Physicochemical and solution properties of quaternary-ammonium-salt-type amphiphilic trimeric ionic liquids

Article abstract of DOI:10.1039/c9cp05541j

The quaternary-ammonium-salt-type amphiphilic compounds 3C_ntris-s-Q X (star-type; carbon number between the central amino and ammonium groups s = 2, 3) and 3C_nlin-3-Q X (linear-type; carbon number between the hydrophilic groups s = 3), where n represents the alkyl chain length (n = 8, 10, 12, 14) and X represents a counterion [hexafluorophosphate, trifluoromethanesulfonate (OTf), bis(fluorosulfonyl)amide (FSA), and bis(trifluoromethanesulfonyl)amide (NTf₂)], were synthesized. Except for 3C₁₂tris-3-Q OTf, these trimeric compounds presented melting points lower than 100 °C and therefore are defined as ionic liquids. Among them, 3C_ntris-3-Q NTf₂ (n = 8, 12, 14) and 3C_nlin-3-Q NTf₂ (n = 8, 10) presented melting points lower than 0 °C. The melting points of the amphiphilic trimeric ionic liquids (n = 8, 10), which were lower for the star- than for the linear-type compounds, were higher than those of the corresponding monomeric compounds but lower than those of the corresponding gemini samples. Moreover, the amphiphilic trimeric ionic liquids exhibited higher conductivities and lower viscosities than the corresponding gemini ionic liquids, while the star-type trimeric ionic liquids presented lower conductivities and higher viscosities than those of the linear-type compounds. The amphiphilic trimeric ionic liquids also readily adsorbed at the air/water interface and oriented themselves to form micelles in aqueous solution. This aggregation behavior was not observed in the monomeric and gemini ionic liquids.

Full text of DOI:10.1039/c9cp05541j

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Physicochemical and solution properties of
quaternary-ammonium-salt-type amphiphilic
trimeric ionic liquids†
Cite this: Phys.Chem.Chem.Phys.,
2019, 21, 25065
Risa Kawai, Shiho Yada and Tomokazu Yoshimura
*
The quaternary-ammonium-salt-type amphiphilic compounds 3Cntris-s-Q X (star-type; carbon number
between the central amino and ammonium groups s = 2, 3) and 3Cnlin-3-Q X (linear-type; carbon
number between the hydrophilic groups s = 3), where n represents the alkyl chain length (n = 8, 10, 12, 14)
and X represents a counterion [hexafluorophosphate, trifluoromethanesulfonate (OTf), bis(fluorosulfonyl)amide
(FSA), and bis(trifluoromethanesulfonyl)amide (NTf2)], were synthesized. Except for 3C12tris-3-Q OTf, these
trimeric compounds presented melting points lower than 100 1C and therefore are defined as ionic liquids.
Among them, 3Cntris-3-Q NTf2 (n = 8, 12, 14) and 3Cnlin-3-Q NTf2 (n = 8, 10) presented melting points
lower than 0 1C. The melting points of the amphiphilic trimeric ionic liquids (n = 8, 10), which were lower for
the star- than for the linear-type compounds, were higher than those of the corresponding monomeric
compounds but lower than those of the corresponding gemini samples. Moreover, the amphiphilic trimeric
ionic liquids exhibited higher conductivities and lower viscosities than the corresponding gemini ionic liquids,
while the star-type trimeric ionic liquids presented lower conductivities and higher viscosities than those of
the linear-type compounds. The amphiphilic trimeric ionic liquids also readily adsorbed at the air/water
interface and oriented themselves to form micelles in aqueous solution. This aggregation behavior was not
observed in the monomeric and gemini ionic liquids.
Received 10th October 2019,
Accepted 23rd October 2019
DOI: 10.1039/c9cp05541j
rsc.li/pccp
concentrations (CMCs) and a greater ability to lower the surface
tension than those of the corresponding monomeric surfactants.7–11
1. Introduction
Ionic liquids, which only comprise cations and anions, display We have previously developed star- and linear-type trimeric
several properties such as low vapor pressure, high thermal surfactants to further improve the performance and express the
stability, high conductivity, unique solubility, and wide electro- functionality of gemini surfactants.12–14 The afforded products
chemical windows. Thus, they are attracting significant interest presented excellent surface-active properties, which exceeded
as novel alternatives to water and organic solvents.1–4 Bulky those of the corresponding monomeric and gemini surfactants.
