Transition from a Sponge-Like to an Onion-Like Nanostructure in the L Phase - Part I.

Hypothesis: In the phase diagram of the system HO/NaCl - AOT (sodium bis(2-ethylhexyl) sulfosuccinate) a homogeneous L channel can be detected at T = 25 °C as a function of the surfactant and the NaCl mass fraction. Our hypothesis is that a structural transition from a sponge-like to an onion-like structure occurs in this channel.

Experiments: We (a) determined the relevant part of the phase diagram, (b) carried out electrical conductivity and viscosity measurements, (c) visualized the nanostructure with freeze-fracture electron microscopy (FFEM) and freeze fracture direct imaging (FFDI), (d) determined the characteristic length scales from Small Angle Neutron Scattering (SANS) curves.

Findings: We detected a narrow one-phase L channel in which the conductivities decrease, while the viscosities increase with increasing surfactant and salt mass fractions. Together with the FFEM/FFDI images and the SANS curves we provide experimental evidence for a structural transition from a sponge-like to an onion-like structure: all structures as well as their coexistences are thermodynamically stable.