Effect of Temperature and Alkyl Chain Length on the Physicochemical Properties of Pyridinium Bis(Trifluoromethylsulfonyl)Imide-Based Ionic Liquids

Densities, refractive indices, viscosities, surface tensions and speeds of sounds were measured and discussed for 1-Ethylpyridinium bis(trifluoromethylsulfonyl)imide ([C 2 py][NTf 2 ]), 1-Ethyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide ([C 2 mpy][NTf 2 ]), 1-Propyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide ([C 3 mpy][NTf 2 ]), 1-Butylpyridinium bis(trifluoromethylsulfonyl)imide ([C 4 py][NTf 2 ]), 1-Butyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide ([C 4 mpy][NTf 2 ]) and 1-Hexylpyridinium bis(trifluoromethylsulfonyl)imide ([C 6 py][NTf 2 ]) as a function of temperature and atmospheric pressure. As expected, all properties decrease with increasing temperature, with viscosity being the property most affected by temperature, while surface tension and refractive index being the properties least affected by temperature. Linear correlation was used to calculate the density, speed of sound, surface tension and refractive index as a function of temperature. The viscosity is correlated using Vogel-Flutcher-Tamman (VFT) equation. From the experimental density data, the coefficients of thermal expansion and the isentropic compressibility coefficients were calculated. The Guggenheim equation was used to calculate the critical temperatures. Moreover, the effect of the alkyl chain length of the investigated ionic liquids on the measured thermophysical properties is discussed.