Ion exchange synthesis and thermal characteristics of some [N +4444] based ionic liquids

Article abstract of DOI:10.1016/j.tca.2013.01.003

Eight salts, derived from tetrabutylammonium cation [N⁺ ₄₄₄₄] and inorganic anions like BF₄^-, NO ₃^-, NO₂^-, SCN^-, BrO ₃^-, IO₃^-, PF₆^- and HCO₃^- were synthesized using the ion exchange method. These ionic liquids (ILs) were characterized using thermogravimetry, differential scanning calorimetry and infrared spectroscopy. Thermophysical properties such as density, volume expansion, heat of fusion, heat of solid-solid transitions, specific heat capacity and thermal energy storage capacity were determined. The total of heat of solid-solid transitions observed below the melting points exceeded the heat of fusion in some cases. The thermal conductivity of the samples was determined both in solid and liquid phases. High values of thermal energy storage capacity and handsome liquid phase thermal conductivities made many of the ionic liquids under investigation were recommended as Thermal Energy Storage Devices (TESDs) as well as heat transfer fluids.

Full text of DOI:10.1016/j.tca.2013.01.003

Thermochimica Acta 556 (2013) 23–29
Contents lists available at SciVerse ScienceDirect
Thermochimica Acta
journal homepage: www.elsevier.com/locate/tca
Ion exchange synthesis and thermal characteristics of some [N+4444] based
ionic liquids
Vasishta D. Bhatt∗, Kuldip Gohil
Department of Chemical Sciences, NVPAS, Vallabh Vidyanagar 388 120, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Eight salts, derived from tetrabutylammonium cation [N+4444] and inorganic anions like BF4−, NO3
,
−
Article history:
Received 18 July 2012
Received in revised form 2 January 2013
Accepted 8 January 2013
Available online 24 January 2013
NO2−, SCN−, BrO3−, IO3−, PF6 and HCO3 were synthesized using the ion exchange method. These
ionic liquids (ILs) were characterized using thermogravimetry, differential scanning calorimetry and
infrared spectroscopy. Thermophysical properties such as density, volume expansion, heat of fusion, heat
of solid–solid transitions, specific heat capacity and thermal energy storage capacity were determined.
The total of heat of solid–solid transitions observed below the melting points exceeded the heat of fusion
in some cases. The thermal conductivity of the samples was determined both in solid and liquid phases.
High values of thermal energy storage capacity and handsome liquid phase thermal conductivities made
many of the ionic liquids under investigation were recommended as Thermal Energy Storage Devices
(TESDs) as well as heat transfer fluids.
−
−
Keywords:
Ionic liquid
Thermal energy storage
Ion exchange synthesis
Heat transfer fluid
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
as compared to that of liquids [10]. Several organic materials like
paraffins, polyethylene glycol and caprylic acid are used for ther-
In recent years, various materials are investigated for appli-
cations related to thermal energy storage and heat transfer [1].
Properties like melting point, decomposition temperature, vol-
ume expansion, density, heat of fusion, heat capacity and thermal
conductivity are most important in determining the suitability of
materials to be used for thermal applications [1–3]. The usable
temperature range of a Thermal Energy Storage Device (TESD) is
believed to be between its melting point and decomposition tem-
perature. Significant heat changes occurring prior to melting in
certain materials can also be utilized to expand the usable tem-
perature range. On the other hand, phenomena like undercooling
and volume expansion with rise in temperature can also contract
the usable temperature range [4,5]. Thus, it is a genuine challenge to
find out an ultimate TESD and a heat transfer fluid for specific appli-
cations as the overall suitability of materials to be used is governed
by a multifaceted interplay between several properties.