ions and delocalized ions as well as ionic structure dissymmetry Further, the results revealed that the melting point of the
lower the melting point of these liquids. Moreover, ionic liquids quaternary-ammonium-salt-type gemini ionic liquids could be
can be prepared from different cation and anion combinations. lowered to B40 1C by introducing bulky counterions and changing
Hence, their physicochemical properties, such as the melting the dissymmetry of the alkyl chains. These surfactants also exhibited
point, density, viscosity, polarity, and hydrophobicity, can be excellent adsorption properties at the air/water interface.15 We
easily controlled. The application potential of these liquids as therefore predicted that the melting point could be lowered by
reaction solvents,1,2 electrolytes,2 catalysts,1,3 and lubricants4 as the introduction of a bulky counterion into the star- and linear-
well as in drug delivery systems5 and solubilization processes6 is type trimeric surfactants, thereby leading to the formation of
therefore being explored in a wide range of fields.
ionic liquids. Thus, novel amphiphilic ionic liquids that exhibit
Gemini surfactants, in which two conventional monomeric both ionic liquid and surfactant properties could be developed.
surfactants are connected by a spacer, have attracted significant Notably, few reports on imidazolium-, phosphonium-, and
interest because of their excellent surface-active properties. pyridinium-type trimeric ionic liquids16–18 and no reports on
Particularly, these surfactants display lower critical micelle quaternary-ammonium-salt-type amphiphilic trimeric ionic
liquids are available in the literature.
With this in mind, in this study, we synthesized the quaternary-
Department of Chemistry, Faculty of Science, Nara Women’s University,
ammonium-salt-containing star- and linear-type amphiphilic
Kitauoyanishi-machi, Nara 630-8506, Japan. E-mail: yoshimura@cc.nara-wu.ac.jp
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9cp05541j trimeric compounds 3Cntris-s-Q X and 3Cnlin-3-Q X, respectively,
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kindly supplied by Nippon Shokubai Co., Ltd (Osaka, Japan).
Deuterated chloroform NMR solvent was purchased from
Cambridge Isotope Laboratories Inc. (Andover, USA). All chemicals
were used as received without further purification. Water from a
Merck KGaA Direct-Q UV system (resistivity = 18.2 MO cm;
Darmstadt, Germany) was used in all the experiments.
2.2. Synthesis of tris(N-alkyl-N,N-dimethyl-2-ammonioethyl)-
amine tribromide (3Cntris-2-Q Br) and triiodide (3Cntris-2-Q I)13
Tris(2-aminoethyl)amine (10.0 g, 1.0 equiv.) in methanol was
added to a stirred mixture of 37% formaldehyde (1.2 equiv.)
and 98% formic acid (1.1 equiv.), and the resulting mixture was
refluxed for 12 h. Next, 1 mol dmꢀ3 HCl was added to the
solution, which was then stirred with heating for 3 h. Subsequently,
the solvent was evaporated and the residue was washed twice with
methanol. Finally, the product was dried under vacuum to
recover tris(N,N-dimethyl-2-aminoethyl)amine hydrochloride as
a white solid.
The afforded product was added to 200 mL of methanol
containing 8 g of sodium hydroxide and subsequently stirred
with heating for 2–3 h. After the solvent was removed by
evaporation, acetone was added to the residue and the solution
was filtered to remove the inorganic salts. The solvent was then
evaporated under reduced pressure to recover tris(N,N-dimethyl-
2-aminoethyl)amine as a brown viscous liquid.
Fig. 1 Chemical structures of the quaternary-ammonium-salt-type trimeric
amphiphilic compounds (a) 3Cntris-s-Q X (star-type) and (b) 3Cnlin-3-Q NTf2
(linear) and (c) of the counterions X.
[alkyl chain length n = 8, 10, 12, 14; s represents the carbon
number between the central amino group and ammonium
group for the star-type samples (s = 2, 3) and the carbon number
between the hydrophilic groups for linear-type samples (s = 3)]
with the bulky counterions (X) hexafluorophosphate (PF6), tri-
fluoromethanesulfonate (OTf), bis(fluorosulfonyl)amide (FSA),
and bis(trifluoromethanesulfonyl)amide (NTf2) (X = NTf2 for
3Cntris-2-Q X and 3Cnlin-3-Q X and PF6, OTf, FSA, NTf2 for
3Cntris-3-Q X; Fig. 1). Subsequently, their ionic liquid and
surface-active properties in aqueous solutions were investigated
and compared to those of the corresponding monomeric and
gemini compounds. Finally, we discussed the effects of the alkyl
and spacer chain lengths, molecular skeleton, number of chains,
and ionic liquid counterion structure on these properties.
n-Octyl iodide, n-decyl iodide, n-dodecyl iodide, or n-tetradecyl
bromide (6.0 equiv.) was next added slowly to stirred solutions of
tris(N,N-dimethyl-2-aminoethyl)amine in ethanol. The mixtures
were refluxed for at least 40 h. After the solvent in each sample
was evaporated under reduced pressure, the residues were washed
repeatedly, first with ethyl acetate and then with hexane. Sub-
sequently, the products were recrystallized from ethyl acetate/
ethanol mixtures (2 : 1, vol/vol, for n = 8, 10, 12 and 1 : 1, vol/vol,
for n = 14) to obtain 3C14tris-2-Q Br and 3Cntris-2-Q I (n = 8, 10,
1
and 12) as white solids (Scheme S1; yields, H NMR data, and
elemental analysis data in the ESI†).
