Thermophysical Properties of Trihexyltetradecyl Phosphonium Octylsulfosuccinate Ionic Liquid
In this study, trihexyltetradecylphosphonium octylsulfosuccinate [P6,6,6,14][docusate] was synthesised by anion metathesis using sodium octylsulfosuccinate salt. The molecular structure of the synthesised IL was confirmed by using 1H NMR and elemental analysis and also physical properties such as density, viscosity and refractive index were studied as a function of temperature at the atmospheric pressure. The experimental values of density and refractive index decrease linearly with increasing temperature. The density, refractive index and viscosity of the present IL at 298.15 K are 0.9631 g cm-3, 1.47190 and 1806.1 mPa.s respectively. The results show that this IL possesses higher viscosity, similar density and refractive index compared to the other trihexyltetradecylphosphonium ILs.
Received: October 18, 2010;
Accepted: December 09, 2010;
Published: March 09, 2011
Ionic liquids (ILs) are a common name given to the organic salts where the
molecules composed of ions and having melting points below 100°C and negligible
vapor pressure (Seddon and Earle, 2000 and Welton
et al., 2007). ILs composed exclusively of organic cations and inorganic
or organic anions, they vary in size and can be either hydrophilic or hydrophobic
(Canongia et al., 2005; Freire
et al., 2007). The unique combination of the inherent physical and
chemical properties namely, low melting points, high thermal stability, liquidity
over a wide temperature range, negligible vapor pressures, low inflammability,
highly solvating capacity for both polar and non polar compounds, high electrical
conductivity (Freire et al., 2007; Pieraccini
and Chiappe, 2004), easy recycling, make these compounds attracting a considerable
attention in many fields (Pieraccini and Chiappe, 2004).
The tenability properties make them obtain increasing and continuing attention
in many important areas of researches and commercial applications such as absorption
media for gas separations, solvents for reactions, heat transfer fluids, separating
agent in extractive distillation, for processing biomass, as the working fluid
in a variety of electrochemical applications (batteries, capacitors, solar cells,
etc.) (Vila et al., 2006), as lubricants (Yu
et al., 2001) and in biocatalysts (Zhong et
al., 2007) with great advantages. The diverse cationic and anionic components
of the ILs will facilitate the choice of ionic liquid with task-specific properties
(Canongia et al., 2005).
Quaternary phosphonium based ILs have been receiving a great deal of attention
in terms of application to electrochemical systems (Hagiwara
et al., 2009). Compared to the other ILs, the remarkable features
of phosphonium ILs are their chemical, thermal and electrochemical stabilities
and also some phosphonium ionic liquids exhibit lower melting points and lower
viscosities which are practical advantages for various applications (Hagiwara
et al., 2009; Sugiya and Tsunashima, 2007).
However, reliable experimental data of physical properties are required for
a better understanding of the IL behavior and also are related to the engineering
components associated with a process (densities and viscosities will determine
important parameters including mass transfer, rates of liquid-liquid phase separation,
power requirements for mixing and pumping) (Huddleston et
al., 2001). Many researchers have synthesised and studied the physiochemical
properties of trihexyltetradecylphosphonium-based Ils with several anions and
reported their physiochemical properties, but thermophysical properties of rihexyltetradecylphosphonium
octylsulfosuccinate [P6,6,6,14][docusate] IL has not been studied.
Hence, an attempt was made to by ou r group to synthesise and study the thermophysical
properties of trihexyltetradecylphosphonium octylsulfosuccinate.
|| Synthesis of Trihexyltetradecylphosphonium octylsulfosuccinate
[P6, 6, 6, 14][docusate]
The present study involves the synthesis of a trihexyltetradecylphosphonium
octylsulfosuccinate [P6,6,6,14][docusate] ionic liquid. The IL was
synthesised by reacting trihexyltetradecylphosphonium chloride with sodium octylsulfosuccinate
(Fig. 1). The structure of the products was verified with
1H-NMR and elemental analysis.
The physical properties of the ionic liquid such as density, viscosity and
refractive index were carried out. In addition to this, thermal expansion coefficient,
molar refraction, molar volume, entropy and crystal energy of the synthesised
IL were estimated.
MATERIALS AND METHODS
Materials: The source and grades of the chemicals used for the synthesis
of the IL are: trihexyltetradecylphosphonium chloride (Aldrich 95%), sodium
octylsulfosuccinate (Aldrich 98%), anhydrous diethylether (Sigma-Aldrich 99%)
and acetone (Sigma-Aldrich 99.9%).
