Characterization and solution properties of adamantane-containing quaternary-ammonium-salt-type amphiphilic ionic liquids

Article abstract of DOI:10.1016/j.molliq.2019.111586

Quaternary-ammonium-salt-type amphiphilic compounds ([C_nAdA][X], where n represents the alkyl chain length (n = 1, 2, 4, 6, 8, or 10), X represents a counterion such as [BF₄]^?, [PF₆]^?, trifluoromethanesulfonate ([OTf]^?), bis(fluorosulfonyl)amide ([FSA]^?), or bis(trifluoromethanesulfonyl)amide ([NTf₂]^?), and Ad and A of AdA represent the adamantane structure and quaternary ammonium group, respectively) were synthesized. The melting points of the prepared compounds were determined by differential scanning calorimetry, and the derivatives with melting points lower than 100 °C were classified as ionic liquids for subsequent analyses. The amphiphilic ionic liquids, [C_nAdA][NTf₂] (n = 6 and 8), exhibited the lowest melting points (30.6 and 38.7 °C, respectively). Further, the [C_nAdA][NTf₂] series of amphiphilic ionic liquids exhibited significantly lower conductivities and higher viscosities than the corresponding ionic liquids without the adamantane moiety, [C_n][NTf₂]. The viscosities of [C_nAdA][NTf₂] (n = 6 and 8) decreased significantly with increasing temperature, and showed a larger temperature dependence than that of the viscosities of the [C_n][X] series. The amphiphilic ionic liquids, [C₈AdA][X] readily adsorbed at the air/water interface and oriented themselves but did not show the critical micelle concentration in the concentration range over which they could be dissolved in water. The amphiphilic ionic liquids and compounds tended to form ion pairs or premicelles, such as dimers or trimers, in aqueous solutions.

Full text of DOI:10.1016/j.molliq.2019.111586

Journal of Molecular Liquids 294 (2019) 111586
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Characterization and solution properties of adamantane-containing
quaternary-ammonium-salt-type amphiphilic ionic liquids
⁎
Risa Kawai, Shiho Yada, Tomokazu Yoshimura
Department of Chemistry, Faculty of Science, Nara Women's University, Kitauoyanishi-machi, Nara 630-8506, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 June 2019
Received in revised form 8 August 2019
Accepted 19 August 2019
Available online 20 August 2019
Quaternary-ammonium-salt-type amphiphilic compounds ([CnAdA][X], where n represents the alkyl chain
length (n = 1, 2, 4, 6, 8, or 10), X represents a counterion such as [BF4]−, [PF6]−, trifluoromethanesulfonate
([OTf]−), bis(fluorosulfonyl)amide ([FSA]−), or bis(trifluoromethanesulfonyl)amide ([NTf2]−), and Ad and A of
AdA represent the adamantane structure and quaternary ammonium group, respectively) were synthesized.
The melting points of the prepared compounds were determined by differential scanning calorimetry, and the
derivatives with melting points lower than 100 °C were classified as ionic liquids for subsequent analyses. The
amphiphilic ionic liquids, [CnAdA][NTf2] (n = 6 and 8), exhibited the lowest melting points (30.6 and 38.7 °C, re-
spectively). Further, the [CnAdA][NTf2] series of amphiphilic ionic liquids exhibited significantly lower conductiv-
ities and higher viscosities than the corresponding ionic liquids without the adamantane moiety, [Cn][NTf2]. The
viscosities of [CnAdA][NTf2] (n = 6 and 8) decreased significantly with increasing temperature, and showed a
larger temperature dependence than that of the viscosities of the [Cn][X] series. The amphiphilic ionic liquids,
[C8AdA][X] readily adsorbed at the air/water interface and oriented themselves but did not show the critical mi-
celle concentration in the concentration range over which they could be dissolved in water. The amphiphilic ionic
liquids and compounds tended to form ion pairs or premicelles, such as dimers or trimers, in aqueous solutions.
© 2019 Elsevier B.V. All rights reserved.
Keywords:
Ionic liquid
Amphiphilic ionic liquid
Quaternary-ammonium salt
Adamantane
Ion-pair formation
Adsorption
1. Introduction
and the development of other novel ionic liquids with better perfor-
mance and greater functionality is expected in the future.
Ionic liquids are salts that have melting points lower than 100 °C.