2.3. Synthesis of tris(N-alkyl-N,N-dimethyl-3-ammoniopropyl)-
amine tribromide (3Cntris-3-Q Br)
n-Octyl bromide, n-decyl bromide, n-dodecyl bromide, or n-tetra-
decyl bromide (6.0 equiv.) was added slowly to stirred solutions of
tris(N,N-dimethyl-3-aminopropyl)amine (1.0 equiv.) in aceto-
2. Experimental
2.1. Materials
Tris(2-aminoethyl)amine hydrochloride, N,N0,N0,N00,N00-pentamethyl- nitrile. The mixtures were refluxed for at least 40 h. Subsequently,
dipropylenetriamine, n-octyl bromide, n-octyl iodide, n-decyl the solvent was evaporated under reduced pressure and the
bromide, n-decyl iodide, n-dodecyl bromide, n-decyl iodide, collected residues were washed repeatedly, first with ethyl acetate
and n-tetradecyl bromide were purchased from Tokyo Chemical and then with acetone. The products were recrystallized from
Ind. Co., Ltd (Tokyo, Japan). Potassium hexafluorophosphate ethyl acetate/ethanol mixtures (4 :1, vol/vol) to form 3Cntris-3-Q
(KPF6), potassium trifluoromethanesulfonate (KOTf), acetone, Br (n = 8, 10, 12, 14) as a white solid (Scheme S1; yields, 1H NMR
acetonitrile, chloroform, dimethyl sulfoxide (DMSO), ethanol, data, and elemental analysis data in the ESI†).
ethyl acetate, formaldehyde solution (37%), formic acid solution
(98%), hexane, hydrochloric acid (1 mol dmꢀ3), methanol, and
sodium hydroxide were purchased from FUJIFILM Wako Pure
2.4. Synthesis of methylalkylbis[3-(dimethylalkylammonio)-
propyl]ammonium tribromide (3Cnlin-3-Q Br)
Chemical Co., Ltd (Osaka, Japan). Tris(N,N-dimethyl-3-amino- n-Octyl bromide, n-decyl bromide, n-dodecyl bromide, or n-tetra-
propyl)amine was kindly supplied by Tosoh Co., Ltd (Tokyo, decyl bromide (6.0 equiv.) was added slowly to stirred solutions
Japan). KNTf2 was purchased from Kanto Chemicals Co., Inc. of N,N,N0,N00,N00-pentamethyldipropylenetriamine (1.0 equiv.)
(Tokyo, Japan). Potassium bis(fluorosulfonyl)amide (KFSA) was in acetonitrile. The mixtures were refluxed for 45 h. After the
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solvent in each sample was evaporated under reduced pressure, scanning calorimetry (DSC; Shimadzu DSC-50 system, Kyoto,
the residues were washed repeatedly, first with hexane and then Japan). For the measurements, 2 mg of the compound was
with ethyl acetate. The products were recrystallized from a placed in a hermetically sealed aluminum pan; an empty
mixture of ethyl acetate and methanol (6 : 1, vol/vol) to form aluminum pan was used as the reference. The measurements
3Cnlin-3-Q Br (n = 8, 10, 12, 14) as a yellow-white solid (Scheme S1; were performed at a heating rate of 0.2 1C minꢀ1 under a
yields, 1H NMR data, and elemental analysis data in the ESI†).
nitrogen atmosphere and the acquired data were corrected and
analyzed using Shimadzu TA-60WS (Kyoto, Japan) software.
The water contents of the amphiphilic ionic liquids were
determined using a coulometric titration system (Hiranuma
2.5. Anion-exchange of the halide ions with the counter
anions PF6, OTf, FSA, and NTf2
Exactly 1.0 equiv. 3C12tris-3-Q Br aqueous solution was added to AQV-200, Karl Fischer, Tokyo, Japan). The viscosities were
3.6 equiv. KPF6, KOTf, and KFSA aqueous solutions, while measured at 50 and 60 1C using a Brookfield DV-2T system
aqueous solutions (1.0 equiv.) of 3Cntris-2-Q I (n = 8, 10, 12), (Middleborough, MA, USA). The solubilities were determined
3C14tris-2-Q Br, 3Cntris-3-Q Br (n = 8, 10, 12, 14), and 3Cnlin-3-Q by mixing 0.01–50 mL of water or organic solvent with
Br were added to KNTf2 aqueous solution. The mixtures were 0.01–0.1 g of ionic liquid or compound.
stirred with heating for 10 h. The purification procedures for
the derivative with each counterion are as follows.
