Ionic liquid crystals with novel thermal properties formed by the gemini surfactants containing four hydroxyl groups

Article abstract of DOI:10.1039/c6ra23235c

Modification of the molecular structures of surfactants is a feasible way to construct ionic liquid crystals (ILCs) with novel properties. Five quaternary ammonium gemini surfactants with different spacers, whose head groups were substituted by four hydroxyethyl groups, abbreviated as 12(2OH)-s-12-(2OH) (s = 3, 4, 5, 6, 8), were prepared. The mesomorphic behaviors of these gemini surfactants were investigated by polarized optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and temperature-dependent FT-IR spectroscopy. These surfactants form smectic A (SmA) phases, the region of which is dependent on the spacer length. The presence of suitable spacer length and multiple hydroxyl groups is helpful in stabilizing the mesophases. Noticeably, because of synergistic effect of the spacer and the hydroxyl groups, the phase enthalpy change of SmA to crystal transitions can be slowly released below the phase transition temperature. This interesting discovery, which has never been reported in ILC systems formed by small molecular weight surfactants, could pave the way for special applications of ILCs in the area of sustained-release materials of heat and could widen the application of ionic liquid crystals.

Full text of DOI:10.1039/c6ra23235c

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Ionic liquid crystals with novel thermal properties
formed by the gemini surfactants containing four
hydroxyl groups†
Cite this: RSC Adv., 2016, 6, 99361
a
a
a
b
b
b
*
*
Lan Lei, Lin Feng, Binglei Song, Zhaolan Zhai, Shibin Shang and Zhanqian Song
Modification of the molecular structures of surfactants is a feasible way to construct ionic liquid crystals (ILCs)
with novel properties. Five quaternary ammonium gemini surfactants with different spacers, whose head
groups were substituted by four hydroxyethyl groups, abbreviated as 12(2OH)-s-12-(2OH) (s ¼ 3, 4, 5, 6,
8), were prepared. The mesomorphic behaviors of these gemini surfactants were investigated by polarized
optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and
temperature-dependent FT-IR spectroscopy. These surfactants form smectic A (SmA) phases, the region of
which is dependent on the spacer length. The presence of suitable spacer length and multiple hydroxyl
groups is helpful in stabilizing the mesophases. Noticeably, because of synergistic effect of the spacer and
the hydroxyl groups, the phase enthalpy change of SmA to crystal transitions can be slowly released below
the phase transition temperature. This interesting discovery, which has never been reported in ILC systems
formed by small molecular weight surfactants, could pave the way for special applications of ILCs in the
area of sustained-release materials of heat and could widen the application of ionic liquid crystals.
Received 18th September 2016
Accepted 12th October 2016
DOI: 10.1039/c6ra23235c
www.rsc.org/advances
near their head groups. Compared with those of the conven-
tional surfactants, the complex molecular structures of gemini
surfactants bring out novel aggregates in solutions.15,16 Some
1 Introduction
Liquid crystals are important anisotropic so materials and are
considered as the fourth state of matter. Ionic liquids, melting at
temperatures below 100 ꢀC, are liquid salts that consist of cations
and anions. They have wide applications as catalysts for chemical
reactions, solvents for chemical synthesis and heat transfer
uids.1–3 In recent years, ionic liquid crystals (ILCs), which are
usually regarded as the combination of ionic liquids and liquid
crystals, emerged and achieved a great deal of attention4,5 because
of their prospective applications as ion-conductive materials,6–8
optoelectronics,9–11 ordered reaction media,12,13 and self-assembled
nano-materials.14 Ionic surfactants are one of a number of impor-
tant compounds that can form ILCs upon heating. The interactions
between the ionic head groups and their corresponding opposite
ions are the key factors that stabilize the thermotropic meso-
phases.4 Modication of the molecular structure of surfactants is
a helpful method used to improve the properties of resultant ILCs.
