Positional order in Langmuir monolayers: An x-ray diffraction study

The structural phase transition from the hexagonal to a distorted-hexagonal (centered rectangular) phase (the LS-S transition) in Langmuir monolayers of octadecanol is studied in a grazing incidence x-ray diffraction experime nt. We find algebraic decay of positional correlations, which suggests soli d-like order in both phases at the distances accessible in the present x-ra y diffraction experiment. The transition is accompanied by strong positiona l disorder, which is evident from the drastic increase of the exponent eta to values eta>2 close to the transition. Remarkably, on approaching the tra nsition, the continuous increase of eta is apparent already at 20 degrees C above it. The positional disorder is attributed to elastic distortions aro und pretransitional fluctuations. [S1063-651X(99)10102-8].