Near-critical two-dimensional smectic-C to solid-like phase transition in azobenzene-derivative Langmuir monolayers

We describe the results of a detailed study of two-dimensional (2D) smectic -C to solid-like phase transition in azobenzene-derivative Langmuir monolay ers by the surface pressure vs area-per-molecule isotherm measurement, pola rizing optical microscopy, and the grazing incidence synchrotron x-ray diff raction. The phase transition was observed to be weakly first order up to s lightly above room temperature with a transition entropy less than 1.0 k(B) per molecule, accompanied by proper characteristics of a nearly ideal hori zontal coexistence line on the isotherm, an abrupt change in optical textur e, and the emergence of a conspicuous x-ray diffraction peak in the solid-l ike phase, which indicates a weak positional order with a correlation lengt h of 10-20 nm. Analysis of the x-ray diffraction data within the framework of distorted hexagonal lattice suggests that the solid-like phase may be re garded as a 2D analog of smectic-L phase that is a hexatic smectic phase wi th the molecules tilted toward a direction between the nearest and the next -nearest bonds. At higher temperatures, the transition became less disconti nuous and entirely disappeared above 40 degreesC in all these experimental aspects. We argue that the transition is viewed as a 2D induced hexatic-hex atic transition (analog of smectic-C to smectic-L transition), which has an isolated critical point where the first-order transition comes to an end. (C) 2001 American Institute of Physics.