Phase changing materials (PCMs) are conventional TESDs as they
offer smooth on and off in thermal circuits [6]. The application
range of such materials is spread between refrigeration systems and
solar energy collection [7,8]. Phase changing behaviour of water
allows thermal applications in the temperature range of 0–100 ◦C
but corrosiveness and volume expansion at high temperature are
its drawbacks [9]. Gases are not preferred TESDs due to their
small values of heat capacity, thermal conductivity and density
mal energy storage. However, high volatility and inflammability
has restricted the popularity of organic phase changing materials
in thermal applications [11]. Recently, organoclay composite with a
remarkable energy storage capacity has been reported [12]. Variety
of inorganic salts of alkali and alkaline earth metals find a place in
thermal energy storage. Major disadvantages of inorganic materi-
als are corrosiveness, low thermal stability and high undercooling
[13,14].
Ionic liquids (ILs) are organic salts with low melting point. Low
vapour pressure and ability to recycle are the major green function-
alities of ILs. High density, high viscosity, high thermal conductivity
along with high thermal stability and non-inflammability are the
characteristic features of ILs [15]. ILs of organic cations like sub-
stituted imidazolium, pyridinium and ammonium with anions like
halides, tetrafluoroborate, hexafluorophosphate and bistriflimide
have been mainly investigated for their roles as catalysts, solvent
and extracting agent [16–23]. Recently, they are being studied for
their uses in the fields of thermal energy storage, solar power plant
[10], solar cell [24] and heat transfer fluids [25,26]. Since, the ILs
tend to exhibit a fair degree of undercooling; nucleating agents such
as copper and graphite powder have been used to unlock the energy
from undercooled ILs [27].
The present paper seeks to report, synthesis and thermal char-
acterization of a series of ILs obtained from tetrabutylammonium
bromide with various anions using silver free ion exchange method
[28]. The suitability of using these ILs as TESD and heat transfer
fluid was determined by studying the properties like heat storage
capacity and thermal conductivity.
∗
Corresponding author. Tel.: +91 9824173185.
E-mail address: vdishq@yahoo.co.in (V.D. Bhatt).
0040-6031/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.tca.2013.01.003

24
V.D. Bhatt, K. Gohil / Thermochimica Acta 556 (2013) 23–29
2. Experimental
2.1. Materials
2.3. Measurements
Melting points were measured using capillary tube apparatus,
and are quoted as the visual observation onset of the melt. The ther-
mometers used in the entire study had an uncertainty of 0.5 ◦C.
The uncertainties in measurement of density, volume expansion
and thermal conductivity mainly come from the errors in measure-
ments of temperatures. The relative error in measurement of these
quantities was within a limit of 0.11% [5]. Heat capacity and heat of
fusion were measured from DSC thermograms for which the mea-
surement uncertainties were not available. The water content in the
ILs was determined using Karl-Fischer universal titrator Systronics-
353. Infrared spectra were recorded on a GX FT-IR PERKIN ELMER
instrument. All samples were examined as KBr pellets. Differen-
tial scanning calorimeter (DSC) was carried out on Mettler STAR
SW-8.10 in the temperature range of −50 ◦C to 200 ◦C in a nitrogen
atmosphere with the scanning rate of 10 ◦C min−1. Thermogravi-
metric analysis (TGA) was conducted using a Perkin Elmer between
30 ◦C and 300 ◦C with the temperature rate 10 ◦C min−1. The instru-
ment was calibrated using melting points of indium, tin, lead and
zinc. Aluminium pans were used in all the experiments.
Tetrabutylammonium bromide (≥99.0%) was procured
from Sigma–Aldrich. Ion exchanger Amberlite IRA-67 (weakly
basic anion exchanger hydroxide form) resin was obtained
from Merck. Potassium bicarbonate, sodium tetrafluorobo-
rate, potassium hexafluorophosphate, potassium bromate,
potassium nitrate, potassium nitrite, potassium thiocyanate
and potassium iodate (≥99. 5%) obtained from Sigma–Aldrich
were used directly. A column containing ion-exchange resin
was previously activated by 1 N NaOH solution. After the
activation, the column was washed with deionised distilled
water.
