X-ray-diffraction study of the superliquid region of the phase diagram of a Langmuir monolayer

Lipid-monolayer isotherms often show a kink that changes to a flat section and back to a kink within a narrow temperature range. In heneicosanoic acid monolayers, along a high-pressure isobar, we have observed a slightly distorted rotator-I phase between the known distorted hexagonal and hexagonal phases. Isotherms show a flat section only where this new phase occurs. The diffraction peaks are broader in this phase than in the adjacent phases; this reentrant behavior occurs in the same region of the phase diagram where the viscosity shows anomalous trends (first reported, we believe, by Copeland, Harkins, and Boyd [J. Chem. Phys. 10, 357 (1942)] and thought to indicate a superliquid phase).