Hydrophilic quaternary ammonium type ionic liquids. Systematic study of the relationship among molecular structures, osmotic pressures, and water-solubility

Article abstract of DOI:10.1021/la201070n

This Letter examines the relationship between the structures of ionic liquids and their water-solubility or osmotic pressure with a number of synthesized quaternary ammonium type ionic liquids and organic salts containing a hydroxyl group as hydrophilic substituted groups on ammonium group cations, and bromide or methylsulfonate as anions. The study found a linear relation between the amount and osmotic pressure of the water-soluble ionic liquids synthesized here, strongly indicating that these water-soluble ionic liquids are perfectly ionized in water like inorganic salts with small diameter ions.

Full text of DOI:10.1021/la201070n

LETTER
pubs.acs.org/Langmuir
Hydrophilic Quaternary Ammonium Type Ionic Liquids. Systematic
Study of the Relationship among Molecular Structures, Osmotic
Pressures, and Water-Solubility
Koji Kawai,†,‡ Kotaro Kaneko,‡ and Tetsu Yonezawa*,†
†Division of Material Science and Engineering, Faculty of Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-Ku, Sapporo,
Hokkaido, 060-8628, Japan
‡Miyoshi Oil and Fat Co., Ltd, 4-66-1, Horikiri, Katsushika-ku, Tokyo, 124-8510, Japan
S Supporting Information
b
ABSTRACT: This Letter examines the relationship between the
structures of ionic liquids and their water-solubility or osmotic
pressure with a number of synthesized quaternary ammonium
type ionic liquids and organic salts containing a hydroxyl group
as hydrophilic substituted groups on ammonium group cations,
and bromide or methylsulfonate as anions. The study found a
linear relation between the amount and osmotic pressure of the
water-soluble ionic liquids synthesized here, strongly indicating
that these water-soluble ionic liquids are perfectly ionized in
water like inorganic salts with small diameter ions.
’ INTRODUCTION
necessary to consider their osmotic pressures because cells are
covered by cell membranes which are semipermeable. However,
there have been few reports on the correlation between ionic
liquid and osmotic pressure,11 and especially no report of
systematic studies on the relationship among structures, coun-
terions and osmotic pressure has been located. This study
systematically synthesized a number of hydrophilic ionic liquids
with hydroxyl groups, and the osmotic pressures were measured
to discuss the relationships among molecular structure, water
solubility, and osmotic pressures.
A room temperature ionic liquid (RTIL) is an organic salt
which is in liquid form at room temperature and which has
specific physical properties such as noncombustibility, no vapor
pressure, high heat resistance, and high ionic conductivity. With
such unique physical features, RTILs have attracted interest in
a number of scientific fields including electronics, chemistry,
analysis, as catalysts, and others.1ꢀ10 For example, an RTIL is
used as an organic reaction solvent instead of conventional
organic solvents due to better safety and the potential for
recycling.4 In lithium-ion rechargeable batteries, the usage of
RTIL as the electrolyte is being investigated due to its non-
combustibility, nonvolatility, as well as its high conductivity,
allowing portability and ensuring the safety of the batteries.5
Further, it is considered to offer promising characteristics as an
electrolyte in dye-sensitized solar cells.6 Also, an RTIL is now a
highly important agent contributing to reductions in environ-
mental loads. For the applications mentioned here, one require-
ment is chemical inactivity of the RTILs. Thus, most RTILs do
not have reactive groups in the molecular structures. However,
quite recently, preparation and applications of functionalized
ionic liquids have been reported. For example, a terminal
hydroxyl group of quaternary ammonium type RTILs with a
hydroxyl group and an alkyl group has been used as a Lewis acid
catalyst.7 The application of imidazolium type RTILs with
hydroxyalkyl groups has been anticipated to increase in the field
of biochemistry and medicine. These can be used as the basic
compounds for transdermal drug delivery systems due to their
skin permeability,8 cellulose decomposition,9 as well as protein
refolding agents.10 To apply RTILs to bio-related fields, it is
’ EXPERIMENTAL SECTION
Detailed information of the synthesis is described in the Supporting
Information, and here one such process will be described: 2-Bromoetha-
nol (TCI) and N,N-dimethylethylamine (Kokusan) were mixed in
acetonitrile and kept stirred for 24 h at room temperature. The
generated 2-hydroxyethylethyldimethylammonium bromide was ob-
tained as a white solid. Bromide anion was exchanged by mixing with
methanesulfonic acid (CH3SO3H, Kanto) or tetrafluoroborate acid
(HBF4, Kanto) in water at room temperature and the mixture was kept
stirred for 24 h. The corresponding methanesulfonate salt and tetra-
fluorobrate salt were successfully obtained as products of this process.
1
The products were characterized with the H NMR, 13C NMR, and
FT-IR spectra, and elemental analyses; the water content, melting tem-
perature, and decomposition temperature of each product were mea-
sured with DSC or TG/DTA; and the osmotic pressure was measured
Received: March 22, 2011
Revised:
May 11, 2011
Published: May 23, 2011
r
2011 American Chemical Society
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LETTER
by a freezing-point depression method with an osmotic-pressure meter.
