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

摘要

Grazing incidence x-ray diffraction and Brewster-angle microscopy measurements have been undertaken for a mixed Langmuir monolayer of octadecanioic acid and methyl octadecanota. For the compositon studied (49.7mol% ester)there are tow noncrystaline tilted phases, one at low pressure in which the chains point to thier nearest neighbors (L_2 phase)and a high-pressure phase in which the tilt is toward next-nearest neighbors. The higher-pressure phase arises from a merger of the L_2' and Ov phases, whith are separted in the pure acid. A continuous change between the tow regions is observed rather than a first-order transition, as had been suggested. The results are discussed with reference to different proppsals 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.Longinov. Phys. Rev.E 51,2237(1995)]and a coupling between tilt and distortion [E.Sirota, Langmuir 13,3849 (1997)].