These were first reported by Wilkes et al. in 1992 [1]. Ionic liquids are
stable in both air and water, and generally exist in liquid state at room
temperature. Recently, they have been the subject of extensive research
efforts [2], because of their advantageous properties such as
nonvolatility, nonflammability, and environmental friendliness, and
they are attracting interest as novel solvents that are different from
water and conventional organic solvents [2–7]. Ionic liquids can be pre-
pared with different combination of cations and anions. Hence, their
physicochemical properties, such as their melting point, density, viscos-
ity, polarity, and hydrophobicity can be controlled readily. They are
therefore being extensively explored for use as reaction solvents [2,3],
electrolytes [2,4], catalysts [2,4,5], lubricants [8], and drug delivery sys-
tems [9]. Ionic liquids with different structures and functions, such as
external-stimuli-responsive ionic liquids [10], magnetic ionic liquids
[11], and cellulose-dissolving ionic liquids [12] have been reported,
Adamantane (tricyclo[3,3,1,1]decane, C10H16) has a saturated hy-
drocarbon cyclic tetrahedral structure with its 10 carbons bonded as
in diamond. Adamantane has many useful chemical and physical prop-
erties owing to its peculiar structure. Some examples of the expected
properties are its transparency, rigidity, high hydrophobicity, lubricity,
and heat resistance due to the high CH density and weak interstitial in-
teraction. Adamantane and its derivatives are widely used in the re-
search and development of various pharmaceutical products, display
materials, and photoresists. Recently, we molecularly designed and syn-
thesized monomeric and bola-type cationic surfactants possessing an
adamantane moiety, and revealed an increase in the hydrophobicity
and decrease in the critical micelle concentration (CMC) of the mole-
cules upon introducing the adamantane unit, as well as the solubiliza-
tion of naphthalene and fatty acids [13,14].
It is known that amphiphilic ionic liquids containing an alkyl chain
show surface activities and form aggregates in an aqueous solution, sim-
ilar to surfactants [15]. However, these amphiphilic ionic liquids are
mostly based on imidazole [16,17] and amine-salt-containing protic-
type compounds [18], and there have been very few reports on
quaternary-ammonium-salt-type amphiphilic ionic liquids [19,20].
Chujo et al. synthesized adamantane-containing imidazolium-based
amphiphilic ionic liquids and reported the lowest melting point (21
Abbreviations: DMSO, dimethyl sulfoxide; OTf, trifluoromethanesulfonate; FSA, bis
(fluorosulfonyl)amide; NTf2, bis(trifluoromethanesulfonyl)amide.
⁎
Corresponding author.
E-mail address: yoshimura@cc.nara-wu.ac.jp (T. Yoshimura).
https://doi.org/10.1016/j.molliq.2019.111586
0167-7322/© 2019 Elsevier B.V. All rights reserved.

2
R. Kawai et al. / Journal of Molecular Liquids 294 (2019) 111586
°C) for an ionic liquid with an alkyl chain length of 12 [21]. Further, in-
troduction of the bulky adamantane unit into the structure of ionic liq-
uids instead of the imidazolium unit is expected to further lower the
melting point of the ionic liquid. There have been no reports on the
preparation of adamantane derivatives of quaternary-ammonium-salt-
type amphiphilic compounds or the evaluation of their properties as
ionic liquids and their surface activity in aqueous solutions.
In this study, quaternary-ammonium-salt-type amphiphilic com-
pounds with an adamantane moiety were designed and synthesized.
The amphiphiles have the structure of [CnAdA][X], where n represents
the alkyl chain length; n = 1, 2, 4, 6, 8, 10, or 12, (here, for n = 1, com-
mercially available trimethyladamantylammonium bromide was used),
X represents the counterion ([BF4]−, [PF6]−, [OTf]−, [FSA]−, or [NTf2]−),
and Ad and A of AdA represent an adamantane moiety and a quaternary
ammonium group, respectively (see Fig. 1a). We investigated their
properties as ionic liquids as well as their surface activity in aqueous so-
lutions in comparison with those of the corresponding parent com-
pounds without the adamantane moiety ([Cn][X], n = 2, 4, 6, 8, 10;
see Fig. 1b). Further, we discuss the effects of the adamantane structure,
alkyl chain length, and structure of the counterion of the derivatives on
the properties of [CnAdA][X].
2.2. Synthesis of N, N-dimethyl-N-alkyladamantylammonium bromide
([CnAdA][Br]) [13]
1-Aminoadamantane hydrochloride was added to 200 mL of metha-
nol containing 8 g of sodium hydroxide, and the solution was stirred
with heating for 2 h. Then, the solvent was removed by rotary evapora-
tion and the residue was washed at least thrice with water to recover 1-
aminoadamantane as a white solid.
