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Despite the similarity of ionic liquid and zwitterion, their some properties such as conductivity and hydrophobicity are different. Here we developed a novel phosphonium salt having a dynamic covalent bond between anion and cation, enabling reversible structural shift between free ion pair and zwitterion. With the phosphonium salt, control of the macroscopic phase behavior in aqueous solution, i.e. forming mono- or bi-phasic systems, upon exposure to an acid or base was reversibly achieved, based on the structural shift of the phosphonium salt. CO2/N2 bubbling reversibly changed the phase behavior as well as acid/base, leading to green separation systems. Keywords: Ionic liquid, Zwitterion, Phase separation, Dynamic covalent bond