(275d) A Study on Thermodynamic Properties of Binary Mixtures of Sesame OIL with Aliquat 336 and Tributyl Phosphate

Binary mixtures of sesame oil with Aliquat 336 and Tributyl Phosphate (TBP) have immense application as extractant in liquid membrane based separation processes. This research work is intended to investigate the molecular interactions and transport properties of miscible binary mixtures of sesame oil + Aliquat 336 (Mixture A) and sesame oil + TBP (Mixture B) by determining the density and viscosity over a temperature range of 25°C-70°C under atmospheric pressure. Basic parameters such as density and viscosity of mixtures lead to an understanding of solute-solvent interactions through the calculations of excess molar volume and viscosity deviation. The data of excess molar volume and viscosity deviation further throw light on molecular interactions as they have applications in solution theory and molecular dynamics. Yousra Litaiem and Mahmoud Dhahbi studied the physicochemical and transport properties of binary mixtures of Aliquat 336 and formamide [1]. Srirachat et al., carried out similar experimental work to study the effect of polarity and temperature on binary mixtures of Di-(2-ethylhexyl) phosphoric acid (D2EHPA) with organic solvents (kerosene, n-heptane, chlorobenzene and 1-octanol) [2].

The formation of ideal mixture hardly causes any change in volume however; real mixture undergoes either an increase or decrease in volume due to molecular interactions between the components of mixtures. Excess molar volume and apparent molar volume indicate the changes or differences in the binary mixtures and specific interactions between them. Apparent molar volume is usually less than the molar volume. A positive deviation in excess molar volume indicates volume expansion on mixing thus repulsive interaction of mixing solvents or weaker interactions than the interactions of molecules in pure solvents. While a negative trend in the data of excess molar volume shows strong interactions of molecules in the mixture than individual molecules before mixing. Similarly, a negative deviation in apparent molar volume indicates a good molecular interaction between the solute and solvent. As the molecules of solute are strongly attracted to the solvent molecules they occupy less space owing to the decrease in apparent molar volume and vice versa. As temperature increases the apparent molar volume increases due to volume expansion caused by weak bonding leading to lesser molecular interactions.

From the measurement of density, the excess molar volume and apparent molar volume have been calculated for mixture A and mixture B. The viscosity data has been used to determine the viscosity deviation of the mixtures (A and B) to understand the attractive-repulsive forces between the interacting molecules of the solvent mixtures. An empirical relation given by Parthasarathy and Bakshi [3] and Vogel-Fulcher-Tammann equation [2] have validated the density and viscosity data, respectively. The excess molar volume and viscosity deviation data have been fitted into the Redlich-Kister polynomial equation [2]. Further, thermal expansion coefficient, excess Gibbs’ free energy (ΔG^E), change in enthalpy (ΔH) and change in entropy (ΔS) of activation of the liquids have also been calculated. The volumetric thermal expansion coefficient shows the change in the volume of the liquid with a change in temperature at constant pressure. With increase in bond energy, the value of the coefficient decreases. While it increases with increase in temperature due to weak bonding. A decreasing trend in the values of the volumetric thermal expansion coefficient with increase in mole fraction further indicates that the intermolecular forces in the mixtures are stronger than in pure solvents.

The density (ρ) of sesame oil lies in between Aliquat 336 and TBP (ρ_TBP {0.973 g/mL} > ρ_{Sesame oil} {0.914 g/mL} > ρ_Aliquat {0.886 g/mL} at 25°C). In mixture A, density decreases with increase in mole fraction of Aliquat and temperature. Whereas in mixture B, density increases with increase in mole fraction of TBP and declines with rise in temperature. The expansion in volume due to increase in kinetic energy and weak bonding with rise in temperature leads to decrease in density. The excess molar volume (V_M^E) data for aliquat mixture is negative. This negative trend indicates strong interactions of molecules in the mixture than individual molecules before mixing. While in mixture B, the excess molar volume data changes from negative to positive with increase in mole fraction showing that the strong interactions of molecules in the mixture shift to repulsive interaction or weaker interactions as the concentration of TBP increases. The apparent molar volume trend is same for both the mixtures. It increases with temperature and mole fraction. This is because rise in temperature leads to volume expansion due to weak bonding leading to lesser molecular interactions. The viscosity (η) of Aliquat is more than that of sesame oil and TBP (η_Aliquat {1.048 Pa.s}> η_{Sesame oil} {0.039 Pa.s}> η_TBP {0.0025 Pa.s} at 25°C). While viscosity of mixture A decreases with temperature, it increases with increasing mole fraction. In case of mixture B, viscosity decreases for both, temperature and increasing mole fraction of TBP. The viscosity deviation (Δη) data is negative and decreases with increase in temperature for both the mixtures. The volumetric thermal expansion coefficient (β) is almost constant with temperature and mole fraction for both the binary mixtures. For pure aliquat, β = 0.0007°C^-1 and β value for pure TBP is 0.0009°C^-1. The trend of ΔG^E data is same for both aliquat and TBP mixtures. ΔG^E increases with temperature to give a maximum value at 0.5 mole fraction of Aliquat for mixture A while in mixture B, ΔG^E value is maximum at 0.58 mole fraction of TBP. ΔH increases while ΔS decreases with mole fraction for mixture A whereas in mixture B, ΔH decreases while ΔS increases with mole fraction. Further, V_M^E and Δη is minimum at 0.47 - 0.76 mole fraction of Aliquat showing that the molecular interaction is comparatively stronger in binary mixture than in pure solvents. From this, it can be concluded that the binary mixture of Aliquat and sesame oil (mixture A) may be applied in future for separation or extraction processes.

Keywords: Sesame oil, Excess molar volume, Viscosity deviation.

References

[1] Y. Litaiem and M. Dhahbi, Physicochemical Properties of an hydrophobic ionic liquid (Aliquat 336) in a polar protic solvent (formamide) at different temperatures. J. Disper. Sci. Technol 36 (5) (2015) 641-651.

[2] W. Srirachat, T. Wannachod, U. Pancharoen S. Kheawhom, Effect of polarity and temperature on the binary interaction between D2EHPA extractant and organic solvents (kerosene, n-heptane, chlorobenzene and 1-octanol): Experimental and thermodynamics. Fluid Ph. Equilibria 434 (2017) 117-129.

[3] C. M. Rodenbush, F. H. Hsieh, D. S. Viswanath, Density and viscosity of vegetable oils. J. Am. Oil Chem.' Soc 76 (12) (1999) 1415-1419.