Liquid Crystal Stationary Phases Research Articles

Original Mesogenic Citronellol-Based Stationary Phase for Both Normal- and Reversed-Phase HPLC Modes: Properties and Applications

In this work, the synthesis, characterization and chromatographic performance of a silica material chemically bonded with a liquid crystal based on terminal citronellol for HPLC separation were investigated. This bonded liquid crystal stationary phase denoted as LC-LCC8 resulted from the grafting of a mesogenic carboxylic acid, 3′-((3,7-dimethyloct-6-en-1-y1)oxy)-[1,1′-biphenyl]-4-carboxylic acid (LCC8), on 4-(1,1-dimethyldisiloxanyl)butan-1-amine. LCC8 was previously synthesized and characterized by proton NMR, and a nematic mesophase at 127 °C was confirmed by differential scanning calorimetry and microscopy. The resulting stationary phase LC-LCC8 was characterized by using diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and elemental analysis. The granulometric distribution was evaluated by scanning electron microscopy (SEM) before and after grafting. An analytical column (150 mm × 2.1 mm × 5 µm) was packed with the stationary phase for the chromatographic study. The chromatographic behaviour of the stationary phase was first characterized using the Tanaka test, exhibiting an exceptionally high shape selectivity (αT/O = 3.9). In normal-phase mode, using a 92/8 cyclohexane/tetrahydrofuran (THF) (v/v) mixture as the mobile phase, the isomeric mixture (Z and E) of crotamiton was well separated. In reversed-phase mode, good selectivity was observed towards organic pollutants [polycyclic aromatic hydrocarbons (PAHs)] and pharmaceutical compounds (xanthines, steroids and antihistamines). The new bonded phase showed satisfactory results both in normal- and reversed-phases modes except in terms of peak shape. The temperature effect on the selectivity and resolution of LC-LCC8 was estimated by injecting different isomers at various temperatures ranging from 293 to 333 K, demonstrating an improved resolution of solutes at high temperatures, which was probably due to a change in orientation order of the grafted ligands.

A promising "metastable" liquid crystal stationary phase for gas chromatography

The liquid crystal state is an ordered physical state between a solid and a liquid. Previous research, in gas chromatography, proved that it provides a geometric selectivity, which allows the separation of geometric position isomers and cis-trans isomers that are difficult to separate on conventional gas chromatography stationary phases (polydimethyl siloxane derived and polyethylene glycol stationary phases). However, their use was generally very limited by the rather high temperature at which they must be operated, normally above the solid-liquid crystal transition temperature. In the present study we are interested in a new synthesized material, 1,4- bis (4-bromohexyloxy benzoate) phenyl (BHOBP). The first characterizations of BHOBP were carried out by thermogravimetric analysis, hot-stage optical microscopy and differential scanning calorimetry to control the thermal stability of the BHOBP as well as the nematic texture of the mesophase highlighted in a well-defined temperature range (120 °C-200 °C). When heated, the solid compound led to a stable liquid crystal state. Its cooling has revealed "a new metastable physical state, which is the supercooled liquid crystal phase". After these first characterizations, the new material was used as a stationary phase for gas chromatography. The BHOBP was deposited in a capillary column by the dynamic method. The inverse gas chromatography study of the column revealed a solid-stable nematic phase transition temperature, in agreement with the first characterization methods. The stable liquid crystal phase showed good resolutions in the analysis of some geometric isomers of low volatility as PAHs. The presence of the supercooled liquid crystal state in the chromatographic column has also been confirmed. This new metastable state is particularly interesting because it enlarged the scope of this material by improving the resolution of several mixtures. Thus, the separation of highly volatile mixtures of geometric isomers (e.g. cis and trans-decalin) was achieved only through this metastable mesophase confirming its unique selectivity. The metastable liquid crystal, used at 80 °C, has also exhibited an original behavior by its stability after several weeks of use at the same temperature, maintaining constant retention factors and selectivity.

Thermodynamic study by gas chromatography of a homologous Series of Alkanes Using New Synthesized Liquid Crystals as Stationary Phases

Thermodynamic parameters of enthalpy ΔH, entropy ΔS, and differential free energy (ΔG) for alkanes were determined for a new liquid-crystal stationary phase. The liquid-crystal was synthesized by successive steps and used as the stationary phase in inverse gas chromatography (IGC), IGC uses for determine their interactions of alkanes with the liquid-crystal in the different mesophases thermal. Two methods were used to assess the estimations of thermodynamic parameters

Historical Development of Liquid Crystalline Stationary Phases: An Overview

Nematic liquid crystals have shown particular selectivity and sensitivity as stationary phases for the separation of isomers having similar volatilities. Because of their unique selectivity towards rigid solute isomers, liquid crystal stationary phases were considered at one time to be a very promising class of materials that give the gas chromatographic separations a great deal of attention. In this overview, we present a brief historical view of the liquid crystal stationary phases development, which were successfully used in gas chromatography for separation of isomers.