Gemini surfactants (dimeric surfactants) are comprised of
two head groups and two alkyl chains linked by a spacer at or
ionic gemini surfactants are also investigated for their abilities
in forming thermotropic ILCs with fascinating mesophases and
properties.17–22 The spacer is the featured structure in a gemini
surfactant molecule. It was recently reported that the spacer
type was responsible for the temperature range of the meso-
phase formed by a series of imidazolium-based gemini
compounds.23 Obviously, a spacer, which is located in the head
group region, can affect the interactions between the head
groups and their counter ions and thus the crystalline state of
a gemini surfactant with its steric chemistry. However, the
detailed relationship between the spacer structure and the
mesophase state of gemini surfactants still needs to be discov-
ered. Moreover, the introduction of hydrogen bonds is
becoming an effective method10,20,24–26 to stabilize or optimize
the thermotropic liquid crystalline structures. The work carried
out by Wei et al.20 indicates that a series of quaternary ammo-
nium gemini surfactants with a hydroxyl group at the spacer
have relatively low melting points and form thermotropic
mesophases over a broad temperature range. Since hydrogen
bonds are important molecular interactions, it is thus expected
that the existence of multiple hydroxyl groups in one surfactant
molecule would bring out interesting properties in formed ILCs.
To learn more about the ILC behavior formed by gemini
surfactants and to produce more ILC systems with novel prop-
erties, a series of quaternary ammonium gemini surfactants
containing four hydroxyl groups, namely alkanediyl-a,u-bis
aKey Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of
Chemical & Materials Engineering, Jiangnan University, Wuxi, Jiangsu 214122, China.
E-mail: ccfsbl@jiangnan.edu.cn
bKey Laboratory of Biomass Energy and Materials, Institute of Chemical Industry of
Forestry Products, CAF, Nanjing, Jiangsu 210042, China. E-mail: lhssxly@hotmail.
com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra23235c
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5 ꢀC minꢁ1 over the investigated temperature range under a N2
atmosphere. The time to nish a heating or cooling DSC process
is about 32 min for 12(2OH)-s-12(2OH), s ¼ 3, 5, 6, 8 and 34 min
for 12(2OH)-4-12(2OH).
2.4 The polarizing optical microscope (POM) observation
The liquid crystalline textures of 12(2OH)-s-12(2OH) were
observed with a polarizing optical microscope (Leica DMLP)
equipped with a hot stage (Linkam THM 600/S) and a digital
Fig. 1 The molecular structures of 12(2OH)-s-12(2OH), s ¼ 3, 4, 5, 6, 8.
ꢁ1
ꢀ
camera. The heating or cooling rate was 2 C min
.
[di(2-hydroxylethyl) dodecylammonium bromide], abbreviated
as 12(2OH)-s-12(2OH), where s ¼ 3, 4, 5, 6, and 8, were inves-
tigated for their thermotropic ionic liquid crystal behavior. The
molecular structure of these surfactants is shown in Fig. 1.
The four hydroxyl groups are located near the quaternary
ammonium head groups and are responsible for the formed
mesophase structures. Aer detailed research with thermogra-
vimetric analysis (TGA), differential scanning calorimetry (DSC),
polarizing optical microscopy (POM), temperature-dependent
X-ray powder diffraction (XRD) and temperature-dependent
FT-IR, the synergistic effects of the spacer structure and the
hydroxyl groups are revealed.
2.5 XRD measurements
The XRD analysis at room temperature was carried out
with a Bruker D8 Focus diffractometer equipped with a small-
angle attachment. The step size was 0.01ꢀ. The temperature-
dependent XRD patterns were recorded using a Rigaku
D/MAX-RC X-ray powder diffractometer with Cu Ka radiation
˚
(l ¼ 1.5408 A). The samples were tested at the chosen temperature
under vacuum aer waiting for 10 min. The step size was 0.02ꢀ.
2.6 The temperature-dependent infrared spectroscopy
The temperature-dependent infrared spectroscopy was
measured by a Perkin-Elmer Spectrum 2000 FT-IR spectrometer
equipped with a high-temperature attachment. Each sample
was kept for 10 min at the chosen temperature before test.