The electrical conductivities of the neat amphiphilic ionic
liquids and their aqueous solutions were measured using a
2.5.1 3C12tris-3-Q PF6. After the solution was filtered, the TOA CM-30R system (Tokyo, Japan). The Krafft temperature
residue was washed fivefold with water and then dried. Next, (TK) and CMC values of the ionic liquids in aqueous solution
acetone was added to the dried residue and the resultant were then determined from the afforded data. To determine the
mixture was filtered to remove the inorganic salt. The solvent Krafft temperature of the amphiphilic trimeric ionic liquids,
in the filtrate was then removed by evaporation. Next, chloroform their clear aqueous solutions (0.0010 wt%) were prepared by
was added to the residue and the resultant mixture was filtered to dissolving them in hot water and incubating the obtained
remove the inorganic salt. This was followed by evaporation to solution in a refrigerator at B5 1C for at least 24 h. The surface
remove the solvent in the filtrate.
tensions of the neat ionic liquids and their aqueous solutions
2.5.2 3C12tris-3-Q OTf. After the solvent in the solution was were measured using a Teclis Tracker tensiometer (Lyon,
evaporated under reduced pressure, acetone was added to the France) using the pendant drop technique. The surface excess
residue to remove the inorganic salt and the filtrate was concentration (G/mol mꢀ2) and occupied area per molecule (A)
evaporated. Chloroform was then added to the residue and of the amphiphilic ionic liquids at the air/water interface were
the mixture was filtered to remove the inorganic salt. This was calculated using the Gibbs adsorption isotherm equations:
followed by evaporation to remove the solvent in the filtrate.
G = ꢀ(1/iRT)(dg/dln C) and A = 1/(NG), where g is the surface
2.5.3 3C12tris-3-Q FSA, 3Cntris-2-Q NTf2, 3Cntris-3-Q NTf2, tension, C is the ionic liquid concentration, R is the gas
and 3Cnlin-3-Q NTf2. After the upper phase was removed by constant (8.31 J Kꢀ1 molꢀ1), T is the absolute temperature,
decantation, the residues were washed fivefold with water and and N is Avogadro’s constant. The value of i, which is the
subsequently dried. Acetone was then added to each residue number of ionic species assumed to be completely dissociated
and the resultant mixtures were filtered to remove the inorganic salt. in aqueous solution, was taken to be 4 for the amphiphilic
Next, the solvent in each filtrate was removed by evaporation and trimeric ionic liquids investigated in this study. The fluores-
chloroform was added to each residue. Each resultant mixture was cence of pyrene in the amphiphilic ionic liquid solutions was
filtered to remove the inorganic salt and the solvent in each filtrate measured using a JASCO FP-6300 system (Tokyo, Japan). The
was removed by evaporation. Further, ethyl acetate was added to the concentration of pyrene in the ionic liquid solutions was 1 ꢁ
residues produced from 3Cntris-2-Q I (n = 8, 10, 12) and 3C14tris-2-Q 10ꢀ6 mol dmꢀ3. The measurements of the solution properties
Br and subsequently filtered to remove the inorganic salt. The were performed at 25.0 ꢂ 0.5 1C.
solvent in each filtrate was removed by evaporation.
All the residues were then dried under reduced pressure to
afford tris(N-alkyl-N,N-dimethyl-2-ammonioethyl)amine, tris(N-
alkyl-N,N-dimethyl-3-ammoniopropyl)amine, and methylalkylbis[3-
(dimethylalkylammonio)propyl]ammonium [PF6, OTf, FSA, and
3. Results and discussion
3.1. Melting points of the amphiphilic derivatives
NTf2] as a white solid (3C12tris-3-Q PF6), a yellow-white solid Fig. 2 illustrates the relationship between the melting point and
(3C12tris-3-Q OTf), a yellow viscous material (3C12tris-3-Q FSA), alkyl chain length n for the stꢀar- and linear-type amphiphilic
clear and orange viscous materials (3Cntris-2-Q NTf2), yellow trimeric compounds with NTf2 counterions (3Cntris-2-Q NTf2,
viscous materials and solids (3Cntris-3-Q NTf2), and clear and 3Cntris-3-Q NTf2, and 3Cnlin-3-Q NTf2). These trimeric com-
yellow viscous liquids (3Cnlin-3-Q NTf2) (Scheme S1, yields, pounds displayed melting points of less than 100 1C and could
1H NMR data, and elemental analysis data in the ESI†).
therefore be defined as ionic liquids. The melting points of the
trimeric ionic liquids differed according to their molecular
skeleton. Thus, for the star-type 3Cntris-2-Q NTf2 and 3Cntris-
2.6. Characterization
The melting points of the quaternary-ammonium-salt-type 3-Q NTf2 (except for 3C8tris-2-Q NTf2), the melting point slightly
amphiphilic trimeric compounds were determined by differential decreased with increasing alkyl chain length. On the other hand,
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3.2. Characterization of the amphiphilic trimeric ionic liquids
The melting points of the amphiphilic star-type trimeric com-
ꢀ
pounds 3C12tris-3-Q X (X = PF6 and FSAꢀ) were also less than
0 1C and thus they were also defined as ionic liquids. Further,
because the water contents of these amphiphilic trimeric ionic
liquids were less than 400 ppm, the properties of these ionic
liquids could be easily investigated.
