Thermophysical properties of sulfonium- and ammonium-based ionic liquids

Experimental data for the density, viscosity, refractive index and surface tension of four sulfonium- and ammonium-based Ionic Liquids (ILs) with the common bis(trifluoromethylsulfonyl)imide anion were measured in the temperature range between 288.15 and 353.15 K and at atmospheric pressure. The ILs considered include butyltrimethylammonium bis(trifluoromethylsulfonyl)imide, [N₄₁₁₁][NTf₂], tributylmethylammonium bis(trifluoromethylsulfonyl)imide, [N₄₄₄₁][NTf₂], diethylmethylsulfonium bis(trifluoromethylsulfonyl)imide, [S₂₂₁][NTf₂], and triethylsulfonium bis(trifluoromethylsulfonyl)imide, [S₂₂₂][NTf₂]. Based on the gathered results and on data taken from literature, the impact of the cation isomerism and of the size of the aliphatic tails, as well as the effect resulting from the substitution of a nitrogen by a sulfur atom as the cation central atom, on the thermophysical properties of sulfonium- and ammonium-based ILs is here discussed. Remarkably, more symmetric cations present a lower viscosity for the same, and sometimes even for higher, alkyl chain lengths at the cation. Additional derivative properties, such as the isobaric thermal expansion coefficient, the surface thermodynamic properties and the critical temperature for the investigated ILs were also estimated and are presented and discussed.