Synthesis, Characterization and Evaluation of Surface and Thermal Properties of 3-Cyclohexyloxy-2-Hydroxypropyl Pyridinium and Imidazolium Surface-Active Ionic Liquids

Article abstract of DOI:10.1002/jsde.12002

In this work, 1-halo-3-(cyclohexyloxy)propan-2-ol (3a/3b) were reacted with N-methylimidazole (4) or pyridine (5) to yield the respective 3-(3-(cyclohexyloxy)-2-hydroxypropyl)-1-methyl-1H-imidazol-3-ium (6a/6b) or pyridinium (7a/7b) surface-active ionic l

Full text of DOI:10.1002/jsde.12002

J Surfact Deterg (2018) 21: 43–52
DOI 10.1002/jsde.12002
ORIGINAL ARTICLE
Synthesis, Characterization and Evaluation of Surface and
Thermal Properties of 3-Cyclohexyloxy-2-Hydroxypropyl
Pyridinium and Imidazolium Surface-Active Ionic Liquids
Rajni Aggarwal1 · Sukhprit Singh1
Received: 21 July 2017 / Revised: 2 September 2017 / Accepted: 6 September 2017
© 2018 AOCS
Abstract In this work, 1-halo-3-(cyclohexyloxy)propan-
2-ol (3a/3b) were reacted with N-methylimidazole (4) or
pyridine (5) to yield the respective 3-(3-(cyclohexyloxy)-2-
hydroxypropyl)-1-methyl-1H-imidazol-3-ium (6a/6b) or
pyridinium (7a/7b) surface-active ionic liquids (SAIL). The
self-aggregation behavior of these ionic liquids (IL) was
evaluated by conductometric and tensiometric methods. The
thermal stability and size of the micelles were determined by
thermogravimetric analysis and dynamic light scattering
studies, respectively. The investigated IL were found to
exhibit very low cytotoxicity as evaluated by MTT (3-(4, 5-
dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide)
assay on the C6 glioma cell line, indicating that the investi-
gated SAIL can be considered for biological applications
like drug and gene delivery. The conventional IL 3-methyl-
1-octyl imidazolium bromide (C8mimBr) was used for com-
parison in property evaluations.
polluting organic solvents. Ionic liquids (IL) contain only
ions and exhibit melting points up to 100 C. Pyridinium
ꢁ
and imidazolium are important classes of IL. IL have
recently gained much attention as benign solvents because
of their unique properties such as lower melting points, non-
volatility, nonflammability, and the wide range of the liquid
state with negligible vapor pressure (Cadena et al., 2004;
Welton, 1999). In the past few years, different types of IL
have been synthesized and applied in different chemical
fields such as synthesis (Howarth, 2000; Kitazume, Zulfi-
qar, & Tanaka, 2000), biochemical reactions (McClements,
2004), catalysis (Peng & Deng, 2001), and electrochemistry
(Hanabusa, Fukui, Suzuki, & Shirai, 2005; Sanders, Ward, &
Hussey, 1986). Because of their unique properties, IL attract
much attention from both academia and chemical industries.
IL cover applications in chemical industries such as plasti-
cizers, lubricants, electrolyte in batteries (He, Li, Simone, &
Lodge, 2006; Huddleston et al., 2001; Lu et al., 2002), cata-
lysts in synthesis (Suarez, Dullius, Einloft, de Souza, &
Dupont, 1996; Zhao & Malhotra, 2002) solvents to manu-
facture nanomaterials (Antonietti, Kuang, Smarsly, & Yong,
2004; Javadian et al., 2013; Zhou, 2005), stationary phases
for chromatography (Poole, 2004), and selective extraction
of metals (Wellens, Thijs, & Binnemans, 2012). IL have the
capability of being reprocessed; therefore they can easily
replace traditional organic solvents (Seth, Chakraborty,
Setua, & Sarkar, 2006). Owing to their nonvolatility and non-
flammability, IL offer great potential for applications in
organic synthesis (Avery, Jenkins, Kimber, Lupton, & Taylor,
2002; Huddleston, Willauer, Swatloski, Visser, & Rogers,
1998), electrochemistry (Fukumoto, Yoshizawa, & Ohno,
2005; Leone, Weatherly, Williams, Thorp, & Murray, 2001),
and material preparation (Brezesinski, Erpen, Iimura, &
Smarsly, 2005; Taubert, 2004; Zhou & Antonietti, 2003). By
Keywords Cationic surfactants ꢀ Interfacial science
J Surfact Deterg (2018) 21: 43–52.
Introduction
Environmental concerns have compelled the search for the
synthesis of the molecules that can replace hazardous and
Electronic supplementary material The online version of this article
(doi:10.1002/jsde.12002) contains supplementary material, which is
available to authorized users.
* Rajni Aggarwal
rajni.aggarwal33@yahoo.com
1
Department of Chemistry, UGC Sponsored Centre for Advanced
Studies-I, Guru Nanak Dev University, Amritsar 143005, India
Published online: 21 February 2018
J Surfact Deterg (2018) 21: 43–52

44
J Surfact Deterg
changing the cation and anion combinations and inserting a
suitable functionality at the carbon chain, many new IL with
distinct physicochemical properties can be synthesized and
investigated for different industrial and biomedical applica-
tions (Scheeren, Machado, Dupont, Fichtner, & Texeira,
2003; Wang & Voth, 2005).
On going through the literature, we found very few pub-
lications reporting the preparation, self-assembly, and cyto-
toxicity of functionalized and flexible IL. Recently, our
group had synthesized ester-based (Aggarwal, Singh, &
Hundal, 2013) and alcohol-based (Chauhan, Singh, &
Bhadani, 2012) single-tailed and gemini amphiphiles
(Aggarwal & Singh, 2014) and evaluated their surface and
other physical properties.
In continuation of this work, we were curious to look at the
effect of the presence of a cycloalkyl group with the ether and
alcohol functional groups in the hydrophobic chain. Hence, in
this work, our group has synthesized cyclohexyloxy-2-
hydroxypropyl containing SAIL and evaluated them for their
micellar, thermal, and cytotoxic properties.
average of three measurements was taken for each solution.