2.2. Synthesis
A short column with 0.2 m length and 0.02 m diameter was
loaded with 20 g of activated resin. This column was then washed
with deionised distilled water. It was then treated with 100 ml
of unimolar salt solution containing desired anion. The column
was again washed 3–4 times with 100 ml portions of deionised
distilled water. 100 ml of 0.1 M aqueous solution of tetrabutyl-
ammonium bromide was then introduced in the column. Few
drops of eluent were tested with silver nitrate solution and
the absence of bromide ions was confirmed by nonappear-
ance of yellowish white precipitates. Water from the eluent
was removed under vacuum (15 mm/Hg) at 60–70 ◦C temper-
ature. Finally compounds were dried in a vacuum desiccator
for 72 h. The ion-exchange cycle for IL synthesis is shown in
Fig. 1.
3. Results and discussion
3.1. Infrared spectra
The structural characterization of all the samples was carried out
using infrared spectroscopy. Since no absorptions were observed
in the range of 3400 cm−1 in any of the spectra, the absence of
significant amounts of water in the samples was confirmed. The
stretching and bending modes of vibration due to tetrabutylamm-
onium cation are shown in Table 1. The bands ꢀ1 corresponding
Fig. 1. Ion-exchange approach in IL synthesis.

V.D. Bhatt, K. Gohil / Thermochimica Acta 556 (2013) 23–29
25
Table 1
Infrared spectral data of ILs.
Sample number
1
Name and mol. formula of IL
Abbreviation
ꢀ1 (cm−1
)
ꢀ2 (cm−1
)
ꢀ3 (cm−1
)
ꢀ4 (cm−1
)
−
Tetrabutylammonium bicarbonate
(C17H37NO3)
[N+4444] [HCO3
]
1630
1477
1310
1039
1114
1155
−
−
2
3
4
5
6
7
8
Tetrabutylammonium tetrafluroborate
(C16H36NBF4)
[N+4444] [BF4
]
]
1630
1630
1624
1630
1626
1625
1630
1477
1477
1476
1477
1474
1476
1477
1310
1310
1308
1310
1315
1305
1310
1039
1114
1155
Tetrabutylammonium hexaflurophosphate
(C16H36NPF6)
[N+4444] [PF6
1040
1116
1155
−
Tetrabutylammonium bromate
(C16H36NBrO3)
[N+4444] [BrO3
]
1032
1112
1145
−
Tetrabutylammonium nitrate
(C16H36N2O3)
[N+4444] [NO3
[N+4444] [NO2
]
1036
1114
1155
−
Tetrabutylammonium nitrite
(C16H36N2O2)
]
]
1042
1108
1157
Tetrabutylammonium thiocyanate
(C17H36N2S)
[N+4444] [SCN−
1044
1110
1158
Tetrabutylammonium iodate
(C16H36NIO3)
[N+4444] [IO3
]
1034
1114
1150
−
to the C N stretching frequency was observed in all the samples
in the vicinity of 1625 cm−1. The bands, ꢀ2 and ꢀ3 corresponding
to the CH3 stretching and symmetric CH2 stretching modes of
vibration were observed in the range of 1475 cm−1 and 1310 cm−1
respectively. The absorptions, ꢀ4 consists of a group of bands arising
system and ethanol (99.9 in mass fraction) obtained from Merck.
The uncertainty of measurement of the tube was 0.0001 g/ml. The
linear volume expansion in the temperature interval of 10 ◦C above
and below the melting point was determined. The linear volume
expansion coefficient (˛) was then calculated using Eq. (3.1).
ꢀ
ꢁ
due to
C
C
stretching vibrations and are seen in the spec-
1
V
∂V
∂T
tra around 1040, 1110 and 1150 cm−1. Characteristic bands at
˛ =
(3.1)
1020 cm−1 and 1065 cm−1 corresponding to stretching of HCO3
ion and CO3 were observed in ILs containing bicarbonate ion.
