The physicochemical features of the Winsor II reverse-micelle system, using long-chain ionic liquid (ILRM) as the surfactant, and their behavior toward the partitioning ratio (D) of penicillin were investigated in this work. It was determined that log D increases proportionally with the reciprocal of W-0 (which is the molar ratio of water to the ionic liquids surfactant in the organic phase of ILRM), and the slope and intercept of the graph of log D vs 1/W-0 are functions of the architectural properties in ILRM. For example, when the hydrophobicity in the poor district in ionic liquids microemulsion is enhanced using a chaotropic inorganic salt or C-2-methylated ionic liquids, the slope rises evidently, although the intercept remains almost constant. In contrast, the intercept increases in microemulsions that contain organic solvent with small molecular size, ionic liquids with high concentration, or short side-chains, all of which contribute to the highly ordered nature of the hydrocarbon chain of ionic liquids, despite the slope remaining constant. The respective relationships of the head-phile and tail-phile parameters on the slope and intercept of log D vs 1/W-0 could be understood with their roles on the lipophilic-hydrophilic balance in ILRM. Those parameters close to the "water pool" intend to change the compactness in an interfacial film, but leave the chain conformation constant. Nevertheless, the tail-philic groups, because of their isolation from the water pool, do not change the slope of the log D vs 1/W-0 line but primarily changes the intercept. The intrinsic relationships between the enthalpic/entropic energies in the interface and the slope/intercept of log D vs 1/W-0 could contribute a better platform to understand the intrinsic mechanism of the partitioning behavior in ILRM.