2 Experimental section
2.1 Materials
1-Bromododecane (98%), diethanolamine (99.5%), 1,3-dibromo-
propane (99%), 1,4-dibromobutane (98%), 1,5-dibromopentane
(98%), 1,6-dibromohexane (97%), and 1,8-dibromooctane (98%)
were obtained from Aladdin Reagent Co., Ltd. (Shanghai, China)
and were used as received. Other common solvents or reagents
used were analytical grade.
3 Results and discussion
3.1 Thermal studies
The decomposition temperatures of the ve compounds are
212.8 ꢀC, 225.8 ꢀC, 220.4 ꢀC, 220.1 ꢀC, and 213.2 ꢀC, according to
the TGA results (see ESI Fig. S2†). The highest temperatures in
the DSC measurements are thus lower than these values to
avoid any decomposition of these surfactants. According to the
rst heating curve of the DSC traces (ESI Fig. S3†), the solid
phase transitions of the ve compounds with the temperature
are shown in the simple Bar-graph of Fig. 2 for convenient
comparison. For 12(2OH)-3-12(2OH) with the spacer containing
the three carbon atoms, there exist two endothermic peaks in
the rst heating curve in the temperature range examined. The
rst peak corresponds to the crystal to crystal transition. The
second peak at 92.8 ꢀC indicates the crystal to mesophase (SmA)
transition as identied by the XRD measurements. Wei et al.20
have investigated a quaternary ammonium gemini surfactant
2.2 Synthesis
The detailed synthetic process is included in the ESI.† The brief
synthetic procedure is described as follows.
1-Bromododecane and diethanolamine were rst mixed and
reacted at 60 ꢀC to obtain N-dodecyldiethanolamine, which was
puried by vacuum distillation. N-Dodecyldiethanolamine, a,u-
dibromoalkane and ethanol were mixed in a sealed system. The
reaction was carried out at 110–120 ꢀC for 72 h. Aer the cooling
and removal of ethanol, the residue was recrystallized with
ethanol and ethyl acetate three times. The nal product was
obtained as a white solid aer being dried under vacuum.
C
12-3(OH)–C12, whose spacer is decorated with only one
hydroxyl group. Although the alkyl chain length is the same as
12(2OH)-3-12(2OH), C12-3(OH)–C12 forms an SmA phase until
The thermogravimetric analysis (TGA) was carried out with 120 ꢀC. This comparison reveals that rationally adjusting the
a NETZSCH TG 209F1 Iris instrument with the scanning rate of hydroxyl group location or increasing the number of hydroxyl
5 C min under a N2 atmosphere. The differential scanning groups is helpful to further decrease the temperature of meso-
calorimetry (DSC) was performed with a Pyris Diamond DSC phase formation and expand the mesophase region. The
system under a N2 atmosphere. The highest temperature for the temperature range for the SmA phase of 12(2OH)-3-12(2OH) is
DSC tests was set to 160–170 ꢀC for 12(2OH)-s-12(2OH), s ¼ 3, 4, at least 67 ꢀC, which is rare to nd among the ILC systems
5, 6, 8 to make sure that no decomposition occurred. The formed by simple quaternary ammonium surfactants.27
2.3 Thermal analysis
ꢁ1
ꢀ
sample was held for 2 min at the beginning and the end of
The spacer of 12(2OH)-4-12(2OH) is only one carbon atom
a DSC scanning process, respectively. The scanning rate was more than that of 12(2OH)-3-12(2OH). However, the behavior
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ratio of 1 : 2 : 3 : 4 at low angle region. This indicates again that
12(2OH)-4-12(2OH) forms more uniform and stable lamellar
structures compared with the other compounds. XRD
measurements were then performed at the temperatures cor-
responding to their liquid crystalline region to identify the type
of the mesophases. For 12(2OH)-s-12(2OH) (s ¼ 3, 4, 5, 6), only
one sharp peak appeared in the small-angle region of the XRD
patterns. Such a pattern can be assigned to the typical SmA
phase. Additionally, a diffuse peak, which is caused by the
melting of alkyl chains, appears at approximately 120 ꢀC for s ¼
ꢀ
3, 5, 6 and at 165 C for s ¼ 4. No sharp peaks at small angle
Fig. 2 Phase transitions of 12(2OH)-s-12(2OH). Abbreviations: Iso ¼
isotropic liquid; Cr, Cr1 ¼ crystal; SmA ¼ smectic A phase.
