Main and tilt transition in octadecylamine monolayers

Monolayer isotherms of ocadecylamine at the air–water interface are investigated, in dependence on temperature and pH. We find that these do show more phase transitions than is usually found in literature. These phase transitions look similar to the monolayer transitions of fatty acids and similar compounds, whose phase diagrams can be related to each other. However, the phase diagram of ocadecylamine shows distinct differences and might be an example of a different class although measurements of mixed films of the material with stearic acid show miscibility of some phases and therefore suggest the same molecular structure of the phases involved. The measurement is complicated by the rather high solubility of the compound in pure water, which increases not only with temperature but also varies very strongly with the pH with a distinct maximum at pH 4.5. The overall behavior with changing pH is opposite to the behavior of fatty acid films, as might be expected from the opposite charge of the headgroup.