Table 1 lists the conductivity (k), zero-shear viscosity (Z0) and
density (d) of theꢀamphiphilic trimeric ionic liquids (n = 8, 10, 12)
comprising NTf2 counterions along with those of the corres-
ponding monomeric ionic liquids (CnNTf2, n = 8, 10, 12). These
trimeric ionic liquid parameters were measured at 50 1C
because of their significantly high viscosity. Moreover, because
the melting point of 3C10tris-2-Q NTf2 was greater than 50 1C, its
parameters were measured at 60 1C. The conductivities of the
star-type 3Cntris-s-Q NTf2 were higher than those of the linear-
type 3Cnlin-3-Q NTf2, thereby demonstrating the difference in
their molecular skeletons. This occurred because the star-type
ionic liquids comprising tris(2-aminoethyl)amine skeletons
have a flexible structure comprising three alkyl chains that
extend radially from the central amino group. Thus, the dis-
tances between the ammonium cations are longer than those of
the linear-type ionic liquids with diethylenetriamine skeletons.
The viscosities were higher for the linear- than the star-type
ionic liquids. This was attributed to the strong interaction and
tight entanglement between the alkyl chains of the former
compounds because of the short distance between them. The
increase in spacer chain length from 2 to 3 led to an increase in
the conductivity and a decrease in the viscosity. As discussed in
Section 3.1, this occurred because of the decrease in the van der
Waals forces and flexibility of the molecular structure with
increasing spacer chain length. The star- and linear-type tri-
meric ionic liquids presented lower conductivities (1/123- to
1/26-fold) and higher viscosities (10- to 100-fold) compared to
those of the corresponding monomeric ionic liquids. They also
displayed lower conductivities and higher viscosities than those
of the gemini ionic liquids C8-2-C6 NTf2 (21.7 mS mꢀ1 at 65 1C,
Fig. 2 Relationship between the melting points and alkyl chain lengths n
for the quaternary-ammonium-salt-type trimeric ionic liquids: 3Cntris-2-Q
NTf2 (K), 3Cntris-3-Q NTf2 (’), and 3Cnlin-3-Q NTf2 (m).
the melting points of the linear-type 3Cnlin-3-Q NTf2 were less
than 0 1C for n = 8, 10 and significantly increased as the alkyl
chain length was increased from 8 to 14. This suggested that
the linear-type trimeric ionic liquids were more affected by van
der Waals forces than the star-type ionic liquids, especially
when the alkyl chain length was increased. The melting point of
the star-type 3Cntris-s-Q NTf2 significantly decreased to less
than 0 1C for all the alkyl chain lengths when the spacer chain
length was increased from 2 to 3. This was more marked for the
melting temperatures in the range of 40–50 1C for n = 10, 12, 14.
The decrease in the melting temperature was attributed to the
increased flexibility of the cation and the decrease in the van
der Waals forces with increasing distance between the alkyl
chains as the spacer chain was enlarged. The melting points of
the linear-type 3Cnlin-3-Q NTf2 with n = 8, 10 were lower than
those of the corresponding star-type 3Cntris-3-Q NTf2, while
those of 3Cnlin-3-Q NTf2 with n = 12, 14 were higher. Further,
based on the counterion, the melting point of the star-type
3C12tris-3-Q X was lowered in the order: OTfꢀ 4 PF6ꢀ 4 FSAꢀ 4
NTf2ꢀ. Notably, the melting points of the ionic liquids comprising
Table 1 Melting point (Tm), conductivity (k), viscosity (Z0), and density (d)
values of the amphiphilic trimeric ionic liquids at 50 1C
ꢀ
FSAꢀ and NTf2 ions were lower than room temperature
Ionic liquid
Tm/1C
k/mS mꢀ1
Z0/Pa s
d/g cmꢀ3
(Table S1, ESI†).
The melting points of the star-type amphiphilic trimeric
ionic liquids (n = 8, 10) were higher than those of the corres-
ponding monomeric ionic liquids (C8 NTf2 and C10 NTf2; lower
than ꢀ20 1C)15 but lower than those of the amphiphilic gemini
ionic liquids (C8-2-C8 NTf2, 76.2 1C and C10-2-C10 NTf2,
85.8 1C).15 Based on the molecular skeleton, the melting
points were in the order: gemini 4 trimeric 4 monomeric.
This trend was attributed to the structures of the three types of
compounds: the trimeric ionic liquids have stronger van der
Waals forces than the monomeric ionic liquids, owing to
the presence of the three alkyl chains connected by spacers.
