Two-dimensional films of discotic molecules at an air-water interface

Abstract

2014 On montre pour la première fois que des molécules discotiques de benzène-hexa-alkanoates peuvent être étalées en monocouches à un interface eau-air. Les mesures de pression superficielles donnent des résultats très analogues à ceux des films d'acides gras. Le noyau benzénique joue le rôle de la tête polaire grâce à six groupes carboxyliques périphériques, et se met à plat sur l'interface. Les chaines alkanoates jouent le rôle des chaines aliphatiques. Les isothermes de pression permettent une estimation des dimensions moléculaires dans le plan de l'interface. Ces dimensions sont pratiquement identiques à celles déduites des mesures de rayons X dans les phases à colonnes formées par les benzène-hexa-alkanoates (cristaux liquides thermotropes). Ces expériences permettent de réfuter l'interprétation de la transition liquide expansé-liquide condensé comme une transition du second ordre entre une phase isotrope et une phase cristal liquide nématique. Abstract. 2014 The possibility of forming Langmuir monolayers with disc-like molecules of benzene-hexa-alkanoates has been investigated for the first time. Surface pressure measurements show that these monolayers behave in many respects as the well-known films of fatty acids. The benzene ring plays the role of the polar head group while the alkanoates chains are the long aliphatic tails. Molecular dimensions can be derived from the surface pressureconcentration isotherms. The results indicate that the benzene rings lie flat at the interface. The projection of the molecular diameter onto the interface is practically identical to the lattice spacing measured in the liquid-crystalline columnar mesophases formed by benzene-hexa-alkanoates in bulk. These experiments allow to refute the interpretation of the liquid expanded-liquid condensed transition in terms of a second order isotropic ~ nematic phase transition, as recently proposed by several theoretical groups.

Monolayers of rod-shaped and disc-shaped liquid crystalline compounds at the air-water interface

“…At temperatures above 5°C, the curves show the formation of a highly compressible film, which collapses at about 2.5 nmVmolecule at 5 mN/m. This spreading behaviour agrees well with n-A diagrams reported previously [5]. As already discussed by Rondelez [5], the collapse area would fit well with a flat orientation of the benzene ring at the water interface.…”

“…This spreading behaviour agrees well with n-A diagrams reported previously [5]. As already discussed by Rondelez [5], the collapse area would fit well with a flat orientation of the benzene ring at the water interface. This arrangement of the discotic LC 7 would formally agree with the arrangement of a conventional amphiphile: The benzene ring with all the six hydrophilic ester groups seems to act as the polar head group and is placed on the water surface, analogously as shown for 10 in Figure 12.…”

“…7). Analogous phase transitions were reported for BHAs with pentanoyl, hexanoyl and heptanoyl chains at higher temperatures [5]. Considering the collapse area, the minimal area necessary for the packing of the six hydrocarbon chains would be 1.1 nmVmolecule.…”

“…So far there have only been a few reports about the spreading behaviour of discotic LCs [5,7], but much work has been done on disc-shaped molecules such as porphyrins or phthalocyanines [32][33][34][35][36][37][38][39][40][41] which do not form thermotropic mesophases. However, the formation of defined monomolecular films at the gas-water interface is uncertain in some cases, considering the published n-A diagrams.…”

“…Recently, the combination of amphiphilic and mesogenic properties of thermotropic LCs has been realized, and spreading experiments at the air-water interface were performed [3][4][5][6][7]. In addition, the functionalization of amphiphilic compounds led to the incorporation of rod-shaped mesogenic units in amphiphiles, such as biphenyl [6,8], stilbene [9,10], azobenzene [11][12][13][14], benzylidenaniline [15] or bipyridine [16,17].…”