Characterization of the Phase Behavior and Complexation in the Heptanoic Acid−Heptylamine−Water System

The consequence of mixing an alkanoic acid with an alkylamine is a proton transfer between the acid and the amine leading to a complexation, which is dependent on the mixing ratio between the two components. If water is added, the formed ammonium alkanoate will have surfactant properties. In this work the complexation between heptanoic acid and heptylamine in the presence of small amounts of water is investigated using 13C NMR and FT-Raman techniques. The results are compared with ab initio calculations, performed on the propanoic acid−propylamine system. Several water-containing complexes are considered. The added water does not interfere with the complexation observed experimentally in the absence of water. Instead it breaks the acid dimers, forming acid monomer−water complexes. Furthermore the lamellar liquid crystalline phase formed in the heptanoic acid−heptylamine−water system was characterized using SAXS. The swelling behavior at an equimolecular ratio between the acid and the amine deviates from that for other binary amine mass fractions.