Positional order in Langmuir monolayers

We find that a structural solid-solid phase transition in a two-dimensional Langmuir film is accompanied by strong positional disorder. Specifically, we find by a grazing-incidence x-ray diffraction experiment that in monolayers of octadecanol both the hexagonal phase LS and the centered rectangular (distorted hexagonal) phase S have algebraic decay of positional correlations. The positional disorder is evidenced by variations of the exponent eta, which drastically increases at the transition. The disorder is attributed to elastic distortions around pretransitional fluctuations.