Thermotropic Liquid Crystalline Side Group Polymers as Stationary Phases in High Performance Liquid Chromatography. II. the Mechanism of Separation

Abstract

Liquid crystalline side group polymers support coated on silica gels have been applied as stationary phases in high performance liquid chromatography. It has been possible to show that also in liquid chromatography, separations based on the mesophase structure can be observed in analogy to gas chromatography. From results of separations in which temperature, flow rate, sample concentration and the solvent strength of the mobile phase were varied, this work derives views on the fundamental mechanisms involved. In addition, it will be shown that different mechanisms are probably involved in the separation of steroids and dinitrobenzene isomers on these stationary phases.     

Liquid Crystals: A Tool for Studies on Chirality

Nematic liquid crystals are extremely sensitive to optically active molecules, which, at very low concentration, convert them into cholesteric liquid crystals. The purpose of this report is to review recent applications of liquid crystals to chirality studies. A brief discussion of the structure and properties of liquid crystals is followed by a survey of the methods currently being employed for the determination of the pitch and handedness of cholesteric liquid crystals. Of special interest in the formation of induced-cholestric phases are the twisting powers of optically active compounds and the structural relationships between dopants and nematics. Liquid crystals can be used, inter alia, for the detection of small optical activities and for the determination of thermal racemization barriers and absolute configurations.     

The Determination of Metribuzin and Its Metabolites by High Pressure Liquid Chromatography

Abstract

A method for the determination of metribuzin and its metabolites in plant tissues has been developed using High Pressure Liquid Chromatography (HPLC). The system used involves reversed-phase chromatography on a C-18 HPLC column and a 62:38 methanol/0.05 M acetic acid mobile phase. Under these conditions, metribuzin and the three known metabolites [deaminated metribuzin (DA), deaminated diketometribuzin (DADK) and diketometribuzin (DK)] are completely resolved from each other. Detection is by UV absorbance at 254 nm.     

Electrochemically Controlled Liquid Chromatography on Conducting Polymer Stationary Phases

Abstract

Electrochemically controlled high performance liquid chromatography (EC/HPLC) using a conducting polymer stationary phase, has been investigated in the present paper. Polypyrrole coated reticulated vitreous carbon (RVC) particles have been employed as the stationary phase. Chromatographic characterization has been carried out using various molecular probes. The results indicate that chromatographic retention can be altered by the imposition of small electrical potentials on the stationary phase and that such effects can be used to improve the separation of certain compounds.     

Investigation of the chirality dependence of thermal properties in chiral-racemic mixtures of the cholesteric ester CE6

Abstract

A scanning adiabatic calorimetric technique has been used to study the thermal properties of the chiral and racemic liquid crystal CE6 and the phase diagram covering the cholesteric phase, the three blue phases and the isotropic phase. The purpose of this investigation is to study thermal properties of liquid crystals as a function of chirality, while all other parameters remain constant. Results for the temperature and the chirality dependence of the enthalpy and of the heat capacity are reported. The latent heats between the cholesteric phase and BPI and between the different blue phases change slightly as a function of the chirality. The total heat of transition at the isotropic phase boundary is independent of the chirality, but with decreasing chirality, we observe a large increase in the latent heat and, correspondingly, a decrease in the pretransitional contribution. These experimental facts are in qualitative agreement with the predictions of a Landau–de Gennes theory for blue phases.     

New Stationary Phase Prepared by Immobilization of a Copper-Amine Complex on Silica and Its Use for High Performance Liquid Chromatography

Chromatographic silica (10 μm) was chemically modified with the silylating agent: [3-(2-aminoethyl)aminopropyl]trimethoxysilane (AEAPTS). The reaction product was characterized by elemental analysis and infrared and ¹³C and ²⁹Si NMR spectra. The chemically modified silica was treated with Cu(II) in methanol medium. This cation was strongly adsorbed through complexation by the pendant ethylenediamine groups attached to the silica surface. The complex formed on the silica surface was shown to be stable in both aqueous and non-aqueous media. The aim of Cu(II) immobilization is to use this new material as a stationary phase in High Performance Liquid Chromatography (HPLC). Separations of synthetic mixtures of aromatic amines and of polyaromatic hydrocarbons were undertaken using 150×3.9 mm HPLC columns packed with the modified silica, with and without copper ions, to follow the influence of the cation on the chromatographic separation and to verify the efficiency of the new stationary phase for HPLC.     