The degree of counterion binding (β) was determined from
the ratio of the linear slopes before and after CMC, given by
(β = 1 – α), where α is the slope of micellar region/premicel-
lar region (Bordes, Tropsch, & Holmberg, 2010).
Surface Tension Measurements
A du Nöuy interfacial tensiometer (Central Scientific Co.,
Inc., Chicago, USA) equipped with a platinum–iridium ring
and of circumference 5.992 cm was used for the measure-
ment of surface tensions to obtain the CMC and other sur-
face properties. Double-distilled water of surface tension
72.6 mN m−1 at 25.0 ꢂ 0.1 ꢁC was used for the calibration
of the instrument.
Dynamic Light Scattering
A light-scattering apparatus (Zeta-sizer Nanoseries, nano-
ZS; Malvern Instrument Ltd., Malvern, UK) was used for
the dynamic light scattering (DLS) measurements at
298.15 K. For the measurement of dimension of aggregates
of pure IL, a 20 mM aqueous solution was prepared.
A Millipore membrane filter of pore size 0.45 μm was used
for the filtration of all solutions prior to measurements. The
average of 10 measurements was taken for each solution.
The temperature was maintained with the help of an in-built
Peltier device with an accuracy of ꢂ0.1 K, and the scattering
angle was 173ꢁ. The data were analyzed using standard algo-
rithms, and are reported with an uncertainty of less than 8%.
Experimental Section
Materials
Cyclohexanol (≥98%), epichlorohydrin (≥98%), and epibro-
mohydrin (≥98%) were purchased from Central Drug House,
Mumbai, India, and were used without further refinement.
Zinc perchlorate hexahydrate (≥98%) and pyridine (≥99.5%)
were purchased from Sigma-Aldrich, St. Louis, Missouri,
United States. N-Methylimidazole (≥99%) was an Acros
product, Janssen-Pharmaceuticalaan 3, 2440 Geel, Belgium.
The conventional IL C8mimBr was synthesized by the qua-
ternization of n-octyl bromide (≥98%) with n-methyl imidaz-
ole (≥99%) and reference IL by an earlier reported procedure
(Chauhan, Singh, & Kamboj, 2014). Chemical structures of
conventional IL C8mimBr and reference IL are shown in
Fig. S2 in File S1 (Supporting Information).
Thermal Stability Measurements
A Perkin Elmer Pyris 1 thermogravimetric analyzer (TGA)
was used for the measurement of thermal stability of the
synthesized IL. All the experiments were run in aluminum
pans under nitrogen atmosphere at the heating rate of
10 ꢁC/min.
Cytotoxicity Analysis
Methods
Cytotoxicity tests were carried out on C6 glioma (cancer-
ous brain cell line, passage number 65) by using MTT
(3-(4, 5-dimethylthiazole-2-yl)-2,5-diphenyltetrazolium bro-
mide) to evaluate the IC50 values of all the synthesized
IL. Cells were seeded at a density of 10 × 103 cells/mL in
96-well microtiter plates. After 24 h of seeding, cells were
treated with the test compounds in the concentration range
20–500 μM. After 24 h of treatment with the test com-
Conductivity Measurements
The instrument used for conductivity measurements was a
conductivity meter (model EQ661; Equip-Tronics, Mumbai,
Maharastra, India). A conductivity cell of cell constant
K = 1.00 was used for all measurements. The aqueous solu-
tions prepared for the experiments were thermostated in the
ꢁ
pounds, the cells were incubated at 37 C with MTT con-
ꢁ
cell at 25.0 ꢂ 0.1 C. For the measurement of the critical
taining 10 mg/10 mL of serum-free medium. The blue
formazan crystals formed by viable cells were solubilized by
100 μL DMSO, and absorbance was read at 595 nm using a
micelle concentration (CMC), double-distilled water of spe-
cific conductivity in the range 1–2 μS cm−1 was used. The
J Surfact Deterg (2018) 21: 43–52

J Surfact Deterg
45
Scheme 1 Synthesis of ionic liquids
ꢁ
Multiskan PLUS plate reader (Thermo Scientific, Waltham,
Massachusetts, USA). The conventional IL C8mimBr was
also tested for cytotoxicity as a reference.
pyridine (5; 1.18 g, 15 mmol) at 80 C for 4 h to give the
respective cationic imidazolium and pyridinium amphiphiles
(Scheme 1). These amphiphiles were purified by recrystalliza-
tion from ethyl acetate and acetone and then dried under
vacuum to get the final product 6a/6b or 7a/7b. The charac-
terization of IL 6a/6b and 7a/7b was done by using various
spectroscopic techniques. IR spectra were recorded in chloro-
form on a Perkin Elmer FT-IR spectrometer. A Bruker
LC-MS system MicroTOF-Q II 10356 was used for mass
spectrometry measurements. An FT-NMR spectrometer
(Bruker, Avance II, Billerica, Masachusetts, USA) was used
Synthesis and Analysis of Surface-Active IL
A two-step methodology was used for the synthesis of
hydroxyl group-functionalized SAIL. The first step included
the synthesis of the intermediates 1-halo-3-(cyclohexyloxy)
propan-2-ol (3a/3b). Cyclohexanol (1; 2 g, 20 mmol) was
stirred magnetically with epichlorohydrin (2a; 3.68ꢁg,
40 mmol) or epibromohydrin (2b; 5.44 g, 40 mmol) at 80 C
for 1 h under solvent-free conditions. Zinc perchlorate hexa-
hydrate was used as catalyst in this reaction. The progress of
the reaction was measured by thin-layer chromatography
using silica gel G-coated 0.25-mm-thick glass plates with
hexane/ethyl acetate (90:10) as the mobile phase. Then
1-halo-3-(cyclohexyloxy)propan-2-ol (3a/3b) were separated
by stepwise fractionation by column chromatography using
silica gel (60–120 mesh) and hexane/ethyl acetate (95:5) as
the solvent system. In the next step, 1-chloro-3-(cyclohexy-
loxy)propan-2-ol (3a; 3.09 g, 15 mmol) and 1-bromo-3-
(cyclohexyloxy)propan-2-ol (3b; 3.75 g, 15 mmol) were
quaternized with N-methylimidazole (4; 1.23 g, 15 mmol) or
for H and 13C/DEPT NMR analysis. Samples for the NMR
1
analysis were prepared in CDCl3 using the internal standard
tetramethylsilane (TMS). CHNS analyses were carried out by
a Flash EA 1112 series elemental analyzer (Thermo Electron
Corporation, Waltham, Massachusetts, USA).