However, these spacers are more flexible than those of the
corresponding gemini ionic liquids.
3C8tris-2-Q NTf2
3C10tris-2-Q NTf2
3C12tris-2-Q NTf2
4.4
50.1
48.6
1.91
1.18a
1.01
8.93
13.9a
11.8
1.33
1.28a
1.24
3C8tris-3-Q NTf2
3C10tris-3-Q NTf2
3C12tris-3-Q NTf2
ꢀ0.7
0.6
ꢀ3.8
1.75
1.53
1.30
5.17
5.49
6.73
1.33
1.29
1.24
3C8lin-3-Q NTf2
3C10lin-3-Q NTf2
3C12lin-3-Q NTf2
ꢀ19.9
ꢀ5.2
46.6
0.624
0.560
0.493
15.7
16.5
17.7
1.33
1.28
1.24
C8NTf2
C10NTf2
C12NTf2
oꢀ2015
oꢀ2015
29.2
86.8
73.4
60.8
0.0466
0.0746
0.0943
1.31b
1.22b
1.22
a
b
60 1C. 25 1C.
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2.10 Pa s at 50 1C).15 This was attributed to their larger number
of alkyl chains and thus, the stronger van der Waals forces
between them. The densities of the trimeric ionic liquids were
in the range of 1.24–1.34 g cmꢀ3. Generally, ionic liquids
present high densities (41.0 g cmꢀ3) because they solely consist
of ions.19,20 Notably, the densities of both the amphiphilic
trimeric and monomeric ionic liquids were higher than those
of conventional ionic liquids and decreased with increasing
alkyl chain length. On the other hand, no difference in the
densities of the star- and linear-type ionic liquids was observed.
3.3. Solubility in various organic solvents
Table 2 lists the solubilities of the amphiphilic trimeric ionic
liquids [3C12tris-2-Q NTf2, 3C12tris-3-Q X (X = PF6, FSA, NTf2),
and 3C12lin-3-Q NTf2] and compounds (3C12tris-3-Q OTf) in
water and the organic solvents DMSO, methanol, acetone, ethyl
acetate, chloroform, benzene, and hexane. The amphiphilic
trimeric ionic liquids displayed very low solubility in water
but were highly soluble (445 wt%) in organic solvents such as
DMSO, methanol, and acetone. The soluꢀbilities of the trimeric
ionic liquids comprising FSAꢀ and NTf2 counterions in ethyl
acetate were significantly higher than those of the ionic liquids
comprising PF6ꢀ and OTfꢀ ions. The gemini ionic liquids were
insoluble in chloroform and benzene.15 On the other hand, the
trimeric ionic liquids were soluble in these two solvents, thereby
indicating that these compounds display excellent solubilities in
a wide range of organic solvents of different polarities (DMSO,
methanol, acetone, ethyl acetate, chloroform, and benzene); this
was attributed to the increased number of alkyl chains. Particularly,
compared to that of the linear-type 3C12lin-3-Q NTf2, the solubility of
the star-type 3C12tris-2-Q NTf2 was higher in chloroform and slightly
higher in benzene. Notably, all the samples were insoluble in
hexane. These results revealed that the solubilities of the
amphiphilic trimeric ionic liquids and compounds in various
organic solvents were affected by the molecular skeleton, counter-
ion, and number of alkyl chains.
Fig. 3 Variations in the (a) conductivity, (b) surface tension, and (c) pyrene
fluorescence intensity ratio (I1/I3) with different concentrations of the
trimeric ionic liquids 3C12tris-2-Q NTf2 (K), 3C12tris-3-Q NTf2 (’), and
3C12lin-3-Q NTf2 (m) at 25 1C.
of the ionic liquids were estimated to be less than 5 1C. Therefore,
their solution properties were evaluated at 25 1C.
Fig. 3 presents the plots of the conductivity, surface tension,
and pyrene fluorescence intensity ratio (I1/I3) as functions of
the concentration for the amphiphilic trimeric ionic liquids
3C12tris-2-Q NTf2, 3C12tris-3-Q NTf2, and 3C12lin-3-Q NTf2. The
data revealed that an increase in the concentration increased
the conductivity and decreased the surface tension of the
trimeric ionic liquids, while both plots displayed a clear break
point, which was considered as the CMC. Further, the pyrene
intensity ratio decreased from 1.8 to B1.5 with increasing
concentration. These results indicated that amphiphilic trimeric
ionic liquids adsorbed at the air/water interface and formed
micelles in aqueous solutions.