A solution of 1-aminoadamantane (1.0 equiv.) in methanol was
added to a stirred mixture of 37% formaldehyde (4.0 equiv.) and 98%
formic acid (3.0 equiv.), and the resulting mixture was refluxed for
12 h. After that, the solvent was removed by evaporation, water was
added to the residue, and the pH of the solution was adjusted to 1–2
with 1 mol dm−3 HCl. Then, the solvent was evaporated, the residue
was washed repeatedly, first with hexane and then with ethyl acetate,
and the product was dried under vacuum to recover N, N-
dimethylaminoadamantane hydrochloride as a white solid (Scheme
S1). The structure was confirmed by 1H NMR spectroscopy (see Supple-
mentary material).
The obtained product was added to 200 mL of methanol containing
8 g of sodium hydroxide, and stirred with heating for 2 h. After the sol-
vent was removed by evaporation, ethyl acetate was added to the resi-
due, and the solution was heated and filtered to remove inorganic salts.
The solvent was evaporated under reduced pressure to recover N, N-
dimethylaminoadamantane as a yellow liquid.
2. Experimental
2.1. Materials
Ethyl bromide, n-butyl bromide, n-hexyl bromide, n-octyl bromide,
n-decyl bromide, or n-dodecyl bromide (2.0 equiv.) was added slowly
to a stirred solution of N, N-dimethylaminoadamantane (1.0 equiv.) in
acetonitrile. The mixture was refluxed for 30 h. After the solvent was
evaporated under reduced pressure, the residue was washed repeat-
edly, first with hexane and then with ethyl acetate, and the product
was recrystallized from 2-propanol to obtain N, N-dimethyl-N-
alkyladamantylammonium bromide [CnAdA][Br] (n = 2, 4, 6, 8, 10, or
12) as a white solid (Scheme S1) (see Supplementary material for
yields, 1H NMR data, and elemental analysis).
1-Aminoadamantane hydrochloride, ethyl bromide, n-butyl bro-
mide, n-hexyl bromide, n-octyl bromide, n-decyl bromide, and n-
dodecyl bromide were purchased from Tokyo Chemical Ind. Co., Ltd.
(Tokyo, Japan). Silver tetrafluoroborate (Ag[BF4]), potassium
hexafluorophosphate (K[PF6]), potassium trifluoromethanesulfonate
(K[OTf]), acetone, acetonitrile, chloroform, dimethyl sulfoxide
(DMSO), ethanol, ethyl acetate, formaldehyde solution (37%), formic
acid solution (98%), hexane, 1 mol dm−3 hydrochloric acid, methanol,
2-propanol, and sodium hydroxide were obtained from FUJIFILM
Wako Pure Chemical Co., Ltd. (Osaka, Japan). Potassium bis
(trifluoromethanesulfonyl)amide (K[NTf2]) was purchased from Kanto
Chemicals Co., Inc. (Tokyo, Japan). Potassium bis(fluorosulfonyl)amide
(K[FSA]) was kindly supplied by Nippon Shokubai Co., Ltd. (Osaka,
Japan). Trimethyladamantylammonium bromide was supplied by
Idemitsu Kosan Co. Ltd. (Tokyo, Japan). Deuterated chloroform and deu-
terated dimethyl sulfoxide used as NMR solvents were 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 MΩ cm, Darmstadt, Germany)
was used in all the experiments.
2.3. Ion-exchange of Br− in [CnAdA][Br] with [BF4]−, [PF6]−, [OTf]−, [FSA]−,
and K[NTf2]−
The aqueous solution of trimethyladamantylammonium bromide or
N, N-dimethyl-N-alkyladamantylammonium bromide (1.0 equiv.) was
added to an aqueous solution of Ag[BF4], K[PF6], K[OTf], K[FSA], or K
[NTf2] (1.1 equiv.), and the mixture was stirred with heating for 10 h.
The purification procedure for the derivative with each counterion is
described below.
[CnAdA][BF4]: After the precipitated gray solid was removed by fil-
tration, the solvent in the filtrate was evaporated under reduced
Fig. 1. Chemical structures of (a) adamantane-containing quaternary-ammonium-salt-type amphiphilic compounds, [CnAdA][X] (n = 1–12), (b) amphiphilic compounds without the
adamantane moiety, [Cn][X] (n = 2–10), and (c) counterions, X.

R. Kawai et al. / Journal of Molecular Liquids 294 (2019) 111586
3
pressure. The residue was dissolved in acetone for n = 1 and in chloro-
3. Results and discussion
form for n = 2, 4, 6, 8, 10, and 12. The insoluble inorganic salt was re-
moved by filtration and the solvent in the filtrate was evaporated. The
residue was washed repeatedly with ethyl acetate and the product
was recrystallized from a mixture of ethyl acetate and methanol (3:1,
vol/vol) to obtain the target compound.
[CnAdA][PF6]: After the solution was filtered, the residue was
washed five times with water and then dried. Hot acetone was added
to the dried residue, and then it was filtered to remove the inorganic
salt. The solvent in the filtrate was removed by evaporation, and the res-
idue was recrystallized from acetone.
