A series of ionic liquids based on trifluoroacetic acid, namely, [C_nmim][TFA] (n = 2, 3, 4, 5, 6) (1-alkyl-3-methylimidazolium trifluoroacetate), were designed and synthesized. The density, surface tension and refractive index were measured in the temperature range of 293.15 to 343.15 ± 0.05 K, and some physicochemical properties of the ILs were calculated. Using the concept of molar surface Gibbs free energy, the traditional Eötvös equation was improved into a modified Eötvös equation, in which the intercept and the slope represented the molar surface enthalpy and the molar surface entropy, respectively, for [C_nmim][TFA] (n = 2, 3, 4, 5, 6). The thermal expansion coefficient (α) of [C_nmim][TFA] was calculated according to the interstitial model, and the order of magnitude of the calculated values was in good agreement with the corresponding experimental values. A new hypothesis was proposed, stating that the interstitial molar surface Gibbs free energy (g_s) is not determined by the type of IL. From the refractive index and the molar surface Gibbs free energy, an equation to predict the surface tension of ILs was derived and the predicted values were highly correlated with the corresponding experimental values. Finally, a new polarity scale for ILs was developed, and the polarity order of the [C_nmim][TFA] (n = 2, 3, 4, 5, 6) ILs was estimated.