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Biobutanol obtained via an acetone–butanol–ethanol(ABE) fermentation process is now considered a potential biofuel.In recent times low density phosphonium cations were found to removebutanol from aqueous solutions. In this regard, low density phosphonium-basedionic liquids (ILs) trihexyl(tetradecyl)phosphonium dicyanamide [TDTHP][DCA]and trihexyl(tetradecyl)phosphonium decanoate [TDTHP][DEC] have beenused for the separation of 1-butanol from aqueous solution. Ternaryliquid–liquid equilibrium data for IL(1)–1-butanol(2)–water(3)are measured at T= 298.15 K and p= 1 atm. Butanol partition coefficients are obtained in the rangeof 25–85 and 20–290 for [TDTHP][DCA] and [TDTHP][DEC],respectively. 1H NMR spectra indicates the absence of ILand water in the raffinate and extract phase, respectively. This experimentallyconfirms that there will be negligible cross-contamination of waterand IL in either phase. The separation factor of butanol over waterapproaches infinity for both systems. A wider spread of binodal curveindicated a higher recovery of butanol at different feed concentrations.The experimental data were compared with excess Gibb’s freeenergy models, namely the nonrandom two liquid (NRTL) and the universalquasichemical (UNIQUAC) models. NRTL and UNIQUAC gave root mean squaredeviation (RMSD) values in the range of 0.12–0.14% and 0.48–0.55%,respectively, for both ILs. Further the predictive ability of a statisticalmechanical framework was also performed using the well-known COSMO-RSmodel. The COSMO-RS model gave RMSD values of 18.67% and 16.21% forthe systems containing the ILs respectively, [TDTHP][DCA] and [TDTHP][DEC]. [ABSTRACT FROM AUTHOR]