Results and Discussion
Analysis
The infrared spectra of these IL exhibited the OH stretching
band at 3435 cm−1. The C–H stretching frequencies were
J Surfact Deterg (2018) 21: 43–52

46
J Surfact Deterg
Table 1 Surface and micellar parameters of IL in aqueous systems
IL
CMC (mM)
Conductancea
β
γCMC
Γmax × 106
Amin
πCMC
ΔGꢁ
ΔGꢁ
m
ad
(mN m−1
)
(mol m−2
)
(nm2)
(mN m−1
)
Surface
tensionb
(kJ/mol)
6a
7.98
6.86
7.62
7.33
0.30
0.38
0.28
0.37
0.49
51.87 ꢂ 0.01
49.01 ꢂ 0.01
45.12 ꢂ 0.01
44.91 ꢂ 0.01
29.33 ꢂ 0.01
1.56
2.01
2.05
2.69
3.065
1.04
0.82
0.79
0.61
0.53
20.33
23.19
27.08
27.29
42.87
−21.55
−28.77
−27.94
−28.18
−22.5
−34.10
−42.50
−38.00
−41.58
−36.5
6b
7a
8.43
8.27
7b
7.00
7.16
C8mimBr
137.36
119.29
a
Electrical conductivity measurements with error estimate ꢂ0.1 mM
Surface tension measurements with error estimate ꢂ0.1
b
observed at 2858 and 2934 cm−1. The C N stretching fre-
quency was observed in the range 1633–1635 cm−1. The
C C stretching frequency for the aromatic imidazolium
and pyridinium ring was observed at 1452 cm−1. The
stretching frequency of C–O was observed in the range
1085–1093 cm−1. The structures of these IL were further
for 6a and δ 4.07 ppm for 6b. The methylene (–CH2) protons
directly attached to the positively charged quaternized nitro-
gen are diastereotopic in nature and were observed as two
multiplets at δ 4.33–4.37 ppm and δ 4.51–4.55 ppm for
6a; δ 4.34–4.38 ppm and δ 4.53–4.58 ppm for 6b; δ
4.31–4.34 ppm and δ 4.83–4.90 ppm for 7a; and δ
5.12–5.15 ppm and δ 4.84–4.89 ppm for 7b. The signal for
–CHOH protons was observed at δ 4.14 for 6a, δ 4.04 for
6b, δ 4.31 for 7a, and δ 4.34 for 7b. Further, the methylene
protons –OCH2CHOH were observed as multiplets at δ
3.33–3.61 ppm for 6a, at δ 3.39–3.65 ppm for 6b, and at δ
3.37–3.71 ppm for 7a/7b. The (–CH) proton of the cyclo-
hexyl ring carbon directly attached to the ether oxygen was
observed as a multiplet at δ 3.26–3.33 ppm and δ 3.28 ppm
for 6(a/b) and 7(a/b), respectively.
1
been confirmed by H, 13C, DEPT (distortionless enhanced
polarization transfer) and 2D HETCOR (heteronuclear chem-
ical shift correlation) experiments. The methylene (–CH2)
protons of cyclohexyl ring appeared as a multiplet in the
range 1.22–1.89 ppm in all IL. The methylene protons H-40,
H-30, and H-50 of cyclohexyl ring of the IL 6a (Fig. S1 in File
S1) appeared in the range δ 1.22–1.53 ppm, whereas H-20
and H-60 appeared in the range δ 1.71–1.88 ppm. However,
these signals were observed in the range δ 1.22–1.89 ppm
for all the other IL. The chemical shift for the heterocyclic
protons –N+CHN– was observed at δ 9.60 ppm for 6a and δ
9.52 ppm for 6b, and the heterocyclic protons –N+CHCHN–
were observed as a doublet at δ 7.43 ppm and δ 7.55 ppm
for 6a and δ 7.47 ppm and δ 7.53 ppm for 6b, respectively.
All the pyridinium ring protons were observed in the range δ
8.10–9.20 ppm. The methyl (–CH3) group directly attached
to the heterocyclic moiety was observed as a singlet at δ 4.05
13C DEPT spectra also helped in assigning the sp3 ring
methylene carbons at δ 23.98–32.16 for all the amphiphiles.
The carbon directly attached to the quaternized nitrogen atom
was observed at δ 52.98 ppm for both 6a and 6b. The hetero-
cyclic carbon –N+CHN– was observed at δ 137.77 ppm and
δ 137.43 ppm for 6a and 6b, respectively. The resonance for
the heterocyclic carbon –N+CHCHN– was observed in the
range δ 123.06–123.21 ppm for 6a and 6b. The methyl
Fig. 1 (a) Variation of specific conductivity as a function of the concentration of ionic liquids 6a/6b and 7a/7b. (b) Variation of surface tension
as a function of the log of concentration of ionic liquids 6a/6b and 7a/7b
J Surfact Deterg (2018) 21: 43–52

J Surfact Deterg
47
carbon directly attached to the imidazolium moiety was
observed at δ 36.65 ppm for 6a and δ 36.93 for 6b ppm. The
signal for the carbon of the cyclohexyl moiety directly
Surface tension measurement provides essential informa-
tion about the adsorption of IL at the air/water interface;
results for these IL are shown in Fig. 1. According to the
Gibbs adsorption isotherm, Γmax (maximum surface excess
concentration) was estimated by using the following equa-
tion (Rosen, 2004):
attached to the ether oxygen was observed at
δ
78.21–78.22 ppm and δ 78.30–78.41 ppm for 6a/6b and 7a/
7b, respectively.