Separation of Isomers on Nematic Liquid Crystal Stationary Phases in Gas Chromatography: A Review

Understanding the composition of complex hydrocarbon mixtures is important for environmental studies in diverse fields, but many prevalent compounds cannot be confidently identified using traditional gas chromatography (GC) techniques. Increasing requirements on analyses of isomeric compounds and the problems encountered in their separation demand a study of more efficient systems which exhibit a high selectivity. Kelker and Fresenius first used nematic liquid crystals as stereospecific stationary phases in GC. Nematic liquid crystal has shown this particular selectivity and sensitivity as stationary phases for the separation of isomers having similar volatilities. Because of their unique selectivity towards rigid solute isomers, liquid crystal stationary phases were considered at one time to be a very promising class of materials that give gas chromatographic separations very different from those that can be obtained with any other stationary phase. Since then, a great deal of attention has been paid to the separation properties of this relatively wide group of substances. Liquid crystal can be used to separate a variety of compounds including isomer mixtures which cannot be separated on conventional stationary phases. This paper aims to review all specific experimental results and presents a comparative analytical study of monomeric nematic liquid crystal stationary phases used in GC. A further contribution of this review is in the field of isomeric compounds separation.

Gas Chromatographic Separation of Structural Isomers on Cholesteric Liquid Crystal Stationary Phase Materials

Gas Chromatographic Separation of Structural Isomers on Cholesteric Liquid Crystal Stationary Phase Materials

FAST ANALYSIS OF PAH BY GC AND GC/MS USING ISOMER SPECIFIC GC CAPILLARY COLUMNS

The required separation of polycyclic aromatic hydrocarbons (PAH) by GC, to meet regulatory requirements, depends largely on column selectivity. Liquid crystal (LC) capillary columns are highly selective for PAH separation. In the current investigation analysis and separation of PAH on columns coated with only the LC stationary phase, the combination of LC and non-polar stationary phase and only non-polar stationary phase, have been studied. The complete separation of isomeric PAH in the mixture of 16 priority PAH is achieved on the LC column. The inherent advantage of higher efficiency for microbore columns compared to conventional 0.25 mm internal diameter (ID) columns of the same length, in combination with the selectivity of the LC stationary phase, resulted in the fast analysis of 16 PAH on the columns with the combination of LC and non-polar stationary phases. Using this column, designated as the ICB-PAH column, separation of the USEPA 16 priority PAH in addition to other isomeric PAH (except benzo[j]fluoranthene) is achieved. Complete separation of 16 priority PAH was achieved in less than 5 minutes on a 6 m × 0.10 mm ID column, and in less than 7 minutes on a 15 m × 0.25 mm ID column. Separations of 16 priority PAH plus five additional isomeric PAH on LC, non-polar, and a combination of LC and non-polar stationary phase columns are discussed.

Electrochromatographic studies of etched capillaries modified with a cyano pentoxy biphenyl liquid crystal

A liquid crystal stationary phase for open tubular capillary electrochromatograpy (OTCEC) is fabricated by etching a fused silica tube and then bonding 4,4′-cyanopentoxy biphenyl by a silanization/hydrosilation process. The versatility of this electrophoretic capillary is demonstrated by separations of proteins, peptides, basic pharmaceuticals and the metabolites of tryptophan. Chromatographic interactions are verified by resolution of two neutral peptides. Variable temperature studies are used to understand the liquid crystal properties of the bonded moiety. EOF measurements as a function of pH and temperature further characterize this unique separation media.

Electrochromatographic studies of etched capillaries modified with a cyano pentoxy biphenyl liquid crystal

A liquid crystal stationary phase for open tubular capillary electrochromatograpy (OTCEC) is fabricated by etching a fused silica tube and then bonding 4,4′-cyanopentoxy biphenyl by a silanization/hydrosilation process. The versatility of this electrophoretic capillary is demonstrated by separations of proteins, peptides, basic pharmaceuticals and the metabolites of tryptophan. Chromatographic interactions are verified by resolution of two neutral peptides. Variable temperature studies are used to understand the liquid crystal properties of the bonded moiety. EOF measurements as a function of pH and temperature further characterize this unique separation media.

