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 remove butanol from aqueous solutions. In this regard, low density phosphonium-based ionic liquids (ILs) trihexyl(tetradecyl)phosphonium dicyanamide [TDTHP][DCA] and trihexyl(tetradecyl)phosphonium decanoate [TDTHP][DEC] have been used for the separation of 1-butanol from aqueous solution. Ternary liquid-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 range of 25-85 and 20-290 for [TDTHP][DCA] and [TDTHP][DEC], respectively. H-1 NMR spectra indicates the absence of IL and water in the raffinate and extract phase, respectively. This experimentally confirms that there will be negligible cross-contamination of water and IL in either phase. The separation factor of butanol over water approaches infinity for both systems. A wider spread of binodal curve indicated a higher recovery of butanol at different feed concentrations. The experimental data were compared with excess Gibbs free energy models, namely the nonrandom two liquid (NRTL) and the universal quasichemical (UNIQUAC) models. NRTL and UNIQUAC gave root mean square deviation (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 statistical mechanical framework was also performed using the well-known COSMO-RS model. The COSMO-RS model gave RMSD values of 18.67% and 16.21% for the systems containing the ILs respectively, [TDTHP][DCA] and [TDTHP][DEC].