ESI-MS (positive ion) mass spectroscopy was found to
be a very valuable technique for the elucidation of the
structures of the synthesized compounds. The parent ion
peaks were observed for the imidazolium (6a/6b) and pyri-
dinium (7a/7b) IL at m/z 239.15 and 236.17, respectively.
These intensities were due to the loss of the counterions,
i.e., the chloride ion (Cl−) and bromide ion (Br−) from the
molecule, i.e, the formation of the positively charged parent
ion [M+– Cl or M+– Br].
Γmax = −1=2:303nRT ð∂γ=∂lnCÞT
ð1Þ
where R, T, and C are the gas constant, absolute tempera-
ture, and the IL concentration, respectively. The value of
n here is 2, because there is one counterion linked to a sin-
gle cationic head group. Γmax is the amount of IL adsorbed
per unit area at the air/water interface after the development
of an absolute monolayer and is determined from the slope
of the log of CMC versus surface tension plot when the
concentration is approaching the CMC. The minimum area
engaged by a single IL molecule at the air/water interface,
Amin, was also determined via the following equation:
Micellization and Surface Activity
À
Á
Amin = 1023 =NA × Γmax
ð2Þ
The CMC values and other surface properties of the IL in
an aqueous system were determined from the conductivity
and surface-tension measurements and are summarized in
Table 1. Figure 1a shows the change of specific conduc-
tance of the aqueous solution versus the concentration of
the IL in the aqueous system. The specific conductivity
values fall into two linear regions of different slopes. The
position of the abrupt changes of the slopes was taken as
the CMC.
Figure 1b shows the surface tension (γ) versus the loga-
rithm of concentration plots of the IL in the aqueous system
at 298 K. At low concentration, the surface tension gradu-
ally decreased on incremental addition of IL to the aqueous
system, but after some additions, the surface tension (γ)
reached a point near the CMC value where the adsorption
of IL tends to a limiting value, so the surface tension curve
appears to be linear. This region indicates the formation of
micelles. The concentration corresponding to the break
point was taken as CMC.
where NA is the Avogadro number and Amin is in nm2. The
micellization parameters, i.e, CMC, γCMC, Γmax, and Amin
,
for the IL 6a/6b and 7a/7b are listed in Table 1. However,
according to the studies of Menger and Rizvi (2011), Men-
ger, Rizvi, and Shi (2011), the determination of area from
the surface-tension curve is limited by the effects of the
finite width of micellization. With the start of micelle for-
mation, the surfactant molecules prefer to go in to the
micelle rather than for the saturation of the interface. There-
fore, the decrease in the surface tension value ceases at the
CMC. A detailed survey of the literature suggests that
adsorption is a cooperative and spontaneous process, and
the applicability of the Gibbs adsorption equations is a
topic of debate (Li, Thomas, & Penfold, 2014; Mukherjee,
Moulik, & Rakshit, 2013). A comprehensive investigation
of the state of adsorption is beyond this work.
The reduction in surface tension at the CMC was deter-
mined by the equation
The CMC values obtained from the specific conductivity
(κ) versus concentration plots are in good agreement with
the values obtained from surface tension (γ) versus log con-
centration plots. From Fig. 1a, it can be clearly observed
that the slope of the linear region below the CMC is greater
than the linear region above the CMC. The values of the
counter-ion binding, β, of the IL are listed in Table 1. Fur-
ther, the CMC values of the IL with chloride (Cl−) as the
counter-ion are more than with Br¯ as the counterion; it is
well known that the affinity of Br¯ is stronger for the
air/water interface. As a result, the respective hydrophobic
tail adsorbed more densely at the air/water interface and the
value of γCMC decreased at the interface. The above results
established that the counterion affects the γCMC values of
SAIL with the same hydrophobic tail.
πCMC = γo – γCMC
ð3Þ
where π is the reduction in the value of surface tension, γo
is the surface tension of pure solvent, and γCMC is the mea-
sured surface tension at the CMC.
The micellization parameter, i.e., CMC, of the bromide-
containing IL, was less than that of IL with the chloride
counterion, and the degree of counterion binding (β) was
found to be higher for IL containing the Br− counterion.
The repulsion in case of chloride ions from the interface is
higher than that with the bromide ions due to their higher
polarizability and van der Waals interaction with the inter-
face. The value of Γmax is higher in case of bromide-
containing IL, which is ascribed to the weak hydration of
Br−. In addition, Amin is lower for the bromide-containing
J Surfact Deterg (2018) 21: 43–52

48
J Surfact Deterg
(b)100
(a)100
Tstart
Tonset
7a
7b
Tstart
Tonset
6a
6b
80
60
40
20
0
80
60
40
20
0
Tstart
Tstart
Tonset
Tonset
0
100
200
300
400
500
0
100
200
300
400
500
Temperature 0C
Temperature 0C
Fig. 2 Thermal decomposition curves of ionic liquids: (a) 6a/6b and (b) 7a/7b, showing the start (Tstart) and the onset (Tonset) temperature of
degradation
IL, suggesting that the Br− counterion assembles more
densely at the air/water interface compared to Cl−-
containing IL.
between ΔGꢁad and ΔGꢁm means a more favorable transfer
of the surfactant molecules from the surface to the micellar
phase and that the surfactant undergoes aggregation more
readily in water (Rebelo, Lopes, Esperança, & Filipe,
2005). Our experimental results show a smaller free energy
gap between ΔGꢁad and ΔGꢁm, and therefore these IL have
a greater tendency to aggregate in solution. The low values
of Amin also indicate the formation of premicellar aggre-
gates more readily in water. As the value of free energy of
adsorption (ΔGꢁad) is more negative for bromide ions com-
pared to chloride ions, adsorption is more favorable in case
of 6b and 7b. But the difference in the values of free ener-
gies of adsorption (ΔGꢁad) and free energy of micellization
(ΔGꢁm) is also small in case of 6b and 7b. Therefore, 6b
and 7b require less amount of work to transfer to bulk solu-
tion from the air/water interface to form premicellar aggre-
gates. The lower Amin values of 6b and 7b compared to 6a
and 7a are also in concurrence with the above outcomes.
