Effect of alkyl chain length in cation on thermophysical properties of two homologous series: 1-alkyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imides and 1-alkyl-3-methylimidazolium trifluoromethanesulfonates

Abstract

The two homologous series of ionic liquids: 1-alkyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imides [CnC1pyr][NTf2] (n = 3, 4, 6, and 8) and 1-alkyl-3-methylimidazolium trifluoromethanesulfonates [CnC1im][TFO] (n = 2, 4, 6, 8, and 10) were examined. The speed of sound, density, isobaric heat capacity, and surface tension were determined as a function of temperature at atmospheric pressure. From these experimental data, the isentropic compressibility, isobaric thermal expansion, isothermal compressibility, isochoric heat capacity, internal pressure, thermal conductivity as well as, the surface entropy and surface enthalpy were calculated. The minimum of the speed of sound dependence on the alkyl chain length in the cation u(n) is observed for [C6C1pyr][NTf2]. This effect is another confirmation of the presence of the minimum of u(n) dependence in ionic liquids with the large, weakly coordinating [NTf2]− anion. At the same time, the minimum of u(n) dependence for the series [CnC1im][TFO] was not observed.