Hydrocarbon Type Separation of Lubricating Base Oil in Multigram Quantity by Preparative HPLC

Abstract

A preparative High Pressure Liquid Chromatography (HPLC) method has been developed to separate lubricating base oil into its three major hydrocarbon fractions: saturates, aromatics, and polars. The results are directly comparable to ASTM Method D2007, hydrocarbon type analysis by gradient elution liquid chromatography. The new method employs a preparative HPLC unit with dual, radially compressed columns consisting of clay and alumina/silica gel columns. Multigram quantities of minor components (1 to 2% by wt.) of a base oil can be isolated for further study.     

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.     

Phase diagrams of cholesteric liquid crystals obtained with a generalized Landau-de Gennes theory

Abstract

Phase diagrams of chiral nematic liquid crystals are studied within the framework of a generalized Landau-Ginzburg-de Gennes theory. Using the parametrization of Grebel, Hornreich, and Shtrikman for the tensor order parameter Q, all relevant elastic terms are included for the helicoidal phase and the blue phases of chiral nematic liquid crystals up to fourth order in Q and its gradient ?Q. The influence of the additional elastic terms on the phase diagrams of the chiral nematic phases is then investigated. The theory correctly describes the variation of the pitch with temperature and the induced biaxiality of the cholesteric phase. The results resolve the discrepancies encountered by Hornreich and Shtrikman in the comparison of experiment and theory. New features in the topology of the phase diagrams of blue phases, like re-entrant phase transitions, are predicted.     

Anomalous light scattering in photonic cholesteric liquid crystals

ABSTRACT

We have carried out polarisation and angle-resolved measurements of the light scattered from photonic cholesteric liquid crystals. Both in samples doped with laser dyes and in inactive (non-doped) samples we have observed pronounced directional dependences of the scattered light, finding angular ranges where the scattering is greatly enhanced and regions where the effect is almost suppressed. Moreover, the total amount of scattered light has also been found to depend strongly on the polarisation and direction of the incident beam. All the results have been interpreted successfully in terms of a simple expression proposed for the scattering cross section, in which the density of states of the ingoing and outgoing beams plays a major role. The expression would be applicable not only to cholesteric liquid crystals but to any one-dimensional photonic material.     

A Simplified Extraction for Mexiletine from Serum and Analysis by High Performance Liquid Chromatography

Abstract

The analysis of mexiletine by High Performance Liquid Chromatography (HPLC) requires organic extraction followed by evaporation of the organic solvent. Calibration curves must be prepared using standards in drug-free serum. This procedure has been simplified by using a back extraction of the mexiletine from serum. Calibration curves can then be more quickly prepared since good agreement can be obtained using standards in hydrochloric acid (HCl) instead of having to prepare standards in serum with the time-consuming necessity of having to extract these standards.     

Responsive Photonic Liquid Marbles

Liquid marbles have potential to serve as mini-reactors for fabricating new materials, but this has been exploited little and mostly for conventional chemical reactions. Here, we uncover the unparalleled capability of liquid marbles to act as platforms for controlling the self-assembly of a bio-derived polymer, hydroxypropyl cellulose, into a cholesteric liquid crystalline phase showing structural coloration by Bragg reflection. By adjusting the cholesteric pitch via quantitative water extraction, we achieve liquid marbles that we can tailor for structural color anywhere in the visible range. Liquid marbles respond with color change that can be detected by eye, to changes in temperature, exposure to toxic chemicals and mechanical deformation. Our concept demonstrates the advantages of using liquid marbles as a miniature platform for controlling the liquid crystal self-assembly of bio-derived polymers, and their exploitation to fabricate sustainable, responsive soft photonic objects.     