On comparing the CMC values of the cyclohexyloxy IL
with the earlier synthesized alkyloxy IL (Bordes et al.,
2010), the CMC value of the former is lower. This proves
that the modification of the structure of hydrophobic tail
from open chain to cyclic structure induces more hydropho-
bicity to the moiety and affects the interfacial and micellar
parameters (Mukherjee, 1967). Therefore, the cyclohexy-
loxy IL are more surface active compared to open-chain
alkyloxy IL.
Thermodynamic Studies
The standard Gibbs free energy of micellization (ΔGꢁ
and the standard Gibbs free energy of adsorption (ΔGꢁ
)
)
m
ad
(Callaghan, Doyle, Alexander, & Palepu, 1993; Rosen,
2004) were evaluated by using the following equation:
Thermal Stability Assessment by TGA
ΔGꢁ m = ð1 + βÞRT lnXCMC = ΔGꢁ ad + ðπCMC=Γmax
Þ
ð4Þ
IL possess negligible vapor pressure and have high thermal
stability (Monteiro, Camilo, Ribeiro, & Torresi, 2010).
Therefore, the thermal stability of the synthesized IL was
analyzed by using a thermogravimetric analyzer. All these
new IL possess reasonably good thermal stability. The
decomposition range of these IL was found to be between
where XCMC is the mole fraction at CMC and β is the
counterion binding. The free energy of adsorption (ΔGꢁ
)
ad
signifies the free energy change when 1 mol of the surfac-
tant in solution is transferred to the surface, and the free
energy of micellization (ΔGꢁm) is defined as the amount of
work done when the surfactant molecules from the mono-
meric form are transferred to the micellar phase from the
surface (Wang, Du, Li, & Zhang, 2010). The values of
ΔGꢁm and ΔGꢁad determine the affinity to form micelles in
solution and to adsorb at the air/water interface
(Yoshimura, Bong, Matsuoka, Honda, & Endo, 2009). If
ꢁ
215 and 242 C. Figure 2 shows the Tonset and Tstart of the
IL 6a/6b and 7a/7b. The start temperature (Tstart) is the
Table 2 Onset and start temperatures for thermal decomposition of
ionic liquids
ΔGꢁ is less negative than the free energy of adsorption
IL
6a
6b
7a
7b
m
ΔGꢁad, the adsorption of surfactant molecule at the air/-
water interface becomes more favorable than in the interior
of the micelle. However, a smaller energy difference
Tstart (ꢁC)
Tonset (ꢁC)
213.81
220.82
226.60
242.53
201.81
215.72
221.52
236.01
J Surfact Deterg (2018) 21: 43–52

J Surfact Deterg
49
Table 3 Melting points of ionic liquids
Table 4 Hydrodynamic diameter (in nm) of ionic liquids (6a/6b and
7a/7b)
IL
6a
6b
7a
7b
Sample
Pure IL
6a
6b
7a
7b
Mp (ꢁC)
84.79
69.38
67.93
64.31
1.72
1.45
0.65
0.62
temperature at which the degradation of the compound
starts, while the onset temperature (Tonset) is the point of
intersection of baseline weight from the start of the experi-
ment and the tangent of the weight versus temperature
curve where the disintegration of the compound takes
place.
These results are in agreement with the earlier reported
results (Gusain et al., 2014; Ise, 2010; Monteiro
et al., 2010).
Melting Point
Among these IL (Table 2), 6b possesses the highest ther-
mal stability whereas 7a has the least. Furthermore, these
IL show better thermal stability than the earlier synthesized
ester-based IL (Mukherjee, 1967), likely because of the
higher thermal stability of the ether linkage (Gusain, Gupta,
Saran, & Khatri, 2014). The corresponding differential
thermogravimetric analysis (DTA) profiles are also shown
in Fig. S3 in File S1. All these IL show a two-step weight
Melting point is the temperature at which there is equilib-
rium between the solid and liquid state at a fixed pressure.
There are many factors on which the melting point of any
compound depends, like symmetry of the hydrophobic
chain or cation (Wasserscheid & Welton, 2003), or inter-
molecular forces like hydrogen bonding, electrostatic or
van der Waals forces, distribution of charge, and size of the
anion (Elaiwi et al., 1995). With increase in the size of the
anion, the melting point decreases. The weaker electrostatic
interaction is stimulated by larger anions with heterocyclic
moieties like pyridine and imidazolium. The melting points
ꢁ
loss in the range 240–265 C (≈ 25% weight loss) and
ꢁ
320–366 C (≈ 56% weight loss). The thermal degrada-
tion of the investigated compounds was observed as a
ꢁ
large weight loss (50%) at 366.50ꢁ C for 6a, (55%) at
ꢁ
ꢁ
of the present IL were found to be in the range 70–80 C
(Table 3).
346.20 Cꢁfor 6b, (56%) at 320.81 C for 7a, and (59%)
at 345.43 C for 7b, due to loss of the ether-linked group
(C6H11O)ꢀ along with counterion. The weight loss
ꢁ
ꢁ
(13.76%) at 139 C for 6a, (25.06%) at 243.2 C for 6b,
DLS Experiments
ꢁ
ꢁ
(13.04%) at 197.58 C for 7a, and (25.26%) at 242 C
for 7b is due to the loss of counterions (Cl− in 6a and 7a
and Br− in 6b and 7b).
DLS provides useful information on the size distribution of
the micelles of amphiphiles in an aqueous solution. The
above physicochemical studies, i.e., surface tension and
conductometry, indicate that these hydroxyl group-
functionalized pyridinium/imidazolium cationic IL have the
ability to form aggregates above the CMC. The aggregate
size of the investigated IL in the aqueous system was mea-
sured by DLS experiments. Intensity-weighted light scatter-
ing graphs (Fig. 3) show two size distributions, with
Further, the IL with bromide as the counterion has
higher thermal stability than those with chloride as the
counterion. Our experimental results also show that the IL
with imidazolium head group are more thermally stable
than pyridinium core-containing IL. This is due to the more
packed structure of the imidazolium ring, higher intermole-
cular interactions, and low steric hindrance (Ise, 2010).
