A mechanism was postulated as follo

A mechanism was postulated as follows: new oleic acid molecules are added both as protonated and as ionized forms to the outer leaflet of the vesicle bilayer. Predominantly the protonated species are translocated into the inner monolayer by a flip-flop process, delivering a proton to the entrapped aqueous phase. The generated pH gradients decayed rapidly in the presence of alkali-metal cations, but could be maintained, if arginine was used as the counter ion of oleic acid, agreeing well with the relatively low membrane permeability for arginine. Intriguingly, it was observed that the generated pH gradient retarded further growth of vesicles due to the energetic cost associated with the transport of the protons into the interior.