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 (C-ASW, 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 X-SA = 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. (C) 2013 Elsevier B.V. All rights reserved.