J. P. Bayle

Comparison of two azobenzene liquid crystal stationary phases in open tubular column gas chromatography

Abstract Comparative applications of two liquid crystals used as stationary phases in gas-liquid chromatography were investigated using different kinds of solutes. These liquid crystal molecules are: 4-(4-trans-pentylcyclohexanecarboxyloxy)-2′-methoxy-4′-(4-trans-pentylcyclohexanecarboxyloxy)-azobenzene (LC1) and 4-(4-trans-pentylcyclohexanecarboxyloxy)-2′-butoxy-3′-methyl-(4-butoxybenzoyloxy)-azobenzene (LC2). Their thermal properties were established with differential scanning calorimetry (DSC). The chromatographic separation abilities of LC1 and LC2 were studied using capillary glass columns. Interesting analytical performances were obtained in different fields: isomeric separation of alkanes, aromatics, polyaromatics, volatile aromatic compounds and cis- and trans-isomers. Comparison between LC1 and LC2 is presented. Elution properties of LC1 and LC2 showed some differences. LC2 exhibited a higher separation efficiency especially for alkanes, light aromatics and polyaromatics, and the elution orders of some solutes were reversed compared with LC1. Then, the two liquid crystals are efficient before and after solid-solid or solid-nematic transitions. Nevertheless, they present higher plate numbers in the nematic state. Nice separations of volatile aromatic compounds are obtained by the two liquid crystals.

Investigation of thermal and analytical properties of two nematic liquid crystals by gas chromatography

SummaryThermal and analytical properties of two liquid crystals containing internal [2-nitro, 3-methyl 4-(4-heptylbenzoyloxy) 4′-methylazobenzene] and external NO2 [2,3-dimethyl 4-(4-heptylbenzoyloxy) 4′-nitroazobenzene] have been investigated by gas chromatography. Transitions temperatures are determined by inverse gas chromatography using standard columns. Good agreement with values determined by differential scanning calorimetry are found. Analytical capabilities of the two liquid crystals are shown in glass capillary columns. Compared to examples in the literature, the crystals used separated different kinds of solutes. In addition, high molecular mass solutes were eluted at relatively low temperatures. Typical separations are observed for hydrocarbon positional isomers, aromatic hydrocarbons, geometric isomers, and volatile aroma compounds. Comparison between the performance of internal and external, NO2-liquid crystals is made.

Nematogens with more flexible chains than aromatic rings in the core

Mesogens containing four rings in the main core can accommodate one terminal and two nearby lateral chains on each outside aromatic ring. These compounds containing six chains present an enantiotropic nematic range which is influenced by the rigidity of the links. The conformational behaviour of the first methyleneoxy group within the chains was investigated by one and two dimensional C-13 NMR. The sign of the jump in chemical shifts when entering the nematic phase indicates the folding of each lateral branch. Dipolar oscillations during cross-polarization contact provide the values of the bond order parameter. The two First lateral fragments do not behave in the same way, demonstrating the influence of the fragment along which the chain is back: folded.

Gas chromatographic properties of some liquid crystals containing dioxyethylene ether terminal chains

SummaryComparative gas chromatographic properties of four rod-like liquid crystals coated on capillary columns are studied. These four liquid crystals contain the same number of atoms in their backbones. They differ by the partial or total replacement of the two terminal heptyloxy chains by a 2-(2-methoxyethoxy)ethoxy (called also dioxyethylene ether) chain. Synthesis and thermal properties of these four liquid crystals are presented. It appears that the introduction of dioxyethylene ether decreases the two transition temperatures as well as the nematic range. With regards to the chromatographic separation abilities, the four liquid crystals are efficient before and after solid-nematic or nematic-liquid transitions; however, the plate numbers are higher in the nematic state. Interesting analytical applications are demonstrated in different fields: isomeric separation of alkanes, aromatics, polyaromatics, volatile aroma compounds, cis and trans isomers and phenols. The increase of the number of oxyethylene units permits to drastically increase the separation efficiency for polar solutes.

