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 experiment. We find algebraic decay of positional correlations, which suggests solid-like order in both phases at the distances accessible in the present x-ray diffraction experiment. The transition is accompanied by strong positional disorder, which is evident from the drastic increase of the exponent η to values η>2 close to the transition. Remarkably, on approaching the transition, the continuous increase of η is apparent already at 20^∘C above it. The positional disorder is attributed to elastic distortions around pretransitional fluctuations.