Synthesis of ionic liquid: Trihexyltetradecylphosphonium octylsulfosuccinate
was synthesised by mixing stoichiometric amounts of trihexyltetradecylphosphonium
chloride and sodium octylsulfosuccinate in diethyl ether and stirred for 48
h followed by separation of the solid. The product was washed with acetone and
the remaining solvent was removed at 70°C under vacuum and then dried in
a vacuum oven for at 80°C for 48 h to afford the clear viscous gel product
Characterisation: The synthesised IL was characterized by using Bruker
Avance 300 spectrometer, 1H NMR spectra was taken in CDCl3
solvent. CHNS-932 (LECO instruments) was used for elemental analysis (Murugesan
et al., 2010).
A coulometric Karl Fischer titrator, DL 39 (Mettler Toledo) was used to determine
the water content of the synthesised IL, using Hydranal coulomat AG reagent
(Riedel-de Haen) (Huddleston et al., 2001). All
measurements were made for IL in triplicate to ensure their reproducibility.
DL-55 autotitrator (Mettler Toledo) with 0.005 M AgNO3 as the titrant
was used to determine the chloride content of the IL (Muhammad
et al., 2008).
Perkin-Elmer, Pyris V-3.81 thermal gravimetric analyzer was used to measure
the start and onset temperatures. The samples (5.0-10.0) mg were placed in aluminum
pans under a nitrogen atmosphere at a heating rate of 10°C. min-1
(Murugesan et al., 2010; Muhammad
et al., 2008; Wilfred et al., 2009).
Stabinger viscometer (Anton-Paar model SVM3000) (Xiaa et
al., 2010) was used for the measurements of density and viscosity of
the present IL at a temperature range (293.15 to 353.15) K. The temperature
was controlled to within ±0.01°C. The repeatability of measurements
were 0.35%, ±5x10-4gcm-3and ±0.02°C
for viscosity, density and temperature respectively (Murugesan
et al., 2010; Muhammad et al., 2008;
Wilfred et al., 2009).
ATAGO programmable digital refractometer (RX-5000 alpha) with measuring accuracy
of±4x10-5 and a controlled temperature to within±0.05°C
was used to measure the refractive index of the synthesised IL in the temperature
range (298.15 to 333.15) K (Murugesan et al., 2010;
Wilfred et al., 2009). Dried samples kept in
desiccators were directly placed into the measuring cell. The apparatus was
calibrated before each series of measurements and checked using pure organic
solvents with known refractive indices (Muhammad et al.,
2008). Reproducibility of the results was confirmed by performing at least
three experiments for each sample.
RESULTS AND DISCUSSIONS
1H NMR and elemental analysis (CHNS) were used to confirm the compound. The
results confirmed the desired structure. The 1H NMR and elemental
analysis results are as follows: 1H NMR (CDCl3): δ
0.89 (24H, t, CH3); 1.26-1.41 (64H, br, CH2); 2.2-2.3
(8H, br, CH2-P); 3.10 (2H, t, CH2COO); 3.90-4.20 (4H,
br, CH2-O); 5.10 (1H, s, CO-CH). Elemental analysis: % found (% calculated)
C, 68.23 (68.98); H, 11.57 (11.69): S, 3.47 (3.54). Chloride content is 69 ppm.
In consideration of ILs for use in processes where it would be in contact with
another phase, ILs impurities (water and halide) may drastically affect the
physical properties. The presence of water may have a rather dramatic affect
on density, viscosity, refractive index and thermal stability. Also it has a
remarkable affect on reactivity, not only in the new biotechnology applications
but also in many synthetic schemes using IL as reaction media (Huddleston
et al., 2001).
The water content value of trihexyltetradecyl phosphonium octylsulfosuccinate
[P6,6,6,14][docusate] synthesised is presented in Table
1. The water content value is comparable with the phosphonium ILs reported
by Tarig et al. (2009), where the water content of trihexyltetradecylphosphoniumbis
(trifluoromethylsulfnyl) imide [P6,6,6,14][NTf2], trihexyltetradecylphosphonium
acetate [P6,6,6,14][OAc] and trihexyltetradecylphosphonium trifluoromethanesulfonate
[P6,6, 6,14][OTf] was in the range of 20-150 ppm.
Thermal stability of ILs is of practical importance for various applications.
