Fatty acid-cationic surfactant vesicles: counter-ion self-encapsulation

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 (ΔpH ≈ 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.