A thermodynamic approach to understanding liquid crystal selectivity in gas chromatography

SummaryThe thermodynamics of four new liquid crystals were investigated in order to understand their selectivity as stationary phases in gas chromatography. In this case study, liquid crystals with a benzoyloxy azobenzene mesogenic core substituted with heptyloxy (C7) and/or dioxyethylene ether (DOE) groups, were used. The chromatographic separations of linalool and citronellal, and of xylene, tetraethylbenzene and cresol isomers, which were achieved with the liquid crystal stationary phases, have been related to the dissolution thermodynamics of the solutes. The results gave us an insight into the mechanism of the molecular recognition involved in the separation processes.

Ordering of a lateral crown ether and terminal short POE chains in some symmetrical nematogens by 13 C NMR

A lateral crown ether fragment can be introduced on symmetrical mesogens containing only three aromatic rings. The replacement of the terminal alkoxy chains by chains containing oxyethylene units decreases the melting and clearing temperatures allowing one to obtain nematic compounds near room temperature. These compounds dissolve LiBF4 salt only to a small extent, but the nematic arrangement, is thereby destroyed. The carbon chemical shift anisotropy and the local C-H bond order parameters were obtained for the nematic phase for the crown ether fragment and the terminal chains. This study indicates that the crown ether average conformation changes insignificantly on decreasing the temperature. The lateral crown ether protrudes markedly from the core with the consequence that the molecular shape is far from rod-like in geometry.

Nitrophenyl liquid crystals with intramolecular hydrogen-bonding in the mesogenic core

Abstract A liquid crystalline homologous series of 4-nitrophenyl-4′-alkoxyphenyl diazene and their 2′-hydroxy analogs were synthesized, and their mesogenic properties were studied. The formation of intramolecular hydrogen bonding in the 2′-hydroxy analog series increased the rigidity of the mesogenic core and enhances the ranges of the liquid crystalline phases.

New chemically bonded liquid crystal for high-performance liquid chromatography: Synthesis, characterization and chromatographic behaviour

SummaryA new bonded liquid-crystal stationary phase has been developed for high-performance liquid chromatography. The method includes the synthesis of 4-(4-carboxybenzyloxy)-3-methyl-2-dodecyloxy-4′-(2-(4′-dodecyloxybenzyl)-ethylene-azobenzene (ALC) which reacts with LiChrospher Si 100 NH2 to give the bonded stationary phase (denoted BLC). Characterization of ALC and the BLC were performed by differential scanning calorimetry (DSC), proton NMR, solid-state13C NMR and elemental analysis. The specific surface area of the BLC was determined by BET. The chromatographic behaviour of the BLC was investigated in both normal-and reversed-phase modes. As a normal phase, a temperature transition is found in the 315–330 K range which leads to remarkable analytical properties. The retention factors of phenanthrene and anthracene increased when the temperature was increased from below to above the transition temperature. In the reversed-phase mode, the behaviour of the BLC seemed more complex, two temperature transitions being apparent. Separation of xylene and diethylbenzene isomers was possible at temperatures above the temperature range of the transitions.

Ordering of some liquid crystals containing the 2-phenylindazole core

Molecules containing the 2-phenylindazole core present liquid crystalline properties even if the two terminal chains do not point along the same axis. 13CNMR in the liquid crystalline phase shows that the molecular long axis is nearly aligned with the para-axis of the phenyl moiety of the 2-phenylindazole core. This implies that the first fragments of the chain belonging to the indazole moiety do not lie along the molecular long axis. To promote liquid crystal properties, this chain needs to possess at least six carbon atoms.

Nematogens incorporating a lateral flexible ring

A lateral flexible ring can be introduced on a mesogenic core containing four rings. Lateral aliphatic rings containing up to 12 atoms have been synthesized. Despite this large lateral protruding substituent, a large nematic range is obtained. A compound with a benzo15-crown-5 lateral ring has been successfully obtained and shows the possibility of designing nematic liquid crystals with a crown ether moiety at a lateral position.