Temperature dependence of the surface tension and 0.1 MPa density for 1-Cn-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate with n = 2, 4, and 6

► Measurements of the 0.1 MPa densities and the air-liquid interfacial tension for ILs. ► Three imidazolium-based tris(pentafluoroethyl)trifluorophosphate ionic liquids. ► Efficiency with which Cl− reduces the density of an IL depends strongly on its anion. ► A QSPR indicates an anomalously high surface tension for hexafluorophosphate IL's.