region were observed at the XRD pattern of 12(2OH)-8-12(2OH).
This means that 12(2OH)-8-12(2OH) transforms to the isotropic
state aer the phase transition at 104 C. The smectic state of
ꢀ
these surfactants can be further conrmed by POM observa-
tions. As shown in Fig. 4, some fan-like textures can be clearly
observed for 12(2OH)-3-12(OH) at 120 ꢀC and 12(2OH)-4-12(OH)
at 165 C, indicating the presence of the SmA phase. Because
the samples are very viscous, the textures of 12(2OH)-5-12(OH)
and 12(2OH)-6-12(OH) are not well-developed.
The layer thickness of the SmA phase can be calculated from
the 001 line of the XRD pattern as shown in Fig. 3. The variation
of the layer spacing with the spacer length of these compounds,
except for 12(2OH)-8-12(2OH), is listed in Table 1. Although the
carbon atoms contained in the spacer have been varied from 3
to 6, the layer spacing is kept almost the same. In other words,
the length of the spacer has little effect on the layer thickness of
the formed liquid crystalline phase. This information reveals
shows obvious differences. Although with the positive effect of
hydroxyl groups in forming mesophases, the crystal to SmA
phase transition was observed at 159.3 ꢀC. As a result, the SmA
phase region was sharply narrowed. This indicates that
12(2OH)-4-12(2OH) can form stable crystals, whose lattice is
strong enough to resist the inuence of high temperatures. A
further increase in the spacer length again reduces the lattice
strength of the surfactant crystals. 12(2OH)-5-12(2OH) forms an
SmA phase at 95 ꢀC, and 12(2OH)-6-12(2OH) forms an SmA
phase at 90 ꢀC and melts at 140 ꢀC. 12(2OH)-8-12(2OH) directly
forms an isotropic liquid aer two overlapped phase transitions
at approximately 104 ꢀC. This indicates that the extremely long
spacer in a gemini surfactant molecule is unable to keep the
order of the SmA state. They may fold and entangle with each
other in the solid state, as in their aqueous solutions.28
ꢀ
In summary, it is clearly observed that the solid phase tran-
sitions of these gemini surfactants are affected by their spacer
length. With the increase of spacer length from s ¼ 3 to s ¼ 6, the
formation temperature of the SmA phase was rst increased and
then generally decreased. The length of the spacer in a gemini
surfactant molecule is able to modulate the distance between the
two head groups and affect the crystal strength. As a result,
different phase transition behavior was observed.
3.2 XRD investigations
The smectic phases formed by surfactants are usually lamellar
structures which can be divided into a hydrophobic sublayer
and hydrophilic ionic sublayers. The hydrophobic sublayer is
composed of the hydrophobic chains held together by van der
Waals interactions. The ionic head groups together with their
bounded opposite ions form the hydrophilic ionic sublayer. The
two sublayers with different polarity are periodically arranged.
Such smectic structures are widely recognized in the mesophase
state formed by ionic surfactants.29–31 To gain more details on
the molecular arrangements of 12(2OH)-s-12(2OH) in the liquid
crystalline state, variable-temperature X-ray diffraction was
performed. The results are shown in Fig. 3. For their solid state
at room temperature, sets of Bragg diffractions can be found
both in the small-angle and medium angle region, indicating
their well-developed three-dimensional crystal structures. It is
worth mentioning that the XRD pattern of 12(2OH)-4-12(2OH)
Fig. 3 X-ray diffraction patterns of 12(2OH)-s-12(2OH) (s ¼ 3, 4, 5, 6,
at room temperature clearly shows reciprocal spacing in the 8) at different temperatures.