Chemistry and Applications of Liquid Crystals

Approximately 5% of all organic compounds are transformed at their melting point into liquid crystals—thermodynamically stable, anisotropic liquids which in contrast to isotropic melts appear turbid and are also known as mesophases. Such melts are classed as smectic, nematic, and cholesteric liquid crystalline phases, depending upon the arrangement of the constituent molecules. The discovery of numerous potential applications during the past ten years has awakened the study of liquid crystals from its former slumber as a physical curiosity and placed it in the limelight of the scientific stage. Uses in display systems for measured values and for computer and process data, as well as for remote controlled timetables, for windows of variable light-transmission, etc., appear particularly promising. Not only black-and-white contrasts are now possible but also color production.     

Hexa-Coordinated Silicon Complexes Derived from Regioisomeric Aminotriphenylethanols and Their use as Dopants for Liquid Crystals

Abstract

Tridentate imine ligands that are obtained from the chiral, regioisomeric amino alcohols 2-amino-1,1,2-triphenyethanol and 2-amino-1,2,2-triphenylethanol serve for the formation of bis-chelated silicon complexes. Whereas the complex based on the former amino alcohol is obtained as a diastereomeric mixture, the complex that is derived from 2-amino-1,2,2-triphenylethanol forms in a completely diastereoselective manner, and its configuration is determined as (A,R,R), according to a crystal structure analysis. The new silicon complexes are found to be efficient dopants for the conversion of nematic liquid crystals into cholesteric phases.

GRAPHICAL ABSTRACT     

Liquid Crystals as Stationary Phases in Chromatography

The most correct analysis of the compositions of diverse analytes mixtures is significant for analytical studies in different fields; however, many prevalent analytes cannot be identified employing traditional partition gas chromatographic methods. Thus, the increasing requirements on analytes of isomeric compounds and the problems encountered in their separation demand a study of more diverse analytical systems which are characterised by higher selectivity. Therefore, the selectivity and polarities of various liquid crystals (rod-like, banana-shape, biforked, oxygen, sulphur, nitrogen, and metal containing molecules, Schiff-base, and polymeric dendrimers) employed as liquid crystalline stationary phases (LCSPs) have been discussed from both points of views, namely, their analytical applications and thermodynamic characteristics of infinitely diluted probes with different acceptor–donor properties. Extreme particular effort has been paid to the different interdependencies between the bound up chemical structures of liquid crystal molecules with their different acceptor–donor properties and the connected resolution capabilities in the interpretation of the probe—LCSP systems, on the basis of the \( { \ln }V_{g\left( T \right)} = f\left( {\frac{1}{T}} \right) \) and \( { \ln }\left( {\frac{{a_{1} }}{{w_{1} }}} \right)^{\infty } = f\left( {\frac{1}{T}} \right) \) dependencies, with regard to the LCSP compositions, which have been controlled by the counterbalancing of the enthalpy and entropy factors. The properties of binary systems composed of liquid crystalline poly(propyleneimine) dendrimers—rod-like molecules of liquid crystals and effects of the dendrimer structure, the chemical nature, and molecular size of the non-mesogens on the ability to dissolve in the liquid crystalline phases, have been interpreted. Practical applications of metallomesogenes and chiral stationary phases for analytical separation of different organic substances have also been taken into consideration.

     

一种评价固定相表面硅羟基残余的新方法

常规的反相色谱柱评价方法未涉及固定相表面残余硅羟基活性的评价.本文以阿米替林的不对称度作为指标,发展了一种评价固定相表面残余硅羟基的新方法,并对十种不同品牌的反相色谱柱进行评价.基于评价结果,选取其中两支色谱柱对碱性抗茵药物进行色谱分离.结果表明,发展的评价方法能够很好地反映反相固定相的硅羟基残余情况,可以为选择色谱柱分离有机碱提供有价值的信息.     

