Transition between two next-nearest-neighbor phases in a mixed Langmuir monolayer. A study by grazing-incidence x-ray diffraction and Brewster-angle microscopy

Grazing incidence x-ray diffraction and Brewster-angle microscopy measurements have been undertaken for a mixed Langmuir monolayer of octadecanoic acid and methyl octadecanoate. For the composition studied (49.7 mol % ester) there are two noncrystalline tilted phases, one at low pressure in which the chains point to their nearest neighbors (L2 phase) and a higher-pressure phase in which the tilt is toward next-nearest neighbors. The higher-pressure phase arises from a merger of the L2′ and Ov phases, which are separated in the pure acid. A continuous change between the two regions is observed rather than a first-order transition, as had been suggested. The results are discussed with reference to different proposals for the origin of two distinct next-nearest neighbor tilted mesophases: the presence of one-dimensional chain backbone order [V. M. Kaganer and E. B. Loginov, Phys. Rev. E 51, 2237 (1995)] and a coupling between tilt and distortion [E. Sirota, Langmuir 13, 3849 (1997)].