The interfacial behaviour of tetraethylammoniurn cations (TEA(+)) at a liquid-liquid interface modified with an anionic phospholipid layer, distearoyl phosphatidic acid (DSPA), is analysed, with the purpose of characterising the permeation properties of the film. The TEA(+) concentration in aqueous solution and the amount of DSPA solution employed to generate the lipidic layer, were varied. The results indicate that the layer is tightly compact and the transfer of TEA(+) cations by permeation does not take place. Instead of this, TEA(+) ions adsorb at the polar head groups of DSPA, and these adsorbed cations could be acting as nucleation centres of DSPA molecules when the DSPA amount is low. (c) 2007 Elsevier Ltd. All rights reserved.