A thermodynamic approach to understanding liquid crystal selectivity in gas chromatography

The 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.

Comparison of two liquid crystals as stationary phases in capillary gas chromatography

The comparative gas chromatographic behavior has been investigated for two new liquid-crystal stationary phases, 2-(3-chlorobenzyloxy)-3-hexanoxy-4-(4-chlorobenzoyloxy)-4′-(4-methylbenzoyloxy)azobenzene (denoted 3-Cl) and 2-(3-methylbenzyloxy)-3-hexanoxy-4-(4-chlorobenzoyloxy)-4′-(4-methylbenzoyloxy)azobenzene (denoted 3-CH3). The structures of the main core of the two liquid crystals are the same but they differ by lateral aromatic branch— 3-chlorobenzyloxy and 3-methylbenzoyloxy. Their thermal properties were established by use of differential scanning calorimetry (DSC) and their chromatographic separation properties by use of glass capillary columns. Interesting analytical performance was obtained in the separation of isomeric aromatic compounds, polyaromatic compounds, volatile aroma compounds, andcis andtrans isomers. Separation efficiency was higher for 3-Cl, especially for volatile aroma compounds, light aromatic compounds, and polyaromatic compounds, although the phases were similar in otherways.

Synthesis, characterization and liquid chromatographic behaviours of a new chemically bonded liquid crystal

A new bonded liquid crystal stationary phase (2OC 12) for high-performance liquid chromatography was studied. It resulted from coupling of LiChrospher Si 100 NH 2 and a mesogenic carboxylic acid, 4-(4-(4-(3,4-didoceyloxy-styrenyl)phenyl-diazenyl)phenyloxy-methylene) benzoic acid (ILC). ILC was characterized with proton NMR and differential scanning calorimetry, while 2OC 12 was characterized by solid state 13C NMR and elemental analysis. 2OC 12 surface area was determined by the BET method. The chromatographic behaviour of 2OC 12 was investigated under both normal- and reversed-phase conditions. The plots of ln k against 1/ T showed transition temperatures at 325 and 337 K. Polyaromatic hydrocarbons (PAHs) were separated using hexane, isooctane or hexane-chloroform. Above the transition temperatures, the bonded material exhibited a liquid crystal-like behaviour: (i) the plate number N was always highest possible, and (ii) the more retained the solute the more elongated it was (anthracene is eluted after phenanthrene, chrysene before tetracene, pentacene after dibenzo-a,h-anthracene). Using acetonitrile/water (60/40), reversed-phase data of aromatic hydrocarbons are similar (highest values of N, better resolution below than during the transitions).

High performance liquid chromatography of aromatic and polyaromatic hydrocarbons on a new chemically bonded liquid crystal phase

The chromatographic behaviour of 15 aromatic and 20 polyaromatic hydrocarbons (PAHs) on a new HPLC bonded liquid crystal stationary phase called BLC is presented. BLC is 4-(4-carboxy-benzyloxy)-3-methyl-2-dodecyloxy-4′-(2-(4′-dodecyloxy-benzyl)-ethylene-azobenzene bonded to LiChrospher Si 100 NH2. The method for PAH separation involved n-hexane in normal-phase HPLC. In reversed phase HPLC, which was applied to the aromatics, the mobile phase was acetonitrile/water (60/40, v/v). UV detection was carried out at 254 nm. The temperature of the chromatographic system was varied from 285 to 343 K. For the normal phase system, two transition temperatures were observed for all the PAHs studied. The first transition occurred at 317 K, and may be induced by the motion of the terminal O-dodecyl chain. When the lateral O-dodecyl moves, the second transition appears at 326 K. Compared to the normal phase separation of polyaromatic hydrocarbons presented in a previous study, the behaviour on the reversed phase seems to be more complex and only one large transition appears at 317 K. The chromatographic behaviour was investigated below, during and above the transition range between 315–330 K. Interesting analytical properties were observed. In the normal phase, the PAH retention factors were enhanced when the temperature was increased from below to above the temperature transitions. A better separation was obtained above the transition range. In the reversed phase, xylene and diethyl benzene isomers were clearly separated above the transitions. The xylene isomers were not resolved below the transitions, but good separation was found for the diethylbenzene isomers. During the transition, the xylenes and diethylbenzenes were not separated. The bonded material exhibited liquid crystal-like behaviour in both (normal and reversed) phases.

