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 measureme nts have been undertaken for a mixed Langmuir monolayer of octadecanoic aci d and methyl octadecanoate. For the composition studied (49.7 mol % ester) there are two noncrystalline tilted phases, one at low pressure in which th e chains point to their nearest neighbors (L-2 phase) and a higher-pressure phase in which the tilt is toward next-nearest neighbors. The higher-press ure phase arises from a merger of the L-2' and Ov phases, which are separat ed in the pure acid. A continuous change between the two regions is observe d rather than a first-order transition, as had been suggested. The results are discussed with reference to different proposals for the origin of two d istinct next-nearest neighbor tilted mesophases: the presence of one-dimens ional chain backbone order [V. M. Kaganer and E. B. Loginov, Phys. Rev. E 5 1, 2237 (1995)] and a coupling between tilt and distortion [E. Sirota, Lang muir 13, 3849 (1997)]. (C) 2000 American Institute of Physics. [S0021-9606( 00)70631-3].