Phase behaviour of the system triethanolamine-decanoic acid-water

The aqueous phase behaviour of alkylamine and alkanoic acid mixtures is largely dictated by proton transfer from the acid to the amine. At equimolecular ratio between the acid and the amine, a compound with surfactant properties will be formed. With asymmetric distribution of carbon atoms in the alkyl chains, the compound will be an ionic surfactant with an organic counterion. The ternary phase diagram at 298.2 K for the system triethanolamine-decanoic acid-water and the binary phase diagram for the system triethanolammonium decanoate-water at different temperatures have been determined. With an excess of amine, the ternary phase diagram is dominated by a large isotropic solution region, owing to the complete solubility of triethanolamine in water. With excess acid, five phases appear; a lamellar, a hexagonal, a reverse hexagonal, a reverse cubic liquid crystalline phase and a reverse micellar solution phase. This part of the ternary phase diagram is reminiscent of phase diagrams of ordinary soaps-alkanoic acids-water. Triethanolammonium decanoate forms a solution phase and a hexagonal liquid crystalline phase with water. The phase behaviour of triethanolammonium decanoate in water differs from that of ordinary soaps, and is ascribed to the occurrence of both ionized and unionized acid and to the difference in the size and the chemical structure of triethanolammonium and alkali counterions.