Characterising the phase behaviour of stearic acid and its triethanolamine soap and acid–soap by infrared spectroscopy

The behaviour of stearic acid neutralised by triethanolamine to form soap and its acid-soap has been examined by infrared spectroscopy. It was found that not only could the neutralisation behaviour be characterised, but the thermotropic behaviour could also be followed. The neutralisation confirmed the formation of a fixed stoichiometeric ratio, 2 : 1, acid-soap. When following the thermotropic behaviour the break up of the acid-soap could be followed along with various disordering and melting transitions of the alkyl chain tail. This allowed all the thermal transitions that have been observed to be characterised in terms of the type of molecular rearrangement that was occurring and also the transition temperature at which they occurred. This allowed the binary phase diagram to be plotted and understood for this system. This is the first time IR has been used to measure a whole phase diagram of this type. The nature of the acid-soap complex itself was also characterised, with very short hydrogen bonds being present as well as a free, non-hydrogen bonded, hydroxyl group.