[CnAdA][OTf]: After the solvent in the solution was evaporated
under reduced pressure, chloroform was added to the residue. The inor-
ganic salt was removed by filtration, and the filtrate was evaporated.
[CnAdA][FSA] and [CnAdA][NTf2]: Once the white solid precipi-
tated, the solution was filtered to recover the solid. On the other
hand, in the case where the solution separated into two phases,
the upper phase was removed by decantation to recover the bottom
liquid phase. The residue was washed five times with water and
then dried. Chloroform was added to the dried residue, and it was
then filtered to remove the inorganic salt, and the solvent in the fil-
trate was removed by evaporation.
3.1. Melting points of the amphiphilic derivatives
Fig. 2 shows the relationship between the melting point (see
Table S1) and alkyl chain length n for the amphiphilic compounds,
[CnAdA][X] (X = [BF4]−, [PF6]−, [OTf]−, [FSA] −, and [NTf2]−). The melt-
ing points of adamantane-containing amphiphilic compounds decrease
in the following order based on the type of counterion:[Br]− N [PF6]−
N
[BF4]− N [OTf]− N [FSA]− N [NTf2]− except for [C8AdA][BF4] and [C8AdA]
[OTf]. The series of [CnAdA][FSA] compounds (with the exception of n
= 2 and 12) and [CnAdA][NTf2] compounds (with the exception of n
= 2) that contain bulky counterions have melting points lower than
100 °C. That is, they can be classified as ionic liquids. In general, ionic liq-
uids are salts consisting of cations and anions with weak electrostatic in-
teractions [1].
Ionic liquids have low melting points than inorganic salts because
they contain organic ions with a bulky structure [25]. In particular,
[C6AdA][NTf2] and [C8AdA][NTf2] are almost in the liquid state at room
temperature (their melting points are 30.6 and 38.7 °C, respectively).
The melting points of adamantane-containing amphiphilic ionic liquids
([CnAdA][X]) are higher than those of the corresponding derivatives
without adamantane ([Cn][X]) (see Fig. S1), indicating that there is no
decrease in the melting point with the introduction of the bulky
adamantane moiety into the structure, which is due to the rigidity and
symmetry of adamantane [26]. The melting point of amphiphilic deriv-
atives [CnAdA][FSA] and [CnAdA][NTf2] first tended to decrease with an
increase in the alkyl chain length from 2 to 4 or 6, respectively, and in-
creased slightly with a further increase in the alkyl chain length. We
speculate that the melting point decreased because of the breakage of
the symmetry of the cation by adamantane and decreased interaction
between the cation and anion in the case of derivatives with a shorter
alkyl chain. Further, as the alkyl chain length increased, the melting
point increased owing to van der Waals forces between the alkyl chains.
On the other hand, the melting points of amphiphilic derivatives with-
out the adamantane moiety, [Cn][FSA] and [Cn][NTf2] are the lowest at
the alkyl chain length of 8 (7 °C for [C8][FSA] and b0 °C for [C8][NTf2]).
This result indicates that the effect of the van der Waals forces in
adamantane-containing ionic liquids manifests at a shorter alkyl chain
length than for the ones without adamantane. The melting point of
[C1][NTf2] containing tetramethylammonium group is 133 °C [27]
while that of [C1AdA][NTf2], in which one of the methyl groups is re-
placed with adamantane, is 96 °C. The melting point of [C1][NTf2] is
high because of the symmetry of the tetramethylammonium cation.
These residues were dried under reduced pressure to obtain N, N-
dimethyl-N-alkyladamantylammonium [BF4, PF6, OTf, FSA, or NTf2] as
a white solid or a clear yellow viscous material (Scheme S1) (see Sup-
plementary material for yields, 1H NMR data, and elemental analysis).
2.4. Characterization
The melting points of the adamantane-containing amphiphilic
compounds were determined by differential scanning calorimetry
(DSC; Shimadzu DSC-50 system, Kyoto, Japan). For the measure-
ments, 2 mg of the compound was placed in a hermetically sealed
aluminum pan; an empty aluminum pan was used as the reference.
The measurements were performed at the heating rate of 0.2
°C min−1 in a nitrogen atmosphere, and the obtained data was
corrected and analyzed using the software, Shimadzu TA-60WS
(Kyoto, Japan). The water contents of the amphiphilic ionic liquids
were determined using a coulometric titration system (Hiranuma
AQV-200, Karl Fischer, Tokyo, Japan). The viscosities were mea-
sured at 35, 45, 55, and 65 °C using a Brookfield DV-2T system
(Middleborough, MA, USA).