The water-solubility of the products was established visually after mixing
with water by stirring at 25 °C for 30 min and leaving the solution for 10
min. In some cases, micelles or other aggregated structures of ILs were
generated in water. Diameters of such products in aqueous solutions
were obtained with a dynamic light scattering method. Some samples
adsorbed water molecules in air, and the concentration of the aqueous
solutions of the products was calculated by considering the water
amount of the neat products obtained by the TG/DTA measurement.
We here attempted to synthesize simple structured RTILs to system-
atically compare the relationship between the molecular structures and
osmotic pressures. Two routes have been reported to synthesize quatern-
ary ammonium salts with hydroxyethyl groups: (i) the reaction of a
secondary aminoalcohol and an alkylhalide12 and (ii) the reaction of a
tertiary amine and 2-bromoethanol.7b,13 The latter route was employed
here, because 2-bromoethanol reagent is cheaper and is simpler to handle.
’ RESULTS AND DISCUSSION
The definition of ILs requires that they are in liquid form
below 100 °C and that RTILs are in liquid form at room
temperature.14 None of the bromides synthesized in this study
were RTILs when they were dried under reduced pressure. Some
bromides (1, 3, 6, and 8) were white solids at room temperature.
However, they instantly absorbed H2O after exposure to air to
change to transparent liquids like RTILs. The DSC or TG/DTA
results of these bromides show their melting points and decom-
position points all corresponding to the reported values
(90ꢀ300 °C).7b,15,16 The melting points of 1, 2, 5, and 9 are
below 100 °C but higher than room temprature. They are
classified in ILs but not in RTILs.17 The obtained ammonium
salts with unsymmetrical structures, ethyldimethylhydroxyethyl-
and diethylmethylhydroxyethyl-methylsulfonate (4 and 7,
respectively), showed lower melting temperatures than the
symmetrical structured methylsulfonates (2 and 9), due to the
weaker intermolecular interactions,18 and they are categorized in
RTILs. However, the melting points of symmetrical quaternary
ammonium bromides (1 and 8) were lower than those of
unsymmetrical quaternary ammonium bromides (3 and 6). This
difference in the melting points may be explained by the
molecular structure of the cations and the bulkiness of the anions.
Bromide anions exhibit a higher negative charge density than
methylsulfonate anions which had a delocalized electron in the
conjugated π-electron orbital and a bulkier structure. Therefore, the
interaction between cationic ammonium and bromide anions is very
strong and the bromide anion would locate near the cation. With
unsymmetrical quaternary ammonium bromides, the location of the
small bromide anion is fixed, that is, it is at the nearest position. With
symmetrical quaternary ammonium bromides, the location of the
bromide anion is flexible, and the melting points of symmetrical
ammonium bromides become lower than those of unsymmetrical
ammonium bromides. The melting temperatures of the ammonium
methylsulfonates (4, 7, and 9) are lower than the melting tempera-
tures or the decomposition points of the corresponding bromides
(3, 6, and 8) and a tetrafluoroborate (5). This situation can be
explained by the bulkiness of the anions. The 1 (bromide) and 2
(methylsulfonate) products each with a small cation (trimet-
hylammonium) showed similar melting temperatures.
Figure 1. Osmotic pressure of ionic liquids and quaternary ammonium
salts. R1R2R3Nþ(CH2)2OH Xꢀ; (O) R1, R2 = CH3, R3 = C2H5, X =
CH3SO3; (b) R1 = CH3, R2, R3 = C2H5, X = CH3SO3; (4) R1, R2 =
CH3, R3 = C2H5, X = Br; (2) R1 = CH3, R2,R3 = C2H5, X = Br; (0) R1,
R2 = CH3, R3 = C2H5, X = BF4; (ꢁ) Imz BF4.
Figure 2. Sizes of the aggregated structures of compounds 3, 5, 6, and
10 in an aqueous solution (100 mmol of salts/100 g of water) measured
by a dynamic scattering method.
compared with those of the bromides (3 and 6) and tetrafluor-
oborate (5). If these ILs are completely ionized in water, the
osmotic pressure at a concentration of 100 mmol of the salts in
100 g of water would be 2 Osmol/kg. However, the osmotic
pressures of 4 and 7, as shown in Figure 1, are above 2. The
higher values may be attributed to the measurement method as
well as to the molecular structures of 4 and 7. The osmotic
Figure 1 is a plot of the osmotic pressures and the concentra-
tions of the ammonium salts and the ILs synthesized here. The
methylsulfonate ILs (4 and 7) show a linear relation like
inorganic salts with small ions, such as NaCl. The osmotic
pressures of these methylsulfonate ILs (4 and 7) were very high
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LETTER
Table 1. Melting Point and Water-Solubility of Ionic Liquids and Quaternary Ammonium Salts
[R1R2R3N(CH2)2OH]þYꢀ
compd
R1
CH3
R2
R3
Y
mp (°C)
water-solubility (g 100 g waterꢀ1, mmol 100 g waterꢀ1
)
1
2
3
CH3
CH3
CH3
CH3
CH3
C2H5
Br
91
96
400ꢀ500, 2173ꢀ2716
>900, >4516
CH3
CH3
CH3SO3
Br
285 (d.c.p.)