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Fig. 5 The proposed molecular arrangement of surfactants in their
SmA state (a ¼ hydrophilic ionic sublayers; b ¼ hydrophobic sublayer;
c ¼ d-spacing).
transitions revealed by two endothermic peaks. However, when
this sample was cooled, no exothermic peaks were observed.
The sequential second heating and second cooling processes
also indicated that a phase transition was absent. During this
phenomenon, it was usually assumed that the rst heating
process eliminated the thermal history of the sample, and the
state without any phase transitions corresponds to the steady
state of the thermodynamics. However, if the sample experi-
encing the above process was stored in a drier for 3 days at room
temperature, the repeated performance of the heating process
indicates that a peak with small enthalpy appear in the DSC
traces. This indicates that the phase recovery of 12(2OH)-3-
12(2OH) from the SmA state to the crystal state could occur at
a slow rate. This sample was then cooled and further stored in
the drier for another 7 days at room temperature. The repeated
heating DSC traces show that two endothermic peaks similar to
the original ones were observed. The total enthalpy change of
the recovered sample indicated that aer waiting for 7 days,
the sample was recovered to about 74.8% of the original
state (estimated from the total enthalpy change DH shown in
Fig. 4 POM texture observations of 12(2OH)-s-12(2OH) (s ¼ 3, 4, 5, 6).
that the orientation of the spacer in the lamellar structures of
the SmA phase is perpendicular to the layer normal. The
arrangement and congurations of the gemini surfactant
molecules in the smectic state are thus possible to speculate.
The proposed molecular arrangements of the gemini surfac-
tants in the SmA phases are shown in Fig. 5. Tanford32 proposed
an equation to calculate the length of a fully extended zigzag
˚
alkyl chain: lc (A) ¼ 1.5 + 1.265nc, where nc denotes the carbon
number. The hydrophobic chain of these compounds with 12
˚
carbons is calculated to be 16.7 A. The hydrophilic sublayer was
unknown until now. According to the reported results on the
ionic liquid crystals formed by quaternary ammonium surfac-
tants,20 the hydrophilic sublayer is estimated to be 8–9 A. The
˚
calculated layer thickness of the liquid crystal phases based on
the molecular arrangement manner is approximately 24.7–25.7
˚
A. This value is smaller than the experimental results of
˚
approximately 30 A. However, this is reasonable when consid-
ering that the interdigitated alkyl chains are melted in the
liquid crystal state and cannot totally overlap with each other.
3.3 An unusual phase transition behavior
While investigating the phase transition behavior of 12(2OH)-s-
12(2OH) with DSC methods, an unexpected phenomenon was
observed. Taking 12(2OH)-3-12(2OH) as an example, whose
DSC curves and DSC time-dependent effect curves are shown
in Fig. 6, at the rst heating curve and upon increasing
the temperature, the sample experiences two solid phase
Table 1 The layer spacing d (A˚) of 12(2OH)-s-12(2OH) with different
spacer length in the SmA phases
t/ꢀC
Phase
d-Spacing (A)
˚
s
3
4
5
6
8
120
165
120
120
120
SmA
SmA
SmA
SmA
Iso
29.7
30.9
29.4
30.4
—
Fig. 6 The DSC curves of 12(2OH)-s-12(2OH) (s ¼ 3, 4, 5, 6) at
different temperatures.