The electrical conductivities of the neat amphiphilic ionic liquids
and their aqueous solutions were measured using a TOA CM-30R
system (Tokyo, Japan). From the data, the Krafft temperatures and
CMC values of the ionic liquids in aqueous solutions were deter-
mined. The surface tensions of neat ionic liquids and their aqueous
solutions were measured with a Teclis Tracker tensiometer (Lyon,
France) by the pendant drop technique. The surface excess concen-
tration (Γ/mol m−2) and occupied area per molecule (A) of the am-
phiphilic ionic liquids at the air/water interface were calculated
using the Gibbs adsorption isotherm equations: Γ = −(1/iRT)(dγ/
d lnC) and A = 1/(NΓ) [22], where γ is the surface tension, C is
the concentration of the ionic liquid, R is the gas constant
(8.31 J K−1 mol−1), T is the absolute temperature, and N is the
Avogadro's constant. The value of i, which is the number of ionic
species assumed to be completely dissociated in the aqueous solu-
tion, was taken to be 2 for the adamantane-containing amphiphilic
ionic liquids investigated in this study. The fluorescence of pyrene
[23,24] in the amphiphilic ionic liquid solutions was measured
using a JASCO FP–6300 system (Tokyo, Japan). The concentration
of pyrene in the ionic liquid solutions was 1 × 10−6 mol dm−3
.
The aqueous solutions of the adamantane-containing amphiphilic
ionic liquids and compounds were prepared using ultra-pure
water from a Direct–Q UV system, and the measurements were per-
Fig. 2. Relationship between the melting point and alkyl chain length n for [CnAdA][X]: X
formed at 25.0
0.5 °C.
= Br− ); [BF4]− ); [PF6]− ); [OTf]− ); [FSA]− ); [NTf2]−
( ( ( ( ( ( ).

4
R. Kawai et al. / Journal of Molecular Liquids 294 (2019) 111586
Further, the melting point of [C1AdA][NTf2] is lower than that of [C1]
[NTf2] because the distance between the cation and anion is increased,
and thus the interaction between them weakens, and they could not
be packed densely owing to the bulkiness of adamantane. Considering
that the melting point of adamantane is 269 °C [26,28], it is interesting
to note that the melting point of adamantane could be decreased to a
value close to room temperature with the introduction of an alkyl
chain into the structure of adamantane; that is, by constructing a
quaternary-ammonium-salt-type amphiphilic structure.
ethyl acetate, chloroform, benzene, and hexane were determined by
mixing 0.01–10 mL of water or an organic solvent with 0.01–0.1 g of
the ionic liquid. The adamantane-containing amphiphilic ionic liquids
and compounds [C8AdA][X] have lower solubilities in water than the
corresponding ionic liquids and compounds [C8][X] without
adamantane (see Table S2), because of the hydrophobicity of
adamantane. The solubility of [C8AdA][X] in water increases in the fol-
lowing order based on the counterion: [NTf2]− b [FSA]− b [PF6]−
b
[OTf]− b [BF4]− (Table S3). Further, the solubility of [CnAdA][NTf2] de-
creases with an increase in the alkyl chain length (see Table S4). The
adamantane-containing amphiphilic ionic liquids with [FSA]− and
[NTf2]− counterions also have excellent solubility in a wide range of or-
ganic solvents with different polarities (DMSO, methanol, acetone, and
ethyl acetate), especially in ethyl acetate (see Table S3). The solubilities
of ionic liquids without adamantane, [Cn][NTf2] in chloroform was de-
termined to be below 0.8 wt% for n ≤ 6, whereas those of [CnAdA]
[NTf2] with adamantane were found be above 45 wt% at all alkyl chain
lengths. The solubilities of adamantane-containing amphiphilic ionic
liquids [CnAdA][NTf2] in benzene increased significantly with an in-
crease in the alkyl chain length from 8 to 10 (see Table S4), and were
higher compared to those of [Cn][NTf2] at the same alkyl chain length.
Further, both types of ionic liquids, with and without adamantane,
were found to be insoluble in non-polar hexane. Thus, the introduction
of adamantane into quaternary-ammonium-salt-type ionic liquids im-
proved their solubilities in various organic solvents. The solubility of
adamantane is 10.9 wt% in benzene and 10.8 wt% in hexane [29],
whereas the solubility of CnAdA X is 0.5–31 wt% in benzene and it is al-
most insoluble in hexane (solubility, b0.01 wt%) (see Table S3). Thus,
the solubility of the compounds in non-polar organic solvents decreased
with the introduction of quaternary ammonium unit into the
adamantane structure.