Fig. 3 (a) Intensity-weighted and (b) number-weighted size distribution in aqueous solution of different ionic liquids (6a/6b and 7a/7b) from
DLS measurement
J Surfact Deterg (2018) 21: 43–52

50
J Surfact Deterg
Fig. 4 (a) Absorbance (nm) versus concentration (μM) plot of IL 6a/6b, 7a/7b, C8mimBr, and reference IL for determination of IC50. The results
shown in the figure represent the average IC50 value of three different trials done in triplicate. (b) Percent cell viability at different concentrations
of IL 6a/6b and 7a/7b
smaller aggregates having an average hydrodynamic diam-
eter in the range 0.62–1.45 nm, and the larger aggregates
with hydrodynamic radius ranging between 123.61 and
215.19 nm, whereas the number-weighted size distribution
graphs show the presence of aggregate with a size
0.62–1.45 nm only. It has been assumed that the collision
of smaller colloidal particles with each other will result in
the larger aggregates (Bohren & Huffman, 1983). Accord-
ing to Rayleigh’s approximation, the intensity of scattered
light varies directly as the sixth power of particle size;
therefore it is considered that larger particles scatter much
more light than the smaller ones (Kamboj, Singh, Bhadani,
Kataria, & Kaur, 2012). The outcomes from Fig. 3 clearly
indicate a higher hydrodynamic diameter (Dh) of the IL
containing the chloro (Cl−) counterion compared to their
bromo (Br−)-containing counterparts. From the data
(Table 4), it can also be seen that the hydrodynamic diame-
ter of pure imidazolium IL (6a/6b) is slightly larger than
that of the pure pyridinium IL (7a/7b).
functionalized IL possess low toxicity and can be used as
carriers in many biomedical applications like gene and drug
delivery. The cytotoxicity of the SAIL was expressed as per-
centage of cell viability in terms of the IC50 value (Fig. 4b),
defined as the concentration of the test compound (in μM)
that causes the death of 50% of the living cells. The conven-
tional IL C8mimBr and the reference IL (3-(2-hydroxy-3-
(pentyloxy)propyl)-1-methyl-1H-imidazol-3-ium chloride)
(ref ) were also tested along with the other amphiphiles. The
experimental results are shown in Figs. 4a, b, which shows
that the investigated cyclohexyloxy SAIL are not cytotoxic
up to a concentration of 250 μM, while at all concentrations
the conventional IL C8mimBr and the reference compound
(ref ) were toxic to the cells. From the above results, it can be
concluded that the modification in the hydrophobic tail from
the open chain to cyclic affects the cytotoxicity value. The
compound becomes less toxic by inducing the cyclohexyloxy
group into the hydrophobic tail. Furthermore, the present
compounds have also found to be less toxic compared to the
earlier synthesized imidazolium and pyridinium amphiphiles
(Chauhan, Singh, Kamboj, Mishra, & Kaur, 2014).
Cytotoxicity Assessment
Hence, the introduction of ether and alcohol functional
groups and the cyclohexyl moiety in SAIL affects the phys-
icochemical properties in a positive way for potential deliv-
ery applications in biological systems.
In the present research work, C6 glioblastoma cells were
treated with 20–500 μM concentrations of the IL by per-
forming MTT assay to evaluate cytotoxicity or cell viability
after 24 h of treatment. The histograms in Fig. S4 in File
S1 represent the percentage of viable cells compared to
control after 24 h of treatment. To determine significance
of results, one-way analysis of variance with Holm–Sidak
post hoc test was used (difference between control and cells
treated with IL). Cells were healthy up to 250 μM concen-
tration of the test compounds. Retraction of processes and
rounding of cells were observed in a culture treated with
higher than 250 μM concentration of IL. The experimental
results clearly show that the ether- and alcohol-
Conclusion
Four new cyclohexyloxy-based SAIL were synthesized and
evaluated for their self-aggregation behavior, thermal sta-
bility, and cytotoxicity. The introduction of the cyclohexy-
loxy moiety and the coexistence of an ether or a hydroxyl
group in these IL affect the physicochemical properties posi-
tively by decreasing the CMC values of the investigated IL
J Surfact Deterg (2018) 21: 43–52

J Surfact Deterg
51
copolymers and ionic liquids as rational templates. Chemistry of
Materials, 17:1683–1690.
Cadena, C., Anthony, J. L., Shah, J. K., Morrow, T. I.,
Brennecke, J. F., & Maginn, E. J. (2004) Why is CO2 so soluble in
imidazolium-based ionic liquids? Journal of the American Chemi-
cal Society, 126:5300–5308.
compared to earlier reported conventional (Cornellas et al.,
2011) and alkyloxy IL (Bordes et al., 2010). Thermal degra-
dation temperatures of investigated IL were determined by
TGA, which showed a dependence on the size of the coun-
terion and cationic moiety (Mukherjee, 1967). The investi-
gated IL containing the bromo counterion possessed higher
thermal stability than the corresponding chloride counterion-
containing ones. Similarly, the imidazolium cations were
found to be more thermally stable, due to their compact
structure and lesser free volume, than the pyridinium ana-
logs. The MTT assay was performed to deduce the cytotox-
icity of the investigated IL. The results showed that the
studied IL were noncytotoxic in the prescribed range below
250 μM. The cytotoxicity of these SAIL was found to be
much lower than that of the conventional IL (C8mimBr)
and some earlier reported cationics (Chauhan, Singh,
Kamboj, Mishra, & Kaur, 2014; Cornellas et al., 2011). DLS
experiments indicated that these new IL form aggregates
above the CMC with Dh in the range 0.62–1.45 nm.
Callaghan, A., Doyle, R., Alexander, E., & Palepu, R. (1993) Ther-
modynamic properties of micellization and adsorption and electro-
chemical studies of hexadecylpyridinium bromide in binary
mixtures of 1,2-ethanediol with water. Langmuir, 9:3422–3426.
