothesis, we proposed that
intra-molecular communication (through-space) is diminishing
or lost past a separation of six carbons, again giving an ion con-
taining two monocations rather than a single dication. As a
model, we consulted photoelectron spectroscopy literature on
lone-pair interactions of diamines to see if a potential through
space communication effect could be leading to the enhanced
salting out ability of the diamine salts with ≤6 carbon spacers.
However, such a through space effect is not likely leading to this
effect as it has been suggested that radical cation species with
terminal nitrogens will have little chance of interacting intra-
molecularly through space when the number of carbons between
the amine groups are greater than four. This has been attributed
to the great number of possible orientations of the nitrogen
groups in space which are unfavorable towards lone-pair inter-
action.14 As the diminishing salting out effect is not noticeably
prevalent until the seven-carbon spaced species, rather than the
We also observed that shorter chain salts at low ionic strength
can still expel organics from aqueous solution, while longer
chain salts at a supposed greater ionic strength are still unable to
salt out these organics, further suggesting that the organic
content of the salt may play a role in its salting out ability. For
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Table 3 Comparison of various loadings of α,ω-diaminoalkane
dihydrochlorides and the ionic strength of those solutions given two
scenarios of solution behavior. Also included is the observed ability of
each species to salt out THF from 1 : 1 (w/w) solutions of H2O and THF
Ionic strength
assuming single assuming two
Ionic strength
% THF
Scheme 2 Newly developed switchable water additives.
Salt Loading
dication
monocations
separated
4
5
0.50 molal 1.5 molal
0.30 molal 0.9 molal
1.0 molal
0.6 molal
0%
46%
Table 4 Comparison of the ability, at 0.80 molal loading, of
switchable water polyamine additives to separate THF from 1 : 1 w/w
solutions of H2O and THF, the resulting retention of the additive in the
aqueous phase when reacted with CO2 and the average number of amine
groups protonated during the treatment
% THF
% Amine
retaineda
Average N atoms
protonateda
Compound separateda
12
12b
13
14
85 1.2%
84 3.5%
76 0.9%
86 1.3%
99.8 0.1%
94.9 2.2%
94.7 0.3%
99.1 0.6%
3.7
3.7
2.8
3.0
1
a Determined by H NMR spectroscopy. b 1.2 molal loading of additive
was used.
Fig. 5 ●: Amount of THF salted out and ■: amount of acetonitrile
salted out from a 1 : 1 w/w solution of THF or acetonitrile and H2O with
a 1.00 molal loading of alkylammonium chloride salts, [H(CH2)xNH3]
Cl, where x is the number of carbons on the alkyl chain.
A rigorous molecular dynamics study of H2O:THF:diammo-
nium dichloride salt interactions in aqueous solution will be pre-
sented in a future publication to further elucidate the role of
carbon content in the salting out ability of the bolaform electro-
lytes. Ultimately, it would seem that even when studying very
similar salts, a slight difference in carbon content results in a
changed ability of the salt to induce organic substrates to salt out
of aqueous solution. This finding is not only important to the
design of switchable water additives but is likely to find use in
other chemical separation technologies.
example (Table 3), a 0.50 molal solution of the 1,8-diaminooc-
tane dihydrochloride salt, 4, provides no salting out of THF
despite an ionic strength of 1.5 molal (if the salt acts as a Z = 2
dication) or an ionic strength of 1.0 molal if it is assumed that
the two ammonium sites are independent of one another (result-
ing in two singly charged cations). However, a 0.30 molal sol-
ution of 1,6-diaminohexane dihydrochloride, 5, provides a lower
ionic strength of 0.90 molal (if the ammonium sites are con-
sidered to be giving a doubly charged cation) and still salts out
roughly 46% of THF in conditions similar to those presented in
Fig. 3. While these calculations can not be considered accurate
because they assume ideality, this result cannot be purely
explained by the lack of electronic communication between the
ammonium groups, but can be explained by the greater organic
content of salt 4.
The notion of increasing the organic character of a salt, result-
ing in a lower salting out ability is not only limited to the bola-
form electrolytes studied. A similar set of salting out studies
using monoamine hydrochloride salts, [H(CH2)xNH3]Cl, (Fig. 5)
again shows a decrease in the salting out efficiency of the alkyl-
ammonium salts as the carbon chain is lengthened.
While the ability of the organic chain in α,ω-diaminoalkane
salts to solubilize organic molecules such as THF is believed to
be the reason for the poor salting out ability, a further hypothesis
might be that the salts, when folded in half, can act as surfac-
tants. A plot of conductivity as a function of salt concentration
in water was found not to have a sharp break, which would have
been indicative of a critical micelle concentration (see Fig. S2 in
the ESI†). Therefore the solubilizing effect of the long-chain
salts is merely by molecular association and not likely to be by
micelle formation.