3.2. Characterization of adamantane-containing amphiphilic ionic liquids
The water contents of the adamantane-containing amphiphilic ionic
liquids [CnAdA][X] (n = 1, 4, 6, 8, and 10) and [CnAdA][NTf2] (n = 1, 4, 6,
8, 10, and 12) were determined to be b100 ppm. Therefore, it was
deemed that investigating the properties of these ionic liquids would
not be problematic. Table 1 shows the conductivity (κ), zero-shear vis-
cosity (η0), density (ρ), and surface tension (γ) of amphiphilic ionic liq-
uids [C6AdA][NTf2] and [C8AdA][NTf2] along with those of the
corresponding amphiphilic ionic liquids without adamantane [C6]
[NTf2] and [C8][NTf2]. The adamantane-containing amphiphilic ionic liq-
uids showed much lower conductivity and higher viscosity compared to
those of the amphiphilic derivatives without adamantane. The viscosity
increases because of the decreased mobility of the cationic portion of
the molecule with the rigid adamantane moiety; the conductivity de-
creases as a result of the decreased ionic mobility. The conductivities
of both the ionic liquids, [C8AdA][NTf2] with adamantane and [C8]
[NTf2] without adamantane, increased with an increase in the tempera-
ture between 45 and 65 °C at a similar rate (see Fig. S2). The viscosity of
[CnAdA][NTf2] and [Cn][NTf2] (n = 6 and 8) decreased with an increase
in the temperature. Further, the rate of decrease in the viscosity with
temperature for [CnAdA][NTf2] is much larger than that for [Cn][NTf2]
(see Fig. S3). A large difference in the viscosity of amphiphilic ionic liq-
uids with and without adamantane is observed at 35 and 45 °C, whereas
their viscosities become almost same at 65 °C. Thus, the amphiphilic
ionic liquids with adamantane show a larger temperature-dependence
of viscosity compared to that of the amphiphilic ionic liquids without
adamantane. At the same alkyl chain length, the density of the
adamantane-containing amphiphilic ionic liquid, [CnAdA][NTf2] is al-
most the same as that of the corresponding derivative, [Cn][NTf2] with-
out adamantane; that is, no difference in density is observed after the
introduction of adamantane into the structure. Further, the surface ten-
sions of [CnAdA][NTf2] are higher than those of the corresponding [Cn]
[NTf2] derivatives.
3.4. Solution properties of adamantane-containing amphiphilic ionic
liquids
The solution properties of the adamantane-containing amphiphilic
ionic liquids [CnAdA][X] (n = 1, 4, 6, and 8, X = [FSA]− and [NTf2]−
)
and the amphiphilic compounds [CnAdA][X] (n = 1 and 8, X = [BF4]−,
[PF6]−, and [OTf]−) that are not ionic liquids, including their Krafft tem-
perature, conductivity, surface tension, and pyrene fluorescence in their
solutions were determined.
3.4.1. Krafft temperature (TK)
Clear aqueous solutions of the adamantane-containing amphiphilic
ionic liquids and compounds (0.050–0.020 wt%) were prepared by dis-
solving them in hot water and incubating the obtained solution in a re-
frigerator at ~5 °C for at least 24 h. As the solutions were clear with no
visible precipitates, the TK value for the ionic liquids and compounds
was estimated to be b5 °C. The amphiphilic ionic liquids, [C1AdA][FSA]
(0.20 wt%), [C6AdA][FSA] (0.050 wt%), [C8AdA][FSA] (0.010 wt%), and
[C1AdA][NTf2] (0.20 wt%) precipitated in the solution. Therefore, the
temperature of the cooled solution was gradually increased under con-
stant stirring, and the conductance (κ) was measured over the temper-
ature range of 0.5 to 5.0 °C. Initially, the conductivity increased rapidly
with increasing temperature because of the gradual dissolution of the
Thus, the introduction of adamantane into the structure of
quaternary-ammonium-salt-type ionic liquids led to remarkable differ-
ences in the surface tension and variation of the conductivity and vis-
cosity with temperature.
3.3. Solubility in various organic solvents
The solubilities of adamantane-containing amphiphilic compounds
[C8AdA][X] (X = [FSA]− and [NTf2]−), which are ionic liquids, and
[C8AdA][X] (X = [BF4]−, [OTf]−, and [NTf2]−), which are not ionic liq-
uids, in various organic solvents such as DMSO, methanol, acetone,
Table 1
Melting point (Tm), conductivity (κ), viscosity (η0), density (ρ), and surface tension (γ) values of amphiphilic ionic liquids [CnAdA][NTf2] and [Cn][NTf2].
