Stationary phases liquid crystal

As with traditional stationary phases, liquid crystalline stationary phases must demonstrate high thermal stability and have low vapour pressures (to ensure eolumn integrity and reproducibility). Furthermore, it is desirable that the liquid crystals exhibit their mesophases over as wide an operating temperature range as possible, so that the columns can be used for more generic separations and to permit their use with common temperature programming techniques. 

The chemical equilibrium in a multiphase system and the corresponding species concentrations in chemical separations are altered when chemical reactions take place. We will illustrate this now for gas-liquid equilibrium (as in gas absorption), vapor-liquid equilibrium (as in distillation), liquid-liquid equilibrium (as in solvent extraction), stationary phase-liquid equilibrium (as in ion exchange, chromatography and crystallization), surface adsorption equilibrium (as in foam fractionation) and Donnan equilibrium. 

Separation of Stereoisomers Osing Liquid Crystal Stationary Phases... 

In most cases the order of elution for a series of isomers on liquid crystalline stationary phases is generally in accord with the solute length-to-breadth ratios with differences in vapor pressure and solute polarity also being of Importance in some cases, leading to an inversion of elution order to that predicted from length-to-breadth ratios [828,829,838]. Long and planar molecules fit better into the ordered structure of the liquid crystal phase whereas nonlinear and nonplanar molecules do not permeate so easily between the liquid crystal molecules of the stationary phase and are more easily eluted from the column. 

Chiralsil-val, 6 96-97 Chiral smectic C liquid crystals, 15 106-107 Chiral stationary phases, 6 79-82 Chiral supramolecular clusters, 24 61 Chiral synthons, 11 5 Chiral titanium complexes, 25 98—99 Chirobiotic phases, for chiral separations, 6 90-91... 

The selection of the solvent is based on the retention mechanism. The retention of analytes on stationary phase material is based on the physicochemical interactions. The molecular interactions in thin-layer chromatography have been extensively discussed, and are related to the solubility of solutes in the solvent. The solubility is explained as the sum of the London dispersion (van der Waals force for non-polar molecules), repulsion, Coulombic forces (compounds form a complex by ion-ion interaction, e.g. ionic crystals dissolve in solvents with a strong conductivity), dipole-dipole interactions, inductive effects, charge-transfer interactions, covalent bonding, hydrogen bonding, and ion-dipole interactions. The steric effect should be included in the above interactions in liquid chromatographic separation. 

Janini, GM., Muschik, G.M., Schroer, J.A., and Zielinski, W.L., Gas-liquid-chromatographic evaluation and gas-chromatography mass spectrometric application of new high-temperature hquid-crystal stationary phases for polycyclic aromatic hydrocarbon separations. Ana/. Chem., 4S, 1879, 1976. 

The synthesis of optically active polymers was tackled with the purpose not only of clarifying the mechanism of polymerization and the conformational state of polymers in solution, but also to explore the potential of these products in many fields as chiral catalysts, as stationary phases for chromatographic resolution of optical antipodes, for the preparation of liquid crystals, and so on. 

Grushka, E. and Solsky, J.F., p-Azoxyanisole liquid crystal as a stationary phase for capillary column gas chromatography, Anal. Chem., 45, 1836, 1973. 

Witkiewicz, Z., Pietrzyk, M., and Dabrowski, R., Structure of liquid crystal molecules and properties of liquid-crystalline stationary phases in gas chromatography, J. Chromatogr., Ill, 189, 1979. 

Dewar, M. and Schroeder, J.P, Liquid-crystals as solvents. II. Further studies of liquid crystals as stationary phases in gas-liquid chromatography, J. Org. Chem., 30, 3485, 1965. 

Janini, G.M., Recent usage of liquid crystal stationary phases in gas chromatography, Adv. Chromatogr., 17, 231, 1979. 

Smith, T.R. and Wozny, M.E., Gas chromatographic separation of underivatized steroids using BPhBT liquid crystal stationary phase, J. High Resolut. Chromatogr. Chromatogr. Commun., 3, 333, 1980. 

Taylor, P.J. and Sherman, P.L., Liquid crystals as stationary phases for high performance liquid chromatography, J. Liq. Chromatogr., 3, 21, 1980. 

A commercially available semetic phase composed of biphenylcarbox-ylate ester attached to the polysiloxane backbone of a fused silica column has been shown to be ideal for the separation of geometric isomers of polycyclic aromatics of various classes of compounds [14,15]. This column exhibits a wide semetic temperature range of 100-300°C and has been shown to be stable to at least 280°C. Liquid-crystal stationary phases have also been employed in SFC [16]. 

L. Sojak, I. Ostrovsky, et al., Separation and identification of C14-C17 alkylbenzenes from dehydrogenation of n-alkanes by capillary gas chromatography using liquid crystals as the stationary phase, Ropa. Uhlie, 25(3) 149-157 (1983). 

S. Rokushika, K. Naikwadi, et al., Liquid crystal stationary phases for gas chromatography and supercritical fluid chromatography, HRC CC J. High Res. Chromatogr. Chromatog. Commun., 5 480-484(1985). 

Betts, T.J., Charmaine, M.M. and Tassone, A.I. (1991) Use of a liquid crystal stationary phase at temperatures below its melting point for the gas chromatographic study of some volatile oil constituents, journal of Chromatography A 547, 335-344. 

To improve the effectiveness of the chromatographic separation, a comparison study has been carried out on cyclodextrin and liquid crystal stationary phases Both materials function as "ordered" media with cyclodextrins the inclusion complex formation predominates, whereas the liquid crystals enable interaction of compounds with the ordered structure of the mesophase ... 

The properties of cyclodextrins were studied using a packed column, whereas the effect of the liquid crystals were enhanced by employing a capillary column The stereoselective properties of these materials as stationary phases were studied with a set of alkylbenzoderivatives The mechanism of the separation is discussed on the basis of the retention data obtained The advantages and drawbacks of these phases are compared with conventional GC stationary phases and analytical applications are discussed ... 

Increasing requirements on analyses of isomeric compounds and the problems encountered in their separation necessitate a study of more efficient systems which exhibit a high selectivity In gas chromatography new, selective stationary phases are studied Attention is also focused on the use of substances with oriented molecules that permit selective separations these properties are exhibited by e g inclusion compounds and liquid crystals ... 

The liquid crystals were deposited in glass capillary columns up to 100 m long, 0 25 mm i.d The columns were whet using the dynamic method, by flowing 1 5 to 20% liquid crystal solutions in chloroform at a flow rate of 1 cm s through the column. The efficiency of the prepared columns was compared with that of common stationary phases (columns 100 m long and 0 25 mm i d exhibited an efficiency of up,to 350 000 theoretical plates for hydrocarbons with k 5) ... 

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