Halogen-free and low-cost alkylcarboxylate-based anionic surface active ionic liquids (SAILs), namely, 1-butyl-3-methylimidazolium alkylcarboxylates ([C(4)mim][CnH2n-1O2], n = 8, 10, 12), were first synthesized through the neutralization of imidazolium hydroxide by alkylcarboxylic acids. A systematic study of their self-aggregation behavior in water was investigated by surface tension, electrical conductivity, steady-state fluorescence quenching, and H-1 NMR. The micellar properties of this series of SAILs in ethylammonium nitrate (EAN) were also studied by surface tensiometry for comparison. A set of surface active parameters and thermodynamic parameters of these compounds in water and EAN was obtained. Surface tension results show that the surface activity of [C(4)mim][CnH2n-1O2] in EAN is inferior to that in water. They exhibit a higher ability to aggregate in water than the traditional anionic surfactants, sodium alkylcarboxylates (SAC), and anionic SAILs, 1-butyl-3-methylimidazolium alkylsulfates ([C(4)mim][CnH2n-1SO4]) with the same hydrocarbon chain length. This demonstrates that the incorporation of carboxylate group and [C(4)mim](+) cation favors micelle formation. To understand the discrepancy in the surface activity of alkylsulfate- and alkylcarboxylate-based SAILs, theoretical calculations were performed to give electrostatic potential of the corresponding anions. The higher surface activity of [C(4)mim] [C12H23O2] mainly originates from the lower electronegativity of its anion. Density functional theory (DFT) calculations manifest that the interaction energy of binary combination SAILs-EAN is larger than that of SAILs-H2O, implying the stronger interaction of the former. Consequently, it is more difficult for [C(4)mim][CnH2n-1O2] to self-aggregate in EAN than in H2O. This work is expected to be of practical value for the environmentally friendly alkylcarboxylate-based SAILs in some potential applications, including nanomaterials synthesis and phase separation, among others.