Ionic liquid
Tm
/°C
κ
η0
/mPa s
ρ
γ
/mS m−1
/g cm−3
/mN m−1
35 °C
5.34
45 °C
55 °C
65 °C
35 °C
865
45 °C
55 °C
66 °C
45 °C
45 °C
[C6AdA][NTf2]
[C8AdA][NTf2]
[C6][NTf2]
30.6
38.7
28.8
b0
503
461
53.8
68.7
276
257
39.0
46.7
186
155
27.8
32.7
1.33
1.27
1.33
1.27
34.2
33.7
30.7
30.6
12.1
71.1
24.2
89.1
33.1
100
112
82.9
[C8][NTf2]

R. Kawai et al. / Journal of Molecular Liquids 294 (2019) 111586
5
amphiphilic ionic liquid (see Fig. S4). Subsequently, it increased gradu-
ally owing to the increase in the ionic mobility. TK was taken as the tem-
perature at which the conductivity versus temperature plot exhibited
an inflection point. The adamantane-containing amphiphilic ionic liq-
uids with the formula [C1AdA][X] (X = [FSA]− and [NTf2]−) do not pos-
sess an alkyl chain, however, their TK values at 0.20 wt% (25.5 and 26.5
°C, respectively) are higher than those of the Cn X (n = 4 and 6) deriva-
tives without adamantane (TK b 5 °C). As mentioned above, the solubil-
ity of the derivatives in water was significantly lowered upon the
introduction of the hydrophobic adamantane moiety. Further, the TK
values of amphiphilic ionic liquids [C1AdA][X] (X = [FSA]− and
[NTf2]−) are higher than those of [C1AdA][X] (X = [BF4]−, [PF6]−, and
[OTf]−) (TK b 5 °C). The TK values of the amphiphilic ionic liquids and
compounds, [C8AdA][X] (X = [BF4]− (0.20 wt%), [PF6]− (0.010 wt%),
[OTf]− (0.10 wt%), [FSA]− (0.0050 wt%), and [NTf2]− (0.0050 wt%))
are b5 °C. Therefore, their other solution properties were evaluated at
25 °C.
Fig. 4. Variation in surface tension and pyrene fluorescence intensity ratio I1/I3 with the
concentration of amphiphilic ionic liquids and compounds [C8AdA][X] at 25 °C: X = X
3.4.2. Conductivity, surface tension, and pyrene fluorescence
= [BF4]−
( ( ( ( ( ); the broken lines shows
); [PF6]− ); [OTf]− ); [FSA]− ); [NTf2]−
Figs. 3 and 4 show the plots of the conductivity, and surface tension
and pyrene fluorescence intensity ratio, I1/I3, as functions of the concen-
tration, respectively, for the adamantane-containing amphiphilic ionic
liquids [C8AdA][X] (X = [FSA]− and [NTf2]−) and compounds [C8AdA]
[X] (X = [BF4]−, [PF6]−, and [OTf]−) with melting points above 100 °C
that are not ionic liquids. The conductivity of [C8AdA][X] increases
with increasing concentration, and the conductivity vs. concentration
curve shows an inflection point. On the other hand, the surface tension
decreases (without remaining constant) with increasing concentration,
while the pyrene I1/I3 ratio remains constant (without any decrease)
with increasing concentration, and no inflection point is observed in
the plots. The broken lines in Fig. 4 show the concentration correspond-
ing to the inflection point in the conductivity vs. concentration plot
(CI.P.) for each solution. These results indicate that [C8AdA][X] deriva-
tives do not form micelles in aqueous solutions, although they adsorb
at the air/water interface in the concentration range over which they
could be dissolved in water. It has been previously reported that amphi-
philic compounds with short alkyl chains (n = 8) exhibit two inflection
points in their conductivity plots [30–32]. The inflection point at the
higher concentration corresponds to the CMC, as confirmed by the re-
sults of the surface tension and fluorescence measurements. Further,
the inflection points at concentrations lower than the CMC are attribut-
able to the formation of an ion pair between the ammonium cation and
its counterion or premicelles such as dimers or trimers. The amphiphilic
CI.P. obtained from the electrical conductivity measurement.
ionic liquids and compounds [C8AdA][X] did not exhibit a second inflec-
tion point in their conductivity plots because of their poor solubility in
water owing to the hydrophobic adamantane in their structure and
the hydrophobic counterion.
