Backbone orientational order in fatty acid monolayers at the air-water interface

We report on the first observation of superlattice x-ray diffraction peaks (indicating herringbone ordering of the molecular cross sections) in a satu rated fatty acid Langmuir monolayer. In the CS phase the "forbidden" (21) p eak is observed with a resolution-limited width, implying that the herringb one order is long range. We also searched for this characteristic peak in t he S, L-2, L-2', and L-2" phases. Although each phase displayed some higher -order peaks, only the lowest-temperature (L-2") phase showed the herringbo ne peak. These results do not preclude the existence of short-range herring bone order in the other phases; a Landau theory of lipid monolayers [V. M. Kaganer and E. B. Loginov, Phys. Rev. Lett. 71, 2599 (1993)] predicts the e xistence of local herringbone order in some tilted phases, manifesting itse lf in the distortion of the molecular lattice from hexagonal. We have studi ed this distortion as a function of temperature along low-pressure isobars. The distortion goes to zero as the temperature is increased, but we do not see a first-order transition. [S1063-651X(98)13412-8].