Characterization of the Liquid-Expanded to Liquid-Condensed Phase Transition of Monolayers by Means of Compressibility

Amphipathic molecules at the air-water interface show a variety of physical states, namely, gas, liquid, and coherent solid monolayer films when subjected to compression. Certain amphiphiles, including phospholipids and fatty acids, display two kinds of liquid states of the films, that is, the liquid-expanded state (LE) and the liquidcondensed state (LC).¹ The latter is sometimes called the solid-expanded phase.² Transformation between the two phases is believed to be related to the reorientation of molecules at the interface and has thus received much attention by many research groups.^3-7 Owing to the growing interest in monolayers by chemists, physicists, and biologists,^3,8-10 proper characterization of these phases and transitions between them is of great importance. In this short note, a new method is proposed to characterize the phase transition from LE to LC.