3.4. Solution properties of the amphiphilic trimeric ionic liquids
Table 3 lists the values of the CMC, degree of micelle
ionization a, surface tension at the CMC (gCMC), surface excess
concentration (G), occupied area per molecule (A), and standard
The solution properties of the amphiphilic trimeric ionic
liquids 3C12tris-2-Q NTf2, 3C12tris-3-Q NTf2, and 3C12lin-3-Q
NTf2, including their Krafft temperature, conductivity, surface
tension, and pyrene fluorescence, were next investigated. Because
the solutions were clear with no visible precipitates, the TK values
À
Á
À
Á
free energies of adsorption DGꢃads and micellization DGmꢃ ic of
the amphiphilic trimeric ionic liquids along with those of the
Table 2 Solubilities (wt%) of the amphiphilic trimeric ionic liquids and compounds in water and various organic solvents at 25 1C
Compound
Water
0.003
DMSO
Methanol
Acetone
Ethyl acetate
Chloroform
Benzene
11
Hexane
a
3C12tris-2-Q NTf2
446
454
456
453
440
—
a
3C12tris-3-Q PF6
3C12tris-3-Q OTf
3C12tris-3-Q FSA
3C12tris-3-Q NTf2
0.006
0.007
0.006
0.006
446
446
446
445
456
456
456
458
456
456
455
450
0.03
5
452
450
1.4
38
20
19
0.1
21
8.6
10
—
a
—
a
—
a
—
a
3C12lin-3-Q NTf2
0.007
450
446
453
450
25
7
—
a
Insoluble.
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Table 3 Values of the Krafft temperature (TK) at 0.0010 wt%, critical micelle concentration (CMC), surface tension at the CMC (gCMC), surface excess
ꢂ
ꢃ
ꢂ
ꢃ
concentration (G), occupied area per molecule (A), and standard free energies of adsorption DGꢃads and micellization DGꢃmic for the amphiphilic
trimeric ionic liquids 3C12tris-s-Q NTf2 and 3C12lin-3-Q NTf2 obtained from the surface tension plots at 25 1C
Ionic liquid
TK/1C
CMC/mmol dmꢀ3
A
g
CMC/mN mꢀ1
G ꢁ 106/mol mꢀ2
A/nm2
DGꢃads/kJ molꢀ1
DGꢃmic/kJ molꢀ1
3C12tris-2-Q NTf2
3C12tris-3-Q NTf2
3C12lin-3-Q NTf2
o5
o5
o5
0.00602
0.0103
0.00608
0.012
0.21
0.18
41.4
48.3
42.1
1.18
0.990
0.428
1.41
1.68
3.88
ꢀ79.4
ꢀ67.9
ꢀ116
ꢀ53.4
ꢀ44.0
ꢀ46.7
3C12tris-2-Q Br
3C12lin-3-Q Br
o5a
0.13913
0.1423
0.22413
0.1923
32.313
0.82013
1.7523
2.0313
1.4723
ꢀ85.013
ꢀ36.313
o5a
ꢀ37.423
a
0.20 wt%.
trimeric surfactant comprising the counterion Brꢀ. The DGaꢃds
and DGꢃmic values of the amphiphilic trimeric ionic liquids were
calculated from the equations:21,22
and surface tension because of their poor water solubility. On the
other hand, the amphiphilic trimeric ionic liquids with three
dodecyl chains and bulky NTf2 counterions exhibited good
ꢀ
water solubility and excellent properties such as micelle forming
ability and interfacial adsorption. This was not observed with the
gemini ionic liquids. The A values of the linear-type trimeriꢀc
compounds were larger in the presence of the counterions NTf2
than Brꢀ, while the reverse trend was observed for the star-type
trimeric compounds. These trends can be explained by the
positions of the bulky counterions. Thus, in the linear-type ionic
liquids, the bulky counterions are mainly present at the interface
because the three ammonium cations are linearly and rigidly
connected by the propylene spacers. On the other hand, in the
star-type ionic liquid, the bulky counterions can exist in the
molecular skeleton because of their flexible structure. The A value
of the linear-type 3C12lin-3-Q NTf2 was also larger than that of the
star-type 3C12tris-3-Q NTf2. The results also revealed that the
linear-type trimeric ionic liquid adsorbed widely at the air/water
interface, while it oriented itself efficiently in aqueous solutions.
ꢀ
ꢁ
ꢀ
ꢁ
ꢀ
ꢁ
1
3
CMC
RT
DGꢃmic ¼ RT
þ b ln
ꢀ
ln 3
(1)
(2)
55:3
3
pCMC
G
DGꢃads ¼ DGꢃmic
ꢀ
where pCMC is the surface pressure at the CMC (pCMC = g0
CMC; g0 and gCMC are the surface tensions of water and the ionic
ꢀ
g
liquid solution at the CMC, respectively) and b is the binding
degree of the counterion with the micelle (b = 1 ꢀ a; a is
obtained from the conductivity plot). The CMCs of the trimeric
ionic liquids comprising the counterion NTf2ꢀ were much lower
than those of the trimeric surfactants containing the counterion
Brꢀ. This suggested that the amphiphilic trimeric ionic liquids
display excellent micelle-forming ability in aqueous solutions. The
CMC of the linear-type 3C12lin-3-Q NTf2 was lower than that of the
star-type 3C12tris-3-Q NTf2, indicating the high micelle-forming
ability of the former. The decrease in spacer chain length for the
star-type ionic liquids led to a slight decrease in the CMC. These
Conclusions
results were supported by the absolute values of DGmꢃ ic
.
