The activity coefficient at infinite dilution, gamma(infinity)(13), was determined by gas-liquid chromatography at temperatures from T = 308.15 to T = 348.15 K for 54 assorted solutes-among them, alkanes (linear, branched, or cyclic), alkenes, alkynes, aromatic hydrocarbons, alcohols, water, thiophene, tetrahydrofuran, 1,4-dioxane, ethers, acetone, ketones, acetonitrile, pyridine, and 1-nitropropane-in the novel bicyclic guanidine superbase-derived protic ionic liquid 1,3,4,6,7,8-hexahydro-1-methyl-2H-pyrimido[1,2-alpha]pyrimidine bis(pentafluoroethyl)sulfonylimide, [MTBDH][BETI]. The partial molar excess Gibbs free energy Delta G(1)(E,infinity), the partial molar excess enthalpy Delta H-1(E,infinity), and entropy at reference temperature T-ref Delta S-1(E,infinity) at infinite dilution were calculated from the experimental gamma(infinity)(13) values obtained over this temperature range. The gas-liquid partition coefficient, K-L, was calculated for each solute and discussed in light of the Abraham solvation parameter model. The density of the IL as a function of temperature was also measured. The selectivities for heptane/benzene, heptane/thiophene, heptane/pyridine, and heptane/nitropropane separation problems were calculated from gamma(infinity)(13) and compared to literature values for select ILs, as these separation problems are central to the petroleum industry. (c) 2012 Elsevier Ltd. All rights reserved.