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Table 2 The phase transition temperatures and the corresponding
total enthalpy changes of 12(2OH)-s-12(2OH) at the heating curves
after different sample storage time
Furthermore, hydroxyl groups are well known to form
hydrogen bonds, which may contribute to the unusual proper-
ties of these systems. To conrm this, temperature-dependent
infrared spectroscopy was carried out. The results are shown
in Fig. 7. At 25 ꢀC, the peaks at 3230 cmꢁ1, 3229 cmꢁ1 and 3285
cmꢁ1 for 12(2OH)-3-12(2OH), 12(2OH)-5-12(2OH) and 12(2OH)-
6-12(2OH), respectively, were attributed to the stretching of the
associated hydroxyl groups. Their shoulder peaks, which almost
merged and indicated with a dotted ellipse, were attributed to
the interactions between hydroxyl groups and bromide
anions.24 It is worth noting that the shoulder peak of 12(2OH)-4-
12(2OH) at 3332 cmꢁ1 was especially obvious, indicating that
the hydroxyl groups interact strongly with the bromide ions in
12(2OH)-4-12(2OH) crystals. This may be responsible for the
strong crystal lattice and high phase transition temperature of
12(2OH)-4-12(2OH). When the temperature was increased to
their liquid crystalline region, the peaks corresponding to the
Total DH
s
Sample storage time
t/ꢀC
(kJ molꢁ1
)
3
Original sample
3 days
68.4, 91.3
82.9
56.3
2.3
7 days
65.2, 87.7
159.3
154.2
84.9, 95
64.4, 92.5
86.9, 93.4
90.0, 140
92.8
42.1
64.2
56.6
82.2
16.5
80.4
37.3
16.4
37.9
4
5
Original sample
About 34 min
Original sample
3 hours
3 days
Original sample
3 days
6
7 days
88.1, 136.3
Table 2). For 12(2OH)-5-12(2OH), two partially overlapped peaks stretching of the associated hydroxyl groups become wider and
appeared at 90 ꢀC. Similar to that of 12(2OH)-3-12(2OH), no shi to higher wavenumbers. No peaks above 3600 cmꢁ1 cor-
peaks were observed at the rst cooling curve. At the sequential responding to the presence of free hydroxyl groups were
second heating curve, two peaks with small enthalpy change at observed. This means that the hydrogen bonds between the
64.4 ꢀC and 92.5 ꢀC were observed (ESI Fig. S3†). The two peaks surfactant molecules still work even in the liquid crystalline
become more obvious when the sample was stored in the drier state at a high temperature. In the SmA state, a polymer-like
for 3 hours at room temperature. The 12(2OH)-5-12(2OH) structure composed of 12(2OH)-s-12(2OH) molecules contain-
sample experiencing the rst heating and the rst cooling ing four hydroxyl groups is expected because of the presence of
processes can almost recover to the original state aer storage multi-connected hydrogen bonds. As a result, the crystallization
in a drier for 3 days at room temperature, indicating that the tendency of 12(2OH)-3-12(2OH), 12(2OH)-5-12(2OH) and
recovery rate of 12(2OH)-5-12(2OH) is obviously faster than that 12(2OH)-6-12(2OH) was depressed.
of 12(2OH)-3-12(2OH). For the 12(2OH)-6-12(2OH), the peaks
The unusual phase transition behavior of 12(2OH)-s-
ꢀ
corresponding to the melting point appeared at 140 C for the 12(2OH), that is, the slow release behavior of the phase
rst heating curve and at 128 ꢀC for the rst cooling curve. The enthalpy, may endow this kind of ILC system with novel
phase recovery from the SmA phase to the crystal is similar to potential applications. It is well known that the phase transition
that of 12(2OH)-3-12(2OH). Aer the sample experiences the heat is usually much larger than the specic heat of the mate-
heating and cooling processes, the original state was regained rials. In the heat storage and heat transfer processes, it is highly
aer 7 days. Among the investigated compounds, 12(2OH)-4-
12(2OH) shows the fastest phase recovery rate. An obvious
endothermic peak appears in the DSC traces of the immediately
implemented second heating process.
