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 monolay ers of octadecanol both the hexagonal phase LS and the centered rectangular (distorted hexagonal) phase S have algebraic decay of positional correlati ons. 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.