The symmetric-asymmetric transition of the sponge phase : I. Effect of the salinity

C. Vinches , C. Coulon , D. Roux

A systematic study of pseudoternary systems made of salted water, pentanol and SDS is presented for salt concentrations ranging from 20 to 80 g/l of NaCl. In the dilute part of the phase diagram, the system with the lowest salt concentration is known to present a continuous phase transition between phases of randomly interconnected membranes, namely a sponge (symmetric) and an asymmetric phase. The same topology is found as the salt concentration is increased and the evolution of the characteristics of the symmetric-asymmetric phase transition is studied. Light scattering experiments are first described. We show that the scattered intensity should be analysed in terms of a product of a structure and a form factor. The latter contribution gives directly the geometrical size of the phase (the mean distance between the membranes) without using any thermodynamical model and therefore gives an unambiguous location of the symmetric-asymmetric phase transition. The structure factor gives the critical behaviour at the phase transition. Electrical conductivity as a function of the frequency (in the range 100 Hz-15 MHz) has also been measured for the first time in this kind of systems. These phases can be considered as composites of a conducting solvent and a dielectric membrane and the data are analysed in terms of a low frequency (DC) conductivity leading to an “ obstruction ” factor and a dielectric constant strongly dependent on the frequency. Far from the S/A phase transition, the obstruction factor is constant and the dielectric constant shows a single Debye-like relaxation. Simple scaling laws are found for the variation of the intensity and of the frequency as a function of dilution. This indicates that the mode probes the “ local structure ” of the sponge phase. Near the S/A phase transition, in addition to this mode, another dielectric relaxation is observed at lower frequencies. At the same time, a departure from the scaling laws of the Debye mode and an increase of the obstruction factor can be interpreted as evidence for defects (holes ?) on the membrane. We show that the role of these defects increases as the salt concentration decreases.

Domaines: Articles anciens

C. Vinches, C. Coulon, D. Roux. The symmetric-asymmetric transition of the sponge phase : I. Effect of the salinity. Journal de Physique II, 1994, 4 (7), pp.1165-1193. ⟨10.1051/jp2:1994193⟩. ⟨jpa-00248036⟩