Density, viscosity, and CO2 solubility in ether-functionalized phosphonium-based bis(trifluoromethanesulfonyl)amide ionic liquids

Abstract

We measured the densities and viscosities of two ionic liquids, triethyl(methoxymethyl)phosphonium bis(trifluoromethanesulfonyl)amide ([P_222(1O1)][TFSA]) and triethyl(2-methoxyethyl)phosphonium bis(trifluoromethanesulfonyl)amide ([P_222(2O1)][TFSA]), at atmospheric pressure and 273.15–363.15 K. The high-pressure density at the pressures up to 50 MPa was also measured at 298.15–353.15 K. Meanwhile, CO₂ solubility in these phosphonium-based ionic liquids was determined using a magnetic suspension balance at 303.15–333.15 K and pressures up to 6 MPa. The experimental density and viscosity at atmospheric pressure were fitted using quadratic and Vogel-Fulcher-Tammann equations, respectively. The high-pressure density was fitted using the Tait equation and Sanchez-Lacombe equation of state. The properties of the two ether-functionalized ionic liquids were compared to those of triethylpentylphosphonium bis(trifluoromethanesulfonyl)amide [P₂₂₂₅][TFSA]. The density at atmospheric pressure increased in the order [P₂₂₂₅][TFSA] < [P_222(2O1)][TFSA] < [P_222(1O1)][TFSA]. The introduction of the ether group effectively reduced viscosity in the following order: [P_222(1O1)][TFSA] < [P_222(2O1)][TFSA] < [P₂₂₂₅][TFSA]. The CO₂ solubilities in [P_222(1O1)][TFSA] and [P_222(2O1)][TFSA] were slightly lower than those in [P₂₂₂₅][TFSA]. In contrast, the molality of CO₂ (m₁) increased in the order [P₂₂₂₅][TFSA] < [P_222(2O1)][TFSA] < [P_222(1O1)][TFSA]. Ether functionalized phosphonium-based cations are effective in enhancing the physical absorption of CO₂, particularly the molality of CO₂.