d) Triamines and tetramines
With a reasonable set of principles for the design of polyamine
additives, we prepared linear and cyclic polyamines from readily
available precursors. A modified N-methylated spermine ana-
logue, 12, was envisioned (Scheme 2). We initially had some
reservations about the presence of only three carbon positions
between N1 and N2 and between N3 and N4, but all four pKaH
values of spermine (pKaH(0.1 N KCl) = 7.95, 8.82, 10.05,10.86)18
fell close to or within the acceptable working range of 8–10.5
and thus synthesis and testing for function as a switchable water
additive was performed. Compound 12 was synthesized using a
modified method developed by Bieber and co-workers19 in a
49% yield. Compound 12 showed the ability to salt out approxi-
mately 85% of THF from our system, the highest we have
1
observed (Table 4), even though H NMR spectroscopy showed
that incomplete protonation of all the amine sites on compound
12 had occurred, as shown in the table. This suggests that a
minimum of four carbon positions between the amine groups is
necessary to allow for full protonation by carbonic acid. Increas-
ing the amount of compound 12 in solution did not yield a
greater THF salting out (Table 4). This was likely due to the
now greater organic content solution helping to solubilize the
THF.
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Table 5 The ability of several sterically-hindered secondary amines to
separate THF from 1 : 1 w/w solutions of H2O and THF and the
resulting retention of the additive in the aqueous phase when reacted
with CO2
In an attempt to force the utilization all amine functional
groups available on a carbon scaffold, we then prepared com-
pound 13, a triamine analogue of N,N-dimethylbenzylamine
(pKaH = 9.03),20 where the amine groups are separated by 5
carbons. This molecular design should minimize the impact of
neighbouring ionization events while maintaining an enhanced
ionic strength effect and avoiding the presence of excess organic
content in the scaffolding. Compound 13 was synthesized using
a modification of the method of Hirsch and co-workers21 in an
84% yield. Compound 13 showed the capacity to salt out
roughly 76% THF from a 1 : 1 w/w solution of THF and H2O.
1H NMR spectroscopy again showed, however, that incomplete
protonation of compound 13 had occurred when it was reacted
with carbonic acid, which likely contributed to the lower than
expected salting out ability. We then moved to the synthesis of a
saturated analogue of compound 13 to eliminate the conjugation
in the carbon backbone. Novel compound 14 was synthesized in
a 54% yield using a modified preparation from Shirai and co-
workers22 followed by a LiAlH4 reduction from the triamide to
triamine. Compound 14 showed an improved ability to salt out
THF from 1 : 1 w/w solutions of H2O and THF, comparable to
compound 12, as shown in Table 4. We were unable to defini-
tively conclude what resulted in the similar salting out effect
between the tetramine 12 and the triamine 14, but we hypoth-
esize that the less conformationally rigid compound 12 may
shield some charge by folding onto itself when solvated, com-
parable to the conclusions in the modeling of Yang and co-
workers,16 while compound 14 cannot undergo conformational
shifts to such an extent. Nonetheless, both compounds 12 and 14
show a greater salting out ability than any amine additive pre-
viously tested and are comparable in effectiveness to a equimolal
loading of NaCl (88% THF removed). Additionally, compounds
12 and 14 at 0.8 molal are as good at salting out THF as a
2.3 molal solution1 of compound 2 but with 65 mol% less addi-
tive. Higher loadings of compound 14 in THF/water systems
were not possible due to insufficient solubility.
% THF
% Amine
retaineda
Base
Loading
separateda
tBuNHCH2CH2OH 1.60 molal 69 0.4%
tBuNHMe
1.60 molal 68 0.8%
99.8 0.1%
99.8 0.1%
However, the CO2-capture literature has shown that carbamate
formation from secondary amines can be prevented by sterically
bulky groups near or attached to the nitrogen.25 This potentially
allows for the development of a secondary amine SW additive.
This would be attractive because secondary amines generally
have faster kinetics for CO2 uptake than tertiary amines26 and
still may afford the desired reverse reaction at moderate
conditions.
Two sterically-hindered secondary amines, N-tert-butylethanol-
amine (tBuNHCH2CH2OH) and N-tert-butyl-methylamine
(tBuNHMe), were found to reversibly salt out THF from 1 : 1
w/w THF/H2O solutions via the introduction and removal of
CO2 and thus could function as switchable water additives
(Table 5). Although these secondary amines are not as effective
in salting out THF as the tertiary amine additives previously
studied, they offer a proof that further exploration of secondary
amines as switchable water additives is not necessarily a fruitless
endeavor. Additionally, the secondary amine additives have
excellent retention in the aqueous phase, which would decrease
losses of amine to the separated organic phase.
Non-sterically hindered primary and secondary amines such
as monoethanolamine are readily switched to ionic species
(bicarbonate salts and carbamate salts) but are less easily con-
verted back to the neutral species. However, non-bulky second-
ary and primary amines are more hydrophilic than tertiary
amines and bulky secondary amines. This additional hydrophili-
city could be advantageous in switchable water systems because
a) the amines could be soluble at higher loadings and b) because
the ability of the salts of these amines to solvate THF may be
lessened, compared to tertiary amines, by the greater hydrophili-
city. We therefore evaluated the use of primary amines as switch-
able water additives for the salting out of THF. MEA, a
monoamine, does salt out THF but not to a great extent. Among
the primary diamines (H2N(CH2)xNH2), the chain length affects
their ability to salt out THF when exposed to CO2 (Fig. 6). As
was found with the diamine dihydrochloride salts (Fig. 3), the
four carbon-spaced diamine (1,4-diaminobutane) performed the
best. While those tests were performed at 0.8 molal loading, we
found that increasing the amount of 1,4-diaminobutane in the
THF/water mixture to 1.2 molal increases the amount of THF
expelled from the aqueous phase but increasing the loading of
diamine further does not help. This suggests that there is a
balance between additive concentration to generate sufficient
ionic strength and the resulting organic content being built up in
the aqueous solution working against the ionic strength effects.