−
p
−
As shown in Table 2, volume expansion coefficients of the ILs
under investigation were found to be in the vicinity of 0.5 × 10−3/◦C.
These values were significantly low and hence are welcomed for the
materials to be potentially used as TESD.
Absorptions in the−regions 793, 893 and 921 cm−1 were found due
to presence of BF4 in the corresponding ILs. The hexafluorophos-
phate and bromate containing ILs were characterized by bands
at 1108 cm−1 and 825 cm−1 respectively. The stretching modes of
vibrations for nitrate containing ILs were observed at 741 cm−1 and
831 cm−1 while for nitrite containing ILs, they were observed in
the region of 1617 and 1678 cm−1. The thiocyanate moiety exhib-
ited characteristic bands at 2053, 2850 and 2935 cm−1. The IL with
iodate anion also showed a typical band at 760 cm−1 [29,30].
3.4. Density
Density (ꢁ) of a material refers to its mass per unit volume and
can readily be measured using specific gravity bottle methods at
their melting temperatures. The bottle was calibrated using ultra-
pure reagent grade type 1 water (conductivity of 5.49 × 10−6 S m−1
)
3.2. Water content and thermal stabilities
prepared using a Millipore water purifier system as well as ethanol
(99.9 in mass fraction) which was obtained from Merck. The uncer-
tainty of measurement for the bottles used was 0.00001 g/ml. A
close proximity in the density values was observed in the samples
under present investigation as shown in Table 2. These values were
found to be in the range of 1050–1291 kg m−3. The IL with bromate
anion reported the least value of density while it was the one with
tetrafluoroborate anion that showed the highest density value of
1291 kg m−3. Materials with high density occupy less space and
can have a high energy storage capacity. However, the linearity of
this relation holds only up to some extent.
The water content in the ILs was determined using Karl–Fischer
titration technique. Each of the samples exhibited water content
below 100 ppm. Melting points of all ILs were measured by direct
melting point methods. A wide range of melting points (47–110 ◦C)
has been observed across the series. This opens a possibility of find-
ing a suitable TESD for different operating temperatures. All the
samples other than that of nitrite registered a thermal stability at
least up to 150–200 ◦C. Table 2 shows the melting points (Tm) and
decomposition temperatures (Tdecomp) of all the samples.
3.3. Volume expansion
3.5. Thermogravimetric analysis
A finely powdered sample with weight equivalent to 10 ml of
the substance was filled in a compact fashion in a graduated glass
tube with a radius of 0.005 m and length of 0.2 m. The tube was
calibrated using ultrapure reagent grade type 1 water (conductiv-
ity of 5.49 × 10−6 S m−1) prepared using a Millipore water purifier
The TGA thermograms of samples other than nitrite did not
show weight loss in the vicinity of 100 ◦C which indicated a lack of
significant amounts of moisture. The nitrite containing IL decom-
posed almost completely in the temperature range of 100–180 ◦C.

26
V.D. Bhatt, K. Gohil / Thermochimica Acta 556 (2013) 23–29
Table 2
Melting points, decomposition temperatures, volume expansion coefficients and densities of the ILs.
a
Sample number
ILs
Tm (◦C)
Tdecomp (◦C)
˛
(×10−3/◦C) (phase)
ꢁ, kg m−3 (phase, ◦C)
−
1
2
3
4
5
6
7
8
[N+4444] [HCO3
]
70
71
71
86
98
47
110
44
180
170
180
180
200
100
230
150
0.62 (l)
0.51 (l)
0.51 (l)
0.68 (l)
0.77 (l)
0.49 (l)
0.54 (l)
0.79 (l)
1287 (l, 70)
1291(l, 71)
1234 (l, 71)
1050 (l, 86)
1195 (l, 98)
1187 (l, 47)
1143 (l, 110)
1279 (l, 44)
[N+4444] [BF4−
[N+4444] [PF6
[N+4444] [BrO3
]
]
−
−
]
−
[N+4444] [NO3−
[N+4444] [NO2
[N+4444] [SCN−
]
]
]
−
[N+4444] [IO3
]
a
˛ is measured in the range of Tm 10 ◦C.