Chauhan, V., Singh, S., & Bhadani, A. (2012) Synthesis, characteriza-
tion and surface properties of long chain β-hydroxy-γ-alkyloxy-
N-methylimidazolium surfactants. Colloids and Surfaces A:
Physicochemical and Engineering Aspects, 395:1–9.
Chauhan, V., Singh, S., & Kamboj, R. (2014) Self-assembly and ther-
mal stability of ether functionalized imidazolium ionic liquids.
Industrial and Engineering Chemistry Research, 53:13247–13255.
Chauhan, V., Singh, S., Kamboj, R., Mishra, R., & Kaur, G. (2014)
Self-assembly, DNA binding and cytotoxicity trends of ether func-
tionalized gemini pyridinium amphiphiles. Journal of Colloid and
Interface Science, 417:385–395.
Cornellas, A., Perez, L., Cornelles, F., Ribosa, I., Manresa, A., &
Garcia, M. T. (2011) Self-aggregation and antimicrobial activity of
imidazolium and pyridinium based ionic liquids in aqueous solu-
tion. Journal of Colloid and Interface Science, 355:164–171.
Elaiwi, A., Hitchcock, P. B., Seddon, K. R., Srinivasan, N.,
Tan, Y. M., Welton, T., & Zora, J. A. (1995) Hydrogen bonding in
imidazolium salts and its implications for ambient-temperature
halogenoaluminate (III) ionic liquids. Journal of the Chemical Soci-
ety Dalton Transactions, 21:3467–3472.
Hence, tailoring of the SAIL structure with suitable func-
tional groups and cyclohexyl ring affects the surface, ther-
mal, and cytotoxicity positively, and these IL can find a
better future in scientific applications such as gene/drug
delivery, medicine, and other areas.
Fukumoto, K., Yoshizawa, M., & Ohno, H. (2005) Room temperature
ionic liquids from 20 natural amino acids. Journal of the American
Chemical Society, 127:2398–2399.
Gusain, R., Gupta, P., Saran, S., & Khatri, O. P. (2014) Halogen free
bis(imidazolium)/bis (ammonium)-di[bis(salicylato)borate] ionic
liquids as energy efficient and environmentally friendly lubricant
additives. ACS Applied Materials & Interfaces, 6:15318–15328.
Hanabusa, K., Fukui, H., Suzuki, M., & Shirai, H. (2005) Specialist
gelator for ionic liquids. Langmuir, 21:10383–10390.
He, Y., Li, Z., Simone, P., & Lodge, T. P. (2006) Self-assembly of
block copolymer micelles in an ionic liquid. Journal of the Ameri-
can Chemical Society, 128:2745–2750.
Howarth, J. (2000) Oxidation of aromatic aldehydes in the ionic liquid
[bmim]PF6. Tetrahedron Letters, 41:6627–6629.
Huddleston, J. G., Visser, A. E., Reichert, W. M., Willauer, H. D.,
Broker, G. A., & Rogers, R. D. (2001) Characterization and com-
parison of hydrophilic and hydrophobic room temperature ionic liq-
uids incorporating the imidazolium cation. Green Chemistry, 4:
156–164.
Huddleston, J. G., Willauer, H. D., Swatloski, R. P., Visser, A. E., &
Rogers, R. D. (1998) Room temperature ionic liquids as novel
media for ‘clean’ liquid–liquid extraction. Chemical Communica-
tions, 16:1765–1766.
Ise, N. (2010) Like likes like: Counterion mediated attraction in
macroionic and colloidal interaction. Physical Chemistry Chemical
Physics, 12:10279–10287.
Javadian, S., Ruhi, V., Heydari, A., Shahir, A. S., Yousefi, A., &
Akbari, J. (2013) Self-assembled CTAB nanostructures in aqueou-
s/ionic liquid systems: Effects of hydrogen bonding. Industrial and
Engineering Chemistry Research, 52:4517–4526.
Acknowledgements The author thanks the UGC BSR for providing
a Research Fellowship. He also thanks the Sophisticated Analytical
Instrumentation Facility (SAIF), Panjab University, Chandigarh,
India, for providing the NMR facilities.
References
Aggarwal, R., & Singh, S. (2014) Synthesis, characterization, and evalu-
ation of surface properties of cyclohexyloxyoxoethylbipyridinium
gemini amphiphiles and a comparison with single-tailed amphiphiles.
Industrial and Engineering Chemistry Research, 53:2549–2557.
Aggarwal, R., Singh, S., & Hundal, G. (2013) Synthesis, characteriza-
tion and evaluation of surface properties of cyclohexanoxycarbo-
nylmethyl pyridinium and cyclohexanoxycarbonyl methyl
imidazolium ionic liquids. Industrial and Engineering Chemistry
Research, 52:1179–1189.
Antonietti, M., Kuang, D., Smarsly, B., & Yong, Z. (2004) Ionic liq-
uids for the convenient synthesis of functional nanoparticles and
other inorganic nanostructures. Angewandte Chemie, International
Edition, 43:4988–4992.
Avery, T. D., Jenkins, N. F., Kimber, M. C., Lupton, D. W., &
Taylor, D. K. (2002) First examples of the catalytic asymmetric
ring-opening of meso 1,2-dioxines utilising cobalt(II) complexes
with optically active tetradentate Schiff base ligands: Formation of
enantio-enriched cyclopropanes. Chemical Communications, 1:
28–29.
Bohren, C. F., & Huffman, D. R. (1983) Absorption and scattering of
light by small particles. New York, NY: Wiley.
Kamboj, R., Singh, S., Bhadani, A., Kataria, H., & Kaur, G. (2012)
Gemini imidazolium surfactants:
Synthesis and their
Bordes, R., Tropsch, J., & Holmberg, K. (2010) Role of an amide
bond for self-assembly of surfactants. Langmuir, 26:3077–3083.
Brezesinski, T., Erpen, C., Iimura, K., & Smarsly, B. (2005) Mesos-
tructured crystalline ceria with a bimodal pore system using block
bio-physiochemical study. Langmuir, 28:11969–11978.
Kitazume, T., Zulfiqar, F., & Tanaka, G. (2000) Molten salts as a
reusable medium for the preparation of heterocyclic compounds.
