The ultralow interfacial tension between two phases of the water and diethylene glycol diethyl ether mixture was measured at many temperatures near the lower critical solution temperature under atmospheric pressure. It was observed that the interfacial tension decreases as the temperature decreases and is reduced to zero at the critical solution temperature. By applying the thermodynamics of interfaces to the experimental results, the entropy and energy of interface formation were evaluated and compared with those of the water and isobutyric acid mixture which has an upper critical solution temperature. The entropy and energy were examined in connection with the partial molar entropy and energy changes of components associated with the transfer of component molecules between the conjugate phases, respectively.
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