Survey and Trends in the Preparation of Chemically Bonded Silica Phases for Liquid Chromatographic Analysis

In order to increase chromatographic selectivity and to extend the analytical capability of reversed phase liquid chromatography (RP HPLC) many investigators have concentrated on the preparation of silica based column packings with chemically bonded phases (CBP). These phases have also been successfully used in sample preparation techniques, mainly in solid phase extraction (SPE). Although alkyl bonded phases (e.g., C₂, C₈, and C₁₈) are the most widely used packings in RP HPLC and SPE, various specific applications require CBPs with polar functional groups (e.g., -NH₂, -NO₂, -CN, and/or -OH). The solution of problems with separation of complicated chiral compounds was attempted by applying stationary phases with chiral selectors (e.g., cyclodextrins, Pirkle phases, crown ethers, etc.). On the other hand, packings with pseudo-membrane or liquid crystal properties have been utilized for the separation of various substances of natural origin. Porous silica is commonly used as a support in the preparation of CBPs. Its physico-chemical characteristics, such as: type and structure of siliceous matrix, porosity, type and concentration of silanol groups, as well as surface purity, strongly influence the density and structure of chemically bonded phases. Recognition of these properties is helpful in optimizing separation processes based on RP HPLC elution and/or extraction of substances with polar character.     

Liquid crystals from hydrogen-bonded amphiphiles

Liquid crystals originating from hydrogen-bonded amphiphiles will be discussed, highlighting the structural features of the amphiphilic components that lead to supramolecular systems exhibiting liquid crystalline character. Liquid crystalline phases derived from two types of hydrogen-bonded amphiphiles will be presented: (a) liquid crystalline phases originating from multihydroxylated amphiphiles; and (b) liquid crystalline phases resulting from combined hydrogen bonding and ionic interactions.     

Chiral,Thermally Irreversible and Quasi-Stealth Photochromic Dopant to Control Selective Reflection Wavelength of Cholesteric Liquid Crystal

A chiral and thermally irreversible photochromic fulgide derivative incorporating an (R)-binaphthol unit in its acid anhydride moiety was used for the photoswitching of the pitch length of cholesteric liquid crystals. Since the absorption maximum wavelengths of both thermally stable photoisomers are nearly in the UV region (quasi-stealth photochromism), it can be exposed to visible light without inducing photochromic reactions. Therefore, when the photoswitching molecule is added to a permanent cholesteric liquid crystal whose reflection light wavelength is in the visible region, the UV light-induced photochromic reaction of the photoswitching molecule changes the wavelength of the reflection light in the visible light region. We have succeeded in regulating the color of cholesteric liquid crystalline cells between red and blue upon UV light irradiation. Attempts to introduce this system in polymer dispersed cholesteric liquid crystals are also described.     

Quantitation of Long Chain Fatty Acids as the Methoxyphenacyl Esters

Abstract

In recent years High Performance Liquid Chromatography has been used for the separation of long chain fatty acids. This study establishes a procedure for the quantitation of the major fatty acids found in oral bacteria. The acids studied were C-10, C-12, C-14, C-16, C-18, C-18:1, C-20, and C-22. The samples were esterified with α-Bromo-m-methoxyacetophenone, separated by reversed phase chromatography and monitored at both 254nm and 280nm. The fatty acids have approximately the same linear absorbance range at 254nm from 100 picomoles to 50 nanomoles. While the linear absorbance range was similar, the response factors varied by more than 40% when using peak heights, compared to less than a 15% variation when using peak areas. Several A-NHI fatty acid reference mixtures were used to assure the reliability (average relative error = 3.72%) of the method. Subsequent analysis of a bacteria sample was made by both High Performance Liquid Chromatography (HPLC) and Gas Liquid Chromatography (GLC) in order to further substantiate the validity of the technique.