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 saturated fatty acid Langmuir monolayer. In the CS phase the ``forbidden'' (21) peak is observed with a resolution-limited width, implying that the herringbone order is long range. We also searched for this characteristic peak in the S, L₂, L₂^', and L₂^'' phases. Although each phase displayed some higher-order peaks, only the lowest-temperature (L₂^'') phase showed the herringbone peak. These results do not preclude the existence of short-range herringbone 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 existence of local herringbone order in some tilted phases, manifesting itself in the distortion of the molecular lattice from hexagonal. We have studied 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.