Optical measurements of the phase diagram of Langmuir monolayers of fatty acid–alcohol mixtures

The surface pressure-temperature diagram of Langmuir monolayers of mixtures of heneicosanoic acid with heneicosanol has been determined from 10 °C to 40 °C by direct observation with Brewster angle microscopy. The measurements focused on the way in which the boundary between the L′2 and L2 phases change with composition. As previously observed by Shih et al. [J. Chem. Phys. 101, 9132 (1994)], it moves to lower pressure and higher temperature with increasing concentration of alcohol. We have discovered that the boundary between the Ov and L2 phases, which had not been studied before, moves in the opposite direction and that the L′2 and Ov phase regions merge, an unanticipated result.