The limiting activity coefficients (gamma(infinity)(13)) for 65 solutes, including polar and non-polar solutes as alkanes, alkenes and alkynes as well as aromatic hydrocarbons, alcohols, ethers, ketones, acetonitrile, pyridine, 1-nitropropane, thiophene, esters and water in the tetraoctylphosphonium bis{(trifluomethyl)sulfonyl} imide ionic liquid (IL), [P-8,P-8,P-8,P-8][NTf2] were determined by gas-liquid chromatography at six temperatures in range of (318.15-368.15) K. The synthesis and DSC data are presented for this IL. Density, p and viscosity, eta as a function of temperature, T in a range of (298.15-398.15) K for [P-8,P-8,P-8,P-8][ENTf2] were measured at pressure p = 101 kPa. The gas-liquid partition coefficients, K-L, at infinite dilution and the thermodynamic functions, the limiting partial molar excess Gibbs energy, enthalpy and entropy were calculated. The values of selectivity and capacity for water/butan-1-ol separation problem were calculated from gamma(infinity)(13) and compared to literature values for phosphonium-based or other selected ionic liquids for the same separation problem. The Abraham solvation parameter model was also presented for all solutes. On the basis of the results, the intermolecular interactions of the chosen IL and solutes were discussed. The data presented here show that [P8,8,8,8][NTf2] reveals very large selectivity for the wateributan-1-ol separation. (C) 2017 Elsevier B.V. All rights reserved.