The phenomenon observed in 12(2OH)-3-12(2OH), 12(2OH)-
5-12(2OH) and 12(2OH)-6-12(2OH) samples indicates that the
SmA phases formed by these compounds can be kept for a long
time at temperatures lower than the mesophase region. The
phase recovery from the SmA phase to the crystal state
happened slowly, and the enthalpy change of phase transition
was also slowly released. This has never been reported in ILC
systems formed by ionic surfactants.33–35 The prerequisite
condition in such systems is that the crystal lattice is weak,
which is related to the spacer length near the head group. In
a 12(2OH)-4-12(2OH) molecule, a spacer containing four carbon
atoms is benecial in forming strong crystals as illustrated
above. As a result, 12(2OH)-4-12(2OH) shows the fastest recovery
rate. Comparatively, the crystal lattices of 12(2OH)-s-12(2OH), s
¼ 3, 5, and 6 are weak as revealed by the fact that they form the
SmA phase at rather lower temperatures. As a result, the SmA
phases formed by these surfactants are stable enough to resist
the effect of low temperature over a period of time.
Fig. 7 The IR spectrum of 12(2OH)-s-12(2OH) (s ¼ 3, 4, 5, 6) at
different temperatures.
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effective at utilizing the phase transition processes. However, 12 T. D. Do and A. R. Schmitzer, RSC Adv., 2015, 5, 635–639.
the phase heat of most materials would be released immediately 13 X. J. Wang, M. Sternberg, F. T. U. Kohler, B. U. Melcher,
when the temperature is lower than the phase transition
temperature, which is hard to control and apply in practical
P. Wasserscheid and K. Meyer, RSC Adv., 2014, 4, 12476–
12481.
industrial processes. The ILC systems presented here reveal that 14 J. W. Gilman, W. H. Awad, R. D. Davis, J. Shields,
the phase transition heat can be slowly released at the
temperatures lower than the phase transition temperatures,
showing good sustained-release properties.
R. H. Harris, C. Davis, A. B. Morgan, T. E. Sutto,
J. Callahan, P. C. Trulove and H. C. DeLong, Chem. Mater.,
2002, 14, 3776–3785.
15 D. P. Acharya, H. Kunieda, Y. Shiba and K. Aratani, J. Phys.
Chem. B, 2004, 108, 1790–1797.
16 K. Jennings, I. Marshall, H. Birrell, A. Edwards, N. Haskins,
O. Sodermann, A. J. Kirby and P. Camilleri, Chem. Commun.,
1998, 1951–1952.
17 S. Fuller, N. N. Shinde, G. J. T. Tiddy, G. S. Attard and
O. Howell, Langmuir, 1996, 12, 1117–1123.
18 M. Sikiric, I. Smit, L. Tusek-Bozic, V. Tomasic, I. Pucic,
I. Primozic and N. Filipovic-Vincekovic, Langmuir, 2003, 19,
10044–10053.
19 Y. J. Wang and E. F. Marques, J. Phys. Chem. B, 2006, 110,
1151–1157.
20 Z. B. Wei, X. L. Wei, D. Z. Sun, J. Q. Liu and X. J. Tang, J.
Colloid Interface Sci., 2011, 354, 677–685.
21 M. Dreja, S. Gramberg and B. Tieke, Chem. Commun., 1998,
1371–1372.
4 Conclusions
The ILC properties of a series of gemini surfactants, whose head
groups contain four hydroxyl groups, were investigated. The spacer
locates near the ionic head groups of these surfactants, affecting
tremendously the ILC properties. The presence of multiple
hydroxyl groups facilitates the ILC phase formation at rather low
temperatures. Unusual phase transition behavior, the slow release
behavior of phase enthalpy below the phase transition tempera-
ture, has been discovered in 12(2OH)-s-12(2OH) samples. This
phenomenon is the synergistic results of spacer length and
multiple hydroxyl groups and is rare to nd in ILC systems formed
by low-molecular weight surfactants. In the future, novel
sustained-release materials of phase transition heat based on ILC
materials, are expected in heat storage and transfer applications.
22 T. H. Zhou and J. X. Zhao, J. Colloid Interface Sci., 2009, 338,
156–162.
