Stability and interactions in mixed monolayers of fatty acid derivatives on artificial sea water

We studied the formation and stability of fatty acid and derivatives films on aqueous sub-phases by means of Langmuir trough experiments. Films were prepared from pure stearic acid (SA), stearyl amine (SAm) and 12-phenyldodecanoic acid (PDA), and from binary systems of SA with either SAm or PDA. For the aqueous sub-phase, multicomponent salt solutions (‘Artificial Sea Water’) at various concentrations (cASW, 0–100%) and pH values (3–7) were explored. SAm and SA differ most strongly at pH 7, where they can dissociate into oppositely charged species. For SA:SAm mixtures at this pH, pressure-area isotherms indicate mixing on a molecular scale. Increase of salt concentration from 0.05 to 0.5 M causes significant contraction of the layer, which is ascribed to (enhanced) electrostatic attractions between the head-groups. Relaxation experiments with these films indicate that dissolution of SAm into the sub-phase is suppressed by SA. The most stable films are formed at xSA = 0.5, in agreement with the calculated excess free energy of mixing. PDA is much less amphiphilic than SA. (Meta-) stable mixed films of SA and PDA are formed only at high salt concentration and low pH, where the solubility of PDA into the sub-phase is the lowest. Under these conditions, SA appears to have a stabilizing effect, which is however not strong enough to prevent expulsion of PDA from interface. Isobars of SA:PDA films corroborate this picture. The weak interactions between SA and PDA are confirmed by the excess free energies of mixing, which are close to those of ideal mixtures.