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We have studied the dialysis of surfactant mixtures of two oppositely charged surfactants (catanionic mixture) by combining HPLC, neutron activation, confocal microscopy, and NMR. In mixtures of n-alkyl trimethylammonium halides and n-fatty acids, we have demonstrated the existence of a specific ratio between both surfactant contents (anionic/cationic almost equal to 2:1) that determines the morphology, the elimination of ions, and the elimination of the soluble cationic surfactant upon dialysis. In mixtures prepared with lower anionic surfactant contents, ill-defined aggregates are formed, and dialysis quickly eliminates the ion pairs (H+X-) formed upon surfactant association and also the cationic surfactant until a limiting 2:1 ratio is reached. By contrast, mixtures prepared above the anionic/cationic 2:1 ratio form micrometer-sized vesicles resistant to dialysis. These closed aggregates retain a significant number of ions (30%) over 1000 hours, and dialysis is unable to eliminate the soluble surfactant. The interactions between surfactants have been estimated by measuring the partitioning of the CTA molecules between the catanionic bilayer, the bulk solution, and mixed micelles when they exist. The mean extraction free energy per CTA in the membrane has been found to increase by 1 kBT to 2 kBT as the soluble surfactant is depleted from the bilayer, which is enough to stop the dialysis. The vesicles produced above the anionic/cationic 2:1 ratio are formed by frozen bilayers and are resistant to extensive dialysis and therefore show an interesting potential for encapsulation as far as durability is concerned.

International audience; Vesicles of fatty acids in the fluid state show interesting biomimetic properties and are potentially versatile substitutes to phospholipid vesicles in materials science. However, their use is hindered by a poor stability against variations in pH, ionic strength, temperature etc., which can be improved by using, for instance, mixtures of fatty acids and other amphiphilic molecules. Here we report an original property of fatty acid vesicles with aliphatic chains in the gel state prepared from mixtures of fatty acids and cationic surfactants. Apart from encapsulating added solutes in high yields and showing a high durability, even against drastic dialysis, dilution, concentration etc., these vesicles also spontaneously encapsulate the counter-ions (H+ and halides) released upon surfactant association. This "self-encapsulation'' leads to large and sustainable pH gradients across the bilayer (Delta pH approximate to 3 over months), and is mediated by the formation of gel-phase bilayers with little defects. Both the formation of the vesicles and their ability for self-encapsulation of counter-ions have a broad generality and can be exploited with other surfactants to generate pH gradients ranging from acid to base.