The IL containing iodate anion started to decompose at 150 ◦C and
the complete decomposition occurred after a further increase of
70 ◦C. The sample with thiocyanate anion registered a maximum
thermal stability across the series. This IL was stable up to 230 ◦C
after which it decomposed completely in a narrow range of 60 ◦C.
All the remaining ILs displayed a weight loss above 180 ◦C and
underwent complete decomposition near 250 ◦C. The TGA thermo-
grams of all ILs are shown in Fig. 2.
broadness of the final melting peak of DSC thermograms in some
cases also indicates the onset of one or more solid–solid transitions
coincident with melting point. In the present study, transitions very
near to the melting points are not distinguished and are considered
to be solely due to phase change.
While, the IL containing iodate anion did not register any addi-
tional endotherm, the nitrite, nitrate and thiocyanate moieties
showed one extra endotherm each at an approximate tempera-
ture interval of 50 ◦C. Two additional endotherms were recorded
in the DSC thermograms of ILs containing bicarbonate and tetra-
fluoroborate anions whereas the other two samples showed three
such endotherms. Heats of these solid–solid transitions (ꢂH1, ꢂH2,
ꢂH3) were calculated from the areas within these endotherms from
respective thermograms.
3.6. Differential scanning calorimetry
Melting endotherms in the DSC thermograms (Fig. 3) were uti-
lized to obtain the heats of fusion (ꢂHm) for the samples under
study. The reproducibility of these thermograms was checked by
repeating them. They were found to be in agreement with an uncer-
tainty of 2 ◦C. Heat of fusion is the amount of energy required for
a phase change from solid to liquid. It is one of the most impor-
tant properties of a potential TESD. Large values of heat of fusion
are indicative of greater effectiveness of material to be used as
TESD. The heat of fusion for samples containing iodate anion was
only 113 kJ kg−1 while the bicarbonate, tetrafluoroborate and thio-
cyanate systems registered values above 250 kJ kg−1. The value of
heat of fusion for IL containing nitrite ion was 201 kJ kg−1 whereas
the remaining three samples registered these values in the vicinity
The total area enclosed by such endotherms (ꢂHTot) in many
cases surpassed even the values o−f the heat of fusion. In particu-
−
lar, the ILs of BF4−, PF6 and BrO3 exhibited enormous values of
total heat of phase transition. It was observed that the heat change
associated with solid–solid transitions increased successively [32].
Following this, the two ILs containing hexafluorophosphate and
bromate anions exhibited somewhat higher heats of solid–solid
transition in the third step as compared to the heat of fusion. Table 3
shows the heats of solid–solid phase transition and heats of fusion
of the ILs. Heats of fusion and total heats of solid–solid phase tran-
sition are plotted in Fig. 4.
of 150 kJ kg−1
.
The latent heat storage devices utilize the phenomenon of phase
change for locking and unlocking the thermal energy [14]. Materi-
als capable of storing and generating heat without phase change are
of particular importance as they can absorb and release heat with-
out any observable change in appearance. Only a slight amount
of contraction and expansion may be associated with the storage
and release of energy. Moreover, undercooling, which is one of the
most critical problem involving the use of phase change material is
eliminated in such kind of devices. In addition to this, the complica-
tion involving the handling of two different phases of the material
also vanishes. Since the last solid-solid transition and the melting
in case of samples 2, 3 and 4 is less than 50 ◦C, applications to be
performed between the temperature range of last solid–solid tran-
sition can enjoy the locking and unlocking of heat in two discrete
steps.