Separation of polynuclear aromatic hydrocarbons and phenols on a new chemically bonded phase for high-performance liquid chromatography

A new bonded liquid-crystal stationary phase for high-performance liquid chromatography is presented. Synthesis entailed reaction of γ-benzyl-L-glutamate-N-carboxyanhydride with aminopropyl LiChrospher 100 NH2. The new stationary phase was characterized by solid-state13C NMR and elemental analysis. Its analytical properties were tested with polynuclear aromatic hydrocarbons (PAH) and phenols. PAH were chromatographed with hexane-chloroform mixtures as mobile phases and were eluted in order of increasing ring number. Total resolution of the 22 PAH studied was achieved by gradient elution. Phenols were chromatographed with a mixture of chloroform and methanol as mobile phase. Small amounts of methanol have a substantial effect on the separation of the phenols. Use of 0.5% (v/v) methanol enabled the separation of 19 phenols and phenol derivatives.

Retention behavior of polycyclic aromatic hydrocarbons in supercritical fluid chromatography on a chemically bonded stationary phases based upon liquid-crystalline polymer

The retention behavior of a set of polycyclic aromatic hydrocarbons in supercritical fluid chromatography have been studied on a chemically bonded stationary phase based upon a side chain liquid crystalline polymer (LCP) with carbon dioxide-based mobile phase. The effects of the mobile phase pressure, column temperature and amount of mobile phase organic modifier have been investigated in order to detect a possible structural change in the liquid crystal polymer linked to the silica support. The influence of these factors on the selectivity coefficients has also been studied. Two distinctive behaviors with temperature are noted at low pressure on the one hand and at higher pressure on the other. This change in behavior is based on the density of the supercritical CO2 and the PAH volatility rather than on any specific stationary phase structural change. Both lower mobile phase pressure and amount of mobile phase modifier are required to obtain better selectivities. Better planarity recognition is observed in SFC than in HPLC with these new bonded liquid crystal stationary phases. The bonded liquid crystal phase is only weakly affected by the addition of organic modifier in the supercritical CO2.

Synthesis and characterization of a new type of chemically bonded liquid crystal stationary phase for HPLC

Two commercially available liquid crystals, 4-cyano-4′-n-pentyl-1,1′-bipheny and 4-cyano-4′-n-pentoxy-1,1′-bipheny, are bonded to a silica hydride surface via hydrosilation in the presence of a free radical iniator, t-butyl peroxide. Elemental analysis, diffuse reflectance Fourier trans-form infrared spectroscopy, and13C and29Si CP-MAS NMR spectroscopy are used to confirm the success of the bonding reaction. The13C CP-MAS spectra suggest a difference in the bonded phase morphology of the two materials. Static hydrolytic stability tests indicate these materials do not degrade significantly in both acidic and basic solutions. Chromatographic tests confirm that these two bonded phase behave differently with respect to their retention of PAHs, alkyl-substituted benzenes and benzodiazepines.

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

A 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.

Synthesis and Characterization of Liquid Crystal Type Stationary Phases on a Silica Hydride Surface

Two liquid crystal materials, 4-methoxyphenyl-4-allyloxybenzoate and cholesteryl-10-undecenoate, containing terminal olefin groups are bonded to a silica hydride surface via a metal complex catalyzed hydrosilation reaction. Bonding is confirmed by DRIFT, carbon-13 CP-MAS NMR and DSC. The effects of different catalysts, reaction temperature, catalyst to olefin ratio, and reaction time were studied to maximize the surface coverage. Chromatographic studies indicate that these bonded materials behave similarly to liquid crystal phases prepared by organosilanization. The hydrosilation method offers the possibility of bonding a wider variety of liquid crystal moieties in order to utilize the unique separation properties of these materials.

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

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.

Variable-temperature, solid-state NMR studies of bonded liquid crystal stationary phases for HPLC

The spin-spin relaxation time measured from the line width (T2*) is used to study bonded phase dynamics of two liquid crystal materials in the solid state as a function of temperature. The motion of these two bonded phases is much more restricted than either C-18 or butyl phenyl bonded to the same substrate. Plots ofT2* vs. 1/T indicate a structural transition of the bonded phase moiety for the two liquid crystal materials but no such change is seen for either C-18 or butyl phenyl over the same temperature range (243–353 K), suggesting a higher degree of association between adjacent bonded moieties for the two liquid crystal materials in comparison to the other two phases. The conclusions from solid-state NMR data coincide with results obtained in earlier chromatographic studies.