The gCMC value of the star-type trimeric ionic liquid 3C12tris- In this study, we designed and synthesized quaternary-
2-Q NTf2 was 8 mN mꢀ1 higher than that of the trimeric ammonium-salt star- and linear-type trimeric amphiphiles.
surfactant 3C12tris-2-Q Br. This indicated that owing to steric We then investigated their physicochemical properties as ionic
hindrance, the introduction of a bulky structure into the liquids as well as their properties in aqueous solutions to develop
counterion reduced the adsorption efficiency at the air/water novel amphiphilic ionic liquids comprising both surfactant and
interface. The gCMC values of the trimeric ionic liquids with ionic liquid properties. Thus, their melting points were lower
spacer chain lengths of 3 were lower for the linear-type 3C12lin- than those of the corresponding gemini ionic liquids, despite the
3-Q NTf2 than for the star-type 3C12tris-3-Q NTf2. This indicated presence of three long alkyl chains. Moreover, the star-type
that the linear-type ionic liquids were highly oriented at the 3Cntris-3-Q NTf2 (n = 8, 10, 12, 14) and linear-type 3Cnlin-3-Q
air/water interface. The three alkyl chains, which exist in the NTf2 (n = 8, 10) exhibited melting points that were lower than
linear conformation of dipropylenetriamine, facilitated the inter- room temperature. The amphiphilic linear-type trimeric ionic
action. On the other hand, in the star-type ionic liquids, the alkyl liquids presented lower conductivities and higher viscosities than
chains exist in a ring around tris(2-aminopropyl)amine. As a their star-type counterparts, indicating that the alkyl chains of the
result, the linear-type 3C12lin-3-Q NTf2 efficiently lowered the linear-type trimeric ionic liquids were tightly entangled. The
surface tension of the water. Further, the decrease in the spacer amphiphilic trimeric ionic liquids were soluble in the low-
chain length for the star-type ionic liquids led to a decrease in the polarity solvents chloroform and benzene. They also displayed
surface tension, as was observed for the linear-type ionic liquid. excellent solubilities in a wider range of organic solvents of
This was attributed to increased interaction as a result of the different polarities than did the gemini ionic liquids. Interest-
close proximity of the alkyl chains. In our previous study,15 the ingly, although the quaternary-ammonium-salt-type gemini and
quaternary-ammonium-salt-type gemini and monomeric ionic monomeric ionic liquids did not exhibit surface-active properties
liquids did not display surface-active properties such as CMC such as CMC, the trimeric ionic liquids adsorbed at the air/water
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4 I. Minami, Molecules, 2009, 14, 2286–2305.
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type trimeric ionic liquids efficiently lowered the surface tension
of water and displayed a higher micelle-forming ability than their
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In this study, we elucidated the effects of the alkyl chain
length, number of alkyl chains, spacer chain length, molecular
skeleton, and counterion structure on the physicochemical and
solution properties of quaternary-ammonium-salt-type amphi-
philic trimeric ionic liquids. Many problems, such as lowering
of the melting points and improvement of their solubility in
water, still need to be addressed for the development of feasible
amphiphilic ionic liquids comprising both ionic liquid and
surfactant properties. However, the melting points of the star-
type trimeric ionic liquid 3C12tris-3-Q NTf2 and linear-type ionic
liquids Cnlin-3-Q NTf2 (n = 8, 10) could notably be lowered to
less than 0 1C and these products also exhibited surface-activity
in aqueous solutions. Thus, we predict the future development
of high-performance functional amphiphilic ionic liquids and
their use in various industrial applications such as new media
for organic reactions, solubilizates, and lubricants as well in
separation and extraction processes.
15 R. Kawai, S. Yada and T. Yoshimura, ACS Omega, 2019, 14,
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16 P. S. Sharma, T. Payagala, E. Wanigasekara, A. B. Wijeratne,
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Conflicts of interest
There are no conflicts to declare.
17 E. Wanigasekara, X. Zhang, Y. Nanayakkara, T. Payagala,
H. Moon and D. W. Armstrong, ACS Appl. Mater. Interfaces,
2009, 1, 2126–2133.
Acknowledgements
´
´
18 O. Nacham, A. Martın-Perez, D. J. Steyer, M. J. Trujillo-
This work was supported by JSPS KAKENHI Grant No. 17K05948.
We would like to thank Editage (www.editage.jp) for the English
language editing.
´
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