The adsorption and orientation of amphiphilic compounds at the air/
water interface are related to their excess surface concentration, Γ, and
the occupied area per molecule, A, which can be calculated from the
slope of the linear curve for the dependence of the surface tension
with the concentration for concentrations lower than the CMC and the
Gibbs adsorption isotherm equations. Table 2 shows the concentration
for ion-pair or premicelle formation (CI.P.) (CMC for [C8AdA][Br]), the
degree of dissociation (α), Γ, A, and pC20 values determined from the
conductivity and surface tension plots. The CI.P. values increase in the
following order based on the counterion: [BF4]− N [OTf]− N [PF6]−
N
[FSA]− N [NTf2]−, indicating that the amphiphilic ionic liquids with
[FSA]− and [NTf2]− counterions form ion pairs or premicelles at low
concentrations. Further, the CI.P. values decreased with an increase in
the hydrophobicity of the counterion. The A values of the amphiphilic
ionic liquids and compounds [C8AdA][X] increased in the following
order based on the counterion: [BF4]− b [PF6]− b [OTf]− b [FSA]−
b
[NTf2]− (see Fig. S5). This order indicates that the larger the radius of
the counterion is, the larger the occupied area is. In particular, the occu-
pied area of amphiphilic ionic liquids with [FSA]− and [NTf2]− counter-
ions was found to be large, which indicates that they adsorb widely at
the air/water interface and orient by themselves. The pC20 of the amphi-
philic ionic liquids and compounds [C8AdA][X] increased in the follow-
ing order based on the counterion: [Br]− b [BF4]− b [PF6]− b [OTf]−
b
[FSA]− b [NTf2]−. The amphiphilic ionic liquids with [FSA]− and
[NTf2]− counterions adsorbed efficiently at the air/water interface de-
spite being bulky. Thus, it can be concluded that the adsorption behav-
iors of [C8AdA][X] at the air/water interface are significantly affected by
the structure of the counterion; the amphiphilic ionic liquids adsorb
widely at the interface but orient themselves efficiently.
4. Conclusions
In this study, we designed and synthesized adamantane-containing
quaternary-ammonium-salt-type amphiphiles using five different
counterions ([BF4]−, [PF6]−, [OTf]−, [FSA]−, or [NTf2]−), and investi-
gated their physicochemical properties as ionic liquids as well as their
properties in aqueous solutions. The melting points of the amphiphilic
ionic liquids increased with the incorporation of the adamantane moi-
ety into their structure. The adamantane-containing amphiphilic ionic
liquids have higher viscosities and lower conductivities than the
Fig. 3. Variation in conductivity with the concentration of amphiphilic ionic liquids and
compounds [C8AdA][X] at 25 °C: X = [BF4]−
( ( ( ( );
); [PF6]− ); [OTf]− ); [FSA]−
[NTf2]−
( ).

6
R. Kawai et al. / Journal of Molecular Liquids 294 (2019) 111586
Table 2
Melting point (Tm), Krafft temperature (TK), concentration for ion-pair formation (CI.P.), CMC, α, surface excess concentration (Γ), occupied area per molecule (A), and adsorption efficiency
(pC20) values of amphiphilic ionic liquids and compounds [C8AdA][X] at 25 °C.
Compound
Tm
/°C
TK
/°C
CI.P.
CMC
α
Γ × 106
A
pC20
/mmol dm−3
/mmol dm−3
/mol m−2
/nm2
[C8AdA][BF4]
[C8AdA][PF6]
[C8AdA][OTf]
[C8AdA][FSA]
[C8AdA][NTf2]
[C8AdA][Br]
139.2
162.3
151.3
76.3
38.7
211.0
b5 (0.20 wt%)
b5 (0.050 wt%)
b5 (0.10 wt%)
b5 (0.0050 wt%)
b5 (0.0050 wt%)
b5 (0.20 wt%)
3.01
0.108
1.67
0.107
0.0941
–
–
–
–
–
–
27.1
0.80
0.83
0.63
0.76
0.84
0.30
1.83
1.45
1.32
1.12
0.889
1.76
0.906
1.15
1.25
1.62
1.87
0.943
2.37
3.10
2.75
3.38
3.61
1.78
CI.P., concentration at which ion pairs or premicelles are formed.
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a large occupied area per molecule at the air/water interface. This re-
sulted in their wide interfacial adsorption, while they oriented them-
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adamantane structure, alkyl chain length, and counterion structure on
the physicochemical and aqueous-solution properties of adamantane-
containing quaternary-ammonium-salt-type ionic liquids. Although
adamantane is a highly crystalline and rigid molecule, it is worth noting
that the melting points of adamantane-containing quaternary-
ammonium-salt-type amphiphilic ionic liquids are relatively low at
~30 °C, and they exhibit unique characteristics, for e.g., their viscosity
depends on the temperature and they adsorb at the air/water interface.
In the future, we expect that it would be possible to synthesize novel
high-performance functional amphiphilic ionic liquids through molecu-
lar design for various industrial applications, for example, as lubricants,
drug delivery agents, new media for organic reactions, and electrolytes
for electrochemical devices.
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Acknowledgment
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This work was supported by JSPS KAKENHI (Grant No. 17K05948).
We thank Editage (www.editage.jp) for English language editing.
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Appendix A. Supplementary data
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