The start temperatures for weight loss (Ts) and onset temperatures
(Td) of trihexyltetradecylphosphonium octylsulfosuccinate are 294
and 368°C respectively. The start and onset temperatures of this series
of ILs are affected slightly by the size of the alkyl chain of the cation, the
decomposition temperature decreases as the alkyl chain increases (Zhao
et al., 2004). The decomposition temperature of the present IL is
lower compared to other phosphonium ILs with short alkyl chain, for [P2,
2, 2 ,8][NTf2] and [P2, 2, 2, 12][NTf2]
are 380 and 400°C, respectively (Sugiya and Tsunashima,
Table 1 and Fig. 2 presents the densities
of trihexyltetradecyl phosphonium octylsulfosuccinate of [P6, 6, 6, 14][docusate]
IL in the temperature range from (293.15 to 353.15) K. The density of the IL
is lower compared with the imidazolium Ils reported by (Zhao
et al., 2004), the densities of [C2CN Mim]BF4,
[C3CN Mim]BF4 and [C4CN Mim]Cl are 2.15, 1.87
and 1.61 g cm-3, respectively.
The measured density of the present ionic liquid in the range from (0.9287
to 0.9664)g cm-3and in agreement with the published values for [P6,6,6,14][NTf2]
and [P6,6,6,14][OTf] (Tarig et al., 2009).
The density of [P6,6,6,14][NTf2] and [P6,6,6,14][OTf]
are 1.0654 and 0.9823 g.cm-3 respectively which indicates that the
effect of the docusate anion on density is similar to that for [NTf2]
and [OTf] anions. The density of the present IL is lower compared to the phosphonium
ILs with short alkyl chain, the density of [P2,2,2,8][NTf2]
and [P2,2,2,12][NTf2] are 1.26, 1.21 and 1.61 g cm-3,
respectively which results from the increases of free volume due to the long
alkyl chain. As expected, the density values for [P6, 6, 6, 14][docusate]
decrease linearly with increasing temperature. The linear behavior is common
to ionic liquids and is a consequence of the large temperature difference between
their working temperature range and their critical temperatures (Tarig
et al., 2009).
Table 1 and Fig. 3 presents the viscosity
for [P6, 6, 6, 14][docusate] IL. The viscosity increases with increasing
molecular weight or alkyl chain (Tarig et al., 2009).
The high viscosity of the present IL when compared to [P6, 6, 6, 14][NTf2]
and [P6, 6, 6, 14][OTf] is due to the long alkyl chain of the docusate
anion which results in increasing the electrostatic interaction between the
cation and anion. Further the higher viscosity of the present IL when compared
to [P2, 2, 2 ,8][NTf2] and [P2, 2, 2, 12][NTf2]
is due to the increased Van der Waals interactions results from the long alkyl
chains of both the phosphonium cation and the docusate anion.
||Density, viscosity and refractive index for Trihexyltetradecylphosphonium
||The density of [P6,6,6,14][docusate] IL as a function
||Log η against T-1 for [P6,6,6,14][docusate]
IL as a function of temperature.
||Refractive index of [P6,6,6,14][docusate] IL as
a function of temperature
In addition, the high viscosity of the present IL compared to the other phosphonium
ILs may due to the large volume of the docusate anion which results in low ion
mobility (Xiaa et al., 2010). Increasing the
alkyl chain length has two contradictory effects:
||Increase the electron donation into the cationic centre which
decreases the electrostatic interaction between the cation and anion and
hence reducing the viscosity
||Increase the Van der Waals interactions between the
alkyl chains which results in increasing the viscosity
The relation between the refraction index and the polarisability constitute
a measure of the importance of the dispersion forces to the cohesion of the
liquid (solvents with a large index of refraction should be capable of enjoying
strong dispersion forces). Also the values of refractive index are regarded
as a measure of the relative extent of the polar domains in the ionic liquid
(Tarig et al., 2009).
The measured refractive index values in the temperature range from (298.15
to 333.15) K for [P6,6,6,14][docusate] is represented in Fig.
4. Table 1 show that the refractive index values of the
present IL is in agreement with other phosphonium ILs, the refractive index
of [P6,6,6,14][NTf2] and [P6,6,6,14][OTf] is
1.4587 and 1.4585 as reported by (Tarig et al., 2009).
As expected, the refractive index values decrease almost linearly with increasing
The experimental values of density and dynamic viscosity at temperature range
from (293.15 to 353.15) K and refractive index from (298.15 to 343.15) K were
measured and reported for the trihexyltetradecylphosphonium octylsulfosuccinate
[P6,6,6,14]][docusate] ionic liquid.
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