DSC thermograms of most of the ILs under present investiga-
tion exhibited one or more additional endotherms prior to melting
temperatures. Such thermograms have been previously reported
for ammonium and pyrrolidinium based ILs [28,31]. Each of these
peaks represents a solid–solid transition occurring in the samples
earlier to melting. The occurrence of such transitions enhances
thermal energy storage properties of phase changing materials. The
3.7. Specific heat capacity and thermal energy storage capacity
Specific heat capacities of all the ILs reported were calculated
from the DSC thermograms [10]. These values were in turn used
to calculate the thermal energy storage capacities. The specific
heat capacities were calculated in the temperature range of first
(T1) and last transitions (T2). The values of heat capacity were
found in the range of 0.20–2.70 kJ kg−1 ◦C−1. The thiocyanate moi-
ety registered the minimum value of heat capacity while nitrite and
iodate containing ILs showed highest values. The remaining sam-
ples exhibited intermediate values for heat capacity as shown in
Table 4. However, the standalone value of heat capacity was not of
Fig. 2. TGA thermograms of ILs.

V.D. Bhatt, K. Gohil / Thermochimica Acta 556 (2013) 23–29
27
Fig. 3. DSC thermograms of ILs.
Table 3
Heats of solid–solid transitions and heats of fusion of ILs.
Sample number
ILs
ꢂH1, kJ kg−1 (T1, ◦C)
ꢂH2, kJ kg−1 (T2, ◦C)
ꢂH3, kJ kg−1 (T3, ◦C)
ꢂHTot (kJ kg−1
)
ꢂHm (kJ kg−1
)
−
1
2
3
4
5
6
7
8
[N+4444] [HCO3
]
109.12 (−44)
100.84 (13)
91.29 (−38)
31.48 (−1)
31.3 (−47)
11.00 (−20)
18.74 (60)
n.a.
87.27 (12)
209.28 (24)
140.09 (10)
66.69 (12)
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
197.91 (27)
187.17 (53)
n.a.
n.a.
n.a.
n.a.
196.39
310.12
429.29
285.34
31.30
11.00
18.74
n.a.
275.17
253.75
155.00
153.71
144.00
201.00
253.00
113.00
−
[N+4444] [BF4−
[N+4444] [PF6
[N+4444] [BrO3
]
]
−
]
−
[N+4444] [NO3−
[N+4444] [NO2
[N+4444] [SCN−
]
]
]
−
[N+4444] [IO3
]
n.a. – not available.
500
450
400
350
300
250
200
150
100
50
Total Heat of fusion[kJ kg-1]
Total Heat of Phase Transiꢀon[kJ kg-1]
0
ILs
Fig. 4. Comparison of heat of fusion and total heat of solid–solid transitions of ILs.

28
V.D. Bhatt, K. Gohil / Thermochimica Acta 556 (2013) 23–29
Table 4
Specific heat capacities, thermal energy storage capacities and thermal conductivities of ILs in usable temperature range.
ILs
Cp, kJ kg−1 ◦C−1
T2 (◦C)
180
170
180
180
110
50
T1 (◦C)
E (×105 kJ m−3
)
tfs (s)
ks, W m−1 ◦C−1 (◦C)
tfl (s)
30
kl, W m−1 ◦C−1 (◦C)
0.384 (71)
(phase, ◦C)
−
[N+4444] [HCO3
]
1.60 (s, −2 to 69)
0.20 (l, 71–180)
−2
4.216
62
0.121 (68)
−
−
[N+4444] [BF4
[N+4444] [PF6
]
]
0.70 (s, 4–70)
0.30 (l) (l, 71–170)
4
2.143
58
0.108 (69)
30
0.383 (72)
0.30 (s, 6–70)
0.10 (l) (l, 71–180)
6
0.859
55
0.062 (69)
45
0.381 (72)
−
[N+4444] [BrO3
]
0.20 (s, −6 to 85)
0.10 (l, 86–180)
−6
0.586
45
0.048 (85)
30
0.659 (87)
−
[N+4444] [NO3
[N+4444] [NO2
]
1.40 (s, −50 to 97)
0.1 (l, 98–110)
−50
−22
−20
0
2.870
47
0.062 (96)
45
0.954 (99)
−
]
]
2.50 (s, −22 to 46)
0.20 (l, 47–50)
2.310
60
0.019 (45)
25
0.592 (46)
[N+4444] [SCN−
[N+4444] [IO3
0.15 (s, −20 to 109)
0.05 (l, 110–180)
180
50
0.457
48
0.054 (109)
0.166 (42)
45
0.936 (111)
0.902 (45)
−
]
2.50 (s, 0–43)
1.730
56
43
0.20 (l, 44–50)
much significance as the more important property thermal energy
storage capacity depends on temperature range and density also.