In a similar salting out test, spermine (the non-methylated ana-
logue of compound 12) also demonstrated an impressive
removal of THF at 0.80 or 1.2 molal loading (Table 6). This was
The time to remove CO2 from the additives (to return the sol-
ution to the original ionic strength) varied between additives
(Fig. S1 in the ESI†). The conversion of the bicarbonate salts to
neutral amines was not 100% even after several hours, but the
conversion of all the additives was sufficient after N2 sparging at
50 °C for 30–60 min to restore the miscibility of the THF/water
mixture.
e) Primary and secondary amines
In our first publication on switchable water, we hypothesized that
only tertiary amine additives would work well as switchable addi-
tives because they react with CO2 in water to exclusively form
bicarbonate salts rather than carbamate salts. Carbamate salt for-
mation is not preferred because it is more difficult to reverse23 and
because it creates fewer ions in solution per mole of amine addi-
tive (compare eqn (4) and (5)). In the case of non-tertiary polya-
mines, there is also the risk that zwitterionic carbamate species
can form24 which would further decrease the ionic strength.
2 NHR2 þ CO2 O ½NH2R2ꢀþ þ ½NR2CðOÞOꢀꢁ
NR3 þ H2O þ CO2 O ½NR3Hꢀþ þ ½HCO3ꢀꢁ
ð4Þ
ð5Þ
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bicarbonate salts can provide solutions of greater ionic strength
at lower loadings in water.
Before the synthesis of a new generation of additives could
take place, we developed a set of principles for future additive
design. These principles evolved from a study of the dependence
of diamine pKaH on the chain length between the functional
groups. We have provided evidence suggesting that the organic
content in the backbone of a bolaform electrolyte plays a large
role in its salting out capabilities, potentially due to enhanced
interactions of the carbon backbone with organic substrates in
aqueous solution. A detailed modeling study of the molecular
interactions of similar aqueous systems will be presented in a
future publication.
We found that polyamines should have a minimum of four
atom carbons between the amine groups (to allow for appropriate
basicity) but otherwise a minimum of hydrophobic organic
content (to allow for maximum salting out capacity). We have
synthesized and tested several new polyamines which showed
improved salting out capabilities compared to our original set of
amine additives. Primary and secondary polyamines are more
effective at salting out than tertiary polyamines, but the reversion
of the primary and secondary polyamines to the neutral form
requires significantly more energy. Further development of
larger, more efficient polyamine additives as well as studies of
new applications of the switchable water solvent systems are cur-
rently being pursued in our lab.
Fig. 6 Percentage of THF salted out from 1 : 1 w/w solutions of THF/
H2O using 0.80 molal loadings of primary diamines with varying
carbon spacer (H2N(CH2)xNH2) when reacted with CO2 for 30 min.
Table 6 The ability of several primary amines to separate THF from
1 : 1 w/w solutions of H2O and THF at various loadings when reacted
with CO2 for 30 min
Base
Loading
% THF separateda
MEA
0.80 molal
0.80 molal
1.20 molal
1.60 molal
0.80 molal
1.20 molal
65 0.8%
79 0.8%
89 0.4%
86 1.2%
91 3.3%
92 1.9%
1,4-Diaminobutane
1,4-Diaminobutane
1,4-Diaminobutane
Spermine
Spermine
a Determined by 1H NMR spectroscopy.
Acknowledgements
The authors thank the National Science Research Council of
Canada, the Walter C. Sumner Foundation, and the Canada
Research Chairs program for funding. PGJ thanks the Killam
Trusts/Canada Council for the Arts for a fellowship. The authors
also acknowledge Dr Ruiyao Wang of Queen’s University for
assistance with crystallographic data and Dr Natalie Cann of
Queen’s University for helpful discussions.
likely due to all the nitrogens in the backbone participating in
salt formation (whether as bicarbonates or carbamates). Sper-
mine, in the presence of CO2, is more effective than NaCl at
equal molality. A future design principle will be to synthesize
additives with greater hydrophilicity so that they may counteract
the effect of increasing organic content in aqueous solution.
While these primary and secondary amines, especially sper-
mine, are impressive in their ability to salt out THF (spermine is
the best so far, at 92% expulsion), they are not easily switched
back to the neutral form. Although MEA, upon treatment with
CO2, does salt out organics, the reverse reaction, removing CO2,
does not occur under our mild conditions of heating to 50 °C
while bubbling an inert gas (N2 or air) through the solution.
Primary amines can be switched back to neutral at higher temp-
eratures, as has already been demonstrated in the literature,27 but
often temperatures over 100 °C are required.
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