THERMOPHYSICAL PROPERTIES AND CARBON DIOXIDE SOLUBILITY OF NOVEL ROOM TEMPERATURE IONIC LIQUIDS

In the present work, a novel series of 1-alkyl-3-propanenitrile imidazolium-based room temperature ionic liquids (RTILs) ([C2CN Rim]+, where R = butyl, hexyl, octyl, decyl, allyl, ethoxyl, benzyl, incorporating sulfonate-based anions (dioctylsulfosuccinate (DOSS), dodecylsulfate (DDS), sulfobenzoic acid (SBA), benzenesulfonate (BS) and triflouromethanesulfonate (TFMS)) were synthesized. Trialkylphosphonium-based monocationic and dicationic RTILs incorporating dioctylsulfosuccinate anion ([Pn,n,n,14][dioctylsulfosuccinate] where n = 6, 8 and [P8,8,8 Cy P8,8,8][dioctylsulfosuccinate] where y = 6, 10) were also synthesized. The molecular structures of the thirty four RTILs synthesized were confirmed using 1H and 13C NMR, FTIR and elemental analysis.
The densities and viscosities of the present RTILs were measured at atmospheric pressure at T = 293.15 to 353.15 K, refractive index was measured at T = 293.15 to 333.15 K, whereas, the start and decomposition temperatures were determined at heating rate 10 Cmin-1. The thermal expansion coefficient, densities at range of temperatures and pressures, molecular volume, molar refraction, standard entropy and lattice energy of these RTILs were also estimated. The present RTILs showed lower densities, similar refractive indices and higher viscosities compared to the other imidazolium and phosphonium-based RTILs. The thermogravimetric results exhibited short-term thermal stability in the range between 470 to 670 K. These RTILs showed a weak temperature dependency on the thermal expansion coefficients, αp = 5.0 × 10−4 to 7.50 × 10−4 K-1. Empirical correlations were proposed to represent the present data on the physical properties. The densities of the present imidazolium-based RTILs are highest when paired with TFMS anion followed by SBA, BS and DDS anion. The lowest densities was observed with DOSS anion. The measured viscosities are higher for the RTILs with DOSS anion, while it was the lowest with TFMS anion.
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The solubility of CO2 in the synthesized RTILs was measured using gravimetric measurement technique (magnetic suspension balance) at T = 298 to 343 K and pressures P = 1 to 20 bar. The results showed that CO2 solubility is influenced by the alkyl chain, functional group and nature of the cation and anion. The solubility of CO2 increases with an increase in pressure and decreases with increasing temperature. The solubility of the present RTILs is described using the Henry‘s law constant along with some thermodynamic properties such as molar enthalpy, entropy and Gibb‘s free energy.
The ILs with the DOSS anion has a considerably higher affinity for CO2 compared with the ILs incorporating DDS, TFMS, SBA and BS anions. The [C2CNDim]DOSS, [C2CNHeim]DOSS and [P8,8,8,14]DOSS ILs have a higher solubility capacity among the studied imidazolium-based nitrile functionalized ILs, imidazolium-based dual functionalized ILs and phosphonium-based ILs respectively. Moreover, [C2CNDim]DOSS IL shows the highest CO2 solubility. The magnitude of ΔH0 decreases from that for moderately strong acid-base bonds at 298 K to that for weak acid-base bonds at 343 K for [C2CNHeim]DOSS and [P8,8,8,14]DOSS ILs while [C2CNDim]DOSS shows a very small decrease. [C2CNHeim]DOSS shows greater IL/CO2 interactions compare to [C2CNDim]DOSS and [P8,8,8,14]DOSS but the effect of temperature on this interaction was lower for [C2CNDim]DOSS. There was a considerable increase in CO2/CH4 solubility selectivity relative to the corresponding non functionalized imidazolium-based ILs. The highest selectivity obtained with non functionalized ILs ([APMim]BF4) is around 47 while for the nitrile functionalized IL ([C2CNDim]DOSS) is 82.7. The CO2/CH4 selectivity for the nitrile functionalized IL ([C2CNDim]DOSS was approximately four times higher than that of the [bmim]BF4,[emim]NTf2 and Sulfolane. Moreover, the recyclability studies indicated that the CO2 solubility capacity of the studied IL could be maintained for several cycles.