Green Chemistry, 2:133–136.
J Surfact Deterg (2018) 21: 43–52

52
J Surfact Deterg
Leone, A. M., Weatherly, S. C., Williams, M. E., Thorp, H. H., &
Murray, R. W. (2001) An ionic liquid form of DNA: Redox active
molten salts of nucleic acids. Journal of the American Chemical
Society, 123:218–222.
imidazolium room temperature ionic liquids: Synthesis from an
organometallic precursor, characterization, and catalytic proper-
ties in hydrogenation reactions. Inorganic Chemistry, 42:
4738–4742.
Li, P. X., Thomas, R. K., & Penfold, J. (2014) Limitations in the use
of surface tension and the Gibbs equation to determine surface
excesses of cationic surfactants. Langmuir, 30:6739–6747.
Lu, W., Fadeev, A. G., Qi, B. H., Smela, E., Mattes, B. R., Ding, J.,
… Forsyth, M. (2002) Use of ionic liquids for π-conjugated poly-
mer electrochemical devices. Science, 297:983–987.
McClements, D. J. (2004) Food emulsions: Principles, practice and
techniques (2nd ed.). Boca Raton, FL: CRC Press.
Menger, F. M., & Rizvi, S. A. A. (2011) Relationship between surface
tension and surface coverage. Langmuir, 27:13975–13977.
Menger, F. M., Rizvi, S. A. A., & Shi, L. (2011) Reply to “Should the
Gibbs analysis be revised?”. Langmuir, 27:7963–7965.
Seth, D., Chakraborty, A., Setua, P., & Sarkar, N. (2006) Interaction
of ionic liquid with water in ternary microemulsions (Triton
X-100/water/1-butyl-3-methylimidazolium hexafluorophosphate)
probed by solvent and rotational relaxation of coumarin 153 and
coumarin 151. Langmuir, 22:7768–7775.
Suarez, P. A. Z., Dullius, J. E. L., Einloft, S., de Souza, R. F., &
Dupont, J. (1996) The use of new ionic liquids in two-phase cata-
lytic hydrogenation reaction by rhodium complexes. Polyhedron,
15:1217–1219.
Taubert, A. (2004) CuCl nanoplatelets from an ionic liquid-crystal
precursor. Angewandte Chemie, International Edition, 43:
5380–5382.
Monteiro, M. J., Camilo, F. F., Ribeiro, M. C. C., & Torresi, R. M.
(2010) Ether bond containing ionic liquids and the relevance of the
ether bond position to transport properties. The Journal of Physical
Chemistry. B, 114:12488–12494.
Wang, G. Y., Du, Z. P., Li, Q. X., & Zhang, W. (2010) Carbohydrate
modified-siloxane surfactants and their adsorption and aggregation
behavior in aqueous solution. The Journal of Physical Chemistry.
B, 114:6872–6877.
Mukherjee, I., Moulik, S. P., & Rakshit, A. K. (2013) Tensiometric
determination of Gibbs surface excess and micelle point: A critical
revisit. Journal of Colloid and Interface Science, 394:329–336.
Mukherjee, P. (1967) The nature of association equilibria and hydro-
phobic bonding in aqueous solutions of association colloids.
Advances in Colloid and Interface Science, 1:242–275.
Peng, J. J., & Deng, Y. Q. (2001) Ionic liquids catalyzed Biginelli
reaction under solvent-free conditions. Tetrahedron Letters, 42:
5917–5919.
Poole, C. F. (2004) Chromatographic and spectroscopic methods for
the determination of solvent properties of room temperature ionic
liquids. Journal of Chromatography. A, 1037:49–82.
Rebelo, L. P. N., Lopes, J. N. C., Esperança, J. M. S. S., & Filipe, E.
(2005) On the critical temperature, normal boiling point, and vapor
pressure of ionic liquids. The Journal of Physical Chemistry. B,
109:6040–6043.
Wang, Y. T., & Voth, G. A. (2005) Unique spatial heterogeneity in
ionic liquids. Journal of the American Chemical Society, 127:
12192–12193.
Wasserscheid, P., & Welton, T. (Eds.) (2003) Ionic liquids in synthe-
sis (pp. 41–55). Weinheim, Germany: Wiley-VCH.
Wellens, S., Thijs, B., & Binnemans, K. (2012) An environmentally
friendlier approach to hydrometallurgy: Highly selective separation
of cobalt from nickel by solvent extraction with undiluted phospho-
nium ionic liquids. Green Chemistry, 14:1657–1665.
Welton, T. (1999) Room temperature ionic liquids: Solvents for syn-
thesis and catalysis. Chemical Reviews, 99:2071–2083.
Yoshimura, T., Bong, M., Matsuoka, K., Honda, C., & Endo, K.
(2009) Surface properties and aggregate morphology of partially
fluorinated carboxylate type anionic gemini surfactants. Journal of
Colloid and Interface Science, 339:230–235.
Zhao, H., & Malhotra, S. V. (2002) Application of ionic liquids in
organic synthesis. Aldrichimica Acta, 35:75–83.
Rosen, M. J. (2004) Surfactants and interfacial phenomenon (3rd
ed.). New York, NY: John Wiley & Sons.
Zhou, Y. (2005) Recent advances in ionic liquids for synthesis of
inorganic nanomaterials. Current Nanoscience, 1:35–42.
Zhou, Y., & Antonietti, M. (2003) Synthesis of very small TiO2 nano-
crystals in a room-temperature ionic liquid and their self-assembly
toward mesoporous spherical aggregates. Journal of the American
Chemical Society, 125:14960–14961.
Sanders, J. R., Ward, E. H., & Hussey, C. L. (1986) Aluminum
bromide-1-methyl-3-ethyl imidazolium bromide ionic liquids: Den-
sities, viscosities, electrical conductivities, and phase transitions.
Journal of the Electrochemical Society, 133:325–330.
Scheeren, C. W., Machado, G., Dupont, J., Fichtner, P. F. P., &
Texeira, S. G. (2003) Nanoscale Pt(0) particles prepared in
J Surfact Deterg (2018) 21: 43–52