OPTICAL MEASUREMENTS OF THE PHASE-DIAGRAM OF LANGMUIR MONOLAYERS OF FATTY ACID-ALCOHOL MIXTURES

The surface pressure-temperature diagram of Langmuir monolayers of mix tures of heneicosanoic acid with heneicosanol has been determined from 10 degrees C to 40 degrees C by direct observation with Brewster angl e microscopy. The measurements focused on the way in which the boundar y between the L(2)' and L(2) phases change with composition. As previo usly observed by Shih et al. [J. Chem. Phys. 101, 9132 (1994)], it mov es to lower pressure and higher temperature with increasing concentrat ion of alcohol. We have discovered that the boundary between the Ov an d L(2) phases, which had not been studied before, moves in the opposit e direction and that the L(2)' and Ov phase regions merge, an unantici pated result. (C) 1995 American Institute of Physics.