282ꢀ285 (d.c.p.)15a
ꢀ87
400ꢀ500, 2019ꢀ2524
4
5
6
CH3
CH3
CH3
CH3
CH3
C2H5
C2H5
C2H5
C2H5
CH3SO3
BF4
>900, >4219
97
100ꢀ200 488ꢀ976
400ꢀ500, 1886ꢀ2357
Br
275(d.c.p.)
252 (d.c.p.)15b
ꢀ75
7
8
CH3
C2H5
C2H5
C2H5
C2H5
C2H5
C2H5
CH3SO3
Br
>900
>3959
C2H5
124
1207b
200ꢀ300, 884ꢀ1327
9
C2H5
CH3SO3
70
>900, >3729
10
ImzBF4
ꢀ89
ꢀ7116
500ꢀ600, 2212ꢀ2655
pressures were measured by the freezing-point depression meth-
od, and when a sample has high viscosity, its osmotic pressure
tends to be slightly higher than the expected 2.19 The viscosity of
quaternary ammonium type ILs is high in appearance. As both
the anions and cations of methylsulfonate ILs (4 and 7) form
hydrogen bonds with water molecules, the interaction between
the ILs and water molecules is relatively strong. Since methyl-
sulfonate anion is more basic than bromide anion (HBr, pKa = ꢀ9;
CH3SO3H, pKa = ꢀ0.6),20 methylsulfonate can hydrate with
water readily compared to bromide. The characteristic molecular
expected that a compound with a higher ionization ratio shows a
higher solubility, and the methylsulfonates are fully ionized in water.
’ CONCLUSION
This study synthesized a number of aliphatic hydrophilic
ammonium salts and ILs and observes their osmotic pressures
and water solubility. The osmotic pressures and degree of
electrolytic dissociation of the ionic liquids can be simply
controlled and adjusted by molecular design. Among the ammo-
nium salts synthesized here, the salts with an asymmetric
hydrophilic cation and methylsulfonate completely dissociated in
water and showed the highest osmotic pressure. This suggests
that molecular design of ILs can control their permeability
through cell membranes. These ILs contain hydrophilic oxya-
nions and cations which are similar to choline with hydroxyl
groups. With dissolution of cellulose with ILs, hydrogen bonds
between the ILs and cellulose are important.22 Because anions
and cations on aliphatic hydrophilic ammonium can form
hydrogen bonds, a contribution to carbon neutrality with bio-
mass would be a possibility. The findings in this study will be
useful in a range of fields including the field of nanotechnology.
ꢀ
structure of the methylsulfonate ILs, a hydrophilic CH3SO3
anion and a cation with a hydroxyl terminal group, must be
considered a factor in their high degree of ionization.
The obtained osmotic pressures of the bromides (3 and 6) and
tetrafluoroborates (5), and 10 (ImzBF4) are much smaller than
those of the methylsulfonates, and they do not show linear
changes against the interaction. The slope of the curve decreased
at 20ꢀ60 mmol of the salts in 100 g of solution. Generally, breaks
in the curves of the osmotic pressure and the concentration occur
at critical micelle concentrations (cmc), indicated by the arrows in
Figure 1, because the osmotic pressure has a colligative property
to the molar concentration of surfactant.21 In order to observe the
formation of aggregated structures of the bromides (3 and 6) and
tetrafluoroborate (5) in aqueous solutions (100 mmol of salts/
100 g of water), dynamic light scatteringmeasurementwas carried
out (Figure 2). The particle diameters of the aggregated mol-
ecules were large, because the molecular sizes of the compounds
are very small. Such aggregate structure formation of the bromide
salts and the tetrafluoroborate salt was also confirmed by the fact
that they did not completely dissolve in water at a concentration
of ions higher than 20ꢀ60 mmol of salts/100 g of water.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details of synth-
b
eses and characterization of the compounds, water-solubility, and
osmotic pressures. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Compared with a general imidazolium type hydrophobic ionic
liquid (10, ImzBF4), the synthesized highly hydrophilic alkylam-
monium ILs (3ꢀ7) showed considerably higher osmotic pres-
sures. The ILs with similar cationic structure, methylsulfonates
(4 and 7), showed higher osmostic pressures than the bromides
(3 and 6) and the tetrafluoroborate (5). This tendency corre-
sponds well to that of the water-solubility (methylsulfonates >
bromides and tetrafluoroborate), as shown in Table 1. It is to be
Corresponding Author
* Fax: þ81-11-706-7881. E-mail: tetsu@eng.hokudai.ac.jp.
’ ACKNOWLEDGMENT
Helpful discussions with Dr. A. Hyono, Dr. S. Abe (Hokkaido
Univ.), K. Kamio, K. Sugiyama, and H. Kawakami (Miyoshi Oil
and Fat Co., Ltd) are gratefully acknowledged.
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LETTER
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