Relationships between center atom species (N, P) and ionic conductivity, viscosity, density, self-diffusion coefficient of quaternary cation room-temperature ionic liquids

The physicochemical properties (ionic conductivity, viscosity, density, and self-diffusion coefficient) of tri-n-ethylpentylphosphonium bis(trifluoromethanesulfonyl)amide (TEPP-TFSA) ionic liquid were compared with those of tri-n-ethylpentylammonium bis(trifluoromethanesulfonyl)amide (TEPA-TFSA). Compared with the TEPA-TFSA ionic liquid, the density and viscosity of the phosphorus ionic liquid are lower, although the ionic conductivity and self-diffusion coefficient are higher. The molar conductivities were compared for the values obtained by the electrochemical impedance method (electrochemical conductivity) and the calculated from the pulsed-gradient spin-echo nuclear magnetic resonance method (diffusive conductivity). The comparison shows that active ionic ratios of the TEPP-TFSA ionic liquid were smaller than those of the TEPA-TFSA ionic liquid in the whole temperature, regardless of the lower viscosity of the TEPP-TFSA ionic liquid, and results with high precision were obtained using Walden's law.