This work addresses surface tension measurements of eight imidazolium-based ionic liquids (ILs) with the bis(trifluoromethylsulfonyl)imide, Tf2N, common anion, and their dependence with temperature, from (293 to 353) K, at atmospheric pressure. The set of selected ionic liquids was chosen to provide a detailed and comprehensive study of the influence of the cation alkyl chain length on the surface tensions of the ionic liquids based on the Tf2N anion. It is shown that, unlike other ionic liquids, the surface tensions of the Tf2N family do not have a linear decrease with the chain length. The surface thermodynamic functions such as surface entropy and enthalpy were derived from the temperature dependence of the surface tension, and their values indicate the importance of the surface ordering in ionic liquids. The use of the Guggenheim and Eo�tvos correlations for the estimation of the critical temperatures of ionic liquids is discussed, and the applicability of the Stefan rule to ionic liquids is analyzed