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

Abstract

Transition between two next-nearest-neighbor phases in a mixed Langmuir monolayer. A study by grazingincidence x-ray diffraction and Brewster-angle microscopyWe describe the results of a detailed study of two-dimensional ͑2D͒ smectic-C to solid-like phase transition in azobenzene-derivative Langmuir monolayers by the surface pressure vs area-per-molecule isotherm measurement, polarizing optical microscopy, and the grazing incidence synchrotron x-ray diffraction. The phase transition was observed to be weakly first order up to slightly above room temperature with a transition entropy less than 1.0 k B per molecule, accompanied by proper characteristics of a nearly ideal horizontal coexistence line on the isotherm, an abrupt change in optical texture, and the emergence of a conspicuous x-ray diffraction peak in the solid-like phase, which indicates a weak positional order with a correlation length 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 regarded as a 2D analog of smectic-L phase that is a hexatic smectic phase with the molecules tilted toward a direction between the nearest and the next-nearest bonds. At higher temperatures, the transition became less discontinuous and entirely disappeared above 40°C in all these experimental aspects. We argue that the transition is viewed as a 2D induced hexatic-hexatic transition ͑analog of smectic-C to smectic-L transition͒, which has an isolated critical point where the first-order transition comes to an end.

Chemical Leslie effect in Langmuir monolayers: a complete experimental characterization

“…BAM images allowed us to determine the local gradient |∇φ 0 | ∼ 4π/0.1 mm and the size of the domain L ∼ 1 mm. In consequence, eqn (11) gives us a value for the ratio K/γ that is of the order of 2 10 −10 m 2 .s −1 , in correct agreement with the previous value found from the analysis of the fluctuations.…”

“…Although targets had been previously observed in Langmuir monolayers of chiral compounds [10][11][12] , it was in the seminal work of Tabe and Yokoyama 5 that such patterns were first attributed to a molecular rotation driven by the evaporation of water in the subphase. This work prompted additional theoretical [13][14][15][16][17] and experimental [18][19][20] studies that featured both evolving target patterns and spirals when the rotation started with defects of strength +1 17,18 .…”

Miscibility Transitions and Lateral Compressibility in Liquid Phases of Lipid Monolayers

“…13 For example, compressibility is used to identify the critical point of a smectic-C to a solid-phase transition in monolayers. 23 On the other hand, although diverging compressibility may be predicted for critical points in traditional monolayer systems, systems with "dipole critical points" like lipids may behave differently. 24 As the critical pressure is approached in lipid monolayers containing immiscible liquids, domains change shape.…”

“…Considering critical points first, the compressibility of a liquid is expected to diverge at a critical point . For example, compressibility is used to identify the critical point of a smectic-C to a solid-phase transition in monolayers . On the other hand, although diverging compressibility may be predicted for critical points in traditional monolayer systems, systems with “dipole critical points” like lipids may behave differently .…”

Texture Changes inside Smectic-C Droplets in Azobenzene Langmuir Monolayers

“…However, a most remarkable feature in this system is the appearance of circular domains (droplets), essentially formed by the trans-isomer (SmC phase) of 8Az3COOH, embedded in an isotropic polar phase mainly constituted by the cis-isomer, when a 8Az3COOH chloroform solution in the photostationary state corresponding to room light is spread on the water subphase. 19 Although the existence of circular domains in Langmuir monolayers of this azobenzene derivative had been reported previously by Tabe et al, 9,13,15 a systematic study and the appropriate experimental conditions to observe them were not discussed in their papers. Our Brewster angle microscopy images 19 show that the inner texture of these droplets is characterized by a continuous change in their reflectivity around either a unique or several point defects (singularities), located at the center or close to the edge of the domain.…”