Thermal energy storage capacities (E) were calculated in the usable
temperature range (T2 − T1) using the following equation [10]:
significant amount of energy in both solid and molten phases. A
number of solid–solid transitions observed in DSC thermograms
indicate their ability to absorb and deliver additional thermal
energy in discrete packets. Three such endotherms in ILs contain-
ing hexafluorophosphate and bromate anions can be used to absorb
and release thermal energy in a narrow temperature window. The
ILs containing nitrate, nitrite and iodate anions are suitable for ther-
mal applications in the temperature range of −50 ◦C to 110 ◦C. These
systems exhibit handsome thermal energy storage capacity and liq-
uid phase thermal conductivity. For a relatively higher temperature
range of −20 ◦C to 180 ◦C, bicarbonate followed by tetrafluorobo-
rate moieties are most attractive as they possess several fold higher
thermal energy storage capacities coupled with moderate solid and
liquid phase thermal conductivities. The thiocyanate containing
IL exhibited the least thermal energy storage capacity in a wide
temperature range restricting popular thermal applications.
An ample database regarding the properties of ILs available
these days offers a tailor made solution for energy storage and
transfer in various thermal applications.
E = ꢁCp(T2 − T1)
(3.2)
Most of the ILs registered a significantly higher value of thermal
energy storage capacity.
3.8. Thermal conductivity
The thermal conductivity of ILs in solid as well as liquid states
was calculated using a known method [5]. A tube with a radius of
0.0052 m and 0.2 m length was used to maintain approximately
one-dimensional heat transfer. The samples with temperature
about 10 ◦C higher than their melting points (Tm) were suddenly
dipped in an oil bath maintained at 30 ◦C (Toil). The time required
for solidification (tfs) along with the other required quantities was
input in the following equation to obtain the thermal conductivity
(ks) in the solid state.
Acknowledgements
[1 + ((Cp(Tm − Toil))/ꢂHm)]
ks =
(3.3)
4[(tfs(Tm − Toil))/(ꢁR2ꢂHm)]
The authors thank the Director, Sophisticated Instrumenta-
tion Centre for Applied Research and Training (SICART), Vallabh
Vidyanagar and Director, Central Salt and Marine Chemicals
Research Institute (CSMCRI), Bhavnagar for analytical support. The
authors also thank Dr. Basudeb Bakshi, Principal, Natubhai V. Patel
College of Pure & Applied Sciences for providing laboratory facili-
ties.
A reverse experiment in which the oil temperature was main-
tained at 120 ◦C (Toil) and the time (tfl) required by solid samples to
achieve the melting temperature (Tm) was conducted to obtain the
thermal conductivity (kl) of ILs in liquid state. In order to remove
the visual uncertainties, the experiments were repeated with fresh
samples until constant values of time were recorded. All the sam-
ples registered several fold high values of thermal conductivity in
a liquid state as compared to that of the solid state. These values
really become important when the material is to be used as a heat
transfer liquid or a TESD. Higher values of thermal conductivity
in liquid state permit efficient heat transfer while lower values in
solid state offer better heat retention. These values showed a con-
siderable variation both in state to state basis and sample to sample
basis. Thermal energy storage capacity, thermal conductivity and
other properties related mathematically are shown in Table 4.
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