Crystalline 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. 

HOST COMMON STRUCTURES OF LIQUID CRYSTALLINE STATIONARY PHASES Yi (CiH4) X(C H4) Y2 with n 1 or acre. Y and X are usually para substituents. 

Witkiewicz, Z. and Stanislaw, P., Separation of close-boiling compounds on liquid-crystalline stationary phases, J. Chromatogr., 154, 60, 1978. 

Naikwadi, K.P., Rokushika, S., and Hatano, H., New hquid crystalline stationary phases for gas chromatography of positional and geometrical isomers having similar volatilities, J. Chromatogr., 331, 69, 1985. 

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. 

Figure 3,4) or a better separation of multicomponent mixtures (see Figure 5,6) results from the use of liquid crystalline stationary phases. 

Fig. 3.8. Chromatographic resolution of Sa//3-androstan-3a/jS-ols on a liquid crystalline stationary phase. Reproduced from [87] with permission of the American Chemical Society.

However, experimental values for log P are sometimes difficult to obtain. Recently, we have investigated the utility of micellar liquid chromatography for assessing the lipophilic character of organic molecules in biological media. In our study we used a liquid crystalline stationary phase to generate retention data for a set of 22 mono-, di-, and tri-substi-tuted benzenes. The retention factor (i.e., log k ) of these compounds was found to be correlated to the log of their octanol/water partition coefficient. 

With respect to the structure of the anisotropic phase, the authors stated the liquid crystals were lamellar. They presented two pieces of evidence to support this contention. First, on the basis of the phase diagram for the dodecanol/SDS/water system, only two liquid crystalline stationary phases will exist (1) hexagonal, or... 

Less common than in GC is the application of LCP as a stationary phase in LC7° Two basic methods to prepare liquid crystalline stationary phases are available, namely ... 

S. Rokushika, K. P. Naikwadi, A. L. Jadhav, and H. Hatano. Polyacrylate liquid crystalline stationary phases in supercritical fluid chromatography with carbon dioxide mobile phase. Chromatographia, 22 209-212,1986. 

Price GJ, Hickling SJ, Shillcock IM. Applications of inverse gas chromatography in the study of liquid crystalline stationary phases. J Chrom-atogr A 2002 969(l-2) 193-205. 

Witkiewicz Z, Oszczudlowski J, Repelewicz M. Liquid-crystalline stationary phases for gas chromatography. J Chromatogr A 2005 1062(2) 155-74. 

Hgure 6 Structure of the SB-smeclic side-chain liquid-crystalline stationary phase (R is 50% liquid crystal substituents). 

GC on liquid-crystalline stationary phases can be used for investigation of the physicochemical... 

MECHANISM OF SEPARATION ON LIQUID-CRYSTALLINE STATIONARY PHASES... 

Liquid-crystalline stationary phases are particularly useful to separate isomers. Isomers that are more elongated in shape (e.g.,p-xylene) are retained in a column filled with LCSPs longer than isomers that are less elongated in shape (e.g., m-xylene). The retention of isomers of chromatographed substances considerably depends on the type of the mesophase of the liquid-crystalline stationary phase. This results from the fact that the type of mesophase affects the dissolution and diffusion of isomers of the chromatographed substance to a different degree. On the whole, nematic liquid crystals show the best separation properties in relation to isomers but, in some cases, very good separation can be obtained on smectic and cholesteric stationary phases. 

Liquid-crystalline stationary phases are used to analyze a great number of chemical compounds such as polycyclic aromatic hydrocarbons, polychlorinated biphenyls, polychlorinated dibenzodioxins, polychlorinated dibenzofur-ans, polychlorinated naphthalenes, terpenes, steroids, pesticides, pheromones, etc. 

Source From Separation of polycyclic aromatic hydrocarbons by capillary gas chromatography using high temperature liquid crystalline stationary phases, in Chem. Anal. ... 

Fig. 6 Chromatogram of the separation of selected hydrocarbons on monomeric liquid-crystalline stationary phases. Temperature of capillary column 50°C. Time of separation 14 min. 1) hexane 2) heptane 3) octane 4) m-xylene 5) p-xylene 6) 0-xylene 7) m-ethyltoluene 8) p-ethyltoluene 9) m-diethylbenzene 10) O-diethylbenzene and 11) p-diethylbenzene.

Witkiewicz, Z. Oszczudlowski, J. Repelewicz, M. Liquid crystalline stationary phases for high-performance liquid chromatography. Chem. Anal. (Warsaw) 2003, 48, 397-412. 

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