Microemulsion sponge phase as a manifestation of the superflexibility critical point of tensionless balanced liquid-liquid interfaces

We have analyzed the mechanical properties of surfactant loaded tensionless balanced liquid/liquid interfaces using self-consistent field theory implementing a coarse grained model. Such a tensionless state, as occurring in microemulsions, signals a first-order interfacial phase transition. Consequently, the interfacial area is set by the amount of surfactant in the system. Near the bulk critical point, such systems suffer an interfacial, so-called superflexibility (SF) critical point as soon as the tensionless state ceases to exist. This happens when the width of the L/L interface becomes comparable to the surfactant size. The bending rigidities vanish upon approaching the SF-point as a power-law. Exclusively near the SF-point, the saddle splay modulus is positive featuring the sponge phase to be its main characteristic.