23 M. Yang, K. Stappert and A. V. Mudring, J. Mater. Chem. C,
2014, 2, 458–473.
24 M. Arkas, D. Tsiourvas, C. M. Paleos and A. Skoulios, Chem.–
Eur. J., 1999, 5, 3202–3207.
25 T. H. Zhou and J. X. Zhao, J. Colloid Interface Sci., 2009, 331,
476–483.
Acknowledgements
Supports from the National Natural Science Foundation of
China (31300486) and the open research fund of Jiangsu Prov-
ince Biomass Energy and Materials Laboratory (JSBEM201501)
are gratefully acknowledged.
26 B. L. Song, S. B. Shang and Z. Q. Song, J. Colloid Interface Sci.,
2012, 382, 53–60.
27 K. Iwamoto, Y. Ohnuki, K. Sawada and M. Sen, Mol. Cryst.
Liq. Cryst., 1981, 73, 95–103.
28 R. Zana, M. Benrraou and R. Rueff, Langmuir, 1991, 7, 1072–
1075.
29 Z. B. Wei, X. L. Wei, X. H. Wang, Z. N. Wang and J. Liu, J.
Mater. Chem., 2011, 21, 6875–6882.
30 A. E. Bradley, C. Hardacre, J. D. Holbrey, S. Johnston,
S. E. J. McMath and M. Nieuwenhuyzen, Chem. Mater.,
2002, 14, 629–635.
31 D. J. Abdallah, A. Robertson, H. F. Hsu and R. G. Weiss, J.
Am. Chem. Soc., 2000, 122, 3053–3062.
Notes and references
1 A. S. Amarasekara, Chem. Rev., 2016, 116, 6133–6183.
2 H. Niedermeyer, J. P. Hallett, I. J. Villar-Garcia, P. A. Hunt
and T. Welton, Chem. Soc. Rev., 2012, 41, 7780–7802.
3 R. Patel, M. Kumari and A. B. Khan, Appl. Biochem.
Biotechnol., 2014, 172, 3701–3720.
4 K. Binnemans, Chem. Rev., 2005, 105, 4148–4204.
5 K. Goossens, K. Lava, C. W. Bielawski and K. Binnemans,
Chem. Rev., 2016, 116, 4643–4807.
6 K. Hoshino, M. Yoshio, T. Mukai, K. Kishimoto, H. Ohno and
T. Kato, J. Polym. Sci., Part A: Polym. Chem., 2003, 41, 3486–
3492.
7 T. Ichikawa, M. Yoshio, A. Hamasaki, T. Mukai, H. Ohno and
T. Kato, J. Am. Chem. Soc., 2007, 129, 10662–10663.
8 M. Yoshio, T. Kato, T. Mukai, M. Yoshizawa and H. Ohno,
Mol. Cryst. Liq. Cryst., 2004, 413, 2235–2244.
9 F. Lo Celso, I. Pibiri, A. Triolo, R. Triolo, A. Pace, S. Buscemi
and N. Vivona, J. Mater. Chem., 2007, 17, 1201–1208.
10 J. Y. Z. Chiou, J. N. Chen, J. S. Lei and I. J. B. Lin, J. Mater.
Chem., 2006, 16, 2972–2977.
32 C. Tanford, J. Phys. Chem., 1972, 76, 3020–3024.
33 R. Klein, H. Dutton, O. Diat, G. J. T. Tiddy and W. Kunz, J.
Phys. Chem. B, 2011, 115, 3838–3847.
34 A. Tolentino, A. Alla, A. M. de Ilarduya, M. Font-Bardia,
S. Leon and S. Munoz-Guerra, RSC Adv., 2014, 4, 10738–
10750.
35 S. Kohmoto, Y. Someya and K. Kishikawa, Liq. Cryst., 2010,
37, 209–216.
11 Q. Zhang, X. Wang, C. J. Barrett and C. G. Bazuin, Chem.
Mater., 2009, 21, 3216–3227.
99366 | RSC Adv., 2016, 6, 99361–99366
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