2.3 T HERMOPHYSICAL PROPERTIES OF RTIL S
2.3.6 Derived thermodynamic properties
The high-precision measurement of density over extended range of temperatures allowed estimating thermal expansion coefficients. In spite of the volumetric ―good- behavior‖ of RTILs, different ILs exhibit different rates of expansion as temperature is progressively distanced from ambient conditions [125]. Moreover, according to Glasser [98, 99] the standard entropy and crystal energy for RTILs can be estimated from the experimental values of density and molecular weight.
The thermal expansion coefficient of the RTILs is lower than most of the molecular organic liquids. For the alkylimidazolium tetrafluoroborate liquids, the thermal expansion coefficient decreases as the length of the substitute chain on the cation decreases. Additionally when comparing the ILs with the same anion and with a butyl and methyl-alkyl chain appended to the cation, the thermal expansion coefficient increases as follows: BMIM+ > BMPy+ > BMPyrr+ [126].
Thermal expansion coefficients of [C4mim]NTf2, [C4mim]dca and [C2mim]EtSO4
ILs in temperature range of 293.15 to 363.15 K-1 are in the range of 4.8 to 6.6 10-4 K-
1 as presented by Carlos and coworkers [126]. Kilaru, P. and coworkers [127]
presented the thermal expansion coefficients of the phosphonium-based ILs, for
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[P6,6,6,14]NTf2, [P6,6,6,14]DBS and [P6,6,6,14]DEP are 5.71, 5.77 and 6.03·10-4 K-1 respectively, and ammonium-based ILs, for [N4,1,1,3]NTf2, , [N6,1,1,3]NTf2 and [N10,1,1,3]NTf2 are 5.79, 5.88. 5.82 10-4 K-1 respectively. In the case of [Bmpy]NTf2
the thermal expansion coefficient varies between 6.3210-4 and 6.34 10-4 K while for [P6,6,6,14]DCA the variation is between 6.51· 10-4 and 6.29 10-4 K-1 in the working temperature range [128]. The values of thermal expansion coefficient for a series of imidazolium-, pyridinium-, phosphonium- and ammonium – based ILs was reported in the range of 5.0 × 10−4 to 6.5 × 10−4 K-1 [94, 127]. Moreover, Guan, W.
and coworkers [125] reported the thermal expansion coefficient of the amino acid IL [C3mim]Glu was 3.28 10-4 K-1.
Absolute standard entropy represents thermodynamic data of special significance, forging the link between enthalpy and Gibbs energy, which is the true arbiter of chemical equilibrium and stability in processes whose outcome is determined by thermodynamic (as opposed to kinetic) considerations[98]. Standard absolute entropies of many materials are unknown which precludes a full understanding of their thermodynamic stabilities. It is useful to estimate standard entropy data for several reasons. First, there is a paucity of standard entropy data for inorganic materials in standard thermochemical tables. Second, experimental determination of absolute entropy calorimetry is both a lengthy and nontrivial procedure; such measurements are no longer fashionable science and, for this reason, increasing reliance has to be placed on estimation techniques for thermochemical data [98].
Glasser, L. and Jenkins, H. D. B. [98, 99] showed that formula unit volume, Vm, can be employed for the general estimation of standard entropy values for materials of varying stoichiometry, through a simple linear correlation between entropy and molar volume. Few literatures are available for the standard entropy of RTILs.
Guan, W. and coworkers [125] reported that the standard molar entropy of the amino acid ILs, [Cnmim][Glu] where n = 1-6, are in varied from 417.9 to 589.3 J·K-1·mol-1, the value was increased with the increase of the alkyl chain length. The standard entropy values for a series of ILs based on, [Cnmim] (where n = 2,3,4,5,6) incorporating alanine and glycine anions were reported by Fang, D. W. and
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coworkers [97]. The values range from 396.9 to 535.5 J·K-1·mol-1 and (360.2 to 498.8) J·K-1·mol-1 for [Cnmim]Ala and [Cnmim]Gly respectively.
Glasser, L. [99] ulstrate that the lattice energy of RTILs can be expressed in terms of the density. It was presented that the densities of the condensed melt and solid phases are very similar, and so the calculated lattice potential energies may well be applied to phase of the material, with the difference between the values generated resulting from the change in density and corresponding to the energy of the phase transition.
The lattice energy for fused CsI is 602.5 kJ.mol-1 which is the lowest crystal energy among alkali-chlorides[97], the low crystal energy is the underlying reason for forming ILs at room temperature as pointed by Krossing [129]. Only few researchers report the crystal energy of the RTILs. Guan, W. and coworkers [125] reported that the crystal energy of the amino acid ILs, [Cnmim][Glu] where n = 1-6, are in varied from 410 to 450 kJ·mol-1, the value was decreased with increasing the alkyl chain length. Fang, D. W. and coworkers [97] presented the crystal energy values for a series of ILs based on, [Cnmim] (where n = 2,3,4,5,6) incorporating alanine and glycine anions The values range from (421 to 456) kJ·mol-1 and (429 to 469) kJ·mol-1 for [Cnmim]Ala and [Cnmim]Gly respectively. All the reported crystal energy values of the RTILs were lower than the lowest crystal energy values among alkali-chlorides.
Molar refraction is a measure of the total polarizability of a mole of a substance and is dependent on the temperature, the refractive index and the pressure, it can be interpreted as the hard-core volume, i.e., an approximate measure of the total volume (without free space) of molecules in one mole of the compound [130]. The molar refraction values for RTILs is generally estimated using Lorentz–Lorenz relationships [102].
The molar refraction of few ILs was reported, for [C4Mim]PF6, [C6Mim]PF6, [C8Mim]PF6, [C4mim]BF4 and [C8mim]Cl the molar refraction is 51.46, 60.69, 70.29, 47.84 and 67.91 cm3.mol-1 respectively [102, 131]. The results showed that an increase in the alkyl chain length ([BEpyr]ESO4, [EMpyr]ESO4 means a slight increase in the values of the molar refractions and the anion effect was in order of Cl-1
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> [PF6- >BF4- [132]. Tariq, M. et al. [102] reported the molar refraction of a number of imidazolium and phosphonium-based ILs. They showed the molar refraction of [Cnmim] incorporating different anions, the effect of these anions is in the order NTf2
> OTf > PF6 > MeSO4 > BF4 > OAc and the molar refraction values increased from 74.91 ([C2mim]NTf2) to 112.02 ([C12mim] NTf2). Moreover, the molar refraction values of the phosphonium-based ILs [P6,6,6,14] incorporating NTf2, OTf and OAc are 192.57, 175.69 and 173.57 cm3.mol-1 respectively.